IEC Guide

contents

A A. contents

A1

B. general - installed power 1. methodology

B1

2. rules and statutory regulations

B3

2.1 definition of voltage ranges

B3

table B1 standard voltages between 100 V and 1000 V (IEC 38-1983) table B2 standard voltages above 1 kV and not exceeding 35 kV (IEC 38-1983)

2.2 regulations 2.3 standards 2.4 quality and safety of an electrical installation 2.5 initial testing of an installation 2.6 periodic check-testing of an installation

B3 B3 B4 B4 B5 B6 B6

table B3 frequency of check-tests commonly recommended for an electrical installation

B6

2.7 conformity (with standards and specifications) of equipment used in the installation

B7

3. motor, heating and lighting loads

B8

3.1 induction motors

B8

table B4 power and current values for typical induction motors

B9

3.2 direct-current motors

B10

table B6 progressive starters with voltage ramp table B7 progressive starters with current limitation

B10 B10

3.3 resistive-type heating appliances and incandescent lamps (conventional or halogen)

B11 table B8 current demands of resistive heating and incandescent lighting (conventional or halogen) appliances B11

3.4 fluorescent lamps and related equipment table B10 current demands and power consumption of commonly-dimensioned fluorescent lighting tubes (at 220 V/240 V - 50 Hz) table B11 current demands and power consumption of compact fluorescent lamps (at 220 V/240 V - 50 Hz)

B11 B12 B12

3.5 discharge lamps

B13

table B12 current demands of discharge lamps

B13

4. power loading of an installation

B14

4.1 installed power (kW) 4.2 installed apparent power (kVA)

B14 B15

table B13 estimation of installed apparent power

B15

4.3 estimation of actual maximum kVA demand

B16

table B14 simultaneity factors in an apartment block table B16 factor of simultaneity for distribution boards (IEC 439) table B17 factor of simultaneity according to circuit function

B16 B17 B17

4.4 example of application of factors ku and ks

B17

table B18 an example in estimating the maximum predicted loading of an installation (the factor values used are for demonstration purposes only)

B17

contents - A1

contents (continued)

A B. general - installed power (continued) 4. power loading of an installation (continued) 4.5 diversity factor 4.6 choice of transformer rating

B18 B18

table B19 IEC-standardized kVA ratings of HV/LV 3-phase distribution transformers and corresponding nominal full-load current values

B18

4.7 choice of power-supply sources

B19

C. HV/LV distribution substations 1. supply of power at high voltage

C1

1.1 power-supply characteristics of high voltage distribution networks

C1

table C1 relating nominal system voltages with corresponding rated system voltages (r.m.s. values) table C2 switchgear rated insulation levels table C3A transformers rated insulation levels in series I (based on current practice other than in the United States of America and some other countries) table C3B transformers rated insulation levels in series II (based on current practice in the United States of America and some other countries) table C4 standard short-circuit current-breaking ratings extracted from table X IEC 56

C3 C3 C4 C4

1.2 different HV service connections 1.3 some operational aspects of HV distribution networks

C11

2. consumers HV substations

C15

2.1 procedures for the establishment of a new substation

C15

3. substation protection schemes

C17

3.1 protection against electric shocks and overvoltages 3.2 electrical protection

C17

table C18 power limits of transformers with a maximum primary current not exceeding 45 A table C19 rated current (A) of HV fuses for transformer protection according to IEC 282-1 table C20 3-phase short-circuit currents of typical distribution transformers

C13

C22 C25 C26 C27

3.3 protection against thermal effects 3.4 interlocks and conditioned manœuvres

C31

4. the consumer substation with LV metering

C34

4.1 general 4.2 choice of panels

C34

table C27 standard short-circuit MVA and current ratings at different levels of nominal voltage

4.3 choice of HV switchgear panel for a transformer circuit 4.4 choice of HV/LV transformer

A2 - contents

C2

C31

C36 C37 C38 C38

table C31 categories of dielectric fluids table C32 safety measures recommended in electrical installations using dielectric liquids of classes 01, K1, K2 or K3

C42

5. a consumer substation with HV metering

C44

5.1 general 5.2 choice of panels 5.3 parallel operation of transformers

C44

6. constitution of HV/LV distribution substations

C49

6.1 different types of substation 6.2 indoor substations equipped with metal-enclosed switchgear 6.3 outdoor substations

C49

C41

C46 C48

C49 C52

A 7. appendix 1␣ : example in coordination of the characteristics of an HV switch-fuse combination protecting an HV/LV transformer

App C1-1

7.1 transfert current and take-over current

App C1-2

7.2 types of faults involved in the transfer region

App C1-3

8. appendix 2␣ : ground-surface potential gradients due to earth-fault currents

App C2-1

9. appendix 3␣ : vector diagram of ferro-resonance at 50Hz (or 60 Hz)

App C3-1

D. low-voltage service connections 1. low-voltage public distribution networks

D1

1.1 low-voltage consumers

D1

table D1 survey of electricity supplies in various countries around the world. table D2

D1 D6

1.2 LV distribution networks 1.3 the consumer-service connection 1.4 quality of supply voltage

D7

2. tariffs and metering

D14

D10 D13

E. power factor improvement and harmonic filtering 1. power factor improvement

E1

1.1 the nature of reactive energy 1.2 plant and appliances requiring reactive current 1.3 the power factor 1.4 tan ϕ 1.5 practical measurement of power factor 1.6 practical values of power factor

E1

table E5 example in the calculation of active and reactive power table E7 values of cos ϕ and tan ϕ for commonly-used plant and equipment

E2 E2 E3 E4 E4 E4 E4

2. why improve the power factor?

E5

2.1 reduction in the cost of electricity 2.2 technical/economic optimization

E5 E5

table E8 multiplying factor for cable size as a function of cos ϕ

E5

3. how to improve the power factor

E6

3.1 theoretical principles 3.2 by using what equipment? 3.3 the choice between a fixed or automatically-regulated bank of capacitors

E6

4. where to install correction capacitors

E9

4.1 global compensation 4.2 compensation by sector 4.3 individual compensation

E9

E7 E8

E9 E10

contents - A3

contents (continued)

A E. power factor improvement and harmonic filtering (continued) 5. how to decide the optimum level of compensation

E11

5.1 general method 5.2 simplified method

E11 E11

table E17 kvar to be installed per kW of load, to improve the power factor of an installation

E12

5.3 method based on the avoidance of tariff penalties 5.4 method based on reduction of declared maximum apparent power (kVA)

E13

6. compensation at the terminals of a transformer

E14

6.1 compensation to increase the available active power output

E14

table E20 active-power capability of fully-loaded transformers, when supplying loads at different values of power factor

E14

6.2 compensation of reactive energy absorbed by the transformer

E15

table E24 reactive power consumption of distribution transformers with 20 kV primary windings

E16

E13

7. compensation at the terminals of an induction motor E17 7.1 connection of a capacitor bank and protection settings

E17

table E26 reduction factor for overcurrent protection after compensation

E17

7.2 how self-excitation of an induction motor can be avoided

E18

table E28 maximum kvar of P.F. correction applicable to motor terminals without risk of self-excitation

E19

8. example of an installation before and after power-factor correction

E20

9. the effect of harmonics on the rating of a capacitor bank

E21

9.1 problems arising from power-system harmonics 9.2 possible solutions 9.3 choosing the optimum solution

E21 E21 E22

table E30 choice of solutions for limiting harmonics associated with a LV capacitor bank

E22

9.4 possible effects of power-factor-correction capacitors on the power-supply system

E23

10. implementation of capacitor banks

E24

10.1 capacitor elements 10.2 choice of protection, control devices, and connecting cables

E24 E25

11. appendix 1␣ : elementary harmonic filters

App E3-1

12. appendix 2␣ : harmonic suppression reactor for a single (power factor correction) capacitor bank

App E4-1

F. distribution within a low-voltage installation

A4 - contents

1. general

F1

1.1 the principal schemes of LV distribution 1.2 the main LV distribution board 1.3 transition from IT to TN

F1 F4 F4

A 2. essential services standby supplies

F5

2.1 continuity of electric-power supply 2.2 quality of electric-power supply

F5 F6

table F10 assumed levels of transient overvoltage possible at different points of a typical installation table F12 typical levels of impulse withstand voltage of industrial circuit breakers labelled Uimp = 8 kV table F18 compatibility levels for installation materials

F8 F8 F13

3. safety and emergency-services installations, and standby power supplies

F15

3.1 safety installations 3.2 standby reserve-power supplies 3.3 choice and characteristics of reserve-power supplies

F15 F15 F16

table F21 table showing the choice of reserve-power supply types according to application requirements and acceptable supply-interruption times

F16

3.4 choice and characteristics of different sources

F17

table F22 table of characteristics of different sources

F17

3.5 local generating sets

F18

4. earthing schemes

F19

4.1 earthing connections

F19

table F25 list of exposed-conductive-parts and extraneous-conductive-parts

F20 F21 F23 F29 F30 F31 F32

4.2 definition of standardized earthing schemes 4.3 earthing schemes characteristics 4.4.1 choice criteria 4.4.2 comparison for each criterion 4.5 choice of earthing method - implementation 4.6 installation and measurements of earth electrodes table F47 resistivity (Ω-m) for different kinds of terrain table F48 mean values of resistivity (Ω-m) for an approximate estimation of an earth-electrode resistance with respect to zero-potential earth

F33 F33

5. distribution boards

F36

5.1 types of distribution board 5.2 the technologies of functional distribution boards 5.3 standards 5.4 centralized control

F36 F37 F38 F38

6. distributors

F39

6.1 description and choice 6.2 conduits, conductors and cables

F39 F41

table F60 selection of wiring systems table F61 erection of wiring systems table F62 some examples of installation methods table F63 designation code for conduits according to the most recent IEC publications table F64 designation of conductors and cables according to CENELEC code for harmonized cables table F66 commonly used conductors and cables

F41 F41 F43 F44 F45 F46

contents - A5

contents (continued)

A F. distribution within a low-voltage installation (continued) 7. external influences

F47

7.1 classification

F47

table F67 concise list of important external influences (taken from Appendix A of IEC 364-3)

7.2 protection by enclosures: IP code

F48 F49

G. protection against electric shocks 1. general

G1

1.1 electric shock 1.2 direct and indirect contact

G1 G1

2. protection against direct contact

G2

2.1 measures of protection against direct contact 2.2 additional measure of protection against direct contact

G2 G3

3. protection against indirect contact

G4

3.1 measure of protection by automatic disconnection of the supply

G4

table G8 maximum safe duration of the assumed values of touch voltage in conditions where UL = 50 V table G9 maximum safe duration of the assumed values of touch voltage in conditions where UL = 25 V

3.2 automatic disconnection for a TT-earthed installation table G11 maximum operating times of RCCBs (IEC 1008)

3.3 automatic disconnection for a TN-earthed installation

G4 G4 G5 G6 G6

table G13 maximum disconnection times specified for TN earthing schemes (IEC 364-4-41)

G7

3.4 automatic disconnection on a second earth fault in an IT-earthed system

G8

table G18 maximum disconnection times specified for an IT-earthed installation (IEC 364-4-41)

G9

3.5 measures of protection against direct or indirect contact without circuit disconnection

G10

4. implementation of the TT system

G13

4.1 protective measures

G13

table G26 the upper limit of resistance for an installation earthing electrode which must not be exceeded, for given sensitivity levels of RCDs at UL voltage limits of 50 V and 25 V

G13

4.2 types of RCD

G14

4.3 coordination of differential protective devices

G15

5. implementation of the TN system

G18

5.1 preliminary conditions 5.2 protection against indirect contact

G18

table G42 correction factor to apply to the lengths given in tables G43 to G46 for TN systems table G43 maximum circuit lengths for different sizes of conductor and instantaneous-tripping-current settings for general-purpose circuit breakers table G44 maximum circuit lengths for different sizes of conductor and rated currents for type B circuit breakers table G45 maximum circuit lengths for different conductor sizes and for rated currents of circuit breakers of type C table G46 maximum circuit lengths for different conductor sizes and for rated currents of circuit breakers of type D or MA Merlin Gerin

5.3 high-sensitivity RCDs 5.4 protection in high fire-risk locations 5.5 when the fault-current-loop impedance is particularly high A6 - contents

G18 G20 G20 G20 G21 G21 G22 G22 G23

A 6. implementation of the IT system

G24

6.1 preliminary conditions

G24

table G53 essential functions in IT schemes

G24

6.2 protection against indirect contact

G25

table G59 correction factors, for IT-earthed systems, to apply to the circuit lengths given in tables G43 to G46

G28

6.3 high-sensitivity RCDs 6.4 in areas of high fire-risk 6.5 when the fault-current-loop impedance is particularly high

G29

7. residual current differential devices (RCDs)

G31

7.1 description 7.2 application of RCDs

G31

G29 G30

G31

table G70 electromagnetic compatibility withstand-level tests for RCDs table G72 means of reducing the ratio I∆n/lph (max.)

G32 G33

7.3 choice of characteristics of a residual-current circuit breaker (RCCB - IEC 1008)

G34

table G74 typical manufacturers coordination table for RCCBs, circuit breakers, and fuses

G34

H. the protection of circuits and the switchgear H1. the protection of circuits 1. general

H1-1

1.1 methodology and definitions

H1-1

table H1-1 logigram for the selection of cable size and protective-device rating for a given circuit

H1-1

1.2 overcurrent protection principles 1.3 practical values for a protection scheme 1.4 location of protective devices

H1-3 H1-4 H1-5

table H1-7 general rules and exceptions concerning the location of protective devices

H1-5

1.5 cables in parallel 1.6 worked example of cable calculations

H1-5 H1-6

table H1-9 calculations carried out with ECODIAL software (Merlin Gerin) table H1-10 example of short-circuit current evaluation

H1-8 H1-9

2. practical method for determining the smallest allowable cross-sectional-area of circuit conductors

H1-10

2.1 general

H1-10

table H1-11 logigram for the determination of minimum conductor size for a circuit

H1-10

2.2 determination of conductor size for unburied circuits

H1-10

table H1-12 code-letter reference, depending on type of conductor and method of installation table H1-13 factor K1 according to method of circuit installation (for further examples refer to IEC 364-5-52 table 52H) table H1-14 correction factor K2 for a group of conductors in a single layer table H1-15 correction factor K3 for ambient temperature other than 30 °C table H1-17 case of an unburied circuit: determination of the minimum cable size (c.s.a.), derived from the code letter; conductor material; insulation material and the fictitious current I'z

H1-10 H1-11 H1-11 H1-12 H1-13

contents - A7

contents (continued)

A H. the protection of circuits and the switchgear (continued) H1. the protection of circuits (continued) 2. practical method for determining the smallest allowable cross-sectional-area of circuit conductors (continued) 2.3 determination of conductor size for buried circuits table H1-19 correction factor K4 related to the method of installation table H1-20 correction factor K5 for the grouping of several circuits in one layer table H1-21 correction factor K6 for the nature of the soil table H1-22 correction factor K7 for soil temperatures different than 20 °C table H1-24 case of a buried circuit: minimum c.s.a. in terms of type of conductor; type of insulation; and value of fictitious current I'z (I'z = Iz) K

H1-14 H1-14 H1-15 H1-15 H1-15

3. determination of voltage drop

H1-17

3.1 maximum voltage-drop limit

H1-17

table H1-26 maximum voltage-drop limits

H1-17

3.2 calculation of voltage drops in steady load conditions

H1-18

table H1-28 voltage-drop formulae table H1-29

H1-18

phase-to-phase voltage drop ∆U for a circuit, in volts per ampere per km

H1-18

4. short-circuit current calculations

H1-20

4.1 short-circuit current at the secondary terminals of a HV/LV distribution transformer

H1-20

table H1-32 typical values of Usc for different kVA ratings of transformers with HV windings i 20 kV table H1-33 Isc at the LV terminals of 3-phase HV/LV transformers supplied from a HV system with a 3-phase fault level of 500 MVA, or 250 MVA

4.2 3-phase short-circuit current (Isc) at any point within a LV installation table H1-36 the impedance of the HV network referred to the LV side of the HV/LV transformer table H1-37 resistance, reactance and impedance values for typical distribution transformers with HV windings i 20 kV table H1-38 recapitulation table of impedances for different parts of a power-supply system table H1-39 example of short-circuit current calculations for a LV installation supplied at 400 V (nominal) from a 1,000 kVA HV/LV transformer

H1-20 H1-20 H1-21 H1-21 H1-22 H1-23 H1-23

4.3 Isc at the receiving end of a feeder in terms of the Isc at its sending end

H1-23

table H1-40 Isc at a point downstream, in terms of a known upstream fault-current value and the length and c.s.a. of the intervening conductors, in a 230/400 V 3-phase system

H1-24

4.4 short-circuit current supplied by an alternator or an inverter

H1-25

5. particular cases of short-circuit current

H1-26

5.1 calculation of minimum levels of short-circuit current

H1-26

table H1-49 maximum circuit lengths in metres for copper conductors (for aluminium, the lengths must be multiplied by 0.62) table H1-50 maximum length of copper-conductored circuits in metres protected by B-type circuit breakers table H1-51 maximum length of copper-conductored circuits in metres protected by C-type circuit breakers table H1-52 maximum length of copper-conductored circuits in metres protected by D-type circuit breakers table H1-53 correction factors to apply to lengths obtained from tables H1-49 to H1-52 A8 - contents

H1-14

H1-28 H1-29 H1-29 H1-29 H1-30

A 5.2 verification of the withstand capabilities of cables under short-circuit conditions table H1-54 value of the constant k2 table H1-55 maximum allowable thermal stress for cables (expressed in amperes2 x seconds x 106)

H1-31 H1-31 H1-31

6. protective earthing conductors (PE)

H1-32

6.1 connection and choice

H1-32

table H1-59 choice of protective conductors (PE)

H1-33

6.2 conductor dimensioning

H1-33

table H1-60 minimum c.s.a.'s for PE conductors and earthing conductors (to the installation earth electrode) table H1-61 k factor values for LV PE conductors, commonly used in national standards and complying with IEC 724

H1-34 H1-34

6.3 protective conductor between the HV/LV transformer and the main general distribution board (MGDB)

H1-35 table H1-63 c.s.a. of PE conductor between the HV/LV transformer and the MGDB, in terms of transformer ratings and fault-clearance times used in France H1-35

6.4 equipotential conductor

H1-35

7. the neutral conductor

H1-36

7.1 dimensioning the neutral conductor

H1-36

7.2 protection of the neutral conductor

H1-36

table H1-65 table of protection schemes for neutral conductors in different earthing systems

H1-37

H2. the switchgear 1. the basic functions of LV switchgear

H2-1

table H2-1 basic functions of LV switchgear

H2-1

1.1 electrical protection 1.2 isolation

H2-1 H2-1

table H2-2 peak value of impulse voltage according to normal service voltage of test specimen

H2-2

1.3 switchgear control

H2-2

2. the switchgear and fusegear

H2-4

2.1 elementary switching devices

H2-4

table H2-7 utilization categories of LV a.c. switches according to IEC 947-3 table H2-8 factor "n" used for peak-to-rms value (IEC 947-part 1) table H2-13 zones of fusing and non-fusing for LV types gG and gM class fuses (IEC 269-1 and 269-2-1)

H2-5 H2-5 H2-7

2.2 combined switchgear elements

H2-9

3. choice of switchgear

H2-11

3.1 tabulated functional capabilities

H2-11

table H2-19 functions fulfilled by different items of switchgear

H2-11

3.2 switchgear selection

H2-11

contents - A9

contents (continued)

A H2. the switchgear (continued) 4. circuit breakers

H2-12

table H2-20 functions performed by a circuit breaker/disconnector

H2-12

4.1 standards and descriptions 4.2 fundamental characteristics of a circuit breaker table H2-28 tripping-current ranges of overload and short-circuit protective devices for LV circuit breakers table H2-31 Icu related to power factor (cos ϕ) of fault-current circuit (IEC 947-2)

H2-12 H2-15 H2-16 H2-17

4.3 other characteristics of a circuit breaker

H2-18 table H2-34 relation between rated breaking capacity Icu and rated making capacity Icm at different power-factor values of short-circuit current, as standardized in IEC 947-2 H2-19

4.4 selection of a circuit breaker table H2-38 examples of tables for the determination of derating/uprating factors to apply to CBs with uncompensated thermal tripping units, according to temperature table H2-40 different tripping units, instantaneous or short-time delayed table H2-43 maximum values of short-circuit current to be interrupted by main and principal circuit breakers (CBM and CBP respectively), for several transformers in parallel

4.5 coordination between circuit breakers table H2-45 example of cascading possibilities on a 230/400 V or 240/415 V 3-phase installation table H2-49 summary of methods and components used in order to achieve discriminative tripping

4.6 discrimination HV/LV in a consumer's substation

H2-20 H2-21 H2-23 H2-25 H2-27 H2-28 H2-29 H2-32

J. particular supply sources and loads 1. protection of circuits supplied by an alternator

J1

1.1 an alternator on short-circuit 1.2 protection of essential services circuits supplied in emergencies from an alternator 1.3 choice of tripping units

J1

1.4 methods of approximate calculation

J6

table J1-7 procedure for the calculation of 3-phase short-circuit current table J1-8 procedure for the calculation of 1-phase to neutral short-circuit current

J4 J5

J6 J7

1.5 the protection of standby and mobile a.c. generating sets

J9

2. inverters and UPS (Uninterruptible Power Supply units)

J10

2.1 what is an inverter? 2.2 types of UPS system

J10

2.3 standards 2.4 choice of a UPS system

J11

2.5 UPS systems and their environment

J14

2.6 putting into service and technology of UPS systems 2.7 earthing schemes

J15

J10 table J2-4 examples of different possibilities and applications of inverters, in decontamination of supplies and in UPS schemes J11

A10 - contents

J12

J17

A 2.8 choice of main-supply and circuit cables, and cables for the battery connection table J2-21 voltage drop in % of 324 V d.c. for a copper-cored cable table J2-22 currents and c.s.a. of copper-cored cables feeding the rectifier, and supplying the load for UPS system Maxipac (cable lengths < 100 m) table J2-23 currents and c.s.a. of copper-cored cables feeding the rectifier, and supplying the load for UPS system EPS 2000 (cable lengths < 100 m). Battery cable data are also included table J2-24 input, output and battery currents for UPS system EPS 5000 (Merlin Gerin)

J20 J21 J21 J21 J22

2.9 choice of protection schemes

J23

2.10 complementary equipments

J24

3. protection of LV/LV transformers

J25

3.1 transformer-energizing in-rush current 3.2 protection for the supply circuit of a LV/LV transformer

J25

3.3 typical electrical characteristics of LV/LV 50 Hz transformers

J26

table J3-5 typical electrical characteristics of LV/LV 50 Hz transformers

J26

3.4 protection of transformers with characteristics as tabled in J3-5 above, using Merlin Gerin circuit breakers

J26

J25

table J3-6 protection of 3-phase LV/LV transformers with 400 V primary windings table J3-7 protection of 3-phase LV/LV transformers with 230 V primary windings table J3-8 protection of 1-phase LV/LV transformers with 400 V primary windings table J3-9

J26 J27 J27

protection of 1-phase LV/LV transformers with 230 V primary windings

J28

4. lighting circuits

J29

4.1 service continuity

J29

4.2 lamps and accessories (luminaires)

J30

table J4-1 analysis of disturbances in fluorescent-lighting circuits

J30

4.3 the circuit and its protection

J31

4.4 determination of the rated current of the circuit breaker

J31

table J4-2 protective circuit breaker ratings for incandescent lamps and resistive-type heating circuits table J4-3 maximum limit of rated current per outgoing lighting circuit, for high-pressure discharge lamps table J4-4 current ratings of circuit breakers related to the number of fluorescent luminaires to be protected

J31 J32 J32

4.5 choice of control-switching devices

J33

table J4-5 types of remote control

J33

4.6 protection of ELV lighting circuits

J34

4.7 supply sources for emergency lighting

J35

5. asynchronous motors

J36

5.1 protective and control functions required

J36

table J5-2 commonly-used types of LV motor-supply circuits

J37

5.2 standards

J38

5.3 basic protection schemes: circuit breaker / contactor / thermal relay

J38

table J5-4 utilization categories for contactors (IEC 947-4)

J39

5.4 preventive or limitative protection

J41 contents - A11

contents (continued)

A J. particular supply sources and loads (continued) 5. asynchronous motors (continued) 5.5 maximum rating of motors installed for consumers supplied at LV table J5-12 maximum permitted values of starting current for direct-on-line LV motors (230/400 V) table J5-13 maximum permitted power ratings for LV direct-on-line-starting motors

J43 J43 J43

5.6 reactive-energy compensation (power-factor correction)

J43

6. protection of direct-current installations

J44

6.1 short-circuit currents 6.2 characteristics of faults due to insulation failure, and of protective switchgear

J44 J45

table J6-4 characteristics of protective switchgear according to type of d.c. system earthing

J45

6.3 choice of protective device

J45

table J6-5 choice of d.c. circuit breakers manufactured by Merlin Gerin

J46

6.4 examples

J46

6.5 protection of persons

J47

7. Appendix␣ : Short-circuit characteristics of an alternator

App J1-1

L. domestic and similar premises and special locations

A12 - contents

1. domestic and similar premises

L1

1.1 general 1.2 distribution-board components 1.3 protection of persons 1.4 circuits

L1 L2 L4 L6

table L1-9 recommended minimum number of lighting and power points in domestic premises table L1-11 c.s.a. of conductors and current rating of the protective devices in domestic installations (the c.s.a. of aluminium conductors are shown in brackets)

L7

2. bathrooms and showers

L8

2.1 classification of zones 2.2 equipotential bonding

L8

2.3 requirements prescribed for each zone

L10

3. recommendations applicable to special installations and locations

L11

L6

L10

1. methodology

B the study of an electrical installation by means of this guide requires the reading of the entire text in the order in which the chapters are presented.

listing of power demands The study of a proposed electrical installation necessitates an adequate understanding of all governing rules and regulations. A knowledge of the operating modes of power-consuming appliances, i.e. "loads" (steady-state demand, starting conditions, non-simultaneous operation, etc.) together with the location and magnitude of each load shown on a building plan, allow a listing of power demands to be compiled. The list will include the total power of the loads installed as well as an estimation of the actual loads to be supplied, as deduced from the operating modes. From these data the power required from the supply source and (where appropriate) the number of sources necessary for an adequate supply to the installation, are readily obtained. Local information regarding tariff structures is also required to permit the best choice of connection arrangement to the power-supply network, e.g. at high voltage or low voltage.

corresponding chapter B - general - installed power

service connection This connection can be made at: c High Voltage: a consumer-type substation will then have to be studied, built and equipped. This substation may be an outdoor or indoor installation conforming to relevant standards and regulations (the low-voltage section may be studied separately if necessary). Metering at high-voltage or low-voltage is possible in this case c Low Voltage: the installation will be connected to the local power network and will (necessarily) be metered according to LV tariffs.

C - HV/LV distribution substations

D - low-voltage service connections

reactive energy The compensation of reactive energy within electrical installations normally concerns only power factor improvement, and is carried out locally, globally or as a combination of both methods.

E - power factor improvement

LV distribution The whole of the installation distribution network is studied as a complete system. The number and characteristics of standby emergency-supply sources are defined. Earth-bonding connections and neutralearthing arrangements are chosen according to local regulations, constraints related to the power-supply, and to the nature of the installation loads. The hardware components of distribution, together with distribution boards and cableways, are determined from building plans and from the location and grouping of loads. The kinds of location, and activities practised in them, can affect their level of resistance to external influences.

F - distribution within a low-voltage installation

protection against electric shock The system of earthing (TT, IT or TN) having been previously determined, it remains, in order to achieve protection of persons against the hazards of direct and indirect contact, to choose an appropriate scheme of protection.

G - protection against electric shock

general - installed power - B1

1. methodology (continued)

B circuits and switchgear Each circuit is then studied in detail. From the rated currents of the loads; the level of short-circuit current; and the type of protective device, the cross-sectional area of circuit conductors can be determined, taking into account the nature of the cableways and their influence on the current rating of conductors. Before adopting the conductor size indicated above, the following requirements must be satisfied: c the voltage drop complies with the relevant standard, c motor starting is satisfactory, c protection against electric shock is assured. The short-circuit current Isc is then determined, and the Isc thermal and electrodynamic withstand capability of the circuit is checked. These calculations may indicate that a different conductor size than that originally chosen is necessary. The performance required by the switchgear will determine its type and characteristics. The use of cascading techniques and the discriminative operation of fuses and tripping of circuit breakers are examined.

H1 - the protection of circuits

H2 - the switchgear

particular supply sources and loads Particular items of plant and equipment are studied: c specific sources such as alternators or inverters, c specific loads with special characteristics, such as induction motors, lighting circuits or LV/LV transformers, or c specific systems, such as direct-current networks.

J - particular supply sources and loads

domestic and similar premises and special locations Certain premises and locations are subject to particularly strict regulations: the most common example being domestic dwellings.

L - domestic and similar premises and special locations

Ecodial 2.2 software Ecodial 2.2 software* provides a complete conception and design package for LV installations, in accordance with IEC standards and recommendations. The following features are included: c construction of one-line diagrams, c calculation of short-circuit currents, c calculation of voltage drops, c optimization of cable sizes, c required ratings of switchgear and fusegear, c discrimination of protective devices, c recommendations for cascading schemes, c verification of the protection of persons, c comprehensive print-out of the foregoing calculated design data. * Ecodial 2.2 is a Merlin Gerin product and is available in French and English versions.

B2 - general - installed power

2. rules and statutory regulations

B Low-voltage installations are governed by a number of regulatory and advisory texts, which may be classified as follows: c statutory regulations (decrees, factory acts, etc.), c codes of practice, regulations issued by professional institutions, job specifications, c national and international standards for installations, c national and international standards for products.

2.1 definition of voltage ranges IEC voltage standards and recommendations three phase, four wire or three wire systems nominal voltage (V) 230/400(1) 277/480(2) 400/690(1) 1000

single phase, three wire systems nominal voltage (V) 120/240 -

table B1: standard voltages between 100 V and 1000 V (IEC 38-1983). 1) The nominal voltage of existing 220/380 V and 240/415 V systems shall evolve towards the recommended value of 230/400 V. The transition period should be as short as possible, and should not exceed 20 years after the issue of this IEC publication. During this period, as a first step, the electricity supply authorities of countries having 220/380 V systems should bring the voltage within the range 230/400 V +6% -10% and those of countries having 240/415 V systems should bring the voltage within the range 230/400 V +10% -6%. At the end of this transition period the tolerance of 230/400 V ±10% should have been achieved; after this the reduction of this range will be considered. All the above considerations apply also to the present 380/660 V value with respect to the recommended value 400/690 V. 2) Not to be utilized together with 230/400 V or 400/690 V.

50 Hz and 60 Hz systems series I highest voltage nominal system for equipment (kV) voltage (kV) 3.6(1) 3.3(1) 3((1) 7.2(1) 6.6(1) 6(1) 12 11 10 (17.5) (15) 24 22 20 36(3) 33(3) 40.5(3) 35(3)

60 Hz systems series II (North American practice) highest voltage nominal system for equipment (kV) voltage (kV) 4.40(1) 4.16(1) 13.2(2) 12.47(2) 13.97(2) 13.2(2) 14.52(1) 13.8(1) 26.4(2) 24.94(2) 36.5(2) 34.5(2) -

table B2: standard voltages above 1 kV and not exceeding 35 kV (IEC 38-1983). * These systems are generally three-wire systems unless otherwise indicated. The values indicated are voltages between phases. The values indicated in parentheses should be considered as non-preferred values. It is recommended that these values should not be used for new systems to be constructed in future. 1) These values should not be used for public distribution systems. 2) These systems are generally four-wire systems. 3) The unification of these values is under consideration.

general - installed power - B3

2. rules and statutory regulations (continued)

B 2.2 regulations In most countries, electrical installations shall comply with more than one set of regulations, issued by National Authorities or by recognised private bodies. It is essential to take into account these local constraints before starting the design.

2.3 standards This Guide is based on relevant IEC standards, in particular IEC 364. IEC 364 has been established by medical and engineering experts of all countries in the world comparing their experience at an international level. Currently, the safety principles of IEC 364 and 479-1 are the fundamentals of most electrical standards in the world. IEC - 38 IEC - 56 IEC - 76-2 IEC - 76-3 IEC - 129 IEC - 146 IEC - 146-4

Standard voltages High-voltage alternating-current circuit breakers Power transformer - Part 2: Temperature rise Power transformer - Part 3: Insulation levels and dielectric tests Alternating current disconnectors and earthing switches General requirements and line commutated converters General requirements and line commutated converters - Part 4: Method of specifying the performance and test requirements of uninterruptible power systems IEC - 265-1 High-voltage switches - Part 1: High-voltage switches for rated voltages above 1 kV and less than 52 kV IEC - 269-1 Low-voltage fuses - Part 1: General requirements IEC - 269-3 Low-voltage fuses - Part 3: Supplementary requirements for fuses for use by unskilled persons (fuses mainly for household and similar applications) IEC - 282-1 High-voltage fuses - Part 1: Current limiting fuses IEC - 287 Calculation of the continuous current rating of cables (100% load factor) IEC - 298 AC metal-enclosed switchgear and controlgear for rated voltages above 1kV and up to and including 52 kV IEC - 364 Electrical installations of buildings IEC - 364-3 Electrical installations of buildings - Part 3: Assessment of general characteristics IEC - 364-4-41 Electrical installations of buildings - Part 4: Protection of safety - Section 41: Protection against electrical shock IEC - 364-4-42 Electrical installations of buildings - Part 4: Protection of safety - Section 42: Protection against thermal effects IEC - 364-4-43 Electrical installations of buildings - Part 4: Protection of safety - Section 43: Protection against overcurrent IEC - 364-4-47 Electrical installations of buildings - Part 4: Application of protective measures for safety - Section 47: Measures of protection against electrical shock IEC - 364-5-51 Electrical installations of buildings - Part 5: Selection and erection of electrical equipment - Section 51: Common rules IEC - 364-5-52 Electrical installations of buildings - Part 5: Selection and erection of electrical equipment - Section 52: Wiring systems IEC - 364-5-53 Electrical installations of buildings - Part 5: Selection and erection of electrical equipment - Section 53: Switchgear and controlgear IEC - 364-6 Electrical installations of buildings - Part 6: Verification IEC - 364-7-701 Electrical installations of buildings - Part 7: Requirements for special installations or locations - Section 701: Electrical installations in bathrooms IEC - 364-7-706 Electrical installations of buildings - Part 7: Requirements for special installations or locations - Section 706: Restrictive conductive locations IEC - 364-7-710 Electrical installations of buildings - Part 7: Requirements for special installations or locations - Section 710: Installation in exhibitions, shows, stands and funfairs IEC - 420 High-voltage alternating current switch-fuse combinations IEC - 439-1 Low-voltage switchgear and controlgear assemblies - Part 1: Types-tested and partially type-tested assemblies IEC - 439-2 Low-voltage switchgear and controlgear assemblies - Part 2: Particular requirements for busbar trunking systems (busways) IEC - 439-3 Low-voltage switchgear and controlgear assemblies - Part 3: Particular requirements for low-voltage switchgear and controlgear assemblies intended to be installed in places where unskilled persons have access for their use Distribution boards IEC - 446 Identification of conductors by colours or numerals IEC - 479-1 Effects of current on human beings and livestock - Part 1: General aspects IEC - 479-2 Effects of current on human beings and livestock - Part 2: Special aspects IEC - 529 Degrees of protection provided by enclosures (IP code) IEC - 644 Specification for high-voltage fuse-links for motor circuit applications

B4 - general - installed power

B IEC - 664 IEC - 694 IEC - 724 IEC - 742 IEC - 755 IEC - 787 IEC - 831-1

Insulation coordination for equipment within low-voltage systems Common clauses for high-voltage switchgear and controlgear standards Guide to the short-circuit temperature limits of electrical cables with a rated voltage not exceeding 0.6/1.0 kV Isolation transformer and safety isolation transformer. Requirements General requirements for residual current operated protective devices Application guide for selection for fuse-links of high-voltage fuses for transformer circuit application Shunt power capacitors of the self-healing type for a.c. systems having a rated voltage up to and including 660 V. - Part 1: General - Performance, testing and rating - Safety requirements - Guide for installation and operation

2.4 quality and safety of an electrical installation Only by c the initial checking of the conformity of the electrical installation, c the verification of the conformity of electrical equipment, c and periodic checking can the permanent safety of persons and security of supply to equipment be achieved.

general - installed power - B5

2. rules and statutory regulations (continued)

B 2.5 initial testing of an installation Before a power-supply authority will connect an installation to its supply network, strict pre-commissioning electrical tests and visual inspections by the authority, or by its appointed agent, must be satisfied. These tests are made according to local (governmental and/or institutional) regulations, which may differ slightly from one country to another. The principles of all such regulations however, are common, and are based on the observance of rigorous safety rules in the design and realization of the installation. IEC 364 and related standards included in this guide are based on an international consensus for such tests, intended to cover all the safety measures and approved installation practices normally required for domestic, commercial and (the majority of) industrial buildings. Many industries however have additional regulations related to a particular product (petroleum, coal, natural gas, etc.). Such additional requirements are beyond the scope of this guide. The pre-commissioning electrical tests and visual-inspection checks for installations in buildings include, typically, all of the following: c insulation tests of all cable and wiring conductors of the fixed installation, between phases and between phases and earth, c continuity and conductivity tests of protective, equipotential and earth-bonding conductors, c resistance tests of earthing electrodes with respect to remote earth, c allowable number of socket-outlets per circuit check, c cross-sectional-area check of all conductors for adequacy at the short-circuit levels prevailing, taking account of the associated protective devices, materials and installation conditions (in air, conduit, etc.), c verification that all exposed- and extraneous metallic parts are properly earthed (where appropriate), c check of clearance distances in bathrooms, etc.

These tests and checks are basic (but not exhaustive) to the majority of installations, while numerous other tests and rules are included in the regulations to cover particular cases, for example: TN-, TT- or IT-earthed installations, installations based on class 2 insulation, SELV circuits, and special locations, etc. The aim of this guide is to draw attention to the particular features of different types of installation, and to indicate the essential rules to be observed in order to achieve a satisfactory level of quality, which will ensure safe and trouble-free performance. The methods recommended in this guide, modified if necessary to comply with any possible variation imposed by a local supply authority, are intended to satisfy all precommissioning test and inspection requirements.

2.6 periodic check-testing of an installation In many countries, all industrial and commercial-building installations, together with installations in buildings used for public gatherings, must be re-tested periodically by authorized agents. Table B3 shows the frequency of testing commonly prescribed according to the kind of installation concerned. installations which require the protection of employees

installations in buildings used for public gatherings, where protection against the risks of fire and panic are required residential

c locations at which a risk of degradation, annually fire or explosion exists c temporary installations at worksites c locations at which HV installations exist c restrictive conducting locations where mobile equipment is used other cases every 3 years according to the type of establishment and its capacity for receiving the public, the re-testing period will vary from one to three years according to local regulations

table B3: frequency of check-tests commonly recommended for an electrical installation.

B6 - general - installed power

B 2.7 conformity (with standards and specifications) of equipment used in the installation conformity of equipment with the relevant standards can be attested in several ways.

attestation of conformity The conformity of equipment with the relevant standards can be attested: c by an official conformity mark granted by the standards organization concerned, or c by a certificate of conformity issued by a laboratory, or c by a declaration of conformity from the manufacturer.

declaration of conformity In cases where the equipment in question is to be used by qualified or experienced persons, the declaration of conformity provided by the manufacturer (included in the technical documentation) together with a conformity mark on the equipment concerned, are generally recognized as a valid attestation. Where the competence of the manufacturer is in doubt, a certificate of conformity can be obtained from an independent accredited laboratory.

the standards define several methods of quality assurance which correspond to different situations rather than to different levels of quality.

mark of conformity Conformity marks are inscribed on appliances and equipment which are generally used by technically inexperienced persons (for example, domestic appliances) and for whom the standards have been established which permit the attribution, by the standardization authority, of a mark of conformity (commonly referred to as a conformity mark).

certification of Quality Assurance A laboratory for testing samples cannot certify the conformity of an entire production run: these tests are called type tests. In some tests for conformity to standards, the samples are destroyed (tests on fuses, for example). Only the manufacturer can certify that the fabricated products have, in fact, the characteristics stated. Quality assurance certification is intended to complete the initial declaration or certification of conformity. As proof that all the necessary measures have been taken for assuring the quality of production, the manufacturer obtains certification of the quality control system which monitors the fabrication of the product concerned. These certificates are issued by organizations specializing in quality control, and are based on the international standard ISO 9000, the equivalent European standard being EN 29000. These standards define three model systems of quality assurance control corresponding to different situations rather than to different levels of quality: c model 3 defines assurance of quality by inspection and checking of final products, c model 2 includes, in addition to checking of the final product, verification of the manufacturing process. This method applies, for example, to the manufacture of fuses where performance characteristics cannot be checked without destroying the fuse, c model 1 corresponds to model 2, but with the additional requirement that the quality of the design process must be rigorously scrutinized; for example, where it is not intended to fabricate and test a prototype (case of a custom-built product made to specification). general - installed power - B7

3. motor, heating and lighting loads

B an examination of the actual apparent-power demands of different loads: a necessary preliminary step in the design of a LV installation.

The examination of actual values of apparent-power required by each load enables the establishment of: c a declared power demand which determines the contract for the supply of energy, c the rating of the HV/LV transformer, where applicable (allowing for expected increases in load), c levels of load current at each distribution board.

3.1 induction motors the nominal power in kW (Pn) of a motor indicates its rated equivalent mechanical power output. The apparent power in kVA (Pa) supplied to the motor is a function of the output, the motor efficiency and the power factor. Pa = Pn η cos ϕ

current demand The full-load current Ia supplied to the motor is given by the following formulae: Pn x 1,000 3-phase motor: Ia = ex U x η x cos ϕ 1-phase motor: Ia = Pn x 1,000 U x η x cos ϕ where Ia: current demand (in amps) Pn: nominal power (in kW of active power) U: voltage between phases for 3-phase motors and voltage between the terminals for single-phase motors (in volts). A single-phase motor may be connected phase-to-neutral or phase-to-phase. η: per-unit efficiency, i.e. output kW input kW cos ϕ: power factor, i.e. kW input kVA input

motor-starting current Starting current (Id) for 3-phase induction motors, according to motor type, will be: c for direct-on-line starting of squirrel-cage motors: v Id = 4.2 to 9 In for 2-pole motors v Id = 4.2 to 7 In for motors with more than 2 poles (mean value = 6 In), where In = nominal full-load current of the motor, c for wound-rotor motors (with slip-rings), and for D.C. motors:

it is generally advantageous for technical and financial reasons to reduce the current supplied to induction motors. This can be achieved by using capacitors without affecting the power output of the motors.

Id depends on the value of starting resistances in the rotor circuits: Id = 1.5 to 3 In (mean value = 2.5 In). c for induction motors controlled by speedchanging variable-frequency devices (for example: Altivar Telemecanique), assume that the control device has the effect of increasing the power (kW) supplied to the circuit motor (i.e. device plus) by 10%.

compensation of reactive-power (kvar) supplied to induction motors The application of this principle to the operation of induction motors is generally referred to as "power-factor improvement" or "power-factor correction". As discussed in chapter E, the apparentpower (kVA) supplied to an induction motor can be significantly reduced by the use of shunt-connected capacitors. Reduction of input kVA means a corresponding reduction of input current (since the voltage remains constant). Compensation of reactive-power is particularly advised for motors that operate for long periods at reduced power.

As noted above cos ϕ = kW input so that a kVA input reduction in kVA input will increase (i.e. improve) the value of cos ϕ. The current supplied to the motor, after power-factor correction, is given by: Ia x cos ϕ cos ϕ' where cos ϕ is the power factor before compensation and cos ϕ' is the power factor after compensation, Ia being the original current.

table of typical values Table B4 shows, as a function of the rated nominal power of motors, the current supplied to them at different voltage levels under normal uncompensated conditions, and the same motors under the same conditions, but compensated to operate at a power factor of 0.93 (tan ϕ = 0.4). These values are averages and will differ to some extent according to the type of motor and the manufacturer concerned. B8 - general - installed power

Note: the rated voltages of certain loads listed in table B4 are still based on 220/380 V. The international standard is now (since 1983) 230/400 V. To convert the current values indicated for a given motor rating in the 220 V and 380 V columns to the currents taken by 230 V and 400 V motors of the same rating, multiply by a factor of 0.95.

B 3.1 induction motors (continued) nominal power Pn kW HP 0.37 0.5 0.55 0.75 0.75 1 1.1 1.5 1.5 2 2.2 3 3 4 3.7 5 4 5.5 5.5 7.5 7.5 10 9 12 10 13.5 11 15 15 20 18.5 25 22 30 25 35 30 40 33 45 37 50 40 54 45 60 51 70 55 75 59 80 63 85 75 100 80 110 90 125 100 136 110 150 129 175 132 180 140 190 147 200 150 205 160 220 180 245 185 250 200 270 220 300 250 340 257 350 280 380 295 400 300 410 315 430 335 450 355 480 375 500 400 545 425 580 445 600 450 610 475 645 500 680 530 720 560 760 600 810 630 855 670 910 710 965 750 1020 800 1090 900 1220 1100 1500

η

% 64 68 72 75 78 79 81 82 82 84 85 86 86 87 88 89 89 89 89 90 90 91 91 91 92 92 92 92 92 92 92 93 93 94 94 94 94 94 94 94 94 94 94 94 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95

without compensation cos ϕ Pa current at different voltages at Pn 1-PH 3-PH 220 V 220 V 380 V 440 V kVA A A A A 0.73 0.79 3.6 1.8 1.03 0.99 0.75 1.1 4.7 2.75 1.6 1.36 0.75 1.4 6 3.5 2 1.68 0.79 1.9 8.5 4.4 2.6 2.37 0.80 2.4 12 6.1 3.5 3.06 0.80 3.5 16 8.7 5 4.42 0.80 4.6 21 11.5 6.6 5.77 0.80 5.6 25 13.5 7.7 7.1 0.80 6.1 26 14.5 8.5 7.9 0.83 7.9 35 20 11.5 10.4 0.83 10.6 47 27 15.5 13.7 0.85 12.3 32 18.5 16.9 0.85 13.7 35 20 17.9 0.86 14.7 39 22 20.1 0.86 19.8 52 30 26.5 0.86 24.2 64 37 32.8 0.86 28.7 75 44 39 0.86 33 85 52 45.3 0.86 39 103 60 51.5 0.86 43 113 68 58 0.86 48 126 72 64 0.86 51 134 79 67 0.86 57 150 85 76 0.86 65 170 98 83 0.86 70 182 105 90 0.87 74 195 112 97 0.87 79 203 117 109 0.87 94 240 138 125 0.87 100 260 147 131 0.87 112 295 170 146 0.87 125 325 188 162 0.87 136 356 205 178 0.87 159 420 242 209 0.87 161 425 245 215 0.87 171 450 260 227 0.87 180 472 273 236 0.87 183 483 280 246 0.87 196 520 300 256 0.87 220 578 333 289 0.87 226 595 342 295 0.88 242 626 370 321 0.88 266 700 408 353 0.88 302 800 460 401 0.88 311 826 475 412 0.88 335 900 510 450 0.88 353 948 546 473 0.88 359 980 565 481 0.88 377 990 584 505 0.88 401 1100 620 518 0.88 425 1150 636 549 0.88 449 1180 670 575 0.88 478 1250 710 611 0.88 508 1330 760 650 0.88 532 1400 790 680 0.88 538 1410 800 690 0.88 568 1490 850 730 0.88 598 1570 900 780 0.88 634 1660 950 825 0.88 670 1760 1000 870 0.88 718 1880 1090 920 0.88 754 1980 1100 965 0.88 801 2100 1200 1020 0.88 849 1260 1075 0.88 897 1350 1160 0.88 957 1450 1250 0.88 1076 1610 1390 0.88 1316 1980 1700

500 V A 0.91 1.21 1.5 2 2.6 3.8 5 5.9 6.5 9 12 13.9 15 18.4 23 28.5 33 39.4 45 50 55 60 65 75 80 85 89 105 112 129 143 156 184 187 200 207 210 220 254 263 281 310 360 365 400 416 420 445 472 500 527 540 574 595 608 645 680 720 760 830 850 910 960 1020 1100 1220 1500

660 V A 0.6 0.9 1.1 1.5 2 2.8 3.8 4.4 4.9 6.6 8.9 10.6 11.5 14 17.3 21.3 25.4 30.3 34.6 39 42 44 49 57 61 66 69 82 86 98 107 118 135 140 145 152 159 170 190 200 215 235 274 280 305 320 325 337 365 370 395 410 445 455 460 485 515 545 575 630 645 690 725 770 830 925 1140

with compensation cos ϕ capa- Pa at Pn citor rating kvar kVA 0.93 0.31 0.62 0.93 0.39 0.87 0.93 0.48 1.1 0.93 0.53 1.6 0.93 0.67 2.1 0.93 0.99 3 0.93 1.31 4 0.93 1.59 4.8 0.93 1.74 5.2 0.93 1.80 7 0.93 2.44 9.5 0.93 2.4 11.3 0.93 2.6 12.5 0.93 2.50 13.6 0.93 3.37 18.3 0.93 4.12 22.4 0.93 4.89 26.6 0.93 5.57 30 0.93 6.68 36 0.93 7.25 39 0.93 8.12 44 0.93 8.72 47 0.93 9.71 53 0.93 11.10 60 0.93 11.89 64 0.93 10.98 69 0.93 11.66 74 0.93 13.89 88 0.93 14.92 93 0.93 16.80 105 0.93 18.69 117 0.93 20.24 127 0.93 23.84 149 0.93 24 151 0.93 25.55 160 0.93 26.75 168 0.93 27.26 172 0.93 29.15 183 0.93 32.76 206 0.93 33.79 212 0.93 30.78 229 0.93 33.81 252 0.93 38.44 286 0.93 39.45 294 0.93 42.63 317 0.93 44.80 334 0.93 45.66 339 0.93 47.98 356 0.93 51 379 0.93 54 402 0.93 57.1 424 0.93 60.84 453 0.93 64.60 481 0.93 67.63 504 0.93 68.50 509 0.93 70.40 538 0.93 72.26 566 0.93 80.64 600 0.93 85.12 634 0.93 91.33 679 0.93 95.81 713 0.93 101.88 758 0.93 107.95 804 0.93 114 849 0.93 121.68 905 0.93 136.86 1019 0.93 167.35 1245

current at different voltages 1-PH 3-PH 220 V 220 V 380 V 440 V A A A A 2.8 1.4 0.8 0.77 3.8 2.2 1.3 1.1 4.8 2.8 1.6 1.3 7.2 3.7 2.2 2 10.3 5.2 3 2.6 13.7 7.5 4.3 3.8 18 9.9 5.7 5 22 11.6 6.6 6.1 22 12.5 7.3 6.8 31 17.8 10.3 9.3 42 24 13.8 12.2 29 16.9 15.4 32 18 16.4 36 20 19 48 28 25 59 34 30 69 41 36 79 48 42 95 55 48 104 63 54 117 67 59 124 73 62 139 79 70 157 91 77 168 97 83 182 105 91 190 109 102 225 129 117 243 138 123 276 159 137 304 176 152 333 192 167 393 226 196 398 229 201 421 243 212 442 255 221 452 262 230 486 281 239 541 312 270 557 320 276 592 350 304 662 386 334 757 435 379 782 449 390 852 483 426 897 517 448 927 535 455 937 553 478 1041 587 490 1088 602 519 1117 634 544 1183 672 578 1258 719 615 1325 748 643 1334 757 653 1410 804 691 1486 852 738 1571 899 781 1665 946 823 1779 1031 871 1874 1041 913 1987 1135 965 1192 1017 1277 1098 1372 1183 1523 1315 1874 1609

500 V A 0.71 1 1.2 1.7 2.2 3.3 4.3 5.1 5.6 8 10.7 12.7 13.7 17 21 26 31 36 42 46 51 55 60 69 74 80 83 98 105 121 134 146 172 175 187 194 196 206 238 246 266 293 341 345 378 394 397 421 447 473 499 511 543 563 575 610 643 681 719 785 804 861 908 965 1041 1154 1419

660 V A 0.47 0.72 0.88 1.3 1.7 2.4 3.3 3.8 4.2 5.9 7.9 9.7 10.5 13 16 20 23 28 32 36 39 41 45 53 56 62 65 77 80 92 100 110 126 131 136 142 149 159 178 187 203 222 259 265 289 303 306 319 336 350 374 388 420 431 435 459 487 516 544 596 610 653 686 729 785 875 1079

table B4: power and current values for typical induction motors. Reminder: some columns refer to 220 and 380 V motors. The international (IEC 38) standard of 230/400 V has been in force since 1983. The conversion factor for current values

for 230 V and 400 V motors is 0.95, as noted on the previous page.

general - installed power - B9

3. motor, heating and lighting loads (continued)

B 3.2. direct-current motors D.C. motors are mainly used for specific applications which require very high torques and/or variable speed control (for example machine tools and crushers, etc.). Power to these motors is provided via speedcontrol converters, fed from 230/400 V 3-phase a.c. sources; for example, Rectivar 4 (Telemecanique). The operating principle of the converter does not allow heavy overloading. The speed controller, the supply line and the protection are therefore based on the duty cycle of the motor (e.g. frequent starting-current peaks) rather than on the steady-state full-load current. For powers i 40 kW, this solution is progressively replaced with a speedchanging variable-frequency device and an asynchronous motor. It is still used for gradual starters and/or retarders. Im

M V power-supply network

In

fig. B5: diagram of a low-power speed controller. motor maximum power 220 V 380 V 415 V kW kW kW 1.5 3 3.3 4 5.5 6 5.5 7.5 8 11 18.5 20 18.5 30 33 22 37 40 55 60 -

440 V (60 Hz) kW 3.5 6.5 8.5 21.5 35 42 63

motor In A 7 7 12 12 16 16 37 37 60 60 72 72 105 105

GRADIVAR Ith A 10 10 20 20 30 30 60 60 100 100 130 130 200 200

catalogue number weight kg VR2-SA2121 VR2-SA2123 VR2-SA2171 VR2-SA2173 VR2-SA2211 VR2-SA2213 VR2-SA2281 VR2-SA2283 VR2-SA2361 VR2-SA2363 VR2-SA2401 VR2-SA2403 VR2-SA2441 VR2-SA2443

1.95 1.95 3.10 3.10 4.90 4.90 5.30 5.30 5.30 5.30 5.40 5.40 10.00 10.00

table B6: progressive starters with voltage ramp. motor maximum power 220 V 380 V 415 V kW kW kW 4 5.5 6 5.5 7.5 8 11 18.5 20 18.5 30 33 22 37 40 55 60 75 80 132 140 -

440 V (60 Hz) kW 6.5 8.5 21.5 35 42 63 90 147

motor In A 12 12 16 16 37 37 60 60 72 72 105 105 140 140 245 245

GRADIVAR Ith A 20 20 30 30 60 60 100 100 130 130 200 200 350 350 530 530

table B7: progressive starters with current limitation. B10 - general - installed power

catalogue number weight kg VR2-SA3171 VR2-SA3173 VR2-SA3211 VR2-SA3213 VR2-SA3281 VR2-SA3283 VR2-SA3361 VR2-SA3363 VR2-SA3401 VR2-SA3403 VR2-SA3441 VR2-SA3443 VR2-SA3481 VR2-SA3483 VR2-SA3521 VR2-SA3523

3.30 3.30 5.10 5.10 5.50 5.50 5.50 5.50 5.60 5.60 11.00 11.00 45.00 45.00 45.00 45.00

B 3.3. resistive-type heating appliances and incandescent lamps (conventional or halogen) the power consumed by a heating appliance or an incandescent lamp is equal to the nominal power Pn quoted by the manufacturer (i.e. cos ø = 1). the currents are given by: c 3-phase case: Ia = Pn* ex U c 1-phase case: Ia = Pn* U where U is the voltage between the terminals of the equipment.

The power consumed by a heating appliance or an incandescent lamp is equal to the nominal power Pn quoted by the manufacturer (i.e. cos ø = 1). The currents are given by: c 3-phase case: Ia = Pn* ex U c 1-phase case: Ia = Pn* U where U is the voltage between the terminals of the equipment. nominal power kW 0.1 0.2 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 6 7 8 9 10

For an incandescent lamp, the use of halogen gas allows a more concentrated light source. The light output is superior and the life of the lamp is doubled. Note: at the instant of switching on, the cold filament gives rise to a very brief but intense peak of current. * Ia in amps; U in volts. Pn is in watts. If Pn is in kW, then multiply the equation by 1,000.

current demand 1-phase 1-phase 127 V 230 V 0.79 0.43 1.58 0.87 3.94 2.17 7.9 4.35 11.8 6.52 15.8 8.70 19.7 10.9 23.6 13 27.6 15.2 31.5 17.4 35.4 19.6 39.4 21.7 47.2 26.1 55.1 30.4 63 34.8 71 39.1 79 43.5

3-phase 230 V 0.25 0.50 1.26 2.51 3.77 5.02 6.28 7.53 8.72 10 11.3 12.6 15.1 17.6 20.1 22.6 25.1

3-phase 400 V 0.14 0.29 0.72 1.44 2.17 2.89 3.61 4.33 5.05 5.77 6.5 7.22 8.66 10.1 11.5 13 14.4

table B8: current demands of resistive heating and incandescent lighting (conventional or halogen) appliances.

3.4. fluorescent lamps and related equipment the power in watts indicated on the tube of a fluorescent lamp does not include the power dissipated in the ballast. the current is given by: Pballast + Pn Ia = U x cos ø If no power-loss value is indicated for the ballast, a figure of 25% of Pn may be used.

standard tubular fluorescent lamps The power Pn (watts) indicated on the tube of a fluorescent lamp does not include the power dissipated in the ballast. The current taken by the complete circuit is given by: Ia = Pballast + Pn U x cos ø where U = the voltage applied to the lamp, complete with its related equipment. with (unless otherwise indicated): c cos ø = 0.6 with no power factor (PF) correction* capacitor, c cos ø = 0.86 with PF correction* (single or twin tubes), c cos ø = 0.96 for electronic ballast. If no power-loss value is indicated for the ballast, a figure of 25% of Pn may be used. Table B8 gives these values for different arrangements of ballast. * "Power-factor correction" is often referred to as "compensation" in discharge-lighting-tube terminology.

general - installed power - B11

3. motor, heating and lighting loads (continued)

B 3.4. fluorescent lamps and related equipment (continued) arrangement of lamps, starters and ballasts single tube with starter

tube power (W) (1) 18 36 58 single tube without 20 starter (2) with 40 external starting strip 65 twin tubes with starter 2 x 18 2 x 36 2 x 58 twin tubes without starter 2 x 40 single tube with 32 high frequency ballast 50 cos ø = 0.96 twin tubes with high2 x 32 frequency ballast 2 x 50 cos ø = 0.96

power consumed (W) 27 45 69 33 54 81 55 90 138 108 36 56

current (A) at 220V/240 V PF not PF electronic corrected corrected ballast 0.37 0.19 0.43 0.24 0.67 0.37 0.41 0.21 0.45 0.26 0.80 0.41 0.27 0.46 0.72 0.49 0.16 0.25

72 112

0.33 0.50

tube length (cm) 60 120 150 60 120 150 60 120 150 120 120 150 120 150

(1) Power in watts marked on tube. (2) Used exclusively during maintenance operations.

table B10: current demands and power consumption of commonly-dimensioned fluorescent lighting tubes (at 220 V/240 V - 50 Hz).

compact fluorescent tubes Compact fluorescent tubes have the same characteristics of economy and long life as classical tubes. They are commonly used in public places which are permanently illuminated (for example: corridors, hallways, bars, etc.) and can be mounted in situations otherwise illuminated by incandescent lamps. type of lamp

lamp power

globe lamps with integral ballast cos ø = 0.5 (1)

9 13 18 25 9 11 15 20 5 7 9 11 10 13 18 26

electronic lamps cos ø = 0.95 (1)

lamps with starter only incorporated (no ballast)

type single "U" form cos ø ≈ 0.35 type double "U" form cos ø ≈ 0.45

power consumed (W) 9 13 18 25 9 11 15 20 10 11 13 15 15 18 23 31

current at 220/240 V (A) 0.090 0.115 0.160 0.205 0.070 0.090 0.135 0.155 0.185 0.175 0.170 0.155 0.190 0.165 0.220 0.315

(1) Cos ø is approximately 0.95 (the zero values of V and I are almost in phase) but the power factor is 0.5 due to the impulsive form of the current, the peak of which occurs "late" in each half cycle.

table B11: current demands and power consumption of compact fluorescent lamps (at 220 V/240 V - 50 Hz).

B12 - general - installed power

B 3.5. discharge lamps the power in watts indicated on the tube of a discharge lamp does not include the power dissipated in the ballast.

These lamps depend on the luminous electrical discharge through a gas or vapour of a metallic compound, which is contained in a hermetically-sealed transparent envelope at a pre-determined pressure. These lamps have a long start-up time, during which the current Ia is greater than the nominal current In. Power and current demands are given for different types of lamp in table B12 (typical average values which may differ slightly from one manufacturer to another). The power in watts indicated on the tube of a discharge lamp does not include the power dissipated in the ballast.

table B12 gives the current taken by a complete unit, including all associated ancillary equipment.

type of power current In(A) starting lamp demand PF not PF Ia/In period (W) at corrected corrected (W) 230V 400V 230V 400V 230V 400V (mins) high-pressure sodium vapour lamps 50 60 0.76 0.3 1.4 4 to 6 to 1.6 70 80 1 0.45 100 115 1.2 0.65 150 168 1.8 0.85 250 274 3 1.4 400 431 4.4 2.2 1000 1055 10.45 4.9 low-pressure sodium vapour lamps standard lamp 18 26.5 0.14 1.1 7 to 15 to 1.3 35 43.5 0.62 0.24 55 72 0.34 90 112 0.84 0.50 135 159 0.73 180 216 0.98 economy lamps 26 34.5 0.45 0.17 1.1 7 to 15 to 1.3 36 46.5 0.22 66 80.5 0.39 91 105.5 0.49 131 154 0.69 mercury vapour + metal halide (also called metaliodide) 70 80.5 1 0.40 1.7 3 to 5 150 172 1.80 0.88 250 276 2.10 1.35 400 425 3.40 2.15 1000 1046 8.25 5.30 2000 2092 2052 16.50 8.60 10.50 6 mercury vapour + fluorescent substance (fluorescent bulb) 50 57 0.6 0.30 1.7 3 to 6 80 90 0.8 0.45 to 2 125 141 1.15 0.70 250 268 2.15 1.35 400 421 3.25 2.15 700 731 5.4 3.85 1000 1046 8.25 5.30 2000 2140 2080 15 11 6.1

luminous efficiency lumens (per watt)

average utilization life of lamp (h)

80 to 120

9000

- lighting of large halls - outdoor spaces - public lighting

100 to 200 8000 to - lighting of 12000 autoroutes - security lighting, station platform, stockage areas

100 to 200 8000 to - new types 12000 more efficient same utilization 70 to 90

6000 6000 6000 6000 6000 2000

- lighting of very large areas by projectors (for example:sports stadiums, etc)

40 to 60

8000 to - workshops 12000 with very high ceilings (halls, hangars) - outdoor lighting - low light output (1)

(1) replaced by sodium vapour lamps. Note: these lamps are sensitive to voltage dips. They extinguish if the voltage falls to less than 50% of their nominal voltage, and will not re-ignite before cooling for approximately 4 minutes. Note: Sodium vapour low-pressure lamps have a light-output efficiency which is superior to that of all other sources. However, use of these lamps is restricted by the fact that the yellow-orange colour emitted makes colour recognition practically impossible.

table B12: current demands of discharge lamps.

general - installed power - B13

4. power* loading of an installation

B In order to design an installation, the actual maximum load demand likely to be imposed on the power-supply system must be assessed. To base the design simply on the arithmetic sum of all the loads existing in the installation would be extravagantly uneconomical, and bad engineering practice. The aim of this chapter is to show how all existing and projected loads can be assigned various factors to account for diversity (nonsimultaneous operation of all appliances of a given group) and utilization (e.g. an electric motor is not generally operated at its full-load capability, etc.). The values given are based on experience and on records taken from actual installations. In addition to providing basic installation-design data on individual circuits, the results will provide a global value

4.1 installed power (kW) the installed power is the sum of the nominal powers of all powerconsuming devices in the installation. This is not the power to be actually supplied in practice.

B14 - general - installed power

Most electrical appliances and equipments are marked to indicate their nominal power rating (Pn). The installed power is the sum of the nominal powers of all power-consuming devices in the installation. This is not the power to be actually supplied in practice. This is the case for electric motors, where the power rating refers to the output power at its driving shaft. The input power consumption will evidently be greater (See 3.1). Fluorescent and discharge lamps associated with stabilizing ballasts, are other cases in which the nominal power indicated on the lamp is less than the power consumed by the lamp and its ballast (See 3.4). Methods of assessing the actual power consumption of motors and lighting appliances are given in Section 3 of this Chapter. The power demand (kW) is necessary to choose the rated power of a generating set or battery, and where the requirements of a prime mover have to be considered. For a power supply from a LV public-supply network, or through a HV/LV transformer, the significant quantity is the apparent power in kVA.

for the installation, from which the requirements of a supply system (distribution network, HV/LV transformer, or generating set) can be specified.

*power: the word "power" in the title has been used in a general sense, covering active power (kW) apparent power (kVA) and reactive power (kvar). Where the word power is used without further qualification in the rest of the text, it means active power (kW). The magnitude of the load is adequately specified by two quantities, viz: c power, c apparent power. power The ratio = power factor apparent power

B 4.2 installed apparent power (kVA) the installed apparent power is commonly assumed to be the arithmetical sum of the kVA of individual loads. The maximum estimated kVA to be supplied however is not equal to the total installed kVA.

The installed apparent power is commonly assumed to be the arithmetical sum of the kVA of individual loads. The maximum estimated kVA to be supplied however is not equal to the total installed kVA. The apparent-power demand of a load (which might be a single appliance) is obtained from its nominal power rating (corrected if necessary, as noted above for motors, etc.) and the application of the following coefficients: output kW η = the per-unit efficiency = input kW kW cos ø = the power factor = kVA The apparent-power kVA demand of the load Pn Pa = η x cos ø

From this value, the full-load current Ia (amps)* taken by the load will be: Pa 103 for single phase-to-neutral c Ia = V connected load Pa 103 c Ia = for three-phase balanced load ex U where: V = phase-to-neutral voltage (volts) U = phase-to-phase voltage (volts) It may be noted that, strictly speaking, the total kVA of apparent power is not the arithmetical sum of the calculated kVA ratings of individual loads (unless all loads are at the same power factor). It is common practice however, to make a simple arithmetical summation, the result of which will give a kVA value that exceeds the true value by an acceptable "design margin". * For greater precision, account must be taken of the factor of maximum utilization as explained below in 4-3.

When some or all of the load characteristics are not known, the values shown in table B13 may be used to give a very approximate estimate of VA demands (individual loads are

generally too small to be expressed in kVA or kW). The estimates for lighting loads are based on floor areas of 500 sq-metres.

fluorescent lighting (corrected to cos ø = 0.86) type of application estimated (VA/m2) fluorescent tube with industrial reflector (1) roads and highways 7 stockage areas, intermittent work heavy-duty works: fabrication and 14 assembly of very large work pieces day-to-day work: 24 office work fine work: 41 drawing offices high-precision assembly workshops power circuits type of application estimated (VA/m2) pumping station compressed air 3 to 6 ventilation of premises 23 electrical convection heaters: private houses 115 to 146 flats and apartments 90 offices 25 dispatching workshop 50 assembly workshop 70 machine shop 300 painting workshop 350 heat-treatment plant 700

average lighting level (lux = Im/m2) 150 300 500 800

(1) example: 65 W tube (ballast not included), flux 5,100 lumens (lm), luminous efficiency of the tube = 78.5 lm/W.

table B13: estimation of installed apparent power.

general - installed power - B15

4. power* loading of an installation (continued)

B 4.3 estimation of actual maximum kVA demand all individual loads are not necessarily operating at full rated nominal power nor necessarily at the same time. Factors ku and ks allow the determination of the maximum power and apparent-power demands actually required to dimension the installation.

All individual loads are not necessarily operating at full rated nominal power nor necessarily at the same time. Factors ku and ks allow the determination of the maximum power and apparent-power demands actually required to dimension the installation.

factor of maximum utilization (ku) In normal operating conditions the power consumption of a load is sometimes less than that indicated as its nominal power rating, a fairly common occurrence that justifies the application of an utilization factor (ku) in the estimation of realistic values. This factor must be applied to each individual load, with particular attention to electric motors, which are very rarely operated at full load.

In an industrial installation this factor may be estimated on an average at 0.75 for motors. For incandescent-lighting loads, the factor always equals 1. For socket-outlet circuits, the factors depend entirely on the type of appliances being supplied from the sockets concerned.

factor of simultaneity (ks) It is a matter of common experience that the simultaneous operation of all installed loads of a given installation never occurs in practice, i.e. there is always some degree of diversity and this fact is taken into account for estimating purposes by the use of a simultaneity factor (ks). The factor ks is applied to each group of loads (e.g. being supplied from a distribution or sub-distribution board). The determination of these factors is the responsibility of the designer, since it requires a detailed knowledge of the installation and the conditions in which the individual circuits are to be exploited. For this reason, it is not possible to give precise values for general application. Factor of simultaneity for an apartment block Some typical values for this case are given in table B14, and are applicable to domestic consumers supplied at 230/400 V (3-phase 4-wires). In the case of consumers using electrical heat-storage units for space heating, a factor of 0.8 is recommended, regardless of the number of consumers. Example: 5 storeys apartment building with 25 consumers, each having 6 kVA of installed load. The total installed load for the building = 36 + 24 + 30 + 36 + 24 = 150 kVA The apparent-power supply required for the building = 150 x 0.46 = 69 kVA From table B 14, it is possible to determine the magnitude of currents in different sections of the common main feeder supplying all floors. For vertical rising mains fed at ground level, the cross-sectional area of the conductors can evidently be progressively reduced from the lower floors towards the upper floors. These changes of conductor size are conventionally spaced by at least 3-floor intervals. In the example, the current entering the rising main at ground level is 150 x 0.46 x 103 = 100 A 400 x e The current entering the third floor is: (36+24) x 0.63 x 103 = 55 A 400 x e

B16 - general - installed power

number of downstream consumers 2 to 4 5 to 9 10 to 14 15 to 19 20 to 24 25 to 29 30 to 34 35 to 39 40 to 49 50 and more

factor of simultaneity (ks) 1 0.78 0.63 0.53 0.49 0.46 0.44 0.42 0.41 0.40

table B14: simultaneity factors in an apartment block.

4th floor

6 consumers 36 kVA

3rd floor

4 consumers 24 kVA

2nd floor

5 consumers 30 kVA

1st floor

6 consumers 36 kVA

ground floor

4 consumers 24 kVA

0.78

0.63

0.53

0.49

0.46

fig. B15: application of the factor of simultaneity (ks) to an apartment block of 5 storeys.

B Factor of simultaneity for distribution boards Table B16 shows hypothetical values of ks for a distribution board supplying a number of circuits for which there is no indication of the manner in which the total load divides between them. If the circuits are mainly for lighting loads, it is prudent to adopt ks values close to unity. number of circuits assemblies entirely tested 2 and 3 4 and 5 6 to 9 10 and more assemblies partially tested in every case choose

factor of simultaneity (ks) 0.9 0.8 0.7 0.6 1.0

Factor of simultaneity according to circuit function ks factors which may be used for circuits supplying commonly-occurring loads, are shown in table B17. circuit function lighting heating and air conditioning socket-outlets lifts and catering hoists (2) - for the most powerful motor - for the second most powerful motor - for all other motors

factor of simultaneity (ks) 1 1 0.1 to 0.2 (1)

1 0.75 0.60

(1) In certain cases, notably in industrial installations, this factor can be higher. (2) The current to take into consideration is equal to the nominal current of the motor, increased by a third of its starting current.

table B16: factor of simultaneity for distribution boards (IEC 439).

table B17: factor of simultaneity according to circuit function.

4.4 example of application of factors ku and ks an example in the estimation of actual maximum kVA demands at all levels of an installation, from each load position to the point of supply.

In this example, the total installed apparent power is 126.6 kVA, which corresponds to an actual (estimated) maximum value at the LV terminals of the HV/LV transformer of 65 kVA only. Note: in order to select cable sizes for the distribution circuits of an installation, the current I (in amps) through a circuit is determined from the equation

kVA x 103 Ue where kVA is the actual maximum 3-phase apparent-power value shown on the diagram for the circuit concerned, and U is the phaseto-phase voltage (in volts). I=

level 1 utilization

workshop A

lathe

pedestaldrill

apparentpower (Pa) kVA

utilization factor max.

apparentsimultaneity power demand factor max. kVA

n°1

5

0.8

4

n°2

5

0.8

4

n°3

5

0,8

4

workshop C

apparentsimultaneity power demand factor kVA

0.75

power circuit

14.4

5

0.8

4

n°1

2

0.8

1.6

2

0.8

1.6

18

1

18

0.2

3

1

3

1

3

0.8

12

1

12 socketoutlets 4,3 lighting circuit

compressor 15 3 socket- 10/16 A 10.6 outlets 1 10 fluorescent lamps

1

10.6

0.4

1

1

1

socketoutlets

apparentpower demand kVA

1

1

2.5

2,5

1

2.5

n°1

15

1

15

n°2

15

1

15

1

18

0.28

5

1

2

1

2

distribution box

1

workshop A distribution board

0,9

18.9

circuit

power circuit

2,5

5 socket10/16 A 18 outlets 20 fluorescent 2 lamps

apparentsimultaneity power demand factor kVA

3,6 lighting

ventilation n°1 fan n°2 oven

level 3

distribution box

n°4

n°2 5 socket10/16 A outlets 30 fluorescent lamps workshop B

level 2

35

power circuit

workshop B distribution board

main general distribution board MGDB

LV/HV

15.6

0.9

65

0.9

workshop C distribution board

0.9

37.8

socketoutlets lighting circuit

table B18: an example in estimating the maximum predicted loading of an installation (the factor values used are for demonstration purposes only). general - installed power - B17

4. power loading of an installation (continued)

B 4.5 diversity factor The term DIVERSITY FACTOR, as defined in IEC standards, is identical to the factor of simultaneity (ks) used in this guide, as described in 4.3. In some English-speaking countries however (at the time of writing) DIVERSITY FACTOR is the inverse of ks i.e. it is always u 1.

4.6 choice of transformer rating When an installation is to be supplied directly from a HV/LV transformer and the maximum apparent-power loading of the installation has been determined, a suitable rating for the transformer can be decided, taking due account of the following considerations: voltage (at no load) rated power (kVA) 50 100 160 250 315 400 500 630 800 1000 1250 1600 2000 2500

c the possibility of improving the power factor of the installation (see chapter E), c anticipated extensions to the installation, c installation constraints (temperature...) standard transformer ratings.

In (A) 400 V

420 V

433 V

480 V

72 144 231 361 455 577 722 909 1155 1443 1804 2309 2887 3608

69 137 220 344 433 550 687 866 1100 1375 1718 2199 2749 3437

67 133 213 333 420 533 667 840 1067 1333 1667 2133 2667 3333

60 120 192 301 379 481 601 758 962 1203 1504 1925 2406 3007

table B19: IEC-standardized kVA ratings of HV/LV 3-phase distribution transformers and corresponding nominal full-load current values.

The nominal full-load current In on the LV side of a 3-phase transformer is given by: Pa 103 In = where Ue Pa = kVA rating of the transformer U = phase-to-phase voltage at no-load* (in volts) In is in amperes. For a single-phase transformer: 3 In = Pa 10 where V V = voltage between LV terminals at no-load* (in volts).

B18 - general - installed power

Simplified equation for 400 V (3-phase load) In = kVA x 1.4 The IEC standard for power transformers is IEC 76. * as given on the transformer-rating nameplate. For table B19 the no-load voltage used is 420 V for the nominal 400 V winding.

B 4.7 choice of power-supply sources The study developed in F2 on the importance of maintaining a continuous supply raises the question of the use of standby-power plant. The choice and characteristics of these alternative sources are described in F3-3. For the main source of supply the choice is generally between a connection to the HV or the LV network of the public power-supply authority. In practice, connection to a HV source may be necessary where the load exceeds (or is planned eventually to exceed) a certain level - generally of the order of 250 kVA, or if the quality of service required is greater than that normally available from a LV network. Moreover, if the installation is likely to cause disturbance to neighbouring consumers, when connected to a LV network, the supply authorities may propose a HV service. Supplies at HV can have certain advantages: in fact, a HV consumer: c is not disturbed by other consumers, which could be the case at LV, c is free to choose any type of LV earthing system, c has a wider choice of economic tariffs, c can accept very large increases in load.

It should be noted, however, that: c the consumer is the proprietor of the HV/LV substation and, in some countries, he must build and equip it at his own expense. The power authority can, in certain circumstances, participate in the investment, at the level of the HV line for example, c a part of the connection costs can, for instance, often be recovered if a second consumer is connected to the HV line within a certain time following the original consumer's own connection, c the consumer has access only to the LV part of the installation, access to the HV part being reserved to the supply-authority personnel (meter reading, operational manœuvres, etc.). However, in certain countries, the HV protective circuit breaker (or fused load-break switch) can be operated by the consumer, c the type and location of the substation are agreed between the consumer and the supply authority.

general - installed power - B19

1. protection of circuits supplied by an alternator

J a major difficulty encountered when an installation may be supplied from alternative sources (e.g. a HV/LV transformer or a LV generator) is the provision of electrical protection which operates satisfactorily on either source. The crux of the problem is the great difference in the source impedances; that of the generator being much higher than that of the transformer, resulting in a corresponding difference in the magnitudes of fault currents.

Most industrial and large commercial electrical installations include certain important loads for which a power supply must be maintained, in the event that the public electricity supply fails: c either, because safety systems are involved (emergency lighting, automatic fire-protection equipment, smoke dispersal fans, alarms and signalization, and so on...) or: c because it concerns priority circuits, such HV

as certain equipment, the stoppage of which would entail a loss of production, or the destruction of a machine tool, etc. One of the current means of maintaining a supply to the so-called “essential” loads, in the event that other sources fail, is to install a diesel-generator set connected, via a changeover switch, to an emergency-power standby switchboard, from which the essential services are fed (figure J1-1).

G

LV

standby supply change-over switch

non essential loads

essential loads

fig. J1-1: example of circuits supplied from a transformer or from an alternator.

1.1 an alternator on short-circuit the establishment of short-circuit current (fig. J1-2) Apart from the limited magnitude of fault current from a standby alternator, a further difficulty (from the electrical-protection point of view) is that during the period in which LV circuit breakers are normally intended to operate, the value of short-circuit current changes drastically. For example, on the occurrence of a shortcircuit at the three phase terminals of an alternator, the r.m.s. value of current will immediately rise to a value of 3 In to 5 In*. An interval of 10 ms to 20 ms following the instant of short-circuit is referred to as the “sub-transient” period, in which the current decreases rapidly from its initial value. The current continues to decrease during the ensuing “transient” interval which may last for 80 ms to 280 ms depending on the machine type, size, etc. The overall phenomenon is referred to as the “a.c. decrement”. The current will finally stabilize in about subtransient period

r.m.s.

0.5 seconds, or more, at a value which depends mainly on the type of excitation system, viz: c manual; c automatic (see figure J1-2). Almost all modern generator sets have automatic voltage regulators, compounded to maintain the terminal voltage sensibly constant, by overcoming the synchronous impedance of the machine as reactive current demand changes. This results in an increase in the level of fault current during the transient period to give a steady fault current in the order of 2.5 In to 4 In* (figure J1-2). In the (rare) case of manual control of the excitation, the synchronous impedance of the machine will reduce the short-circuit current to a value which can be as low as 0.3 In, but is often close to In*.

transient period

alternator with automatic voltage regulator

3 In

alternator with manual excitation control

In 0.3 In instant of fault

10 to 20 ms

0.1 to 0.3 s

t

fig. J1-2: establishment of short-circuit current for a three-phase short circuit at the terminals of an alternator. * depending on the characteristics of the particular machine.

particular supply sources and loads - J1

1. protection of circuits supplied by an alternator (continued)

J 1.1 an alternator on short-circuit (continued) Figure J1-2 shows the r.m.s. values of current, on the assumption that no d.c. transient components exist. In practice, d.c. components of current are always present to some degree in at least two phases, being maximum when the short-circuit occurs at the alternator terminals. This feature would appear to complicate still further the matter of electrical protection, but, in fact, the d.c. component in each phase simply increases the r.m.s. values already mentioned, so that calculations and trippingcurrent settings for protective devices based only on the a.c. components, as indicated below, will be conservative, i.e. the actual currents will always be either equal to or higher than those calculated. The further the point of short-circuit from the generator the lower the fault current, and the more rapidly the transient d.c. components disappear. Furthermore, the a.c. decrement also becomes negligible when the network impedance to the fault position attains ohmic values which are high compared with the reactance values of the alternator (since the overall change in impedance is then relatively small).

alternator impedance data Manufacturers furnish values of the several impedances mentioned below. Resistances are negligibly small compared to the reactances. It can be seen from the constantly-changing value of r.m.s. current that the effective reactance* changes constantly from a low value (sub-transient reactance) to a high value (synchronous reactance) in a smooth progression. The values discussed below are derived from test curves and correspond with current values measured at the instant of shortcircuit. * An explanation of the significance of the fixed reactance values and how they relate to a smooth variation of current is briefly described in Appendix J1. c the sub-transient reactance x”d is expressed in % by the manufacturer (analogous to the short-circuit impedance voltage of a transformer). The ohmic value X”d is therefore calculated as follows: x”d Un2 10-5 X”d (ohms) = Pn where: x”d is in % Un is in volts (phase/phase) Pn is in kVA c the % transient reactance x’d is given in ohms by: x'd Un2 10-5 X'd (ohms) = Pn c the % zero-phase-sequence reactance x’o is given in ohms by: x'o Un2 10-5 X'o (ohms) = Pn In the absence of more precise information, the following representative values may be used: x”d = 20% ; x’d = 30 % ; x’o = 6% Pn and Un being, respectively, the rated 3-phase power (kVA) and the rated phase/phase voltage of the alternator (volts).

J2 - particular supply sources and loads

The sub-transient reactance is used when calculating the short-circuit current-breaking rating for LV circuit breakers which have opening times of 20 ms or less, and also for the electrodynamic stresses to be withstood by CBs and other components (such as busbars, cleated single-core cables, etc.). The transient reactance is used when considering the breaking capacity of LV circuit breakers with an opening time that exceeds 20 ms, and also for the thermal withstand capabilities of switchgear and other system components. Remark: from the instant at which the shortcircuit is established, the alternator reactance will rapidly increase. This means that the currents calculated from the defined fixed values x"d and x'd (for breaking capacity) will always exceed those that will actually occur at the instant of circuit breaker contact separation, i.e. there is an inherent safety factor incorporated in the current-level calculation. These calculations for the circuit breaker short-circuit breaking capacity are based on the symmetrical a.c. components of current only, i.e. no account is taken of the d.c. unidirectional components. For the circuit breaker short-circuit making capacity, the d.c. components are crucial, as discussed in Chapter C, Sub-clause 1.1 (figure C-5).

J short-circuit current magnitude at the terminals of an alternator c the transient 3-phase short-circuit current at the terminals of an alternator is given by: Ig Isc = 100* where: x’d Ig: rated full-load current of the alternator x’d = transient reactance per phase of the alternator in %; c when these values are compared with those for a short-circuit at the LV terminals of 630 kVA 20 kV/400 V Usc = 4%

a transformer of equal kVA rating, the current from the alternator will be found to be of the order of 5 or 6 times less than that from the transformer. The difference will be even greater where (as is generally the case) the alternator rating is lower than that of the transformer. * for CBs with opening time exceeding 20 ms.

250 kVA 400 V X'd = 30%

A

non essential loads

essential loads

fig. J1-3: example of an essential services switchboard supplied (in an emergency) from a standby alternator. Example (figure J1 - 3) What is the value of 3-phase short-circuit current at point A according to the origin of supply? Circuit impedances are negligible compared with those of the sources. c transformer supply 3-phase Isc = 21.5 kA (see table C20 in Chapter C)

c alternator supply 3-phase Isc = Ig x 100 = Pn x 100 x'd x'd eUn where: Pn is expressed in kVA Un is expressed in volts x’d is expressed in % Isc is expressed in kA 3-phase Isc = 250 x 100 = 1.2 kA ex 400 x 30

particular supply sources and loads - J3

1. protection of circuits supplied by an alternator (continued)

J 1.2 protection of essential services circuits supplied in emergencies from an alternator the difficulty is due to the small margin between the rated current and the short-circuit current of the alternator.

The characteristics (s.c. breaking capacity and range of adjustable magnetic tripping unit) of the CBs protecting the circuits of essential loads must be defined as described below: Choice of s.c. breaking capacity This parameter must always be calculated for the condition of supply from the transformer, or other “normal” source. Adjustment of magnetic tripping units In practice, the only circuit breakers concerned are those protecting the essential services circuits at the main general distribution board. The protection of circuits from local distribution or sub-distribution boards is always calibrated at a much lower level than those at the main general distribution board, so that, except in unusual cases, adequate fault currents are available from an alternator to ensure satisfactory protective-gear operation at these lower levels. Two difficulties have to be overcome: c the first is the need for discrimination of circuit protection with the protection scheme for the alternator. For the basic protection requirements of an alternator, viz: overload protection, the curve shown in figure J1-4 is representative (see Note 1). c the second concerns protection of persons against electric shock from indirect contact, when the protection depends on the operation of overcurrent relays (for example, in IT* or TN systems). The operation of these relays must be assured, whether the supply is from the alternator or from the transformer (see Note 2). Instantaneous or short-time delay magneticrelay trip settings of the circuit breakers concerned must therefore be set to operate at minimum fault levels occurring at the extremity of the circuits they protect, when being supplied from the alternator.

Note 1. Sensitive high-speed protection of an alternator against internal faults (i.e. upstream of its CB) is always possible by using a pilot-wire and current-transformers differential scheme of protection, with the advantage that discrimination with circuit protection schemes is absolute. The problem of discriminative overload protection (as noted above) remains, however. A widely-used solution to this problem is provided by a voltage-controlled overcurrent relay, which depends on the following principle: short-circuit currents cause much lower system voltages than overload currents. An inverse-time/current overload relay is used having two operating curves, one of which corresponds to that of fig. J1-4, and is effective when system voltage levels are normal. If the system voltage falls below a pre-set value, the relay is automatically switched to operate much faster and at lower current levels than those shown in fig. J1-4. Modern low-setting magnetic tripping units, however, often provide a simpler solution as noted in 1.3 below. Note 2. Where the level of earth-fault current is not sufficient, in IT* and TN systems, to trip CBs on overcurrent, the protection against indirect-contact hazards can be provided by an appropriate use of RCDs, as indicated in Chapter G Sub-clause 6.5 Suggestion 2 (for IT circuits) and Sub-clause 5.5. Suggestion 2 (for TN circuits). time (s)

1000

100 12 10 7 3 2 1

1.1 1.2 1.5

2

3

4

5 I/IG overload

fig. J1-4: overload protection of an alternator. * Two concurrent earth faults on different phases or on one phase and on a neutral conductor, are necessary on IT systems, to create an indirect-contact hazard.

J4 - particular supply sources and loads

J 1.3 choice of tripping units the calculation of the minimum fault current (in IT or TN schemes) is complex. Software packages for this purpose are available.

calculation of the fault-current loop impedance (Zs) for IT and TN systems The determination of the minimum level of short-circuit current, from the calculation of the fault-loop impedance Zs (by the sum of impedances method) is difficult, mainly because of the uncertainly, in a practical installation, of the accuracy of the zerophase-sequence impedances. When conductor routes are known in sufficient detail, impedances can then be determined by the use of software, currently available commercially. Approximate methods for 3-phase and 1-phase short circuits are presented in Sub-clause 1.4.

types of suitable tripping units The choice of low-setting magnetic tripping units will generally be necessary, such as Compact NS* with STR (magnetic-trip short time delay is adjustable from 1.5 to 10 Ir) or circuit breakers Multi 9* curve B (tripping between 3 and 5 In). In practice, these CBs (or their equivalents) will always be necessary when the current rating of the CB is greater than one third of the alternator current rating and will, in most cases, obviate the need for voltage-controlled overload relays. Switchgear manufacturers often furnish tables showing recommended combinations of circuit breakers for commonly-used standby-generator schemes. * Merlin Gerin products.

characteristics of protection for essential-services circuits type of circuit fault-breaking rating (FBR)

tripping unit adjustment

dieselgenerator protection cabinet power-source changeover switch

main circuits

FBR > Isc with supply from transformer

Im or short-delay trip setting level < the minimum fault current at the far end of the circuit when supplied from the alternator (see Note 2 in Sub-clause 1.2)

suband final circuits

FBR > Isc with supply from transformer

check the protection of persons against indirect-contact hazards, particularly on IT and TN systems (see Note 2 in Sub-clause 1.2)

B

Isc: 3-ph short-circuit current Im: magnetic-tripping-relay current setting loads

fig. J1-5: the protection of essential services circuits.

particular supply sources and loads - J5

1. protection of circuits supplied by an alternator (continued)

J 1.4 methods of approximate calculation An installation on (normal) 630 kVA transformer supply (figure J1-6) includes an essential-services distribution board which can also be supplied from a standby 400 kVA diesel-alternator set.

What circuit breakers should be installed on the out-going ways from the essentialservices board: c if the installation is TN-earthed? c if the installation is IT-earthed?

transformer 630 kVA 20 kV/400 V

alternator 400 kVA 400 V

alternator and diesel protection equipment cabinet

PE essential circuits main distribution board

non essential circuits

NS250N STR22SE 250 A

NS160N TM400D

IB = 220 A 100 m 120 mm2

IB = 92 A 70 m 35 mm2

PE : 70 mm2

PE : 35 mm2

sub-distribution board

fig. J1-6: example.

calculation of the minimum level of 3-phase short-circuit current Table J1-7 shows the procedure for an alternator together with one or several circuits. item of plant alternator circuit total

R mΩ Ra 22.5 L S R

X mΩ X'd 0.08 x L X

Z mΩ

R2 + X 2

table J1-7: procedure for the calculation of 3-phase short-circuit current. S = c.s.a. in mm2 L = length in metres For the calculation of cable impedance, refer to Chapter H1, Sub-clause 4.2.

J6 - particular supply sources and loads

Isc kA

1.05xVn R2 + X 2

J Consider the 220 A circuit in figure J1-6 c alternator Ra = 0 2 2 X’d = Un x 0.30 = 400 x 0.30 = 120 mΩ Pn 400 c circuit Rc = 22.5 x 100 = 18.75 mΩ 120 Xc = 0.08 x 100 = 8 mΩ c application of the method of impedances as indicated in table J1-7; R = Ra + Rc = 0 + 18.75 = 18.75 mΩ X = X’d + Xc = 120 + 8 = 128 mΩ total impedance per phase: Z = R2 + X 2 = (18.75)2 + (128)2 = 129.4 mΩ

Isc = 1.05 Vn = 1.05 x 230 = 1.87 kA (r.m.s.) Z 0.129 Note: In practice there will always be some measure of d.c. transient current in at least two phases, so that the above value will normally be exceeded during the period required to trip the CB.

calculation of the minimum level of 1-phase to earth short-circuit fault current Table J1-8 shows the procedure for an alternator together with one or several circuits. item of plant alternator circuit total

R mΩ Ra 22.5 L (1 + m) Sph R

X mΩ 2 X'd + Xo 3 0.08 x L x 2 X

Z mΩ

R2 + X 2

Isc kA

1.05xVn R2 + X 2

table J1-8: procedure for the calculation of 1-phase to neutral short-circuit current. For the calculation of cable impedance, refer to Chapter H1, Sub-clause 4.2. Consider the 220 A circuit in figure J1-6 c alternator Ra = 0 2 Xa = (2 x 120 + 400 x 0.06) x 1 = 88 mΩ 400 3 c circuit Rc = 22.5 x 100 x (1 + 120 / 70) = 50.89 mΩ 120 Xc = 0.08 x 100 x 2 = 16 mΩ c application of the method of impedances, as for the previous example: R = Ra + Rc = 0 + 50.89 = 50.89 mΩ X = Xa + Xc = 88 + 16 = 104 mΩ The total impedance: Z = R2 + X 2 = 50.892 + 1042 = 115.8 mΩ

and Isc1 (phase/neutral) = 1.05 x 230 = 2.09 kA. 115.8

particular supply sources and loads - J7

1. protection of circuits supplied by an alternator (continued)

J 1.4 methods of approximate calculation (continued) maximum permissible setting of instantaneous or short-time delay tripping units c TN scheme Of the two fault conditions considered (3-phase and 1-phase/neutral) the 3-phase fault was found to give the lower short-circuit current. The setting of the protective relay must therefore be selected to a current level below that calculated. For the 220 A outgoing circuit the trip unit would be rated at 250 A and adjusted (in principle) to Isc/250, i.e. 1,870/250 = 7.4 In. Owing to a ± 20 % manufacturing tolerance however, the maximum permissible setting would be 7.4 = 6.2 In 1.2 A tripping unit type TM250D* set at 6 In on a NS250N* circuit breaker (breaking capacity = 36 kA i.e. > 21.5 kA) would be appropriate; c IT scheme In this case the protection must operate for a second earth fault occurring before the first earth fault is cleared. This condition (only) produces indirect-contact hazards on an IT system. If the neutral conductor is not distributed, then the minimum short-circuit current for the system will be the phase-to-phase value (i.e. concurrent earth faults on two different phases) which is equal to 0.866 Isc (Isc = the 3-phase s.c. current). If the neutral is distributed, the minimum s.c. current occurs when a phase-to-earth fault and a neutral-to-earth fault occur concurrently, and a protective relay setting equal to 0.5 Isc (phase to neutral) i.e. half the value of a phase-to-neutral short-circuit current, is conventionally used to ensure positive relay operation, v for the case of a non-distributed neutral, the minimum s.c. current = 0.5 x 0.866 x 1.87 = 0.81 kA The tripping unit rated at 250 A will be set at 810 x 1 = 2.7 In 250 1.2 (the factor 1.2 accounting for the ± 20 % manufacturing tolerance for tripping units). A TM250D or a STR22SE tripping unit set at 2.5 In would be appropriate, v when the neutral is distributed, the minimum s.c. current relay setting = 0.5 x 2.08 = 1.04 kA The 250 A tripping unit will be set at 1.040 x 1 250 1.2 = 3.5 In (the 1.2 factor covering manufacturing tolerance, as before) A STR22SE tripping unit, set at 3.0 In would be satisfactory. Note: The foregoing method is based on a simplified application of the following formulae: ➀ Isc (3-phase) = V ph Z1 ➁ Isc (phase/phase) = eVph Z1+Z2 ➂ Isc (phase/earth) = 3 Vph Z1+Z2+Z0

J8 - particular supply sources and loads

Where Z1 = positive phase-sequence impedance Z2 = negative phase-sequence impedance Z0 = zero phase-sequence impedance Simplifications: c Z1 is assumed to be equal to Z2 so that formula ➁ becomes eVph = 0.866 Vph or 0.866 Isc (3-phase) 2 Z1 Z1 c In table J1-8 the calculated cable reactance assumes that X1 = X2 = X0 for the cable, so that in formula ③ the total reactance = (X1 + X2 + X0) 1/3 = (3 X1) 1/3 = X1 * Merlin Gerin product.

J 1.5 the protection of standby and mobile a.c. generating sets Practical guides in certain national standards classify generator sets according to three categories, viz: c permanent installations (as discussed in Sub-clauses 1.1 to 1.4); c mobile sets (figure J1-9); c portable power packs (figure J1-10).

mobile sets These are used mainly to provide temporary supplies (on construction sites for example) where protection of persons against electric shock must be ensured by the use of RCDs with an operating threshold not exceeding 30 mA.

non-metallic conduit prividing supplementary insulation

PE

C32N 30 mA

T Vigicompact NS100 TM63G 30 mA

PE load circuits

fig. J1-9: mobile generating set.

portable power packs The use of hand-carried power packs by the general public is becoming more and more popular. When the pack and associated appliances are not of Class II (i.e. double insulation), 30 mA RCDs are required by most national standards. C60N 30 mA T

fig. J1-10: portable power pack with RCD protection.

particular supply sources and loads - J9

2. inverters and UPS (Uninterruptible Power Supply units)

J 2.1 what is an inverter? An inverter produces an a.c. supply of high quality (i.e. an undistorted sine-wave, free from interference) from a d.c. source; its function is the inverse of that of a rectifier (figure J2-1). Its main purpose (when associated with a rectifier which provides its input) is to afford a high-quality power supply to equipment for which the interference and disturbances of a normal power-supply system cannot be tolerated (e.g. to computer systems). Power systems are subjected to many kinds of perturbation which adversely affect the quality of supply: atmospheric phenomena (lightning, freezing), accidental faults (shortcircuits), industrial parasites, the switching of large electric motors (lifts, fluorescent lighting) are among the many causes of poor quality of supplies. Apart from occasional loss of supply, the disturbances take the form of more-or-less severe voltage dips, high- and low-frequency parasites, continuous “noise” from

fluorescent-lamp circuits and (normally undetectable, but totally unacceptable to sensitive electronic systems) of miniinterruptions of several milli-seconds. By the addition of a storage battery at the input terminals of the inverter (and therefore across the output terminals of the associated rectifier), an elementary UPS system is formed. In normal circumstances, the rectifier supplies the load through the inverter, while, at the same time, a trickle charge from the rectifier maintains the battery fully charged. A loss of a.c. power supply from the distribution network would simply result in the battery automatically maintaining the output from the inverter with no discernable interruption. load

d.c. source

sinusoidal a.c. output

inverter

fig. J2-1: inverter function.

2.2 types of UPS system there are two main types of UPS system: c off-line, c on-line.

J10 - particular supply sources and loads

Several types of UPS system exist according to the degree of protection against powernetwork “pollution” required, and whether supply autonomy (automatic standby-supply on the loss of normal power supply) is specified, or not. The two most commonlyused types are described below. An off-line type of UPS system (figure J2-2) is connected in parallel with a supply direct from the public distribution network, as shown in figure J2-2, and is autonomous, within the capacity of its battery, on loss of the a.c. power supply. In normal operation the filter improves the quality of the current while the voltage is maintained sensibly constant at its declared value by appropriate and automatic regulation within the filter unit. When the tolerance limits are exceeded, including a total loss of supply, a contactor, which carries the normal load, changes over rapidly to the UPS unit (in less than 10 ms) the power then being supplied from the battery. On the return of normal power supply, the contactor changes back to its original condition; the battery then recharges to its full capacity. These units are normally of low rating (i 3 kVA) but are capable of passing large

transient currents such as those for motorstarting and switching on of (cold) resistive loads. The most common use for such units is the supply to multi-workstation ITE (information technology equipment) installations, such as cash registers.

An on-line type of UPS system (figure J2-3) is connected directly between the public a.c. supply network and the load, and has an autonomous capability, the period of which depends on the battery capacity and load magnitude. The total load passes through the system, which affords a supply of electrical energy within strict tolerance limits, regardless of the state of the a.c. power supply network. On loss of the latter, the battery automatically, and without interruption, maintains the pollution-free a.c. supply to the load. This system is equally suitable for small loads (i 3 kVA) or large loads (up to several MVA).

a.c. power supply network

a. c. power supply network F sensitive load inverter

rectifier/ charger

filter

battery

fig. J2-2: off-line UPS system.

sensitive load inverter

rectifier charger battery

fig. J2-3: on-line UPS system.

J Other apparatus, not assuring a no-break performance, but which protect sensitive loads from certain disturbances commonly occurring on power distribution network, include the following: c the filter-plug which is simply an a.c. plug for connecting or interconnecting loads, which has built-in HF (high-frequency) filters, in order to reduce HF parasitic interference to acceptable levels. Its principal use is on micro-informatic stand-alone PCs rated at 250 to 1,000 VA, for general office purposes; c the network (or mains) -supply conditioner is a complete system for providing an uncontaminated a.c. power supply, but without autonomy, i.e. no provision against loss of supply from the a.c. distribution network. Its principal functions are to: v filter out HF parasites, v maintain a sensibly-constant voltage level, v isolate (galvanically) the load from the a.c. power network. It is equally applicable to office or industrial systems which do not require a no-break standby supply, up to ratings of 5,000 VA; c the slim-line UPS has integral protection with autonomy for each micro-informatic stand-alone PC and its peripherals, and is installed immediately under the microprocessor. Two outputs, each with back-up from the UPS unit, supply the central processor and screen. Two further outputs, which are filtered, supply other less-sensitive units (e.g. the printer). The slim-line UPS belongs to the class of off-line UPS schemes. types of UPS units, conditioners and filters diagrams of principle

filter plug

mains-supply conditioner

slim-line UPS

off-line UPS

on-line UPS F

F

disturbances considered type of network corrective disturbance measures HF parasites c variations of voltage regulation autonomy 10 to 30 mn (according to battery capacity) rated power i 250 VA c 300 - 1,000 VA 1,000 - 2,500 VA > 2,500 VA applications minimal protection

c c

c c

c c

c c

c

c

c

c c c c

c c

c c c

c c c c

all sensitive loads

microinformatic stand-alone PC

micro-informatic terminals

highly disturbed a.c. power systems and/or heavy loads

table J2-4: examples of different possibilities and applications of inverters, in decontamination of supplies and in UPS schemes.

2.3 standards The international standard presently covering semi-conductor converters is IEC 146-4.

particular supply sources and loads - J11

2. inverters and UPS (Uninterruptible Power Supply units) (continued)

J 2.4 choice of a UPS system The choice of a UPS system is determined mainly by the following parameters: c rated power, based on: v maximum value of actual estimated kVA demand, v transitory current peaks (motor starting, energization of resistive loads, transformers...). Note: in order to obtain satisfactory discrimination of protective devices for all

types of load, it may be necessary to adjust the power rating of the UPS system. c voltage levels upstream (input) and downstream (output) of the UPS unit; c duration of autonomy required (i.e. supply from the battery); c frequencies upstream (input) and downstream (output) of the UPS unit; c level of availability required.

(9)

(5)

(8)

UPS

distribution board (4)

mains 2

(6)

C/S mains 1

(2)

(1) (3)

(7)

fig. J2-5: classical arrangement of a UPS on-line installation, using an inverter. UPS 1. inverter 2. rectifier/charger 3. batteries (usual periods of autonomy 10 - 15 - 30 mn - several hours) 4. static contactor (see “availability” below) 5. isolating transformer, if galvanic isolation from upstream circuits is necessary. 6. outgoing ways 7. transformer for specific downstream-circuits voltage 8. changeover switch 9. transformer to match the upstream voltage to that of the consumer. Note: At first sight, the circuit arrangement in figure J2-5 closely resembles that of the off-line UPS system (of figure J2-2). In fact, however, it is an on-line system, in which the load is normally passing through circuit 1. The static contactor is open in this situation, but closes automatically if the UPS system becomes overloaded, or fails for any reason. In such a case, the load will then be supplied from the (reserve) circuit 2. This action is the converse of that of the off-line scheme.

the power rating of a UPS unit must take account of the peak motorstarting currents, of the possibility of future extensions to the installation, and of the overload capability of the inverter and other UPS-unit components.

J12 - particular supply sources and loads

Conditions will automatically return to normal if the overload, etc. is corrected. In this arrangement, the voltage output of the inverter is always maintained in synchronism with the voltage of the powersupply network (i.e. within close tolerance limits of magnitude and phase difference) thereby minimizing the disturbance in the event of “instantaneous” changeover from circuit 1 to circuit 2 operation.

power (VA) The rated power of the UPS unit must be sufficient to satisfy the steady load demand as well as loads of a transitory nature. The demand will be the sum of the apparent (VA) loads of individual items, for example, the CPU (central processing unit) and will amount to Pa, generally corrected by a factor (1.2 to 2) to allow for future extensions. However, in order to avoid oversizing of the installation, account should be taken of the overload capacity of the UPS components. For example, inverters manufactured by Merlin Gerin can safely withstand the following overloaded condition: c 1.5 In for 1 minute; c 1.25 In for 10 minutes.

Instantaneous variations of load: these variations occur at times of energizing and de-energizing of one or more items of load. For an instantaneous change of load up to 100 % of the nominal rating of the UPS unit, the output voltage generally remains between + 10 % and - 8 % of its rated value.

J Example of a power calculation Choice of a UPS unit suitable for the loads shown in figure J2-6. load circuits no.: 1 : 80 kVA 2 : 10 kVA 3 : 20 kVA 4 : 20 kVA 5 : 30 kVA

fig. J2-6: example. Assumed operating constraints: circuit no. 4 will take a transitory current equal to 4 In for a period of 200 ms when initially energized. This operation will be carried out at least once a day. The peak kVA demand, therefore, represents a supplement (over the steady-state 20 kVA demand) of 3 x 20 kVA = 60 kVA. The remaining circuits require no such transitory peak currents. In all cases the kVA values cited have taken the load power factors into account. Possible future extensions to the installation are estimated to amount to 20% of the existing load. The maximum steady-state power demand presently considered is therefore: P = 80 + 10 + 20 + 20 + 30 = 160 kVA. With allowance for extensions (of 20%) = 160 x 1.2 = 192 kVA. With an additional 200 ms peak of (3 x 20) kVA the total amounts to 192 + 60 = 252 kVA. The total of 252 kVA however, includes the 60 kVA peak current which is easily absorbed by the 1.5 In overload capability of (a M.G) UPS system, so that the rating of a suitable UPS unit would be 252 x 1/1.5 = 168 kVA for the nearest standard rating available above the calculated value, e.g. 200 kVA. For the choice of suitable protective devices, see Sub-clause 2.9.

C/S 200 kVA

fig. J2-7: solution to the example.

availability A UPS system is generally provided with an alternative (unconditioned) emergency source, a situation which affords a relatively high level of availability. By way of example, a UPS alone has a MTBF (mean time between failures) of 50,000 hours. In the usual case, where the supply is doubled as noted above (mains 1 and mains 2 in figure J2-5) the MTBF obtained is in the range 70,000 to 200,000 hours, depending on the availability of the second source. Switching from one source to the other is achieved automatically by a static (solid state) contactor. Configurations having a higher redundancy, e.g. three UPS units each rated at P/2 to supply a load of P (figure J2-8) are also sometimes installed. The calculation of their level of availability can be carried out by specialists, and the manufacturers are able to quote availability levels, relative to their own products and recommended layouts.

C/S P/2

P/2 P P/2

fig. J2-8: 3 UPS P/2 units providing a high level of availability of a power rated P.

particular supply sources and loads - J13

2. inverters and UPS (Uninterruptible Power Supply units) (continued)

J 2.5 UPS systems and their environment UPS system components include the means to communicate with other equipments.

UPS units can communicate with other equipments, notably with IT (information technology) systems, passing data concerning the state of the UPS components (static contactor open or closed, and so on...) and receiving orders controlling its function, in order to: c optimize the protection scheme: the UPS, for example, transmits data (such as: condition normal, supply being maintained by the battery, alarm for period of autonomy almost reached) to the computer it is supplying. The computer deduces the appropriate corrective action, and indicates accordingly; c permit remote control: the UPS transmits data concerning the state of UPS components, together with measured quantities, to the console of an operator, who is then able to carry out operational manœuvres through remote-control channels; c supervise (manage) the installation: the consumer (i.e. the “user”) has a centralized management technique facility which allows him to acquire data from the UPS unit(s) which are then stored and analysed, with anomalies indicated, and the state of the UPS is presented on a mimic board or displayed on a screen, and finally to exercise remote control of UPS functions (figures J2-9 to J2-11).

This evolution towards a general compatibility between diverse systems and related hardware requires the incorporation of new functions in the UPS systems. These functions can be designed to ensure mechanical and electrical compatibility with other equipments: standard versions are now provided with dry contacts and current loops. Interconnection facilities according to the standards RS 232, RS 422 or RS 485 can be incorporated on request. In fact, certain advanced modules include modern cards with integral protocole (JBus for example). Furthermore, they can make use of specialized software for automatic checking and fault diagnosis (e.g. Soft-Monitor on PC) which may be integrated into other systems of overall supervision (figure J2-10).

fig. J2-9: UPS units can communicate with centralized system management terminals.

fig. J2-10: software (e.g. Soft-Monitor) allows remote checking and automatic fault diagnosis of the UPS system.

fig. J2-11: UPS units are readily integrated into centralized management systems.

J14 - particular supply sources and loads

2. inverters and UPS (Uninterruptible Power Supply units) (continued)

J 2.10 complementary equipments transformers A two-winding transformer included on the upstream side of the static contactor of circuit 2 (see figure J2-5) allows: c a change of voltage level when the power network voltage is different to that of the load; c a different arrangement for the neutral on the load-side winding, from that of the power network. Moreover, such a transformer: c reduces the short-circuit current level on the secondary, (i.e. load) side compared with that on the power network side,

c prevents third harmonic currents (and multiples of them) which may be present on the secondary side from passing into the power-system network, providing that the primary winding is connected in delta.

anti-harmonic filter The UPS system includes a battery charger which is controlled by commutated thyristors or transistors. The resulting regularlychopped current cycles “generate” harmonic components in the power-supply network. These indesirable components are filtered at the input of the rectifier and for most cases this reduces the harmonic current level sufficiently for all practical purposes. In certain specific cases however, notably in very large installations, an additional filter circuit may be necessary.

For example, when: c the power rating of the UPS system is large relative to the HV/LV transformer supplying it; c the LV busbars supply loads which are particularly sensitive to harmonics; c a diesel (or gas-turbine, etc.) driven alternator is provided as a standby power supply. In such cases, the manufacturers of the UPS system should be consulted.

communications equipment Communication with equipment associated with informatic systems (see Sub-clause 2.5) may entail the need for suitable facilities within the UPS systems. Such facilities may be incorporated in an original design, or added to existing systems on request.

fig. J2-27: a UPS installation with incorporated communication systems.

J24 - particular supply sources and loads

3. protection of LV/LV transformers

J These transformers are generally in the range of several hundreds of VA to some hundreds of kVA and are frequently used for: c changing the (LV) voltage level for: v auxiliary supplies to control and indication circuits, v lighting circuits (230 V created when the primary system is 400 V 3-phase 3-wires), c changing the method of earthing for certain loads having a relatively high capacitive current to earth (informatic equipment) or resistive leakage current (electric ovens, industrial-heating processes, mass-cooking installations, etc.). LV/LV transformers are generally supplied

with protective systems incorporated, and the manufacturers must be consulted for details. Overcurrent protection must, in any case, be provided on the primary side. The exploitation of these transformers requires a knowledge of their particular function, together with a number of points described below. Note: In the particular cases of LV/LV safety isolating transformers at extra-low voltage, an earthed metal screen between the primary and secondary windings is frequently required, according to circumstances, as recommended in European Standard EN 60742, and as discussed in detail in Sub-clause 3.5 of Chapter G.

3.1 transformer-energizing in-rush current At the moment of energizing a transformer, high values of transient current (which includes a significant d.c. component) occur, and must be taken into account when considering protection schemes. The magnitude of the current peak depends on: c the value of voltage at the instant of energization, c the magnitude and polarity of magnetic flux (if any) existing in the core of the transformer, c characteristics of the load on the transformer. In distribution-type transformers, the first current peak can attain a value equal to 10 to 15 times the full-load r.m.s. current, but for small transformers (< 50 kVA) may reach values of 20 to 25 times the nominal full-load current. This transient current decreases rapidly, with a time constant θ (see figure J3-1) of the order of several milli-seconds to several tens of milli-seconds.

I Î first 10 to 25 In

In t

θ

fig. J3-1: transformer-energizing in-rush current.

3.2 protection for the supply circuit of a LV/LV transformer The protective device on the supply circuit for a LV/LV transformer must avoid the possibility of incorrect operation due to the magnetizing in-rush current surge, noted above in 3.1. It is necessary to use therefore: c selective (i.e. slightly time-delayed) circuit breakers of the type Compact NS STR* (figure J3-2) or c circuit breakers having a very high magnetic-trip setting, of the types Compact NS or Multi 9* curve D (figure J3-3).

t

50 to 70 ms

r.m.s. value of the first peak

* Merlin Gerin.

instantaneous I trip

fig. J3-2: tripping characteristic of a Compact NS STR circuit breaker. t

In r.m.s. value of the first peak

10In 20In

I

fig. J3-3: tripping characteristic of a circuit breaker according to standardized type D curve (for Merlin Gerin 10 to 14 In).

particular supply sources and loads - J25

3. protection of LV/LV transformers (continued)

J 3.2 protection for the supply circuit of a LV/LV transformer (continued) Example (figure J3-4) A 400 V 3-phase circuit is supplying a 125 kVA 400/230 V transformer (In = 180 A) for which the first in-rush current peak can reach 17 In, i.e. 17 x 180 A = 3,067 A. A Compact NS250 circuit breaker with Ir setting of 200 A would therefore be a suitable protective device. A particular case: overload protection installed at the secondary side of the transformer An advantage of overload protection located on the secondary side, is that the short-circuit protection on the primary side can be set at a high value, or alternatively a circuit breaker type MA* may be used. The primary-side short-circuit protection setting must, however, be sufficiently sensitive to ensure its operation in the event of a short-circuit occurring on the secondary side of the transformer (upstream of secondary protective devices).

NS250N tripping unit STR22SE (Ir = 200)

3 x 70 mm2 400/230 V 125 kVA

fig. J3-4: example. Note: The primary protection is sometimes provided by fuses, type a M. This practice has two disadvantages: c the fuses must be largely oversized (at least 4 times the nominal full-load rated current of the transformer); c in order to provide isolating facilities on the primary side, either a load-break switch or a contactor must be associated with the fuses.

* Motor-control circuit breaker, the short-circuit protective relay of which is immune to high transient-current peaks, as shown in figure J5-3.

3.3 typical electrical characteristics of LV/LV 50 Hz transformers 3-phase kVA rating no-load losses (W) full-load losses (W) s.c. voltage (%) 1-phase kVA rating no-load losses (W) full-load losses (W) s.c. voltage (%)

5 100 250 4.5

6.3 110 320 4.5

8 130 390 4.5

10 150 500 5.5

12.5 160 600 5.5

16 170 840 5.5

20 270 800 5.5

8 105 400 5

10 115 530 5

12.5 120 635 5

16 140 730 4.5

20 150 865 4.5

25 31.5 175 200 1065 1200 4 4

25 310 1180 5.5

31.5 350 1240 5

40 350 1530 5

50 410 1650 4.5

63 460 2150 5

80 520 2540 5

100 570 3700 5.5

40 215 1400 5

50 265 1900 5

63 305 2000 4.5

80 450 2450 5.5

100 450 3950 5

125 160 525 635 3950 4335 5

125 680 3700 4.5

160 680 5900 5.5

200 790 5900 5

250 950 6500 5

315 1160 7400 4.5

400 1240 9300 6

500 1485 9400 6

630 1855 11400 5.5

800 2160 11400 5.5

table J3-5: typical electrical characteristics of LV/LV 50 Hz transformers.

3.4 protection of transformers with characteristics as tabled in J3-5 above, using Merlin Gerin circuit breakers 3-phase transformers (400 V primary) P (kVA) In (A) Usc %

circuit breakers type

5 10 16 20 25 31.5 40 50 63

7 14 23 28 35 44 56 70 89

4.5 5.5 5.5 5.5 5.5 5 5 4.5 5

80 100 125

113 141 176

5 5.5 4.5

160

225

5.5

250 315 400 500

352 444 563 704

5 4.5 6 6

630

887

5.5

C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K NC100 D NC100 D NC100 D NC100 D NS100H/L NS160H/L NS160H/L NS250N/H/L NS250N/H/L NS400N/H/L NS250N/H/L NS400N/H/L C801N/H/L C801N/H/L C801N/H/L C801NH/L C1001N/H/L C1001N/H/L C1251N/H

trip-unit current rating (A)/type no. 20 32 63 63 80 80 80 100 MA100 STR22SE STR22SE STR22SE STR22SE STR23SE STR22SE STR23SE STR35SE STR35SE STR35SE STR35SE STR35SE STR35SE STR35SE

table J3-6: protection of 3-phase LV/LV transformers with 400 V primary windings. J26 - particular supply sources and loads

J 3-phase transformers (230 V primary) P (kVA) In (A) Usc %

circuit breakers type

5 10 16 20 25 31.5 40 50 63

12 24 39 49 61 77 97 122 153

4.5 5.5 5.5 5.5 5.5 5 5 4.5 5

80

195

5

100 125 160 250

244 305 390 609

5.5 4.5 5.5 5

315

767

4.5

400

974

6

C60 / NC100 D or K C60 / NC100 D or K NC100 D NC100 D NS100H/L NS100H/L NS100H/L NS100H/L NS250N/H/L NS400N/H/L NS250N/H/L NS400N/H/L NS630N/H/L C801N/H/L C801N/H/L C801N/H/L C1001N/H/L C1001N/H/L C1251N/H C1251N/H

trip-unit current rating (A)/type no. 40 63 80 100 STR22SE STR22SE STR22SE STR22SE STR22SE STR23SE STR22SE STR23SE STR23SE STR35SE STR35SE STR35SE STR35SE STR35SE STR35SE STR35SE

table J3-7: protection of 3-phase LV/LV transformers with 230 V primary windings. 1-phase transformers (400 V primary) P (kVA) In (A) Usc %

circuit breakers type

0.1 0.16 0.25 0.4 0.63 1 1.6 2 2.5 4 5 6.3 8 10 12.5 16 20 25 31.5 40 50 63

0.24 0.39 0.61 0.98 1.54 2.44 3.9 4.88 6.1 9.8 12.2 15.4 19.5 24 30 39 49 61 77 98 122 154

13 10.5 9.5 7.5 7 5.2 4 2.9 3 2.1 1.9 1.6 5 5 5 4.5 4.5 4.5 4 4 4 5

80

195

4.5

100 125 160

244 305 390

5.5 5 5

C60 D or K C60 D or K C60 D or K C60 D or K C60 D or K C60 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K NC100 D NC100 D NS160H/L NS160H/L NS160H/L NS160H/L NS250N/H/L NS400N/H/L NS250N/H/L NS400 NS630 C801N/H/L C801N/H/L

trip-unit current rating (A)/type no. 1 1 1 2 3 6 10 10 16 20 32 40 50 63 63 80 100 STR22SE STR22SE STR22SE STR22SE STR22SE STR23SE STR22SE STR23SE STR23SE STR35SE STR35SE

table J3-8: protection of 1-phase LV/LV transformers with 400 V primary windings.

particular supply sources and loads - J27

3. protection of LV/LV transformers (continued)

J 3.4 protection of transformers with characteristics as tabled in J3-5 above, using Merlin Gerin circuit breakers (continued) 1-phase transformers (230 V primary) P (kVA) In (A) Usc %

circuit breakers type

0.1 0.16 0.25 0.4 0.63 1 1.6 2 2.5 4 5 6.3 8 10 12.5 16 20 25

0.4 0.7 1.1 1.7 2.7 4.2 6.8 8.4 10.5 16.9 21.1 27 34 42 53 68 84 105

13 10.5 9.5 7.5 7 5.2 4 2.9 3 2.1 1.9 1.6 5 5 5 4.5 4.5 4.5

31.5 40

133 169

4 4

50

211

5

63 80 100 125 160

266 338 422 528 675

5 4.5 5.5 5 5

C60 D or K C60 D or K C60 D or K C60 D or K C60 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K NC100 D NC100 D NC100 D NS160H/L NS160H/L NS250N/H/L NS250N/H/L NS250N/H/L NS250N/H/L NS400N/H/L NS250N/H/L NS400N/H/L NS630N/H/L C801N/H/L C801N/H/L C801N/H/L C801N/H/L C1001N/H/L

trip-unit current rating (A)/type no. 1 2 3 4 6 10 16 16 20 40 50 63 80 100 100 STR22SE STR22SE STR22SE STR22SE STR22SE STR22SE STR23SE STR22SE STR23SE STR23SE STR35SE STR35SE STR35SE STR35SE STR35SE

table J3-9: protection of 1-phase LV/LV transformers with 230 V primary windings.

J28 - particular supply sources and loads

4. lighting circuits

J the presence of adequate lighting contributes to the satety of persons.

The planning and realization of a lighting installation requires a sound understanding of the materials installed, together with familiarity with the rules for safety against fire hazards in establishments receiving the public.

emergency lighting is intended to facilitate the evacuation of persons in case of fire or other panic-causing situations, when normal lighting systems may have failed.

definitions Normal lighting refers to the installation designed for everyday use. Emergency lighting must ensure easy evacuation of persons from the premises concerned, in the event that the normal lighting system fails. Furthermore, emergency lighting must be adequate to allow any particular safety manœuvres provided in the premises to be carried out.

In fact, the provision of adequate illumination in the event of fire or other catastrophic circumstances is of great importance in reducing the likelihood of panic, and in permitting the necessary safety manœuvres to be carried out.

Standby lighting is intended to substitute normal lighting, where the latter fails. Standby lighting permits everyday activities to continue more or less normally, depending on the original design specification, and on the extent of the normal lighting failure. Failure of the standby lighting system must automatically switch on the emergency lighting system.

4.1 service continuity continuity of normal lighting service must be sufficient, independent of other supplementary systems.

normal lighting

in emergency lighting circuits, absolute discrimination between protective devices on the different circuits must be provided.

emergency lighting

Regulations governing the minimum requirements for ERP (Establishments Receiving the Public) in most European countries, are as follows: c installations which illuminate areas accessible to the public must be controlled and protected independently from installations providing illumination to other areas; c loss of supply on a final lighting circuit (i.e. fuse blown or CB tripped) must not result in total loss of illumination in an area which is capable of accommodating more than 50 persons; c protection by RCDs (residual current differential devices) must be divided amongst several devices (i.e. more than one device must be used).

These schemes include illuminated emergency exit signs and direction indications, as well as general lighting. c emergency exit indications In areas accommodating more than 50 persons, luminous directional indications to the nearest emergency exits must be provided; c general emergency lighting General lighting is obligatory when an area can accommodate 100 persons or more (50 persons or more in areas below ground level). A fault on a lighting distribution circuit must not affect any other circuit: v the discrimination of overcurrent-protection relays and of RCDs must be absolute, so that only the faulty circuit will be cut off, v the installation must be an IT scheme, or must be entirely class II, i.e. doubly-insulated. Sub-clause 4.7 describes different kinds of suitable power supplies.

particular supply sources and loads - J29

4. lighting circuits (continued)

J 4.2 lamps and accessories (luminaires) fluorescent tubes For normal operation a fluorescent tube requires a ballast and a starter (device for initiating the luminous discharge). c the ballast is an iron-cored inductor, permanently connected in series with the tube; its function is threefold, viz: v to limit the preheating current during the (brief) starting period, v to provide a pulse of high voltage at the end of the starting period to strike the initial arc, v to stabilize the current through the luminous column (hence the term “ballast”).

c the starter is a switch, which, by breaking the (electrode-preheating) current passing through the ballast, causes a high-voltage transient pulse to appear across the tube. This causes an arc (in the form of a gaseous discharge) to be established through the tube. The discharge is then self-sustaining at normal voltage. The ballast, capacitor and the tube, engender disturbances during the periods of starting, steady operation and extinction. These disturbances are analysed in table J4-1 below.

The presence of the ballast means that the power-factor (cos ø) of the circuit is low (of the order 0.6) with the corresponding consumption of reactive energy, which is generally metered. For this reason each fluorescent lamp is normally provided with its own power-factor-correction capacitor. single-phase fluorescent lamp with its individual p.f. correction capacitor 1 2 3

single-phase twin-tube fluorescent lamp with each tube having its own starter and series ballast. One of the tubes has a capacitor connected in series with its ballast. The two sets of equipment are connected in parallel. The arrangement is known internationally as a “duo”-circuit luminaire. The capacitor displaces the phase of the current through its tube, to nullify the flicker effect, as well as correcting the overall p.f. A

ballast

switching-on disturbances c high current peak to charge capacitor; order of magnitude 10 In for 1 sec. A number of lamps on one circuit can result in peaks of 300-400 A for 0.5 ms. This can cause a CB to trip, or the welding of contacts in a contactor. In practice, limit each circuit to 8 tubes per contactor; c moderate overload at the beginning of steady operating condition (1.1-1.5 In for 1 sec) according to type of starter. c no high current peak as noted above; c same order of moderate overload at beginning of steady operating condition as for the single tube noted above. This arrangement is recommended for difficult cases.

switching-off disturbances no particular problems

c can generate a current peak at start; c can cause leakage to earth of HF current (at 30 kHz) via the phase conductor capacitances to earth.

no particular problems

1 2 3

presence of 5th and 7th harmonics at very low level

no particular problems

presence of 3 rd harmonic currents in the neutral, which can reach 70 to 80% of the nominal phase current. In this case, therefore, the c.s.a. of the neutral conductor must equal that of the phase conductors.

B starter

Advantages: Energy savings of the order of 25%. Rapid one-shot start. No flicker or stroboscopic effects.

table J4-1: analysis of disturbances in fluorescent-lighting circuits.

J30 - particular supply sources and loads

c star-connected lamps (3-ph 4-wire 400/230 V system) 1 2 3 N

starter

fluorescent lamp with HF ballast

steady-operating disturbances circulation of harmonic currents (sinusoidal currents at frequencies equal to whole-number multiples of 50 (or 60) Hz: c delta-connected lamps (see Appendix J2) (3-ph 3-wire 230 V system)

J 4.3 the circuit and its protection dimensions and protection of the conductors The maximum currents in the circuits can be estimated using the methods discussed in Chapter B. Accordingly, account must be taken of: c the nominal power rating of the lamp and the ballast; c the power factor. The temperature within the distribution panel also influences the choice of the protective device (see Chapter H2 Sub-clause 4.4). In general tables are available from manufacturers to assist in making a choice.

Note: for circuits in which large peak currents occur (at times of switching on) and their magnitude is such that CB tripping is a possibility, the cable size is chosen after the protective CB (with an instantaneous trip setting sufficient to remain closed during the current peaks) has been selected. See the Note following table J4-2.

factor of simultaneity ks (diversity) A particular feature of large (e.g. factory) lighting circuits is that the whole load is “on” or “off”, i.e. there is no diversity. Furthermore, even among a number of lighting circuits from a given distribution panel, the factor ks is generally near unity.

Consequently, the interior of distribution panels supplying lighting schemes are frequently at an elevated temperature, an important consideration to be taken into account when selecting protective devices.

4.4 determination of the rated current of the circuit breaker The rated current of a circuit breaker is generally chosen according to the rating of the circuit conductors it is protecting (in the particular circumstances in the Note of 4.3 above, however, the reverse procedure was found to be necessary). The circuit conductor ratings are defined by the maximum steady load current of the circuit. power (kW) 1 1.5 2 2.5 3 3.5 4 4.5 5 6 7 8 9 10

230 V 1-phase current rating In (A) 6 10 10 16 16 20 20 25 25 32 32 40 50 50

The following tables allow direct selection of circuit breaker ratings for certain particular cases.

230 V 3-phase current rating In (A) 3 4 6 10 10 10 16 16 16 20 20 25 25 32

400 V 3-phase current rating In (A) 2 3 4 4 6 10 10 10 10 10 16 16 16 20

table J4-2: protective circuit breaker ratings for incandescent lamps and resistive-type heating circuits (see Note below). Note: at room temperature the filament resistance of a 100 W 230 V incandescent lamp is approximately 34 ohms. Some milli-seconds after switching on, the filament resistance rises to 2302/100 = 529 ohms. The initial current peak at the instant of switch closure is therefore practically 15 times its normal operating current. A similar (but generally less severe) transient current peak occurs when energizing any resistivetype heating appliance.

particular supply sources and loads - J31

4. lighting circuits (continued)

J 4.4 determination of the rated current of the circuit breaker (continued) The following table (J4-3) is valid for 230 V and 400 V installations, with or without individual power-factor correcting capacitors. mercury vapour fluorescent lamps P i 700 W 6A P i 1000 W 10 A P i 2000 W 16 A metal-halogen mercury-vapour lamps P 275 W 6A P 1000 W 10 A P 2000 W 16 A high-pressure sodium discharge lamps P 400 W 6A P 1000 W 10 A table J4-3: maximum limit of rated current per outgoing lighting circuit, for high-pressure discharge lamps. single-phase distribution 230 V three-phase distribution + N : 400 V phase/phase types de tube number of luminaires per phase luminaires rating (W) single-phase 18 7 14 21 42 70 112 140 175 with capacitor 36 3 7 10 21 35 56 70 87 58 2 4 6 13 21 34 43 54 duo circuit 2x18= 36 3 7 10 21 35 56 70 87 with 2x36= 72 1 3 5 10 17 28 35 43 capacitor 2x58= 116 1 2 3 6 10 17 21 27 current rating of 1-,2-,3 -or 4- pole CBs 1 2 3 6 10 16 20 25

225 112 69 112 56 34

281 140 87 140 70 43

351 175 109 175 87 54

443 221 137 221 110 68

562 281 174 281 140 87

703 351 218 351 175 109

32

40

50

63

80

100

Calculation for tubes with p.f. capacitor; connected in star number of tubes per phase = 0.8 C x 0.86 V Pu x 1.25 where: C = current rating of C B, V = phase/neutral voltage, 0.86 = cos ø of circuit, 0.8 = derating factor for high temperature in CB housing, 1.25 = factor for watts consumed by ballast, Pu = nominal power rating of tube (W). three-phase 3 wire system(230 V) phase/phase types de tube number of luminaires per phase luminaires rating (W) single-phase 18 4 8 12 24 40 64 81 101 with capacitor 36 2 4 6 12 20 32 40 50 58 1 2 3 7 12 20 25 31 duo circuit 2x18= 36 2 4 6 12 20 32 40 50 with 2x36= 72 1 2 3 6 10 16 20 25 capacitor 2x58= 116 0 1 1 3 6 10 12 15 current rating of 2- or 3- pole CBs 1 2 3 6 10 16 20 25

127 64 40 64 32 20

162 81 50 81 40 25

203 101 63 101 50 31

255 127 79 127 63 39

324 162 100 162 81 50

406 203 126 203 101 63

32

40

50

63

80

100

Calculation for tubes with p.f. capacitor; connected in delta number of tubes per phase = 0.8 C x 0.86 U Pu x 1.25 x e where: U = phase/phase voltage tables J4-4: current ratings of circuit breakers related to the number of fluorescent luminaires to be protected.

J32 - particular supply sources and loads

2. inverters and UPS (Uninterruptible Power Supply units) (continued)

J 2.10 complementary equipments transformers A two-winding transformer included on the upstream side of the static contactor of circuit 2 (see figure J2-5) allows: c a change of voltage level when the power network voltage is different to that of the load; c a different arrangement for the neutral on the load-side winding, from that of the power network. Moreover, such a transformer: c reduces the short-circuit current level on the secondary, (i.e. load) side compared with that on the power network side,

c prevents third harmonic currents (and multiples of them) which may be present on the secondary side from passing into the power-system network, providing that the primary winding is connected in delta.

anti-harmonic filter The UPS system includes a battery charger which is controlled by commutated thyristors or transistors. The resulting regularlychopped current cycles “generate” harmonic components in the power-supply network. These indesirable components are filtered at the input of the rectifier and for most cases this reduces the harmonic current level sufficiently for all practical purposes. In certain specific cases however, notably in very large installations, an additional filter circuit may be necessary.

For example, when: c the power rating of the UPS system is large relative to the HV/LV transformer supplying it; c the LV busbars supply loads which are particularly sensitive to harmonics; c a diesel (or gas-turbine, etc.) driven alternator is provided as a standby power supply. In such cases, the manufacturers of the UPS system should be consulted.

communications equipment Communication with equipment associated with informatic systems (see Sub-clause 2.5) may entail the need for suitable facilities within the UPS systems. Such facilities may be incorporated in an original design, or added to existing systems on request.

fig. J2-27: a UPS installation with incorporated communication systems.

J24 - particular supply sources and loads

3. protection of LV/LV transformers

J These transformers are generally in the range of several hundreds of VA to some hundreds of kVA and are frequently used for: c changing the (LV) voltage level for: v auxiliary supplies to control and indication circuits, v lighting circuits (230 V created when the primary system is 400 V 3-phase 3-wires), c changing the method of earthing for certain loads having a relatively high capacitive current to earth (informatic equipment) or resistive leakage current (electric ovens, industrial-heating processes, mass-cooking installations, etc.). LV/LV transformers are generally supplied

with protective systems incorporated, and the manufacturers must be consulted for details. Overcurrent protection must, in any case, be provided on the primary side. The exploitation of these transformers requires a knowledge of their particular function, together with a number of points described below. Note: In the particular cases of LV/LV safety isolating transformers at extra-low voltage, an earthed metal screen between the primary and secondary windings is frequently required, according to circumstances, as recommended in European Standard EN 60742, and as discussed in detail in Sub-clause 3.5 of Chapter G.

3.1 transformer-energizing in-rush current At the moment of energizing a transformer, high values of transient current (which includes a significant d.c. component) occur, and must be taken into account when considering protection schemes. The magnitude of the current peak depends on: c the value of voltage at the instant of energization, c the magnitude and polarity of magnetic flux (if any) existing in the core of the transformer, c characteristics of the load on the transformer. In distribution-type transformers, the first current peak can attain a value equal to 10 to 15 times the full-load r.m.s. current, but for small transformers (< 50 kVA) may reach values of 20 to 25 times the nominal full-load current. This transient current decreases rapidly, with a time constant θ (see figure J3-1) of the order of several milli-seconds to several tens of milli-seconds.

I Î first 10 to 25 In

In t

θ

fig. J3-1: transformer-energizing in-rush current.

3.2 protection for the supply circuit of a LV/LV transformer The protective device on the supply circuit for a LV/LV transformer must avoid the possibility of incorrect operation due to the magnetizing in-rush current surge, noted above in 3.1. It is necessary to use therefore: c selective (i.e. slightly time-delayed) circuit breakers of the type Compact NS STR* (figure J3-2) or c circuit breakers having a very high magnetic-trip setting, of the types Compact NS or Multi 9* curve D (figure J3-3).

t

50 to 70 ms

r.m.s. value of the first peak

* Merlin Gerin.

instantaneous I trip

fig. J3-2: tripping characteristic of a Compact NS STR circuit breaker. t

In r.m.s. value of the first peak

10In 20In

I

fig. J3-3: tripping characteristic of a circuit breaker according to standardized type D curve (for Merlin Gerin 10 to 14 In).

particular supply sources and loads - J25

3. protection of LV/LV transformers (continued)

J 3.2 protection for the supply circuit of a LV/LV transformer (continued) Example (figure J3-4) A 400 V 3-phase circuit is supplying a 125 kVA 400/230 V transformer (In = 180 A) for which the first in-rush current peak can reach 17 In, i.e. 17 x 180 A = 3,067 A. A Compact NS250 circuit breaker with Ir setting of 200 A would therefore be a suitable protective device. A particular case: overload protection installed at the secondary side of the transformer An advantage of overload protection located on the secondary side, is that the short-circuit protection on the primary side can be set at a high value, or alternatively a circuit breaker type MA* may be used. The primary-side short-circuit protection setting must, however, be sufficiently sensitive to ensure its operation in the event of a short-circuit occurring on the secondary side of the transformer (upstream of secondary protective devices).

NS250N tripping unit STR22SE (Ir = 200)

3 x 70 mm2 400/230 V 125 kVA

fig. J3-4: example. Note: The primary protection is sometimes provided by fuses, type a M. This practice has two disadvantages: c the fuses must be largely oversized (at least 4 times the nominal full-load rated current of the transformer); c in order to provide isolating facilities on the primary side, either a load-break switch or a contactor must be associated with the fuses.

* Motor-control circuit breaker, the short-circuit protective relay of which is immune to high transient-current peaks, as shown in figure J5-3.

3.3 typical electrical characteristics of LV/LV 50 Hz transformers 3-phase kVA rating no-load losses (W) full-load losses (W) s.c. voltage (%) 1-phase kVA rating no-load losses (W) full-load losses (W) s.c. voltage (%)

5 100 250 4.5

6.3 110 320 4.5

8 130 390 4.5

10 150 500 5.5

12.5 160 600 5.5

16 170 840 5.5

20 270 800 5.5

8 105 400 5

10 115 530 5

12.5 120 635 5

16 140 730 4.5

20 150 865 4.5

25 31.5 175 200 1065 1200 4 4

25 310 1180 5.5

31.5 350 1240 5

40 350 1530 5

50 410 1650 4.5

63 460 2150 5

80 520 2540 5

100 570 3700 5.5

40 215 1400 5

50 265 1900 5

63 305 2000 4.5

80 450 2450 5.5

100 450 3950 5

125 160 525 635 3950 4335 5

125 680 3700 4.5

160 680 5900 5.5

200 790 5900 5

250 950 6500 5

315 1160 7400 4.5

400 1240 9300 6

500 1485 9400 6

630 1855 11400 5.5

800 2160 11400 5.5

table J3-5: typical electrical characteristics of LV/LV 50 Hz transformers.

3.4 protection of transformers with characteristics as tabled in J3-5 above, using Merlin Gerin circuit breakers 3-phase transformers (400 V primary) P (kVA) In (A) Usc %

circuit breakers type

5 10 16 20 25 31.5 40 50 63

7 14 23 28 35 44 56 70 89

4.5 5.5 5.5 5.5 5.5 5 5 4.5 5

80 100 125

113 141 176

5 5.5 4.5

160

225

5.5

250 315 400 500

352 444 563 704

5 4.5 6 6

630

887

5.5

C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K NC100 D NC100 D NC100 D NC100 D NS100H/L NS160H/L NS160H/L NS250N/H/L NS250N/H/L NS400N/H/L NS250N/H/L NS400N/H/L C801N/H/L C801N/H/L C801N/H/L C801NH/L C1001N/H/L C1001N/H/L C1251N/H

trip-unit current rating (A)/type no. 20 32 63 63 80 80 80 100 MA100 STR22SE STR22SE STR22SE STR22SE STR23SE STR22SE STR23SE STR35SE STR35SE STR35SE STR35SE STR35SE STR35SE STR35SE

table J3-6: protection of 3-phase LV/LV transformers with 400 V primary windings. J26 - particular supply sources and loads

J 3-phase transformers (230 V primary) P (kVA) In (A) Usc %

circuit breakers type

5 10 16 20 25 31.5 40 50 63

12 24 39 49 61 77 97 122 153

4.5 5.5 5.5 5.5 5.5 5 5 4.5 5

80

195

5

100 125 160 250

244 305 390 609

5.5 4.5 5.5 5

315

767

4.5

400

974

6

C60 / NC100 D or K C60 / NC100 D or K NC100 D NC100 D NS100H/L NS100H/L NS100H/L NS100H/L NS250N/H/L NS400N/H/L NS250N/H/L NS400N/H/L NS630N/H/L C801N/H/L C801N/H/L C801N/H/L C1001N/H/L C1001N/H/L C1251N/H C1251N/H

trip-unit current rating (A)/type no. 40 63 80 100 STR22SE STR22SE STR22SE STR22SE STR22SE STR23SE STR22SE STR23SE STR23SE STR35SE STR35SE STR35SE STR35SE STR35SE STR35SE STR35SE

table J3-7: protection of 3-phase LV/LV transformers with 230 V primary windings. 1-phase transformers (400 V primary) P (kVA) In (A) Usc %

circuit breakers type

0.1 0.16 0.25 0.4 0.63 1 1.6 2 2.5 4 5 6.3 8 10 12.5 16 20 25 31.5 40 50 63

0.24 0.39 0.61 0.98 1.54 2.44 3.9 4.88 6.1 9.8 12.2 15.4 19.5 24 30 39 49 61 77 98 122 154

13 10.5 9.5 7.5 7 5.2 4 2.9 3 2.1 1.9 1.6 5 5 5 4.5 4.5 4.5 4 4 4 5

80

195

4.5

100 125 160

244 305 390

5.5 5 5

C60 D or K C60 D or K C60 D or K C60 D or K C60 D or K C60 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K NC100 D NC100 D NS160H/L NS160H/L NS160H/L NS160H/L NS250N/H/L NS400N/H/L NS250N/H/L NS400 NS630 C801N/H/L C801N/H/L

trip-unit current rating (A)/type no. 1 1 1 2 3 6 10 10 16 20 32 40 50 63 63 80 100 STR22SE STR22SE STR22SE STR22SE STR22SE STR23SE STR22SE STR23SE STR23SE STR35SE STR35SE

table J3-8: protection of 1-phase LV/LV transformers with 400 V primary windings.

particular supply sources and loads - J27

3. protection of LV/LV transformers (continued)

J 3.4 protection of transformers with characteristics as tabled in J3-5 above, using Merlin Gerin circuit breakers (continued) 1-phase transformers (230 V primary) P (kVA) In (A) Usc %

circuit breakers type

0.1 0.16 0.25 0.4 0.63 1 1.6 2 2.5 4 5 6.3 8 10 12.5 16 20 25

0.4 0.7 1.1 1.7 2.7 4.2 6.8 8.4 10.5 16.9 21.1 27 34 42 53 68 84 105

13 10.5 9.5 7.5 7 5.2 4 2.9 3 2.1 1.9 1.6 5 5 5 4.5 4.5 4.5

31.5 40

133 169

4 4

50

211

5

63 80 100 125 160

266 338 422 528 675

5 4.5 5.5 5 5

C60 D or K C60 D or K C60 D or K C60 D or K C60 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K NC100 D NC100 D NC100 D NS160H/L NS160H/L NS250N/H/L NS250N/H/L NS250N/H/L NS250N/H/L NS400N/H/L NS250N/H/L NS400N/H/L NS630N/H/L C801N/H/L C801N/H/L C801N/H/L C801N/H/L C1001N/H/L

trip-unit current rating (A)/type no. 1 2 3 4 6 10 16 16 20 40 50 63 80 100 100 STR22SE STR22SE STR22SE STR22SE STR22SE STR22SE STR23SE STR22SE STR23SE STR23SE STR35SE STR35SE STR35SE STR35SE STR35SE

table J3-9: protection of 1-phase LV/LV transformers with 230 V primary windings.

J28 - particular supply sources and loads

4. lighting circuits

J the presence of adequate lighting contributes to the satety of persons.

The planning and realization of a lighting installation requires a sound understanding of the materials installed, together with familiarity with the rules for safety against fire hazards in establishments receiving the public.

emergency lighting is intended to facilitate the evacuation of persons in case of fire or other panic-causing situations, when normal lighting systems may have failed.

definitions Normal lighting refers to the installation designed for everyday use. Emergency lighting must ensure easy evacuation of persons from the premises concerned, in the event that the normal lighting system fails. Furthermore, emergency lighting must be adequate to allow any particular safety manœuvres provided in the premises to be carried out.

In fact, the provision of adequate illumination in the event of fire or other catastrophic circumstances is of great importance in reducing the likelihood of panic, and in permitting the necessary safety manœuvres to be carried out.

Standby lighting is intended to substitute normal lighting, where the latter fails. Standby lighting permits everyday activities to continue more or less normally, depending on the original design specification, and on the extent of the normal lighting failure. Failure of the standby lighting system must automatically switch on the emergency lighting system.

4.1 service continuity continuity of normal lighting service must be sufficient, independent of other supplementary systems.

normal lighting

in emergency lighting circuits, absolute discrimination between protective devices on the different circuits must be provided.

emergency lighting

Regulations governing the minimum requirements for ERP (Establishments Receiving the Public) in most European countries, are as follows: c installations which illuminate areas accessible to the public must be controlled and protected independently from installations providing illumination to other areas; c loss of supply on a final lighting circuit (i.e. fuse blown or CB tripped) must not result in total loss of illumination in an area which is capable of accommodating more than 50 persons; c protection by RCDs (residual current differential devices) must be divided amongst several devices (i.e. more than one device must be used).

These schemes include illuminated emergency exit signs and direction indications, as well as general lighting. c emergency exit indications In areas accommodating more than 50 persons, luminous directional indications to the nearest emergency exits must be provided; c general emergency lighting General lighting is obligatory when an area can accommodate 100 persons or more (50 persons or more in areas below ground level). A fault on a lighting distribution circuit must not affect any other circuit: v the discrimination of overcurrent-protection relays and of RCDs must be absolute, so that only the faulty circuit will be cut off, v the installation must be an IT scheme, or must be entirely class II, i.e. doubly-insulated. Sub-clause 4.7 describes different kinds of suitable power supplies.

particular supply sources and loads - J29

4. lighting circuits (continued)

J 4.2 lamps and accessories (luminaires) fluorescent tubes For normal operation a fluorescent tube requires a ballast and a starter (device for initiating the luminous discharge). c the ballast is an iron-cored inductor, permanently connected in series with the tube; its function is threefold, viz: v to limit the preheating current during the (brief) starting period, v to provide a pulse of high voltage at the end of the starting period to strike the initial arc, v to stabilize the current through the luminous column (hence the term “ballast”).

c the starter is a switch, which, by breaking the (electrode-preheating) current passing through the ballast, causes a high-voltage transient pulse to appear across the tube. This causes an arc (in the form of a gaseous discharge) to be established through the tube. The discharge is then self-sustaining at normal voltage. The ballast, capacitor and the tube, engender disturbances during the periods of starting, steady operation and extinction. These disturbances are analysed in table J4-1 below.

The presence of the ballast means that the power-factor (cos ø) of the circuit is low (of the order 0.6) with the corresponding consumption of reactive energy, which is generally metered. For this reason each fluorescent lamp is normally provided with its own power-factor-correction capacitor. single-phase fluorescent lamp with its individual p.f. correction capacitor 1 2 3

single-phase twin-tube fluorescent lamp with each tube having its own starter and series ballast. One of the tubes has a capacitor connected in series with its ballast. The two sets of equipment are connected in parallel. The arrangement is known internationally as a “duo”-circuit luminaire. The capacitor displaces the phase of the current through its tube, to nullify the flicker effect, as well as correcting the overall p.f. A

ballast

switching-on disturbances c high current peak to charge capacitor; order of magnitude 10 In for 1 sec. A number of lamps on one circuit can result in peaks of 300-400 A for 0.5 ms. This can cause a CB to trip, or the welding of contacts in a contactor. In practice, limit each circuit to 8 tubes per contactor; c moderate overload at the beginning of steady operating condition (1.1-1.5 In for 1 sec) according to type of starter. c no high current peak as noted above; c same order of moderate overload at beginning of steady operating condition as for the single tube noted above. This arrangement is recommended for difficult cases.

switching-off disturbances no particular problems

c can generate a current peak at start; c can cause leakage to earth of HF current (at 30 kHz) via the phase conductor capacitances to earth.

no particular problems

1 2 3

presence of 5th and 7th harmonics at very low level

no particular problems

presence of 3 rd harmonic currents in the neutral, which can reach 70 to 80% of the nominal phase current. In this case, therefore, the c.s.a. of the neutral conductor must equal that of the phase conductors.

B starter

Advantages: Energy savings of the order of 25%. Rapid one-shot start. No flicker or stroboscopic effects.

table J4-1: analysis of disturbances in fluorescent-lighting circuits.

J30 - particular supply sources and loads

c star-connected lamps (3-ph 4-wire 400/230 V system) 1 2 3 N

starter

fluorescent lamp with HF ballast

steady-operating disturbances circulation of harmonic currents (sinusoidal currents at frequencies equal to whole-number multiples of 50 (or 60) Hz: c delta-connected lamps (see Appendix J2) (3-ph 3-wire 230 V system)

J 4.3 the circuit and its protection dimensions and protection of the conductors The maximum currents in the circuits can be estimated using the methods discussed in Chapter B. Accordingly, account must be taken of: c the nominal power rating of the lamp and the ballast; c the power factor. The temperature within the distribution panel also influences the choice of the protective device (see Chapter H2 Sub-clause 4.4). In general tables are available from manufacturers to assist in making a choice.

Note: for circuits in which large peak currents occur (at times of switching on) and their magnitude is such that CB tripping is a possibility, the cable size is chosen after the protective CB (with an instantaneous trip setting sufficient to remain closed during the current peaks) has been selected. See the Note following table J4-2.

factor of simultaneity ks (diversity) A particular feature of large (e.g. factory) lighting circuits is that the whole load is “on” or “off”, i.e. there is no diversity. Furthermore, even among a number of lighting circuits from a given distribution panel, the factor ks is generally near unity.

Consequently, the interior of distribution panels supplying lighting schemes are frequently at an elevated temperature, an important consideration to be taken into account when selecting protective devices.

4.4 determination of the rated current of the circuit breaker The rated current of a circuit breaker is generally chosen according to the rating of the circuit conductors it is protecting (in the particular circumstances in the Note of 4.3 above, however, the reverse procedure was found to be necessary). The circuit conductor ratings are defined by the maximum steady load current of the circuit. power (kW) 1 1.5 2 2.5 3 3.5 4 4.5 5 6 7 8 9 10

230 V 1-phase current rating In (A) 6 10 10 16 16 20 20 25 25 32 32 40 50 50

The following tables allow direct selection of circuit breaker ratings for certain particular cases.

230 V 3-phase current rating In (A) 3 4 6 10 10 10 16 16 16 20 20 25 25 32

400 V 3-phase current rating In (A) 2 3 4 4 6 10 10 10 10 10 16 16 16 20

table J4-2: protective circuit breaker ratings for incandescent lamps and resistive-type heating circuits (see Note below). Note: at room temperature the filament resistance of a 100 W 230 V incandescent lamp is approximately 34 ohms. Some milli-seconds after switching on, the filament resistance rises to 2302/100 = 529 ohms. The initial current peak at the instant of switch closure is therefore practically 15 times its normal operating current. A similar (but generally less severe) transient current peak occurs when energizing any resistivetype heating appliance.

particular supply sources and loads - J31

4. lighting circuits (continued)

J 4.4 determination of the rated current of the circuit breaker (continued) The following table (J4-3) is valid for 230 V and 400 V installations, with or without individual power-factor correcting capacitors. mercury vapour fluorescent lamps P i 700 W 6A P i 1000 W 10 A P i 2000 W 16 A metal-halogen mercury-vapour lamps P 275 W 6A P 1000 W 10 A P 2000 W 16 A high-pressure sodium discharge lamps P 400 W 6A P 1000 W 10 A table J4-3: maximum limit of rated current per outgoing lighting circuit, for high-pressure discharge lamps. single-phase distribution 230 V three-phase distribution + N : 400 V phase/phase types de tube number of luminaires per phase luminaires rating (W) single-phase 18 7 14 21 42 70 112 140 175 with capacitor 36 3 7 10 21 35 56 70 87 58 2 4 6 13 21 34 43 54 duo circuit 2x18= 36 3 7 10 21 35 56 70 87 with 2x36= 72 1 3 5 10 17 28 35 43 capacitor 2x58= 116 1 2 3 6 10 17 21 27 current rating of 1-,2-,3 -or 4- pole CBs 1 2 3 6 10 16 20 25

225 112 69 112 56 34

281 140 87 140 70 43

351 175 109 175 87 54

443 221 137 221 110 68

562 281 174 281 140 87

703 351 218 351 175 109

32

40

50

63

80

100

Calculation for tubes with p.f. capacitor; connected in star number of tubes per phase = 0.8 C x 0.86 V Pu x 1.25 where: C = current rating of C B, V = phase/neutral voltage, 0.86 = cos ø of circuit, 0.8 = derating factor for high temperature in CB housing, 1.25 = factor for watts consumed by ballast, Pu = nominal power rating of tube (W). three-phase 3 wire system(230 V) phase/phase types de tube number of luminaires per phase luminaires rating (W) single-phase 18 4 8 12 24 40 64 81 101 with capacitor 36 2 4 6 12 20 32 40 50 58 1 2 3 7 12 20 25 31 duo circuit 2x18= 36 2 4 6 12 20 32 40 50 with 2x36= 72 1 2 3 6 10 16 20 25 capacitor 2x58= 116 0 1 1 3 6 10 12 15 current rating of 2- or 3- pole CBs 1 2 3 6 10 16 20 25

127 64 40 64 32 20

162 81 50 81 40 25

203 101 63 101 50 31

255 127 79 127 63 39

324 162 100 162 81 50

406 203 126 203 101 63

32

40

50

63

80

100

Calculation for tubes with p.f. capacitor; connected in delta number of tubes per phase = 0.8 C x 0.86 U Pu x 1.25 x e where: U = phase/phase voltage tables J4-4: current ratings of circuit breakers related to the number of fluorescent luminaires to be protected.

J32 - particular supply sources and loads

J 4.5 choice of control-switching devices The advent of switching devices which combine the functions of remote control and protection, of which the remotely-controllable residual-current circuit breaker is the prototype, simplifies lighting-control circuits considerably, thereby enlarging the scope and diversity of control schemes. remote-control mode

point-to-point remote control centralized remote control point-to-point and centralized remote control control signals over communications bus control signals over timemultiplexing channels

Certain switching devices include control circuitry for operation at ELV (extra-lowvoltage, i.e. < 50 V or < 25 V according to requirements); these control circuits being insulated for 4,000 V with respect to the power circuits. The situation at the time of writing is summarized below in table J4-5.

function of corresponding switchgear and controlled equipment remote control

remote control + overcurrent protection

bistable switch

circuit breaker controlled by hard-wire system

contactor “pilot” bistable switch remote controlled switch remotely controlled switch

remotely controlled circuit breaker over communications bus remotely controlled static contactor/ circuit breaker combination

remote control + overcurrent protection + insulation monitoring and protection residual current circuit breaker controlled by hard-wire system

local control devices

centralized control devices

push-button

stairway time-switch with automatic switch-off automatic photo-electric lighting-control switches movement detectors; central clock relaying

switch push button

residual current circuit breaker controlled over communications bus

according to type

table J4-5: types of remote control.

particular supply sources and loads - J33

4. lighting circuits (continued)

J 4.6 protection of ELV lighting circuits A LV/ELV transformer is often located in an inaccessible position, so that protection installed on the secondary side would be equally difficult to reach. For this reason the protection is commonly provided on the primary circuit. The protective device is therefore chosen: c to provide switching control (Multi 9 type C CB, or type aM fuses); c to ensure protection against short-circuits. It must therefore be verified that: v in the case of a CB, the minimum value of short-circuit current exceeds by a suitable margin the short-circuit magnetic relay setting Im of the CB concerned, v in the case of fuses it is also necessary to ensure that the I2t energy let-through of the fuse(s) at minimum short-circuit current is well below the level of the thermal withstand capacity of the circuit conductors, c if necessary, overload protection must be provided. If the number of lamps on the circuit has been correctly chosen, however, overload protection is not necessary. Example: The s.c. current Isc2 at the secondary terminals of a single-phase LV/ELV transformer is equal to Us where Zs = Us2 x Usc % Zs Pn 100 Pn x 100 so that Isc2 = = 400 x 100 Us x Usc% 12 x 6 = 555 A which gives Isc1 = 29 A in the primary circuit. Circuit breaker type C trips if the primary current u Im1 = 10 In = 20 A, which corresponds to a secondary current of 20 x 230 = 383 A 12 The maximum resistance of the ELV (i.e. secondary) circuit* may be deduced from these two secondary s.c. currents, viz: 555 A and 383 A as follows: Rc = U2 - V2 = 12 - 12 = 0.0313 - 0.0216 Im2 Isc2 383 555 = 9.7 mΩ * from the transformer terminals to the ELV distribution board.

Note: The true value of Rc permitted is, in principle, greater than 9.7 milli-ohms, because the source impedance (i.e. U2/555, will be mainly reactive, not resistive, as (implied) in the example. However, for simplicity, and to automatically provide a safety margin under all circumstances, an arithmetic subtraction, as shown, is recommended. The maximum length of the 12 V circuit based on 9.7 mΩ will therefore be: Rc (mΩ) x S (mm2) in metres = 9.7 x 6 2 x 22.5 (µΩ.mm) 2 x 22.5 for a 6 mm2 copper cable = 1.3 m It is then necessary to check that this length is sufficient to reach the 12 V distribution board, where the outgoing ways are protected with other devices. If the length is insufficient, then an increase in the c.s.a. of the conductors, proportional to the increased length required, will satisfy the constraint for maximum Rc; for example, a conductor of 10 mm2 would allow 1.3 x 10/6 = 2.2 m of circuit length in the above case.

J34 - particular supply sources and loads

2A

LV ELV

230/12 V 400 VA Usc = 6%

secondary circuit

fig. J4-6: example.

J 4.7 supply sources for emergency lighting Supply sources for emergency-lighting systems must be capable of maintaining the supply to all lamps in the most unfavourable circumstances likely to occur, and for a period judged necessary to ensure the total evacuation of the premises concerned, with (in any case) a minimum of one hour.

compatibility between emergency lighting sources and other parts of the installation Emergency-lighting sources must supply exclusively the circuits installed only for operation in emergency situations. Standby lighting systems operate to maintain illumination, on failure of normal lighting circuits (generally in non-emergency circumstances). However, failure of standby lighting must automatically bring the emergency lighting system into operation.

Central sources for emergency supplies may also be used to provide standby supplies, provided that the following conditions are simultaneously fulfilled: c where there are several sources, the failure of one source must leave sufficient capacity in service to maintain supply to all safety systems, with automatic load shedding of non-essential loads (if necessary); c the failure of one source, or one equipment concerned with safety, must leave all other sources and safety equipments unaffected; c any safety equipment must be arranged to receive supply from any source.

classification of emergencylighting schemes Many countries have statutory regulations concerning safety in buildings and areas intended for public gatherings. Classification of such locations leads to the determination of suitable types of solutions, authorized for use in emergency-lighting schemes in the different areas. The following four classifications are typical. Type A The lamps are supplied permanently and totally during the presence of the public by a single central source (battery of storage cells, or a heat-engine-driven generator). These circuits must be independent of any other circuits (1). Type B The lamps are permanently supplied during the presence of the public, either: c by a battery to which the lamps are permanently connected, and which is on permanent trickle charge from a normal lighting source, or, c by a heat-engine-driven generator, the characteristics of which also assure supplies to essential loads within one second (since the set is already running and supplying the emergency lighting) in the event of failure of the normal power supply, or, c by autonomous units which are normally supplied and permanently alight from the normal lighting supply, and which remain alight (for at least one hour), on the loss of normal supply, by virtue of a self-contained battery. The battery is trickle-charged in normal circumstances. These units have fluorescent lamps for general emergency lighting, and fluorescent or incandescent lamps for exit and directionindicating signs. The circuits for all emergency lamps must be independent of any other circuits (1).

Type C The lamps may, or may not, be supplied in normal conditions and, if supplied, may be fed from the normal lighting system, or from the emergency-lighting supply. c the emergency-lighting batteries must be maintained on charge from the normal source by automatically regulated systems, that ensure a minimum of capacity equal to the full emergency-lighting load for one hour; c the heat-engine-driven generator sets must be capable of automatically picking-up the full emergency lighting load from a standby (stationary) condition, in less than 15 seconds, following the failure of normal supply. The engine start-up power is provided by a battery which is capable of six starting attempts, or by a system of compressed air. Minimum reserves of energy in the two systems of start-up must be maintained automatically. c failures in the central emergency supply source must be detected at a sufficient number of points and adequately signalled to supervisory/maintenance personnel; c autonomous units may be of the permanently-lit type or non-permanently-lit type. The circuits for all emergency lamps must be independent of any other circuits (2). Type D This type of emergency lighting comprises hand-carried battery-powered (primary or secondary cells) at the disposal of service personnel or the public. (1) Circuits for types A and B, in the case of a central emergency power source, must also be fire-resistant. Conduit boxes, junction sleeves and so on must satisfy national standard heat tests, or the circuits must be installed in protective cable chases, trunking, etc. capable of assuring satisfactory performance for at least one hour in the event of fire. (2) Cable circuits of type C are not required to comply with the conditions of (1).

particular supply sources and loads - J35

5. asynchronous motors

J the asynchronous (i.e. induction) motor is robust and reliable, and very widely used. 95% of motors installed around the world are asynchronous. The protection of these motors is consequently a matter of great importance in numerous applications.

The consequences of an incorrectly protected motor can include the following: c for persons: v asphyxiation due to the blockage of motor ventilation, v electrocution due to insulation failure in the motor, v accident due to sticking (contact welding) of the controlling contactor; c for the driven machine and the process: v shaft couplings and axles, etc. damaged due to a stalled rotor, v loss of production, v manufacturing time delayed; c for the motor: v motor windings burnt out due to stalled rotor, v cost of dismantling and reinstating or replacement of motor, v cost of repairs to the motor.

specific features of motor performance influence the powersupply circuits required for satisfactory operation.

A motor power-supply circuit presents certain constraints not normally encountered in other (common) distribution circuits, owing to the particular characteristics, specific to motors, such as: c heavy start-up current (see figure J5-1) which is highly reactive, and can therefore be the cause of an important voltage drop; c number and frequency of start-up operations are generally high; c the heavy start-up current means that motor overload protective devices must have operating characteristics which avoid tripping during the starting period.

It is, therefore, the safety of persons and goods, and reliability and availability levels which must influence the choice of protective equipment. In economic terms, it is the overall cost of failure which must be considered; a penalty which is increasingly severe as the size of the motor, and difficulties of access to it increase. Loss of production is a further, and evidently important factor.

t

I" = 8 to 12 In Id = 5 to 8 In In = nominal motor current

td 1 to 10s

20 to 30 ms In

Id

I"

fig. J5-1: direct-on-line starting-current characteristics of an induction motor.

5.1 protective and control functions required functions to be provided generally include: c basic protective devices, c electronic control equipment, c preventive or limitative protection equipment.

J36 - particular supply sources and loads

Functions generally provided are: c basic protection, including: v isolating facility, v manual local and/or remote control, v protection against short-circuits, v protection against overload; c electronic controls consisting of: v progressive “soft-start” motor starter, or, v speed controller; c preventive or limitative protection by means of: v temperature sensors, v multi-function relays, v permanent insulation-resistance monitor or RCD (residual-current differential device). Table J5-2 below, shows diverse motor-circuit configurations commonly used in LV distribution boards.

I

J basic protection

fuse-disconnector + discontactor (using thermal relay)

circuit breaker* motor circuit + discontactor breaker* + contactor (using thermal relay)

contactor circuit breaker* ACPA

standards disconnection (or isolation)

manual control

remote control

short-circuit protection * circuit breaker includes disconnector capability

overload protection

c large power range c allow all types of starting schemes c a well-proven method c suitable for systems having high fault levels

refer also to Chapter H2, Sub-clause 2-2 electronic controls

preventive or limitative protection devices

c large power range c method is simple c avoids need to stock and compact for fuse cartridges low-power motors c disconnection is visible in certain cases c identification of the reason for tripping i.e. short circuit or overload

progressive “soft-start” starter device c limitation v current peaks I v voltage drops U v mechanical constraints during start-up period c thermal protection is incorporated

c low installation costs c no maintenance c high degree of safety and reliability c suitable for systems having high fault levels c long electrical life

speed controller c from 2 to 130 % of nominal speed c thermal protection is incorporated c possibility of communication facilities

thermal sensors Protection against abnormal heating of the motor by thermistance-type sensors in the motor windings, connected to associated relays. multi-function relays Direct and indirect thermal protection against: c the starting period excessively long, or stalled-rotor condition c imbalance, absence or inversion of phase voltages c earth fault or excessive earth-leakage current c motor running on no-load; motor blocked during start-up c pre-alarm overheating indication permanent insulation-to-earth monitor and RCD (residual-current differential relay) Protection against earth-leakage current and short-circuits to earth. Signalled indication of need for motor maintenance or replacement.

table J5-2: commonly-used types of LV motor-supply circuits.

particular supply sources and loads - J37

5. asynchronous motors (continued)

J 5.2 standards The international standards covering materials discussed in this Sub-clause are: IEC 947-2, 947-3, 947-4-1, and 947-6-2. These standards are being adopted (often without any changes) by a number of countries, as national standards.

5.3 basic protection schemes: circuit breaker / contactor / thermal relay functions to be implemented are: c control (start/stop), c isolation (safety during maintenance), c protection against short-circuits, c specific protection as noted in Sub-clause 5.1 Where several different devices are used to provide protection, coordination between them is necessary.

among the many possible methods of protecting a motor, the association of a circuit breaker incorporating an instantaneous magnetic trip for shortcircuit protection and a contactor with a thermal overload relay* provides many advantages.

The control and protection of a motor can be provided by one, two or three devices, which share the required functions of: c control (start/stop); c disconnection (isolation) for safety of personnel during maintenance work; c short-circuit protection; c protection specific to the particular motor (but at least thermal relay overcurrent protection). When these functions are performed by several devices, co-ordination between them is essential. In the case of an electrical fault of any kind, none of the devices involved must be damaged, except items for which minor damage is normal in the particular circumstances, e.g. replaceable arcing contacts in certain contactors, after a given number of service operations, and so on... The kind of co-ordination required depends on the necessary degree of service continuity and on safety levels, etc. t

range 1.05 - 1.20 In

circuit breaker magnetic relay

characteristics of thermal relay

end of start-up period

contactor thermal relay

cable thermal-withstand limit

ts 1 to 10 s

limit of thermalrelay constraint

câble motor (nominal current In)

short-circuit tripping characteristic of the circuit breaker (type MA)

20 to 30 ms In

Is

I"

Imagn. circuit breaker only

l CB plus contactor (see Note) short-circuit-current breaking capacities

fig. J5-3: tripping characteristics of a circuit breaker (type MA)** and thermal-relay / contactor (1) combination. Advantages c interlocking; This combination of devices facilitates c diverse remote indications; installation work, as well as operation and c better protection for the starter for shortmaintenance, by: circuit currents up to about 30 In (see c the reduction of the maintenance work load: figure J5-3). the CB avoids the need to replace blown In the majority of cases short-circuit faults fuses and the necessity of maintaining a occur at the motor, so that the current is stock (of different sizes); limited by the cable and the wiring of the c better continuity performance: a motor starter (e.g. the direct-acting trip coil of the circuit can be re-energized immediately CB). following the elimination of a fault; c possibility of adding RCD: c additional complementary devices v an RCD of 500 mA sensitivity practically sometimes required on a motor circuit are eliminates fire risk due to leakage current, easily accommodated; v protection against destruction of the motor c tripping of all three phases is assured (short-circuiting of laminations) by the early (thereby avoiding the possibility of “singledetection of earth-fault currents (300 mA to phasing”); 30 A); c full-load current switching possibility (by c etc. CB) in the event of contactor failure, e.g. contact welding; * The association of an overload relay and a contactor is referred to as a “discontactor” in some countries. ** Merlin Gerin.

J38 - particular supply sources and loads

J Note: When short-circuit currents are very high, the contacts of some contactors may be momentarily forced open by electro-magnetic repulsion, so that two sets of contacts (i.e. those of the CB and those of the contactor) are acting in series. The combination effectively increases the s.c. current-breaking capacity above that of the CB alone. Conclusion The association circuit breaker / contactor / thermal relay(1) for the control and protection of motor circuits is eminently appropriate where: c the maintenance service for an installation is reduced, which is generally the case in tertiary and small-and medium-sized industrial enterprises; c the job specification calls for complementary functions; c there is an operational requirement for a load-breaking facility in the event of contact welding of the contactor. (1) a contactor in association with a thermal relay is commonly referred to as a discontactor.

standardization of the association of circuit breakers/ discontactors Categories of contactor The standard IEC 947-4 gives utilization categories which considerably facilitate the choice of a suitable contactor for a given service duty. The utilization categories advise on: c a range of functions for which the contactor may be adapted; c its current breaking and making capabilities; c standard test values for expected life duration on load, according to its utilization. The following table gives some typical examples of the utilization categories covered. utilization category AC-1 AC-2 AC-3 AC-4

application characteristics Non-inductive (or slightly inductive) loads: cos ø u 0.95 (heating, distribution) Starting and switching off of slip-ring motors Cage motors: starting, and switching off motors during running Cage motors: starting, plugging, inching

table J5-4: utilization categories for contactors (IEC 947-4). Types of co-ordination For each association of devices, a type of coordination is given, according to the state of the constituant parts following a circuit breaker trip out on fault, or the opening of a contactor on overload. IEC 947-4-1 defines two types of coordination, type 1 and type 2, which set maximum allowable limits of deterioration of switchgear, which must never present a danger to personnel. c type 1: deterioration of the contactor and/or of its relay is acceptable under 2 conditions: v no risk for the operator, v all elements other than the contactor and its relay must remain undamaged; c type 2: burning, and the risk of welding of the contacts of the contactor are the only risks allowed.

Which type to choose? The type of co-ordination to adopt depends on the parameters of exploitation, and must be chosen to satisfy (optimally) the needs of the user and the cost of installation. c type 1: v qualified maintenance service, v volume and cost of switchgear reduced, v continuity of service not demanded, or provided by replacement of motor-starter drawer; c type 2: v continuity of service imperative, v no maintenance service, v specifications stipulating this type of coordination.

particular supply sources and loads - J39

5. asynchronous motors (continued)

J 5.3 basic protection schemes: circuit breaker / contactor / thermal relay (continued) key points in the successful association of a circuit breaker and a discontactor t Compact NS type MA

2

1 CB magnetic-trip performance curve 2 thermal-relay characteristic 3 thermal-withstand limit of the thermal relay 1 3

Isc ext.

I

fig. J5-5: the thermal-withstand limit of the thermal relay must be to the right of the CB magnetic-trip characteristic. Standards define precisely all the elements which must be taken into account to realize a correct co-ordination of type 2: c absolute compatibility between the thermal relay of the discontactor and the magnetic trip of the circuit breaker. In figure J5-5 the thermal relay is protected if its limit boundary for thermal withstand is placed to the right of the CB magnetic trip characteristic curve. In the case of a motor-control circuit breaker incorporating both magnetic and thermal devices, co-ordination is provided in the design;

it is not possible to predict the s.c. current-breaking capability of a CB + contactor combination. Laboratory tests and calculations by manufacturers are necessary to determine which type of CB to associate with which contactor, and to establish the s.c. breaking capacity of the combination. Tables are published by Merlin Gerin giving this information in their “LV Distribution” catalogue.

short-circuit current-breaking capacity of a combination circuit breaker + contactor In the studies, the s.c. current-breaking capacity which must be compared to the prospective short-circuit current is: c either, that of the CB + contactor combination, if these devices are physically close together (e.g. in the same drawer or compartment of a MCC*). A short-circuit downstream of the combination will be limited to some extent by the impedances of the contactor (see previous Note) and the thermal relay. The combination can therefore be used on a circuit for which the prospective short-circuit current level exceeds the rated s.c. current-breaking capacity of the circuit breaker. This feature very often presents a significant economic advantage; c or, that of the CB only, for the case where the contactor is separated from the CB (so that a short-circuit is possible on the intervening circuit). For such a case, IEC 947-4-1 requires the rating of the circuit breaker to be equal to or greater than the prospective short-circuit current at its point of installation. * Motor Control Centre.

J40 - particular supply sources and loads

c the short-circuit current breaking rating of the contactor must be greater than the regulated threshold of the CB magnetic trip relay, since it (the contactor) must be capable of breaking a current which has a value equal to, or slightly less than, the setting of the magnetic relay (as seen from figure J5-5); c a reliable performance of the contactor and its thermal relay when passing short-circuit current, i.e. no excessive deterioration of either device and no welding of contactor contacts.

M

fig. J5-6: circuit breaker and contactor mounted in juxtaposition.

M

fig. J5-7: circuit breaker and contactor separately mounted, with intervening circuit conductors.

J choice of instantaneous magnetic-trip relay for the circuit breaker The operating threshold must never be less than 12 In for this relay, in order to avoid possible tripping due to the first current peak during start-up. This current peak can vary from 8 In to 11 or 12 In.

5.4 preventive or limitative protection preventive or limitative protection devices detect signs of impending failure, so that action can be taken (automatically or by operator intervention) to avoid or limit the otherwise inevitable consequences.

The main protection devices of this type for motor are: c thermal sensors in the motor (windings, bearings, cooling-air ducts, etc.); c multifunction protections; c insulation-failure detection devices on running, or stationary motor.

thermal sensors Thermal sensors are used to detect abnormal temperature rise in the motor by direct measurement. The thermal sensors are generally embedded in the stator windings (for LV motors), the signal being processed by an associated control device acting to trip the circuit breaker (figure J5-8).

fig. J5-8: overheating protection by thermal sensors.

multi-function motor-protection relay The multi-function relay, associated with a number of sensors and indication modules, provides protection for motors, such as: c thermal overload; c rotor stalled, or starting-up period too long; c overheating; c phase current imbalance, loss of one phase, inverse rotation; c earth fault (by RCD); c running on no-load, blocked rotor on start-up. The advantages of this relay are essentially: c a comprehensive protection, providing a reliable, high-performance and permanent monitoring/control function; c efficient surveillance of all motor-operating schedules; c alarm and control indications; c possibility of communication via communication buses.

fig. J5-9: multi-function protection, typified by the Telemecanique relay, type LT8 above.

particular supply sources and loads - J41

5. asynchronous motors (continued)

J 5.4 preventive or limitative protection (continued) preventive protection of stationary motors This protection concerns the monitoring of the level of insulation resistance of a stationary motor, thereby avoiding the undesirable consequences of insulation failure during operation, such as: c for motors used on emergency systems for example: failure to start or to perform correctly; c in manufacturing: loss of production. This type of protection is indispensable for essential-services and emergency-systems motors, especially when installed in humid and/or dusty locations. Such protection avoids the destruction of a motor by short-circuit to earth during start-up (one of the most frequently-occurring incidents) by giving a warning in advance that maintenance work is necessary to restore the motor to a satisfactory operational condition. Examples of application (figure J5-10) Fire-protection system “sprinkler” pumps. Irrigation pumps for seasonal operation, etc. Example: a vigilohm SM 20 (Merlin Gerin) relay monitors the insulation of a motor, and signals audibly and visually any abnormal reduction of the insulation resistance level. Furthermore, this relay can prevent any attempt to start the motor, if necessary.

SM20

MERLIN GERIN SM20

IN

OUT

fig. J5-10: preventive protection of stationary motors.

limitative protection Residual current differential protective devices (RCDs) can be very sensitive and detect low values of leakage current which occur when the insulation to earth of an installation deteriorates (by physical damage, contamination, excessive humidity, and so on). Some versions of RCDs, specially designed for such applications, provide the following possibilities: c to avoid the destruction of a motor (by perforation and short-circuiting of the laminations of the stator) caused by an eventual arcing fault to earth. This protection can detect incipient fault conditions by operating at leakage currents in the range of 300 mA to 30 A, according to the size of the motor (approx. sensitivity: 5 % In). Instantaneous tripping by the RCD will greatly limit the extent of damage at the fault location; c to reduce considerably the risk of fire due to earth-leakage currents (sensitivity i 500 mA). A typical RCD for such duties is type RH328A relay (Merlin Gerin) which provides: c 32 sensitivities (0.03 to 250 A); c possibility of discriminative tripping or to take account of particular operational requirements, by virtue of 8 possible timedelays (instantaneous to 1 s.); c automatic operation if the circuit from the current transformer to the relay is broken; c protected against false operation; c insulation of d.c. circuit components: class A.

J42 - particular supply sources and loads

RH328A

MERLIN GERIN

fig. J5-11: example using relay RH328A.

J voltage drop at the terminals of a motor during starting must never exceed 10% of rated voltage Un.

The importance of limiting voltage drop at the motor during start-up In order that a motor starts and accelerates to its normal speed in the appropriate time, the torque of the motor must exceed the load torque by at least 70%. However, the starting current is much greater than the full-load current of the motor; moreover, it is largely inductive. These two factors are both very unfavourable to the maintenance of voltage at the motor. Failure to provide sufficient voltage will reduce the motor torque significantly (motor torque is proportional to U2) and will result either in an excessively long starting time, or, for extreme cases, in failure to start.

Example: c with 400 V maintained at the terminals of a motor, its torque would be 2.1 times that of the load torque; c for a voltage drop of 10% during start-up, the motor torque would be 2.1 x 0.92 = 1.7 times the load torque, and the motor would accelerate to its rated speed normally; c for a voltage drop of 15% during start-up, the motor torque would be 2.1 x 0.852 = 1.5 times the load torque, so that the motorstarting time would be longer than normal. In general, a maximum allowable voltage drop of 10% Un is recommended during the start-up of a motor.

5.5 maximum rating of motors installed for consumers supplied at LV The disturbances caused on LV distribution networks during the start-up of large DOL (direct-on-line) a.c. motors can occasion considerable nuisance to neighbouring consumers, so that most power-supply authorities have strict rules intended to limit such disturbances to tolerable levels. The amount of disturbance created by a given motor depends on the “strength” of the network, i.e. on the short-circuit fault level at the point concerned. The higher the fault level, the “stronger” the system and the lower the disturbance (principally volt-drop) experienced by neighbouring consumers. For distribution networks in many countries, typical values of maximum allowable starting type of motor single- location or three-phase single phase three phase

dwellings others dwellings others

currents for DOL motors are shown below in table J5-12. Corresponding maximum power ratings of the same motors are shown in table J5-13. Since, even in areas supplied by one power authority only, “weak” areas of the network exist as well as “strong” areas, it is always advisable to secure the agreement of the power supplier before acquiring the motors for a new project. Other (but generally more costly) alternative starting arrangements exist, which reduce the large starting currents of DOL motors to acceptable levels; for example, star-delta starters, slip-ring motors, “soft start” electronic devices, etc. maximum starting current (A) overheadundergroundline network cable network 45 45 100 200 60 60 125 250

table J5-12: maximum permitted values of starting current for direct-on-line LV motors (230/400 V). type of motor singleor three-phase location

single-phase 230 V (kW)

dwellings others overhead line network underground cable network

three-phase 400 V

1.4 3

direct-on-line starting at full load (kW) 5.5 11

other methods of starting (kW) 11 22

5.5

22

45

table J5-13: maximum permitted power ratings for LV direct-on-line-starting motors.

5.6 reactive-energy compensation (power-factor correction) The effect of power factor correction on the amount of current supplied to a motor is indicated in table B4 in Chapter B Sub-clause 3-1, and the method of correction in Chapter E Clause 7.

particular supply sources and loads - J43

6. protection of direct-current installations

J differences between a.c. and d.c. installations Although the basic design principles in each case are similar, there are differences in: c the calculations for short-circuit currents, and; c the choice of protective equipment, since the techniques employed for the interruption of direct current differ in practice from those used for alternating current.

6.1 short-circuit currents in order to calculate the maximum short-circuit current from a battery of storage cells, when the internal resistance of the battery is unknown, the following approximate formula may be used: Isc = kC where C = the rated ampere-hour capacity of the battery, and k is a coefficent close to 10 (and in any case is less than 20).

battery of storage cells (or accumulators) For a short-circuit at its output terminals, a battery will pass a current according to Ohm’s law equal to Isc = Vb/Ri where: Vb = open circuit voltage of the fullycharged battery Ri = the internal resistance of the battery (this value is normally obtained from the manufacturer of the battery, as a function of its ampere-hour capacity) When Ri is not known, an approximate formula may be used, namely: Isc = kC where C is the ampere-hour rating of the battery and k is a coefficient close to 10, and in any case is always less than 20.

Example: What is the short-circuit current level at the terminals of a battery with the following characteristics: c 500 Ah capacity; c fully-charged open-circuit voltage 240 V (110 cells at 2.2 V/cell); c discharge rate 300 A; c autonomy 1/2 hour; c internal resistance is 0.5 milli-ohm/cell so that Ri = 110 x 0.5 = 55 mΩ for the battery, 3 and Isc = 240 x 10 = 4.4 kA 55 The short-circuit currents are seen to be (relatively) low.

Isc

fig. J6-1: battery of storage cells.

direct-current generator If Vg is the open-circuit voltage of the generator and Ri its internal resistance, then: Isc = Vg / Ri. In the absence of precise data, and for a d.c. system of voltage Un, Vg may be taken as 1.1 Un.

Example: A d.c. generator rated at 200 kW, 230 V, and having an internal resistance of 0.032 ohm, will give a terminal short-circuit current of 230 x 1.1 = 7.9 kA 0.032 G =

Icc

fig. J6-2: direct-current generator.

Isc at any point in an installation V Ri + Rl Where Ri is as previously defined, V is either Vb or Vg as previously defined, Rl is the sum of the resistances of the faultcurrent loop conductors. Where motors are included in the system, they will each (initially) contribute a current of approximately 6 In (i.e. six times the nominal full-load current of the motor) so that: V Isc = + 6 (In mot) Ri + Rl where In mot is the sum of the full-load currents of all running motors at the instant of short-circuit.

+

In this case Isc =

J44 - particular supply sources and loads

-

fig. J6-3: short-circuit at any point of an installation.

J 6.2 characteristics of faults due to insulation failure, and of protective switchgear Devices for circuit interruption are sensitive to the level of d.c. voltage at their terminals when breaking short-circuit currents. The table below provides the means for determining these voltages, which depend on the source voltage and on the method of earthing the source.

Note: In the following text the word “pole” has two meanings, viz: 1. Referring to a d.c. source, for example: the positive pole or the negative pole of a battery or generator. 2. Referring to a switch or circuit breaker, for example: a pole of a circuit breaker makes or breaks the current in one conductor. A pole of a circuit breaker may be made up of modules, each of which contains a contact. The pole may therefore consist of one module or (particularly in d.c. circuits) several seriesconnected modules.

Voltage stresses across opening contacts are reduced by the technique of connecting a number of contacts in series per pole, as mentioned in the table below, and in the following text. types of network

earthing schemes and various fault conditions

system earthing one pole earthed at the source i

i

a +

+

U –

R B

A

fault A

fault B

fault C

–

case 1 pole (a) must break maximum Isc at U volts poles (a) and (b) must break the maximum Isc at U volts there is no short-circuit current in this case

i

a

a

+

U/2 + U/2

R B b

b C

analysis of each fault

unearthed system source is not earthed

source with mid-point earthing

U

R

–

A

B b

C

case 2 pole (a) must break maximum Isc* at U/2 volts poles (a) and (b) must break the maximum Isc at U volts as for fault A but concerning pole (b)*

A

C

case 3 there is no short-circuit in this case poles (a) and (b) must break the maximum Isc at U volts as for fault A

* U/2 divided by Ri/2 = Isc (max.)

the most unfavourable case case of a circuit breaker

fault A all the contacts participating in current interruption are series connected in the positiveconductor (or the negative conductor if the positive pole of the source is earthed). Provide an additional pole for inserting in the earthed polarity conductor, to permit circuit isolation (figure J6-6).

A=B=C see Note below the table provide in the CB pole for each conductor the number of contacts necessary to break Isc (max.) at the voltage U/2.

fault B (or faults A and C simultaneously) provide the number of contacts necessary for breaking the current indicated in the CB pole of each conductor.

table J6-4: characteristics of protective switchgear according to type of d.c. system earthing. Note: each pole is equally stressed for faults at A, B or C, since maximum Isc must be broken with U/2 across the CB pole(s) in each case.

6.3 choice of protective device for each type of possible insulation failure, the protective devices against short-circuits must be adequately rated for the voltage levels noted in table J6-4 above.

The choice of protective device depends on: c the voltage appearing across the currentbreaking element. In the case of circuit breakers, this voltage dictates the number of circuit-breaking contacts that must be connected in series for each pole of a circuit breaker, to attain the levels indicated in table J6-4; c the rated current required; c the short-circuit current level at its point of installation (in order to specify its s.c. currentbreaking capacity);

c the time constant of the fault current (L/R in milli-seconds) at the point of installation of the CB. Table J6-5 below gives characteristics (current ratings, s.c. current-breaking capacity, and the number of series-connected contacts per pole required for a given system voltage) for circuit breakers made by Merlin Gerin.

particular supply sources and loads - J45

6. protection of direct-current installations (continued)

J 6.3 choice of protective device (continued) type

ratings (A)

sc current-breaking capacity kA for L/R i 0.015 seconds (the number of series-connected contacts per pole is shown in brackets) 24/48 V 125 V 250 V C32HDC 1 to 40 20 (1p) 10 (1p) 20 (2p) 10 (2p) C60a 10 to 40 10 (1p) 10 (2p) 20 (3p) 25 (4p) C60N 6 to 63 15 (1p) 20 (2p) 30 (3p) 40 (4p) C60H 1 to 63 20 (1p) 25 (2p) 40 (3p) 50 (4p) C60L 1 to 63 25 (1p) 30 (2p) 50 (3p) 60 (4p) NC100H 50 to 100 20 (1p) 30 (2p) 40 (3p) 20 (4p) NC100LH 10 to 63 50 (1p) 50 (1p) 50 (1 p) NS100N 16 to 100 50 (1p) 50 (1p) 50 (1p) NC100H 16 to 100 85 (1p) 85 (1p) 85 (1p) NS100L 16 to 100 100 (1p) 100 (1p) 100 (1p) NS160N 40 to 160 50 (1p) 50 (1p) 50 (1p) NS160H 40 to 160 85 (1p) 85 (1p) 85 (1p) NS160L 40 to 160 100 (1p) 100 (1p) 100 (1p) NS250N 40 to 250 50 (1p) 50 (1p) 50 (1p) NS250H 40 to 250 85 (1p) 85 (1p) 85 (1p) NS250L 40 to 250 100 (1p) 100 (1p) 100 (1p) NS400H MP1/MP2-400 85 (1p) 85 (1p) 85 (1p) NS630H MP1/MP2/MP3-630 85 (1p) 85 (1p) 85 (1p) C1251N-DC P21/P41-1250 50 (1p) 50 (1p) 50 (2p) M10-DC 1000 100 (3p) 100 (3p) 100 (3p) M20-DC 2000 100 (3p) 100 (3p) 100 (3p) M40-DC 4000 100 (3p) 100 (3p) 100 (3p) M60-DC 6000 100 (4p) 100 (4p) 100 (4p) M80-DC 8000 100 (4p) 100 (4p) 100 (4p)

thermal overload protection 500 V

50 (3p) 50 (2p) 85 (2p) 100 (2p) 50 (2p) 85 (2p) 100 (2p) 50 (2p) 85 (2p) 100 (2p) 85 (2p) 85 (2p) 50 (3p) 100 (3p) 100 (3p) 100 (3p)

750 V

25 (3p) 50 (4p) 50 (4p50 (4p)

1000 V

50 (4p) 50 (4p) 50 (4p)

special DC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC no thermal relay; provide an external relay (if necessary)

coefficient for uprating the instantaneous magnetic tripping units* special DC 1.38 1.38 1.38 1.38 1.42 1.42 1.42 1.42

tripping units MP1/MP2/MP3 special for direct current

table J6-5: choice of d.c. circuit breakers manufactured by Merlin Gerin. * These tripping units may be used on a.c. or d.c. circuit breakers, but the operating levels marked on each unit correspond to r.m.s. a.c. values. When used on a d.c. circuit breaker the setting must be changed according to the co-efficient in table J6-5. For example, if it is required that the d.c. circuit breaker should trip at 800 A or more the coefficient given in table J6-5 is 1.42, then the setting required will be 800 x 1.42 = 1,136 A.

6.4 examples Example 1 Choice of protection for an 80 A outgoing d.c. circuit of a 125 V system, of which the negative pole is earthed. The Isc = 15 kA. + 125 V = -

NC100 H 3-pole 80 A load

fig. J6-6: example.

Example 2 Choice of protection for a 100 A outgoing d.c. circuit of a 250 V system, of which the midpoint is earthed. Isc = 15 kA. + 250 V = -

NC100 H 4-pole 100 A load

fig. J6-7: example.

J46 - particular supply sources and loads

Table J6-4 shows that the full system voltage will appear across the contacts of the positive pole. Table J6-5 indicates that circuit breaker NC100H (30 kA 2 contacts/pole 125 V) is an appropriate choice. Preferred practice is to (also) include a contact in the negative conductor of the outgoing circuit, to provide isolation (for maintenance work on the load circuit for example), as shown in figure J6-6. Note: three contacts in series, which open in unison, effectively triple the speed of contact separation. This technique is often necessary for successfully breaking d.c. current. Table J6-4 shows that each pole will be subject to a recovery voltage of U/2, i.e. 125 V for all types of s.c. fault. Table J6-5 indicates that circuit breaker NC100H (30 kA 2 contacts/pole 125 V) is suitable for cases A and C, i.e. 2 contacts in the positive and 2 contacts in the negative pole of the CB. It will be seen in the 250 V column that 4 contacts will break 20 kA at that voltage (case B of table J6-4).

J 6.5 protection of persons

+

The rules for protection are the same as those already covered for a.c. systems. However, the conventional voltage limits and the automatic disconnection times for safety of persons are different (see tables G8 and G9 of Chapter G, Sub-clause 3.1): c all exposed conductive parts are interconnected and earthed; c automatic tripping is achieved in the timeperiod specified. RCDs are not applicable to d.c. circuits, so that in practice: c the principles of the TN scheme are used for cases 1 and 2 of Sub-clause 6.2. It is then sufficient to check that, in the case of a shortcircuit, the current magnitudes will be sufficient to trip the instantaneous magnetic relays. The checking methods are identical to those recommended for an a.c. network. c principles of the IT scheme for case 3 in Sub-clause 6.2, v the insulation level of the installation must be under permanent surveillance and any failure must be immediately indicated and alarmed: this can be achieved by the installation of a suitable monitoring relay as shown in Chapter G, Sub-clause 3.4, v the presence of two concurrent faults to earth (one on each polarity) constitutes a short-circuit, which will be cleared by overcurrent protection. As for the a.c. systems, it is sufficient to verify that the current magnitude exceeds that necessary to operate the magnetic (or short-time delay) circuit breaker tripping units.

U fixed

-

+

U variable ou fixed

-

TR5A

XM200

fig. J6-8: insulation (to earth) monitors for an IT direct-current installation.

particular supply sources and loads - J47

7. short-circuit characteristics of an alternator

J The characteristics of a 3-phase alternator under short-circuit conditions are obtained from oscillogram traces recorded during tests, in which a short-circuit is applied instantaneously to all three terminals of a machine at no load, excited (at a fixed level) to produce nominal rated voltage. The resulting currents in all three phases will normally* include a d.c. component, which reduces exponentially to zero after (commonly) some tens of cycles. The curve shown below in figure AJ1-1 is the current trace, from which the d.c. component has been eliminated, of a recording made during the testing of a 3-phase 230 V 50 kVA machine. The definitions of alternator reactance values are based on such "symmetrical" curves. c b i 0

a t

fig. AJ1-1: short-circuit current of one phase of a 3-phase alternator with the d.c. component eliminated. * unless, by chance, the voltage of a phase happens to be maximum at the instant of short-circuit. In that case, there will be no d.c. transient in the phase concerned.

The reduction of current magnitude from its initial value occurs in the following way. At the instant of short-circuit, the only impedance limiting the magnitude of current is principally** the inherent leakage reactance of the armature (i.e. stator) windings, generally of the order of 10%-15%. The large stator currents are (practically) entirely inductive, so that the synchronously rotating m.m.f. produced by them acts in direct opposition to that of the excitation current in the rotor winding. The result is that the rotor flux starts to reduce, thereby reducing the e.m.f. generated in the stator windings, and consequently reducing the magnitude of the fault current. The effect is cumulative, and the reduced fault current, in turn, now reduces the rotor flux at a slower rate, and so on, i.e. the flux follows the exponential law of natural decay, its reduction rate at any instant depending on the magnitude of the quantity causing the phenomenon. Eventually, a stable state is reached, in which the (greatly reduced) rotor flux produces just enough voltage to maintain the stator current at the level of equilibrium between the three quantities, viz. current, flux and voltage. The reduction of fault current therefore is caused by a diminution of the generated e.m.f. due to armature reaction, and not, in fact, by an increase in impedance of the machine (that is why the term "effective reactance" was used in Chapter J Sub-clause 1.1). ** the sub-transient reactance, which is defined later, is very nearly equal to the leakage reactance.

As shown in figure AJ1-1, the current reduction requires a certain time, and the reason for this is that, as the rotor flux begins to diminish, the change of flux induces a current in the closed rotor circuit in the direction which, in effect, increases the excitation current, i.e. opposes the establishment of a reduced level of magnetic flux. The gradual predominance of the stator m.m.f. depends on the overall effect of rotor and stator time constants, the result of which is the principal factor in the "a.c. current decrement" shown in figure AJ1-1. If, during a short-circuit, there were no eddy currents induced in the unlaminated face of round-rotor alternators, or in damper windings (see note 1) of salient-pole alternators, the envelope of the a.c. current decrement would be similar to that of curve b in figure AJ1-1, i.e. the so-called transient-current envelope. The presence of either of the two features, mentioned above however, gives rise to the sub-transient component of current (curve C). The effect is analogous to that of the closed circuit of the rotor-excitation winding described above (i.e. the induced currents oppose the change), but having a very much shorter time constant. The overall a.c. current decrement is therefore composed of the sum of two exponentially-decaying quantities, viz. the sub-transient and the transient components, as shown in figure AJ1-2. Note 1: Damper windings are made up of heavy gauge copper bars embedded in the pole faces of salient-pole rotors, to form a squirrel-cage "winding" similar to that of an induction motor. Their purpose is to help to maintain synchronous stability of the alternator. With the rotor turning at the same speed as that of the m.m.f. due to the stator currents, no currents will be induced in the damper windings; if a difference in the speed of rotation occurs, due to loss of synchronism, then currents induced in the damper windings will be in a direction that produces a torque which acts to slow (an overspeeding rotor) or to accelerate (an underspeeding rotor). A similar, but much smaller effect occurs due to eddy currents in the surface of solid unlaminated rotors of turbo-alternators.

For advanced analytical studies of alternators, two component axes "direct" and "quadrature" are defined, and subtransient and transient reactances, etc. are derived for each component system. In the simple studies needed for 3-phase symmetrical fault levels and for circuitbreaker performance based on such faults, the direct-axis component system only is required; this accounts for the suffix "d" of reactance values, shown in figure AJ1-2. Suffix "q" is used for quadrature quantities. i" Vo/x"d

ia

enveloppe of the current, ia

i'

Vo/x'd

i t

Vo/xd substransient period

transient period

steady state

x''d = the sub-transient reactance Vo/i'' x'd = the transient reactance Vo/i' xd = the synchronous reactance Vo/i Vo = peak rated voltage of the alternator

fig. AJ1-2: a.c. component of armature current versus time, in a short-circuited alternator (no d.c. transient is shown).

Appendix J1 - 1

J The reactances are generally defined as r.m.s. voltages divided by r.m.s. currents. In the current trace of figure AJ1-2, however, it is simpler to use the projected peak values of current, so that Vo must be the rated peak voltage of the machine. Note 2: in the definition of "i" some authors use the actual voltage measured during the test, instead of Vo. Moreover, xd is generally denoted by Xs and is referred to as "synchronous reactance".

asymmetrical currents As previously noted, in general, all 3-phases of short-circuit current will include a d.c. component. These components give rise to additional electro-dynamic and thermal stresses in the machine itself, and in circuitbreakers protecting a faulted circuit. The worst condition is that of a phase in which the d.c. component is the maximum possible, i.e. the d.c. transient value at zero time (the instant of fault) is equal to the peak value of current given by Vo/xd'', as defined in figure AJ1-2. A typical test trace of this condition is shown in figure AJ1-3. stator phase current d.c. component

time

instant of short circuit The current envelope of an asymmetrical transient has the same dimensions about the d.c. transient curve, as the symmetrical envelope has about the current zero axis.

fig. AJ1-3: a fully-offset asymmetrical transient fault-current trace. The consequence of asymmetrical transient fault currents and the standardized relationship between the symmetrical and asymmetrical quantities for circuit breaker performance ratings are given in Sub-clause 1.1 of Chapter C, and are illustrated in figure C5.

2 - Appendix J1

1. the basic functions of LV switchgear

H2 the role of switchgear is that of: c electrical protection; c safe isolation from live parts; c local or remote switching.

National and international standards define the manner in which electric circuits of LV installations must be realized, and the capabilities and limitations of the various switching devices which are collectively referred to as switchgear. The main functions of switchgear are: c electrical protection; c electrical isolation of sections of an installation; c local or remote switching. These functions are summarized below in table H2-1. Electrical protection at low voltage is (apart from fuses) normally incorporated in circuit electrical protection against overload currents short-circuit currents insulation failure

breakers, in the form of thermal-magnetic devices and/or residual-current-operated tripping devices (less-commonly, residualvoltage-operated devices - acceptable to, but not recommended by IEC). In addition to those functions shown in table H2-1, other functions, namely: c over-voltage protection; c under-voltage protection are provided by specific devices (lightning and various other types of voltage-surge arrester; relays associated with: contactors, remotelycontrolled circuit breakers, and with combined circuit breaker/isolators… and so on).

isolation

control

- isolation clearly indicated by an authorized fail-proof mechanical indicator - a gap or interposed insulating barrier between the open contacts, clearly visible.

- functional switching - emergency switching - emergency stopping - switching off for mechanical maintenance

table H2-1: basic functions of LV switchgear.

1.1 electrical protection electrical protection assures: c protection of circuit elements against the thermal and mechanical stresses of short-circuit currents; c protection of persons in the event of insulation failure; c protection of appliances and apparatus being supplied (e.g motors, etc.).

The aim is to avoid or to limit the destructive or dangerous consequences of excessive (short-circuit) currents, or those due to overloading and insulation failure, and to separate the defective circuit from the rest of the installation. A distinction is made between the protection of: c the elements of the installation (cables, wires, switchgear…); c persons and animals; c equipment and appliances supplied from the installation; c the protection of circuits (see chapter H1): v against overload; a condition of excessive current being drawn from a healthy (unfaulted) installation, v against short-circuit currents due to complete failure of insulation between conductors of different phases or (in TN systems) between a phase and neutral (or PE) conductor. Protection in these cases is provided either by fuses or circuit breaker, at the distribution board from which the final circuit (i.e. the

circuit to which the load is connected) originates. Certain derogations to this rule are authorized in some national standards, as noted in chapter H1 sub-clause 1.4. c the protection of persons against insulation failures (see chapter G). According to the system of earthing for the installation (TN, TT or IT) the protection will be provided by fuses or circuit breakers, residual current devices, and/or permanent monitoring of the insulation resistance of the installation to earth. c the protection of electric motors (see chapter J clause 5) against overheating, due, for example, to long term overloading; stalled rotor; single-phasing, etc. Thermal relays, specially designed to match the particular characteristics of motors are used. Such relays may, if required, also protect the motor-circuit cable against overload. Shortcircuit protection is provided either by type aM fuses or by a circuit breaker from which the thermal (overload) protective element has been removed, or otherwise made inoperative.

The aim of isolation is to separate a circuit or apparatus, or an item of plant (such as a motor, etc.) from the remainder of a system which is energized, in order that personnel may carry out work on the isolated part in perfect safety. In principle, all circuits of an LV installation shall have means to be isolated. In practice, in order to maintain an optimum continuity of service, it is preferred to provide a means of isolation at the origin of each circuit. An isolating device must fulfil the following requirements: c all poles of a circuit, including the neutral (except where the neutral is a PEN conductor) must be open (1);

c it must be provided with a means of locking open with a key (e.g. by means of a padlock) in order to avoid an unauthorized reclosure by inadvertence; c it must conform to a recognized national or international standard (e.g. IEC 947-3) concerning clearance between contacts, creepage distances, overvoltage withstand capability, etc. and also:

1.2 isolation a state of isolation clearly indicated by an approved "fail-proof" indicator, or the visible separation of contacts, are both deemed to satisfy the national standards of many countries.

(1) the concurrent opening of all live conductors, while not always obligatory, is however, strongly recommended (for reasons of greater safety and facility of operation). The neutral contact opens after the phase contacts, and closes before them (IEC 947-1).

the protection of circuits - the switchgear - H2-1

1. the basic functions of LV switchgear (continued)

H2 1.2 isolation

(continued) v verification that the contacts of the isolating device are, in fact, open. The verification may be: - either visual, where the device is suitably designed to allow the contacts to be seen (some national standards impose this condition for an isolating device located at the origin of a LV installation supplied directly from a HV/LV transformer), - or mechanical, by means of an indicator solidly welded to the operating shaft of the device. In this case the construction of the device must be such that, in the eventuality that the contacts become welded together in the closed position, the indicator cannot possibly indicate that it is in the open position. v leakage currents. With the isolating device open, leakage currents between the open contacts of each phase must not exceed: - 0.5 mA for a new device, - 6.0 mA at the end of its useful life. v voltage-surge withstand capability, across open contacts. The isolating device, when open must withstand a 1.2/50 µs impulse, having a peak value of 5, 8 or 10 kV according to its service voltage, as shown in table H2-2. The device must satisfy these conditions for altitudes up to 2,000 metres. Consequently, if tests are carried out at sea level, the test values must be increased by 23% to take into account the effect of altitude. See standard IEC 947 and the Note immediately preceding table F-10. service (nominal) voltage (V) 230/400 400/690 1,000

Industrial LV switchgear which affords isolation when open is marked on the front face by the symbol . This symbol may be combined with those indicating other features where a device also performs other functions as shown in figure H2-4.

fig. H2-3: symbol for a disconnector* also commonly referred to as an isolator. switch-disconnector*, also referred to as a load-break isolating switch

circuit breaker suitable for circuit isolation

fig. H2-4: symbols for circuit isolation capability incorporated in other switching devices. * IEC 617-7 and 947-3.

Note. In this guide the terms "disconnector" and "isolator" have the same meaning.

impulse withstand peak voltage (kV) 5 kV 8 kV 10 kV

table H2-2: peak value of impulse voltage according to normal service voltage of test specimen.

1.3 switchgear control switchgear-control functions allow system operating personnel to modify a loaded system at any moment, according to requirements, and include: c functional control (routine switching, etc.); c emergency switching; c maintenance operations on the power system.

H2-2 - the protection of circuits - the switchgear

In broad terms "control" signifies any facility for safely modifying a load-carrying power system at all levels of an installation. The

operation of switchgear is an important part of power-system control.

functional control This control relates to all switching operations in normal service conditions for energizing or de-energizing a part of a system or installation, or an individual piece of equipment, item of plant, etc. Switchgear intended for such duty must be installed at least: c at the origin of any installation; c at the final load circuit or circuits (one switch may control several loads). Marking (of the circuits being controlled) must be clear and unambiguous. In order to provide the maximum flexibility and continuity of operation, particularly where the switching device also constitutes the protection (e.g. a circuit breaker or switch-fuse) it is preferable to include a switch at each level of distribution, i.e. on

each outgoing way of all distribution and subdistribution boards. The manœuvre may be: c either manual (by means of an operating lever on the switch) or; c electric, by push-button on the switch or at a remote location (load-shedding and reconnection, for example). These switches operate instantaneously (i.e. with no deliberate delay), and those that provide protection are invariably omni-polar*. The main circuit breaker for the entire installation, as well as any circuit breakers used for change-over (from one source to another) must be omni-polar units. * one break in each phase and (where appropriate) one break in the neutral (see table H1-65).

H2 emergency switching emergency stop An emergency switching is intended to de-energize a live circuit which is, or could become, dangerous (electric shock or fire). An emergency stop is intended to arrest a movement which has become dangerous. In the two cases: c the emergency control device or its means of operation (local or at remote location(s)) such as a large red mushroom-headed emergency-stop pushbutton must be recognizable and readily accessible, in proximity to any position at which danger could arise or be seen; c a single action must result in a complete switching-off of all live conductors (1) (2);

c a "break glass" emergency switching initiation device is authorized, but in unmanned installations the re-energizing of the circuit can only be achieved by means of a key held by an authorized person. It should be noted that in certain cases, an emergency system of braking, may require that the auxiliary supply to the braking-system circuits be maintained until final stoppage of the machinery. (1) Taking into account stalled motors. (2) In a TN schema the PEN conductor must never be opened, since it functions as a protective earthing wire as well as the system neutral conductor.

switching-off for mechanical maintenance work This operation assures the stopping of a machine and its impossibility to be inadvertently restarted while mechanical maintenance work is being carried out on the driven machinery. The shutdown is generally carried out at the functional switching device, with the use of a suitable safety lock and warning notice at the switch mechanism.

the protection of circuits - the switchgear - H2-3

2. the switchgear and fusegear

H2 2.1 elementary switching devices disconnector (or isolator) This switch is a manually-operated, lockable, two-position device (open/closed) which provides safe isolation of a circuit when locked in the open position. Its characteristics are defined in IEC 947-3. A disconnector is not designed to make or to break current* and no rated values for these functions are given in standards. It must, however, be capable of withstanding the passage of short-circuit currents and is assigned a rated short-time withstand capability; generally for 1 second, unless otherwise agreed between user and manufacturer. This capability is normally more than adequate for longer periods of (lower-valued) operational overcurrents, such as those of motor-starting. Standardized mechanical-endurance, overvoltage, and leakage-current tests, must also be satisfied. * i.e. a LV disconnector is essentially a deadsystem switching device to be operated with no voltage on either side of it, particularly when closing, because of the possibility of an unsuspected short-circuit on the downstream side. Interlocking with an upstream switch or circuit breaker is frequently used.

load-breaking switch This control switch is generally operated manually (but is sometimes provided with electrical tripping for operator convenience) and is a non-automatic two-position device (open/closed). It is used to close and open loaded circuits under normal unfaulted circuit conditions. It does not consequently, provide any protection for the circuit it controls. IEC standard 947-3 defines: c the frequency of switch operation (600 close/open cycles per hour maximum); c mechanical and electrical endurance (generally less than that of a contactor); c current making and breaking ratings for normal and infrequent situations. IEC 947-3 also recognizes 3 categories of load-breaking switch, each of which is suitable for a different range of load power factors, as shown in table H2-7.

fig. H2-6: symbol for a load-breaking switch. When closing a switch to energize a circuit there is always the possibility that an (unsuspected) short circuit exists on the circuit. For this reason, load-break switches are assigned a fault-current making rating, i.e. successful closure against the electrodynamic forces of short-circuit current is assured. Such switches are commonly referred to as "fault-make load-break" switches. Upstream protective devices are relied upon to clear the short-circuit fault.

H2-4 - the protection of circuits - the switchgear

fig. H2-5: symbol for a disconnector (or isolator).

H2 nature of current alternating current

utilization category frequent infrequent operation operation AC-20A AC-20B AC-21A

AC-21B

AC-22A

AC-22B

AC-23A

AC-23B

typical applications

connecting and disconnecting under no-load conditions switching of resistive loads including moderate overloads switching of mixed resistive and inductive loads, including moderate overloads switching of motor loads or other highly inductive loads

table H2-7: utilization categories of LV a.c. switches according to IEC 947-3. Category AC-23 includes occasional switching of individual motors. The switching of capacitors or of tungsten filament lamps shall be subject to agreement between manufacturer and user. The utilization categories referred to in table H2-7 do not apply to an equipment normally used to start, accelerate and/or stop individual motors. The utilization categories for such an equipment are dealt with in chapter J, table J5-4. Example: A 100 A load-break switch of category AC-23 (inductive load) must be able: c to make a current of 10 In (= 1,000 A) at a power factor of 0.35 lagging; c to break a current of 8 In (= 800 A) at a power factor of 0.35 lagging; c to withstand short-circuit currents (not less than 12 In) passing through it for 1 second, where 12 In equals the r.m.s. value of the a.c. component, while the peak value (expressed in kA) is given by a factor "n" in table XVI of IEC 947- Part 1, reproduced below for reader convenience (table H2-8). test current I (A) I i 01 500 1 500 < I i 3 000 3 000 < I i 4 500 4 500 < I i 6 000 6 000 < I i 10 000 10 000 < I i 20 000 20 000 < I i 50 000 50 000 < I

power-factor (ms) 0.95 0.9 0.8 0.7 0.5 0.3 0.25 0.2

time-constant

n

5 5 5 5 5 10 15 15

1.41 1.42 1.47 1.53 1.7 2.0 2.1 2.2

table H2-8: factor "n" used for peak-to-rms value (IEC 947-part1).

bistable switch (télérupteur) This device is extensively used in the control of lighting circuits where the depression of a pushbutton (at a remote control position) will open an already-closed switch or close an open switch in a bistable sequence. Typical applications are: c two-way switching on stairways of large buildings; c stage-lighting schemes; c factory illumination, etc. Auxiliary devices are available to provide: c remote indication of its state at any instant; c time-delay functions; c maintained-contact features.

fig. H2-9: symbol for a bistable remotelyoperated switch (télérupteur).

the protection of circuits - the switchgear - H2-5

2. the switchgear and fusegear (continued)

H2 2.1 elementary switching devices (continued) contactor The contactor is a solenoid-operated switching device which is generally held closed by (a reduced) current through the closing solenoid (although various mechanically-latched types exist for specific duties). Contactors are designed to carry out numerous close/open cycles and are commonly controlled remotely by on-off pushbuttons. The large number of repetitive operating cycles is standardized in table VIII of IEC 947-4-1 by: c the operating duration: 8 hours; uninterrupted; intermittent; temporary of 3, 10, 30, 60 and 90 minutes; c utilization category: (for definition see table J5-4) for example, a contactor of category AC3 can be used for the starting and stopping of a cage motor; c the start-stop cycles (1 to 1,200 cyles per hour); c mechanical endurance (number of off-load manœuvres); c electrical endurance (number of on-load manœuvres);

c a rated current making and breaking performance according to the category of utilization concerned. Example: A 150 A contactor of category AC3 must have a minimum current-breaking capability of 8 In (= 1,200 A) and a minimum current-making rating of 10 In (= 1,500 A) at a power factor (lagging) of 0.35.

control circuit

power circuit

fig. H2-10: symbol for a contactor.

discontactor* A contactor equipped with a thermal-type relay for protection against overloading defines a "discontactor". Discontactors are used extensively for remote push-button control of lighting circuits, etc., and may also be considered as an essential element in a motor controller, as noted in sub-clause 2.2. "combined switchgear elements". The discontactor is not the equivalent of a

two classes of LV cartridge fuse are very widely used: c for domestic and similar installations type gG c for industrial installations type gG, gM or aM.

H2-6 - the protection of circuits - the switchgear

circuit breaker, since its short-circuit currentbreaking capability is limited to 8 or 10 In. For short-circuit protection therefore, it is necessary to include either fuses or a circuit breaker in series with, and upstream of, the discontactor contacts. *This term is not defined in IEC publications but is commonly used in some countries.

fuses Fuses exist with and without "fuse-blown" mechanical indicators. Fuses break a circuit by controlled melting of the fuse element when a current exceeds a given value for a corresponding period of time; the current/time relationship being presented in the form of a performance curve for each type of fuse. Standards define two classes of fuse: c those intended for domestic installations, manufactured in the form of a cartridge for rated currents up to 100 A and designated type gG in IEC 269-3; c those for industrial use, with cartridge types designated gG (general use); and gM and aM (for motor-circuits) in IEC 269-1 and 2. The main differences between domestic and industrial fuses are the nominal voltage and current levels (which require much larger physical dimensions) and their fault-current breaking capabilities. Type gG fuse-links are often used for the protection of motor circuits, which is possible when their characteristics are capable of withstanding the motor-starting current without deterioration. A more recent development has been the adoption by the IEC of a fuse-type gM for motor protection, designed to cover starting, and short-circuit conditions. This type of fuse is more popular in some countries than in others, but at the present time the aM fuse in combination with a thermal overload relay is more-widely used.

A gM fuse-link, which has a dual rating is characterized by two current values. The first value In denotes both the rated current of the fuse-link and the rated current of the fuseholder; the second value Ich denotes the time-current characteristic of the fuse-link as defined by the gates in Tables II, III and VI of IEC 269-1. These two ratings are separated by a letter which defines the applications. For example: In M Ich denotes a fuse intended to be used for protection of motor circuits and having the characteristic G. The first value In corresponds to the maximum continuous current for the whole fuse and the second value Ich corresponds to the G characteristic of the fuse link. For further details see note at the end of sub-clause 2.1. An aM fuse-link is characterized by one current value In and time-current characteristic as shown in figure H2-14. Important: Some national standards use a gI (industrial) type fuse, similar in all main essentails to type gG fuses. Type gI fuses should never be used, however, in domestic and similar installations.

fig. H2-11: symbol for fuses.

H2 fusing zones conventional currents

gM fuses require a separate overload relay, as described in the note at the end of sub-clause 2.1.

The conditions of fusing (melting) of a fuse are defined by standards, according to their class. c class gG fuses These fuses provide protection against overloads and short-circuits. Conventional non-fusing and fusing currents are standardized, as shown in figure H2-12 and in table H2-13. v the conventional non-fusing current Inf is the value of current that the fusible element can carry for a specified time without melting. Example: a 32 A fuse carrying a current of 1.25 In (i.e. 40 A) must not melt in less than one hour (table H2-13) v the conventional fusing current If (=I2 in fig.H2-12) is the value of current which will cause melting of the fusible element before the expiration of the specified time. Example: a 32 A fuse carrying a current of 1.6 In (i.e. 52.1 A) must melt in one hour or less (table H2-13). IEC 269-1 standardized tests require that a fuse-operating characteristic lies between the two limiting curves (shown in figure H2-12) for the particular fuse under test. This means that two fuses which satisfy the test can have significantly different operating times at low levels of overloading. v the two examples given above for a 32 A fuse, together with the foregoing notes on standard test requirements, explain why class

rated current* In (A)

gG gM

In i 4 A 4 < In < 16 A 16 < In i 63 A 63 < In i 160 A 160 < In i 400 A 400 < In

these fuses have a poor performance in the low overload range. v it is therefore necessary to install a cable larger in ampacity than that normally required for a circuit, in order to avoid the consequences of possible long term overloading (60% overload for up to one hour in the worst case). By way of comparison, a circuit breaker of similar current rating: v which passes 1.05 In must not trip in less than one hour; and v when passing 1.25 In it must trip in one hour, or less (25% overload for up to one hour in the worst case). t minimum pre-arcing time curve

1h.

fuse-blown curve

Inf I2

I

fig. H2-12: zones of fusing and non-fusing for gG and gM fuses.

conventional nonfusing current Inf 1.5 In 1.5 In 1.25 In 1.25 In 1.25 In 1.25 In

conventional fusing current If I2 2.1 In 1.9 In 1.6 In 1.6 In 1.6 In 1.6 In

conventional time h 1 1 1 2 3 4

table H2-13: zones of fusing and non-fusing for LV types gG and gM class fuses (IEC 269-1 and 269-2-1). * Ich for gM fuses

class aM fuses protect against short-circuit currents only, and must always be associated with another device which protects against overload.

c class aM (motor) fuses These fuses afford protection against shortcircuit currents only and must necessarily be associated with other switchgear (such as discontactors or circuit breakers) in order to ensure overload protection < 4 In. They are not therefore autonomous. Since aM fuses are not intended to protect against low values of overload current, no levels of conventional non-fusing and fusing currents are fixed. The characteristic curves for testing these fuses are given for values of fault current exceeding approximately 4 In (see figure H2-14), and fuses tested to IEC 269 must give operating curves which fall within the shaded area. Note: the small "arrowheads" in the diagram indicate the current/time "gate" values for the different fuses to be tested (IEC 269).

t

minimum pre-arcing time curve fuse-blown curve

4In

x In

fig. H2-14: standardized zones of fusing for type aM fuses (all current ratings).

the protection of circuits - the switchgear - H2-7

2. the switchgear and fusegear (continued)

H2 2.1 elementary switching devices (continued) rated short-circuit breaking currents A characteristic of modern cartridge fuses is that, owing to the rapidity of fusion in the case of high short-circuit current levels*, a current cut-off begins before the occurrence of the first major peak, so that the fault current never reaches its prospective peak value (fig. H2-15). This limitation of current reduces significantly the thermal and dynamic stresses which would otherwise occur, thereby minimizing danger and damage at the fault position. The rated short-circuit breaking current of the fuse is therefore based on the r.m.s. value of the a.c. component of the prospective fault current. No short-circuit current-making rating is assigned to fuses. *for currents exceeding a certain level, depending on the fuse nominal current rating, as shown below in figure H2-15A.

Reminder Short-circuit currents initially contain d.c. components, the magnitude and duration of which depend on the XL/R ratio of the faultcurrent loop. Close to the source (HV/LV transformer) the relationship I peak / Irms (of a.c. component) immediately following the instant of fault, can be as high as 2.5 (standardized by IEC, and shown in figure H2-15A). At lower levels of distribution in an installation, as previously noted, XL is small compared with R and so for final circuits I peak / I rms ~ 1.41, a condition which corresponds with figure H2-15 above and with the "n" value corresponding to a power factor of 0.95 in table H2-8.

I prospective fault-current peak rms value of the a.c. component of the prospective fault current current peak limited by the fuse 0.01s Tf Ta Ttc

0.02s

Tf: fuse pre-arc fusing time Ta: arcing time Ttc: total fault-clearance time

fig. H2-15: current limitation by a fuse.

Note on gM fuse ratings. A gM type fuse is essentially a gG fuse, the fusible element of which corresponds to the current value Ich (ch = characteristic) which may be, for example, 63 A. This is the IEC testing value, so that its time/ current characteristic is identical to that of a 63 A gG fuse. This value (63 A) is selected to withstand the high starting currents of a motor, the steadystate operating current (In) of which may be in the 10-20 A range.

H2-8 - the protection of circuits - the switchgear

maximum possible current peak characteristic i.e. 2.5Ir.m.s. (IEC)

prospective fault current (kA) peak 100

(c) 50

160A 20 100A (b)

50A

10

nominal fuse ratings

(a) 5

peak current cut-off characteristic curves

2

1 1

The peak-current-limitation effect occurs only when the prospective r.m.s. a.c. component of fault current attains a certain level. For example, in the above graph the 100 A fuse will begin to cut off the peak at a prospective fault current (r.m.s.) of 2 kA (a). The same fuse for a condition of 20 kA r.m.s. prospective current will limit the peak current to 10 kA (b). Without a current-limiting fuse the peak current could attain 50 kA (c) in this particular case. As already mentioned, at lower distribution levels in an installation, R greatly predominates XL, and fault levels are generally low. This means that the level of fault current may not attain values high enough to cause peakcurrent limitation. On the other hand, the d.c. transients (in this case) have an insignificant effect on the magnitude of the current peak, as previously mentioned.

t

0.005s

2

5

10

20

50

100

a.c. component of prospective fault current (kA) r.m.s.

fig. H2-15A: limited peak current versus prospective r.m.s. values of the a.c. component of fault current for LV fuses.

H2 This means that a physically smaller fuse barrel and metallic parts can be used, since the heat dissipation required in normal service is related to the lower figures (10-20 A). A standard gM fuse, suitable for this situation would be designated 32M63 (i.e. In M Ich). The first current rating In concerns the steady-load thermal performance of the fuselink, while the second current rating (Ich) relates to its (short-time) starting-current performance. It is evident that, although suitable for shortcircuit protection, overload protection for the motor is not provided by the fuse, and so a separate thermal-type relay is always necessary when using gM fuses. The only advantage offered by gM fuses, therefore, when compared with aM fuses, are reduced physical dimensions and slightly lower cost.

2.2 combined switchgear elements Single units of switchgear do not, in general, fulfil all the requirements of the three basic functions, viz: protection, control and isolation. Where the installation of a circuit breaker is not appropriate (notably where the switching rate is high, over extended periods) combinations of units specifically designed for such a performance are employed. The most commonly-used combinations are described below:

switch and fuse combinations Two cases are distinguished: c the type in which the operation of one (or more) fuse(s) causes the switch to open. This is achieved by the use of fuses fitted with striker pins, and a system of switch tripping springs and toggle mechanisms. This type of combination is generally used for current levels exceeding 100 A, and is commonly associated with a thermal-type overcurrent relay for overload protection (for which the fuses alone may not be suitable). If the switch is classified as AC22 or AC23, and associated with a motor-overload type of thermal relay, the ensemble, i.e. switch, striker-pin fuses and overload relay, is suitable for the control and protection of a motor circuit, and: c the type in which a non-automatic switch is associated with a set of fuses in a common enclosure. In some countries, and in IEC 947-3, the terms "switch-fuse" and "fuse-switch" have specific meanings, viz: v a switch-fuse comprises a switch (generally 2 breaks per pole) on the upstream side of three fixed fuse-bases, into which the fuse carriers are inserted (figure H2-17(a)), v a fuse-switch consists of three switch blades each constituting a double-break per phase. These blades are not continuous throughout their length, but each has a gap in the centre which is bridged by the fuse cartridge. Some designs have only a single break per phase, as shown in figures H2-17(a) and (b).

fig. H2-16: symbol for an automatictripping switch-fuse, with a thermal overload relay.

fig. H2-17 (a): symbol for a non-automatic switch-fuse.

fig. H2-17 (b): symbol for a non-automatic fuse-switch.

the protection of circuits - the switchgear - H2-9

2. the switchgear and fusegear (continued)

H2 2.2 combined switchgear elements (continued) The current range for these devices is limited to 100 A maximum at 400 V 3-phase, while their principal use is in domestic and similar installations. To avoid confusion between the first group (i.e. automatic tripping) and the second group, the term "switch-fuse" should be qualified by the adjectives "automatic" or "non-automatic".

fuse - disconnector + discontactor fuse - switch-disconnector + discontactor As previously mentioned, a discontactor does not provide protection against short-circuit faults. It is necessary, therefore, to add fuses (generally of type aM) to perform this function. The combination is used mainly for motorcontrol circuits, where the disconnector or switch-disconnector allows safe operations such as: c the changing of fuse links (with the circuit isolated); c work on the circuit downstream of the discontactor (risk of remote closure of the discontactor). The fuse-disconnector must be interlocked with the discontactor such that no opening or closing manœuvre of the fuse-disconnector is possible unless the discontactor is open (figure H2-18 (a)), since the fusedisconnector has no load-switching capability. A fuse-switch-disconnector (evidently) requires no interlocking (figure H2-18 (b)). The switch must be of class AC22 or AC23 if the circuit supplies a motor.

circuit-breaker + contactor circuit-breaker + discontactor These combinations are used in remotelycontrolled distribution systems in which the rate of switching is high, or for control and protection of a circuit supplying motors. The protection of induction motors is considered in chapter J, clause J5.

H2-10 - the protection of circuits - the switchgear

fig. H2-18 (a): symbol for a fusedisconnector + discontactor.

fig. H2-18 (b): symbol for a fuse-switchdisconnector + discontactor.

3. choice of switchgear

H2 3.1 tabulated functional capabilities After having studied the basic functions of LV switchgear (clause 1, table H2-1) and the different components of switchgear (clause 2), table H2-19 summarizes the capabilities of the various components to perform the basic functions. switchgear item isolator (or disconnector) (4) switch (5) residual device (RCCB) (5) switchdisconnector contactor bistable-switch (telerupteur) fuse circuit breaker (5) circuit breaker disconnector (5) residual and overcurrent circuit breaker (RCBO) (5) point of installation (general principle)

isolation control functional

emergency switching emergency stop switching for (mechanical) mechanical maintenance

electrical protection overload short-circuit differential

c c c

c c

c (1) c (1)

c (1) (2) c (1) (2)

c c

c

c

c (1)

c (1) (2)

c

c c

c (1) c (1)

c (1) (2)

c c

c

c (1)

c (1) (2)

c

c c

c c

c

c

c (1)

c (1) (2)

c

c

c

c

c

c (1)

c (1) (2)

c

c

c

c

origin of each circuit

all points where, for operational reasons it may be necessary to stop the process

in general at the incoming circuit to every distribution board

at the supply point to each machine and/or on the machine concerned

at the supply point to each machine

origin of each circuit

origin of each circuit

origin of circuits where the earthing system is appropriate TN-S, IT, TT

c

c

c (3)

table H2-19: functions fulfilled by different items of switchgear. (1) Where cut-off of all active conductors is provided (2) It may be necessary to maintain supply to a braking system (3) If it is associated with a thermal relay (the combination is commonly referred to as a "discontactor") (4) In certain countries a disconnector with visible contacts is mandatory at the origin of a LV installation supplied directly from a HV/LV transformer (5) Certain items of switchgear are suitable for isolation duties (e.g. RCCBs according to IEC 1008) without being explicitly marked as such.

3.2 switchgear selection Software is being used more and more in the field of optimal selection of switchgear. Each circuit is considered one at a time, and a list is drawn up of the required protection functions and exploitation of the installation, among those mentioned in table H2-19 and summarized in table H2-1. A number of switchgear combinations are studied and compared with each other against relevant criteria, with the aim of achieving: c satisfactory performance; c compatibility among the individual items; from the rated current In to the fault-level rating Icu; c compatibility with upstream switchgear or taking into account its contribution; c conformity with all regulations and specifications concerning safe and reliable circuit performance.

In order to determine the number of poles for an item of switchgear, reference is made to chapter H1, clause 7, table H1-65. Multifunction switchgear, initially more costly, reduces installation costs and problems of installation or exploitation. It is often found that such switchgear provides the best solution.

the protection of circuits - the switchgear - H2-11

4. circuit breakers

H2 the circuit breaker/disconnector fulfills all of the basic switchgear functions, while, by means of accessories, numerous other possibilities exist.

As shown in table H2-19 the circuit breaker/ disconnector is the only item of switchgear capable of simultaneously satisfying all the basic functions necessary in an electrical installation. Moreover, it can, by means of auxiliary units, provide a wide range of other functions, for example: indication (on-off - tripped on fault); undervoltage tripping; remote control… etc. These features make a circuit-breaker/ disconnector the basic unit of switchgear for any electrical installation. functions isolation control

protection

functional emergency switching switching-off for mechanical maintenance overload short-circuit insulation faulty undervoltage

remote control indication and measurement

possible conditions c c c (with the possibility of a tripping coil for remote control) c c c c (with differential-current relay) c (with undervoltage-trip coil) c added or incorporated c (generally optional with an electronic tripping device)

table H2.20: functions performed by a circuit-breaker/disconnector.

4.1 standards and descriptions industrial circuit breakers must conform with IEC 947-1 and 947-2 or other equivalent standards. Corresponding European standards are presently being developed. Domestic-type circuit breakers should conform to IEC standard 898, or an equivalent national standard.

H2-12 - the protection of circuits - the switchgear

standards For industrial LV installations the relevant IEC standards are, or are due to be: c 947-1: general rules; c 947-2: part 2: circuit breakers; c 947-3: part 3: switches, disconnectors, switch-disconnectors and fuse combination units; c 947-4: part 4: contactors and motorstarters; c 947-5: part 5: control-circuit devices and switching elements; c 947-6: part 6: multiple function switching devices; c 947-7: part 7: ancillary equipment. Corresponding European and many national standards are presently in the course of harmonization with the IEC standards, with which they will be in very close agreement. For domestic and similar LV installations, the appropriate standard is IEC 898, or an equivalent national standard.

H2 description Figure H2-21 shows schematically the principal parts of a LV circuit breaker and its four essential functions: 1 - the circuit-breaking components, comprising the fixed and moving contacts and the arc-dividing chamber. 2 - the latching mechanism which becomes unlatched by the tripping device on detection of abnormal current conditions. This mechanism is also linked to the operation handle of the breaker.

3 - a trip-mechanism actuating device: c either: a thermal-magnetic device, in which a thermally-operated bi-metal strip detects an overload condition, while an electromagnetic striker pin operates at current levels reached in short-circuit conditions, or: c an electronic relay operated from current transformers, one of which is installed on each phase. 4 - a space allocated to the several types of terminal currently used for the main powercircuit conductors.

power circuit terminals

contacts and arc-dividing chamber

fool-proof mechanical indicator

latching mechanism

trip mechanism and protective devices

fig. H2-21: principal parts of a circuit breaker.

domestic circuit breakers conforming to IEC 898 and similar national standards perform the basic functions of: c isolation c protection against overcurrent.

fig. H2-22: domestic-type circuit breaker providing overcurrent protection and circuit isolation features.

some models can be adapted to provide sensitive detection (30 mA) of earth-leakage current with CB tripping, by the addition of a modular block, as shown in figure H2-23, while other models (complying with IEC 1009) have this residual-current feature incorporated, viz. RCBOs., and, more recently, IEC 947-2 (appendix B) CBRs. fig. H2-23: domestic-type circuit breaker as above (H2-22) plus protection against electric shocks by the addition of a modular block.

the protection of circuits - the switchgear - H2-13

4. circuit breakers (continued)

H2 4.1 standards and descriptions (continued) apart from the above-mentioned functions further features can be associated with the basic circuit breaker by means of additional modules, as shown in figure H2-24; notably remote control and indication (on-off-fault).

1

2 3 4 5

FF -OFF O O-O O-OFF O-OFF

fig. H2-24: "Multi 9" system* of LV modular switchgear components.

moulded-case type industrial circuit breakers conforming to IEC 947-2 are now available, which, by means of associated adaptable blocks provide a similar range of auxiliary functions to those described above (figure H2-25).

SDE

OF2

SD OF1

OF2 SDE SD OF1

fig. H2-25: example of a modular (Compact NS*) industrial type of circuit breaker capable of numerous auxiliary functions. * Merlin Gerin product.

H2-14 - the protection of circuits - the switchgear

H2 heavy-duty industrial circuit breakers of large current ratings, conforming to IEC 947-2, have numerous built-in communication and electronic functions (figure H2-26).

fig. H2-26: examples of heavy-duty industrial circuit breakers. The "Masterpact"* provides many automation features in its tripping module. These circuit breakers are provided with means to adjust protective-device settings over a wide range, and also with: c a 20 mA output loop; c remote indication contacts; c load indication at the CB.

4.2 fundamental characteristics of a circuit breaker the fundamental characteristics of a circuit breaker are: c its rated voltage Ue c its rated current In c its tripping-current-level adjustment ranges for overload protection (Ir** or Irth**) and for short-circuit protection (Im)** c its short-circuit current breaking rating (Icu for industrial CBs; Icn for domestic-type CBs).

rated operational voltage (Ue) This is the voltage at which the circuit breaker has been designed to operate, in normal (undisturbed) conditions. Other values of voltage are also assigned to the circuit breaker, corresponding to disturbed conditions, as noted in sub-clause 4.3.

rated current (In)

frame-size rating

This is the maximum value of current that a circuit breaker, fitted with a specified overcurrent tripping relay, can carry indefinitely at an ambient temperature stated by the manufacturer, without exceeding the specified temperature limits of the currentcarrying parts. Example: A circuit breaker rated at In = 125 A for an ambient temperature of 40 °C will be equipped with a suitably calibrated overcurrent tripping relay (set at 125 A). The same circuit breaker can be used at higher values of ambient temperature however, if suitably "derated". Thus, the circuit breaker in an ambient temperature of 50 °C could carry only 117 A indefinitely, or again, only 109 A at 60 °C, while complying with the specified temperature limit. Derating a circuit breaker is achieved therefore, by reducing the trip-current setting of its overload relay, and marking the CB accordingly. The use of an electronic-type of tripping unit, designed to withstand high temperatures, allows circuit breakers (derated as described) to operate at 60 °C (or even at 70 °C) ambient.

A circuit breaker which can be fitted with overcurrent tripping units of different currentlevel-setting ranges, is assigned a rating which corresponds with that of the highest current-level-setting tripping unit that can be fitted.

Note: In for circuit breakers (in IEC 947-2) is equal to Iu for switchgear generally, Iu being rated uninterrupted current. * Merlin Gerin products. ** Current-level setting values which refer to the current-operated thermal and "instantaneous" magnetic tripping devices for over-load and short-circuit protection.

the protection of circuits - the switchgear - H2-15

4. circuit breakers (continued)

H2 4.2 fundamental characteristics of a circuit breaker (continued) overload relay trip-current setting (Irth or Ir) Apart from small circuit breakers which are very easily replaced, industrial circuit breakers are equipped with removable, i.e. exchangeable, overcurrent-trip relays. Moreover, in order to adapt a circuit breaker to the requirements of the circuit it controls, and to avoid the need to install over-sized cables, the trip relays are generally adjustable. The trip-current setting Ir or Irth (both designations are in common use) is the current above which the circuit breaker will trip. It also represents the maximum current that the circuit breaker can carry without tripping.

That value must be greater than the maximum load current IB, but less than the maximum current permitted in the circuit Iz (see chapter H1, sub-clause 1.3). The thermal-trip relays are generally adjustable from 0.7 to 1.0 times In, but when electronic devices are used for this duty, the adjustment range is greater; typically 0.4 to 1 times In. Example (figure H2-27): a circuit breaker equipped with a 320 A overcurrent trip relay, set at 0.9, will have a trip-current setting: Ir = 320 x 0.9 = 288 A Note: for circuit breakers equipped with non-adjustable overcurrent-trip relays, Ir = In.

rated current of the tripping unit to suit the circumstances In

0.7 In

adjustment range

overload trip current setting to suit the circuit Ir

224 A

288 A

circuit-breaker frame-size rating

320 A

400 A

I

fig. H2-27: example of a 400 A circuit breaker equipped with a 320 A overload trip unit adjusted to 0.9, to give Ir = 288 A.

short-circuit relay trip-current setting (Im) Short-circuit tripping relays (instantaneous or slightly time-delayed) are intended to trip the circuit breaker rapidly on the occurrence of high values of fault current. Their tripping threshold Im is: c either fixed by standards for domestic type CBs, e.g. IEC 898, or, c indicated by the manufacturer for industrialtype CBs according to related standards, notably IEC 947-2.

domestic breakers IEC 898 modular industrial (2) circuit breakers industrial (2) circuit breakers IEC 947-2

type of protective relay thermalmagnetic

overload protection Ir = In

thermalmagnetic

Ir = In fixed

thermalmagnetic

Ir = In fixed adjustable: 0.7 In i Ir < In

electronic

long delay 0.4 In i Ir < In

For the latter circuit breakers there exists a wide variety of tripping devices which allow a user to adapt the protective performance of the circuit breaker to the particular requirements of a load.

short-circuit protection low setting standard setting type B type C 3 In i Im < 5 In 5 In i Im < 10 In low setting standard setting type B or Z type C 3.2 In < fixed < 4.8 In 7 In < fixed < 10 In fixed: Im ≈ 7 to 10 In adjustable: - low setting : 2 to 5 In - standard setting: 5 to 10 In short-delay, adjustable 1.5 Ir i Im < 10Ir instantaneous (I) fixed I ≈ 12 to 15 In

high setting circuit type D 10 In i Im < 20 In (1) high setting type D or K 10 In < fixed < 14 In

table H2.28: tripping-current ranges of overload and short-circuit protective devices for LV circuit breakers. (1) 50 In in IEC 898, which is considered to be unrealistically high by most European manufacturers (M-G = 10 to 14 In). (2) For industrial use, IEC standards do not specify values. The above values are given only as being those in common use.

H2-16 - the protection of circuits - the switchgear

H2 t (s)

t (s)

I(A)

I(A) Ir

Im

PdC

fig. H2-29: performance curve of a circuit breaker thermal-magnetic protective scheme.

Ir

Im

I

PdC

fig. H2-30 : performance curve of a circuit breaker electronic protective scheme.

Ir: overload (thermal or short-delay) relay trip-current setting. Im: short-circuit (magnetic or long-delay) relay trip-current setting. I: short-circuit instantaneous relay trip-current setting. PdC: breaking capacity.

isolating feature A circuit breaker is suitable for isolating a circuit if it fulfills all the conditions prescribed for a disconnector (at its rated voltage) in the relevant standard (see sub-clause 1.2). In such a case it is referred to as a circuit breaker-disconnector and marked on its front face with the symbol

the short-circuit current-breaking performance of a LV circuit breaker is related (approximately) to the cos ϕ of the fault-current loop. Standard values for this relationship have been established in some standards.

All Multi 9, Compact NS and Masterpact LV switchgear of Merlin Gerin manufacture is in this category.

rated short-circuit breaking capacity (Icu or Icn) The short-circuit current-breaking rating of a CB is the highest (prospective) value of current that the CB is capable of breaking without being damaged. The value of current quoted in the standards is the r.m.s. value of the a.c. component of the fault current, i.e. the d.c. transient component (which is always present in the worst possible case of short-circuit) is assumed to be zero for calculating the standardized value. This rated value (Icu) for industrial CBs and (Icn) for domestic-type CBs is normally given in kA r.m.s. Icu (rated ultimate s.c. breaking capacity) and Ics (rated service s.c. breaking capacity) are defined in IEC 947-2 together with a table relating Ics with Icu for different categories of utilization A (instantaneous tripping) and B (time-delayed tripping) as discussed in subclause 4.3. Tests for proving the rated s.c. breaking capacities of CBs are governed by standards, and include: c operating sequences, comprising a succession of manœuvres, i.e. closing and opening on short-circuit; c current and voltage phase displacement. When the current is in phase with the supply voltage (cos ϕ for the circuit = 1), interruption of the current is easier than that at any other power factor. Breaking a current at low lagging* values of cos ϕ is considerably more difficult to achieve; a zero power-factor circuit being (theoretically) the most onerous case.

In practice, all power-system short-circuit fault currents are (more-or-less) at lagging power factors, and standards are based on values commonly considered to be representative of the majority of power systems. In general, the greater the level of fault current (at a given voltage), the lower the power factor of the fault-current loop, for example, close to generators or large transformers. Table H2-31 below extracted from IEC 947-2 relates standardized values of cos ϕ to industrial circuit breakers according to their rated Icu. Icu 6 kA < Icu i 10 kA 10 kA < Icu i 20 kA 20 kA < Icu i 50 kA 50 kA i Icu

cos ϕ 0.5 0.3 0.25 0.2

table H2-31: Icu related to power factor (cos ϕ) of fault-current circuit. (IEC 947-2). c following an open - time delay - close/open sequence to test the Icu capacity of a CB, further tests are made to ensure that v the dielectric withstand capability; v the disconnection (isolation) performance and v the correct operation of the overload protection, have not been impaired by the test.

the protection of circuits - the switchgear - H2-17

4. circuit breakers (continued)

H2 4.3 other characteristics of a circuit breaker Familiarity with the following less-important characteristics of LV circuit breakers is, however, often necessary when making a final choice.

rated insulation voltage (Ui) This is the value of voltage to which the dielectric tests voltage (generally greater than 2 Ui) and creepage distances are referred. The maximum value of rated operational voltage must never exceed that of the rated insulation voltage, i.e. Ue i Ui.

rated impulse-withstand voltage (Uimp) This characteristic expresses, in kV peak (of a prescribed form and polarity) the value of voltage which the equipment is capable of withstanding without failure, under test conditions. For further details see chapter F, clause 2.

category (A or B) and rated short-time withstand current (Icw) As already briefly mentioned (sub-clause 4.2) there are two categories of LV industrial switchgear, A and B, according to IEC 947-2: c those of category A, for which there is no deliberate delay in the operation of the "instantaneous" short-circuit magnetictripping device (figure H2-32), are generally moulded-case type circuit breakers, and, c those of category B for which, in order to discriminate with other circuit breakers on a time basis, it is possible to delay the tripping of the CB, where the fault-current level is lower than that of the short-time withstand current rating (Icw) of the CB (figure H2-23). This is generally applied to large open-type circuit breakers and to certain heavy-duty moulded-case types. Icw is the maximum current that the B category CB can withstand, thermally and electrodynamically, without sustaining damage, for a period of time given by the manufacturer.

t (s)

I(A) Im

fig. H2-32: category A circuit breaker. t (s)

I(A) Im

I

Icw

fig. H2-33: category B circuit breaker.

H2-18 - the protection of circuits - the switchgear

PdC

H2 rated making capacity (Icm) Icm is the highest instantaneous value of current that the circuit breaker can establish at rated voltage in specified conditions. In a.c. systems this instantaneous peak value is related to Icu (i.e. to the rated breaking current) by the factor k, which depends on the power factor (cos ϕ) of the short-circuit current loop (as shown in table H2-34). Example: a LV circuit breaker has a rated breaking capacity Icu of 100 kA r.m.s. Its rated making capacity Icm will be 100 x 2.2 = 220 kA peak.

in a correctly designed installation, a circuit breaker is never required to operate at its maximum breaking current Icu. For this reason a new characteristic Ics has been introduced. It is expressed in IEC 947-2 as a percentage of Icu (25, 50, 75, 100%).

many designs of LV circuit breakers feature a short-circuit current limitation capability, whereby the current is reduced and prevented from reaching its (otherwise) maximum peak value (figure H2-35). The current-limitation performance of these CBs is presented in the form of graphs, typified by that shown in figure H2-36, diagram (a).

Icu 6 kA < Icu i 10 kA 10 kA < Icu i 20 kA 20 kA < Icu i 50 kA 50 kA i Icu

cos ϕ 0.5 0.3 0.25 0.2

Icm = kIcu 1.7 x Icu 2 x Icu 2.1 x Icu 2.2 x Icu

table H2.34: relation between rated breaking capacity Icu and rated making capacity Icm at different power-factor values of short-circuit current, as standardized in IEC 947-2.

rated service short-circuit breaking capacity (Ics) The rated breaking capacity (Icu) or (Icn) is the maximum fault-current a circuit breaker can successfully interrupt without being damaged. The probability of such a current occurring is extremely low, and in normal circumstances the fault-currents are considerably less than the rated breaking capacity (Icu) of the CB. On the other hand it is important that high currents (of low probability) be interrupted under good conditions, so that the CB is immediately available for reclosure, after the faulty circuit has been repaired. It is for these reasons that a new

characteristic (Ics) has been created, expressed as a percentage of Icu, viz: 25, 50, 75, 100% for industrial circuit breakers. The standard test sequence is as follows: c O - CO - CO* (at Ics); c tests carried out following this sequence are intended to verify that the CB is in a good state and available for normal service. For domestic CBs, Ics = k Icn. The factor k values are given in IEC 898 table XIV. In Europe it is the industrial practice to use a k factor of 100% so that Ics = Icu. Note: O represents an opening operation. CO represents a closing operation followed by an opening operation.

fault-current limitation The fault-current limitation capacity of a CB concerns its ability, more or less effective, in preventing the passage of the maximum prospective fault-current, permitting only a limited amount of current to flow, as shown in figure H2-35. The current-limitation performance is given by the CB manufacturer in the form of curves (figure H2-36 diagrams (a) and (b)). c diagram (a) shows the limited peak value of current plotted against the r.m.s. value of the a.c. component of the prospective fault current ("prospective" fault-current refers to the fault-current which would flow if the CB had no current-limiting capability); c limitation of the current greatly reduces the thermal stresses (proportional I2t) and this is shown by the curve of diagram (b) of figure H2-36, again, versus the r.m.s. value of the a.c. component of the prospective fault current. LV circuit breakers for domestic and similar installations are classified in certain standards (notably European Standard EN 60 898). CBs belonging to a class (of

current limiters) have standardized limiting I2t let-through characteristics defined by that class. In these cases, manufacturers do not normally provide characteristic performance curves. Icc prospectice fault-current peak

prospectice fault-current

limited current peak limited current t

tc

fig. H2-35: prospective and actual currents.

t

n rre cu s c d i ite st m ri -li cte n a no har c

limited peak current (kA)

limited peak current (A2 x s) 4,5.105

22

2.105

(a)

prospective a.c. component (r.m.s.)

(b)

prospective a.c. component (r.m.s.)

150

150 kA

fig. H2-36: performance curves of a typical LV current-limiting circuit breaker.

the protection of circuits - the switchgear - H2-19

4. circuit breakers (continued)

H2 4.3 other characteristics of a circuit breaker (continued) current limitation reduces both thermal and electrodynamic stresses on all circuit elements through which the current passes, thereby prolonging the useful life of these elements. Furthermore, the limitation feature allows "cascading" techniques to be used (see 4.5) thereby significantly reducing design and installation costs.

the advantages of current limitation The use of current-limiting CBs affords numerous advantages: c better conservation of installation networks: current-limiting CBs strongly attenuate all harmful effects associated with short-circuit currents; c reduction of thermal effects: conductors (and therefore insulation) heating is significantly reduced, so that the life of cables is correspondingly increased; c reduction of mechanical effects: forces due to electromagnetic repulsion are lower, with less risk of deformation and possible rupture, excessive burning of contacts, etc.; c reduction of electromagnetic-interference effects: less influence on measuring instruments and associated circuits, telecommunication systems, etc. These circuit breakers therefore contribute towards an improved exploitation of: c cables and wiring; c prefabricated cable-trunking systems; c switchgear, thereby reducing the ageing of the installation.

Example: On a system having a prospective shortcircuit current of 150 kA r.m.s., a circuit breaker limits the peak current to less than 10% of the calculated prospective peak value, and the thermal effects to less than 1% of those calculated. Cascading of the several levels of distribution in an installation, downstream of a limiting CB, will also result in important economies. The technique of cascading, described in sub-clause 4.5 allows, in fact, substantial savings on switchgear (lower performance permissible downstream of the limiting CB(s)) enclosures, and design studies, of up to 20% (overall). Discriminative protection schemes and cascading are compatible, in the range Compact NS*, up to the full short-circuit breaking capacity of the switchgear. * A Merlin Gerin product.

4.4 selection of a circuit breaker the choice of a range of circuit breakers is determined by: the electrical characteristics of the installation, the environment, the loads and a need for remote control, together with the type of telecommunications system envisaged.

choice of a circuit breaker The choice of a CB is made in terms of: c electrical characteristics of the installation for which the CB is destined; c its eventual environment: ambient temperature, in a kiosk or switchboard enclosure, climatic conditions, etc.; c short-circuit current breaking and making requirements; c operational specifications: discriminative tripping, requirements (or not) for remote control and indication and related auxiliary contacts, auxiliary tripping coils, connection into a local network (communication or control and indication) etc.,

c installation regulations; in particular: protection of persons; c load characteristics, such as motors, fluorescent lighting, LV/LV transformers, etc. Problems concerning specific loads are examined in chapter J. The following notes relate to the choice of a LV circuit breaker for use in distribution systems.

choice of rated current in terms of ambient temperature The rated current of a circuit breaker is defined for operation at a given ambient temperature, in general: c 30 °C for domestic-type CBs; c 40 °C for industrial-type CBs. Performance of these CBs in a different ambient temperature depends principally on the technology of their tripping units.

ambient temperature

single CB in free air

temperature of air surrounding the circuit breakers

circuit breakers installed in an enclosure

fig. H2-37: ambient temperature.

H2-20 - the protection of circuits - the switchgear

ambient temperature

H2 circuit breakers with uncompensated thermal tripping units have a tripcurrent level that depends on the surrounding temperature.

uncompensated thermalmagnetic tripping units Circuit breakers with uncompensated thermal tripping elements have a tripping-current level that depends on the surrounding temperature. If the CB is installed in an enclosure, or in a hot location (boiler room, etc.), the current required to trip the CB on overload will be sensibly reduced. When the temperature in which the CB is located exceeds its reference temperature, it will therefore be "derated". For this reason, CB manufacturers provide tables which indicate factors to apply at temperatures different to

the CB reference temperature. It may be noted from typical examples of such tables (tables H2-38) that a lower temperature than the reference value produces an up-rating of the CB. Moreover, small modular-type CBs mounted in juxtaposition, as shown typically in figure H2-24, are usually mounted in a small closed metal case. In this situation, mutual heating, when passing normal load currents, generally requires them to be derated by a factor of 0.8.

C60a. C60H: curve C. C60N: curves B and C (reference temperature: 30 °C) rating (A) 20 °C 25 °C 30 °C 35 °C 40 °C 45 °C 50 °C 55 °C 1 1.05 1.02 1.00 0.98 0.95 0.93 0.90 0.88 2 2.08 2.04 2.00 1.96 1.92 1.88 1.84 1.80 3 3.18 3.09 3.00 2.91 2.82 2.70 2.61 2.49 4 4.24 4.12 4.00 3.88 3.76 3.64 3.52 3.36 6 6.24 6.12 6.00 5.88 5.76 5.64 5.52 5.40 10 10.6 10.3 10.0 9.70 9.30 9.00 8.60 8.20 16 16.8 16.5 16.0 15.5 15.2 14.7 14.2 13.8 20 21.0 20.6 20.0 19.4 19.0 18.4 17.8 17.4 25 26.2 25.7 25.0 24.2 23.7 23.0 22.2 21.5 32 33.5 32.9 32.0 31.4 30.4 29.8 28.4 28.2 40 42.0 41.2 40.0 38.8 38.0 36.8 35.6 34.4 50 52.5 51.5 50.0 48.5 47.4 45.5 44.0 42.5 63 66.2 64.9 63.0 61.1 58.0 56.7 54.2 51.7 NS250N/H/L (reference temperature: 40 °C) rating (A) 40 °C 45 °C TM160D 160 156 TM200D 200 195 TM250D 250 244

50 °C 152 190 238

55 °C 147 185 231

60 °C 0.85 1.74 2.37 3.24 5.30 7.80 13.5 16.8 20.7 27.5 33.2 40.5 49.2 60 °C 144 180 225

tables H2-38: examples of tables for the determination of derating/uprating factors to apply to CBs with uncompensated thermal tripping units, according to temperature. Example What rating (In) should be selected for a CB c protecting a circuit, the maximum load current of which is estimated to be 34 A; c installed side-by-side with other CBs in a closed distribution box; c in an ambient temperature of 50 °C. A circuit breaker rated at 40 A would be derated to 35.6 A in ambient air at 50 °C (see

table H2-38). To allow for mutual heating in the enclosed space, however, the 0.8 factor noted above must be employed, so that, 35.6 x 0.8 = 28.5 A, which is not suitable for the 34 A load. A 50 A circuit breaker would therefore be selected, giving a (derated) current rating of 44 x 0.8 = 35.2 A.

compensated thermal-magnetic tripping units These tripping units include a bi-metal compensating strip which allows the overload trip-current setting (Ir or Irth) to be adjusted, within a specified range, irrespective of the ambient temperature. For example: c in certain countries, the TT system is standard on LV distribution systems, and domestic (and similar) installations are protected at the service position by a circuit breaker provided by the supply authority.

This CB, besides affording protection against indirect-contact hazard, will trip on overload; in this case, if the consumer exceeds the current level stated in his supply contract with the power authority. The circuit breaker (i 60 A) is compensated for a temperature range of - 5 °C to + 40 °C. c LV circuit breakers at ratings i 630 A are commonly equipped with compensated tripping units for this range (- 5 °C to + 40 °C).

the protection of circuits - the switchgear - H2-21

4. circuit breakers (continued)

H2 4.4 selection of a circuit breaker (continued) general note concerning derating of circuit breakers It is evident that a CB rated to carry a current In at its reference ambient temperature (30 °C) would overheat when carrying the same current at (say) 50 °C. Since LV CBs are provided with overcurrent protective devices which (if not compensated) will operate for lower levels of current in higher ambient temperatures, the CB is automatically derated by the overload tripping device, as shown in the tables H2-38. Where the thermal tripping units are temperature-compensated, the tripping current level may be set at any value between 0.7 to 1 x In in the ambient temperature range of - 5 °C to + 40 °C. The reference ambient temperature in this case is 40 °C (i.e. on which the rating In is based). For these compensated units, manufacturers' catalogues generally also give derated values of In for ambient temperatures above the compensated range, e.g. at + 50 °C and + 60 °C; typically, 95 A at + 50 °C and 90 A at + 60 °C, for a 100 A circuit breaker.

H2-22 - the protection of circuits - the switchgear

H2 electronic tripping units are highly stable in changing temperature levels.

electronic tripping units An important advantage with electronic tripping units is their stable performance in changing temperature conditions. However, the switchgear itself often imposes operational limits in elevated temperatures, M25N/H/L circuit breaker A

i 40 °C 2500 1 2500 1

In (A) maximum adjustment Ir In (A) maximum adjustement Ir

circuit breaker B coeff.

as mentioned in the general note above, so that manufacturers generally provide an operating chart relating the maximum values of permissible trip-current levels to the ambient temperature (figure H2-39). 45 °C 2500 1 2500 1

50 °C 2500 1 2500 1

55 °C 2450 0.98 2350 0.94

60 °C 2400 0.96 2200 0.88

In (A)

1 2500

circuit breaker A 0.96 2400 0.94 2350 circuit breaker B

0.88 2200 θ °C 20

25

30

35

40

45

50

55

60

fig. H2-39: derating of two circuit breakers having different characteristics, according to the temperature.

selection of an instantaneous, or short-time-delay, tripping threshold Principal charasteristics of magnetic or shorttime-delay tripping units. Type classification according to IEC 898. See also table H2-28. type t

tripping unit low setting type B

applications c sources producing low-short-circuit-current levels (standby generators) c long lengths of line or cable

standard setting type C

c protection of circuits: general case

high setting type D or K

c protection of circuits having high initial transient current levels (e.g. motors, transformers, resistive loads)

12 In type MA

c protection of motors in association with discontactors (contactors with overload protection)

I t

I t

I t

I

table H2-40: different tripping units, instantaneous or short-time-delayed.

the protection of circuits - the switchgear - H2-23

4. circuit breakers (continued)

H2 4.4 selection of a circuit breaker (continued) the installation of a LV circuit breaker requires that its short-circuit breaking capacity (or that of the CB together with an associated device) be equal to or exceeds the calculated prospective short-circuit current at its point of installation.

the circuit breaker at the output of the smallest transformer must have a short-circuit capacity adequate for a fault current which is higher than that through any of the other transformer LV circuit breakers (fig. H2-42).

selection of a circuit breaker according to the short-circuit breaking capacity requirements The installation of a circuit breaker in a LV installation must fulfil one of the two following conditions: c either have a rated short-circuit breaking capacity Icu (or Icn) which is equal to or exceeds the prospective short-circuit current calculated for its point of installation, or c if this is not the case, be associated with another device which is located upstream, and which has the required short-circuit breaking capacity.

The selection of main and principal circuit breakers c a single transformer Table C-13 (in chapter C) gives the shortcircuit current level on the downstream side of a commonly-used type of HV/LV distribution transformer. If the transformer is located in a consumer's substation, certain national standards require a LV circuit breaker in which the open contacts are clearly visible*. Example (figure H2-41): What type of circuit breaker is suitable for the main circuit breaker of an installation supplied through a 250 kVA HV/LV (400 V) 3-phase transformer in a consumer's substation? In transformer = 360 A Isc (3-phase) = 8.9 kA. A 400 A CB with an adjustable tripping-unit range of 250 A-400 A and a short-circuit breaking capacity (Icu) of 35 kA* would be a suitable choice for this duty.

In the second case, the characteristics of the two devices must be co-ordinated such that the energy permitted to pass through the upstream device must not exceed that which the downstream device and all associated cables, wires and other components can withstand, without being damaged in any way. This technique is profitably employed in: c associations of fuses and circuit breakers; c associations of current-limiting circuit breakers and standard circuit breakers. The technique is known as "cascading" (see sub-clause 4.5 of this chapter).

250 kVA 20 kV/400 V

Visucompact NS400N

fig. H2-41: example of a transformer in a consumer's substation.

* A type Visucompact NS400N of Merlin Gerin manufacture is recommended for the case investigated.

c several transformers in parallel (figure H2-42) v the circuit breakers CBP outgoing from the LV distribution board must each be capable of breaking the total fault current from all transformers connected to the busbars, viz: Isc1 + Isc2 + Isc3, v the circuit breakers CBM, each controlling the output of a transformer, must be capable of dealing with a maximum short-circuit current of (for example) Isc2 + Isc3 only, for a short-circuit located on the upstream side of CBM1. From these considerations, it will be seen that the circuit breaker of the smallest transformer will be subjected to the highest level of fault current in these circumstances, while the circuit breaker of the largest transformer will pass the lowest level of short-circuit current. v the ratings of CBMs must be chosen according to the kVA ratings of the associated transformers. Note: the essential conditions for the successful operation of 3-phase transformers in parallel may be summarized as follows: 1. the phase shift of the voltages, primary to secondary, must be the same in all units to be paralleled. 2. the open-circuit voltage ratios, primary to secondary, must be the same in all units. 3. the short-circuit impedance voltage (Zsc%) must be the same for all units. For example, a 750 kVA transformer with a Zsc = 6% will H2-24 - the protection of circuits - the switchgear

HV

HV

Tr1

Tr2

LV A1

HV Tr3

LV A2

CBM

B1

B2 CBP

CBM

LV A3

CBM

B3 CBP

E

fig. H2-42: transformers in parallel.

share the load correctly with a 1,000 kVA transformer having a Zsc of 6%, i.e. the transformers will be loaded automatically in proportion to their kVA ratings. For transformers having a ratio of kVA ratings exceeding 2, parallel operation is not recommended, since the resistance/ reactance ratios of each transformer will generally be different to the extent that the resulting circulating currrent may overload the smaller transformer.

H2 Table H2-43 indicates, for the most usual arrangement (2 or 3 transformers of equal kVA ratings) the maximum short-circuit currents to which main and principal CBs (CBM and CBP respectively, in figure H2-42) are subjected. The table is based on the following hypotheses: c the short-circuit 3-phase power on the HV side of the transformer is 500 MVA; c the transformers are standard 20/0.4 kV distribution-type units rated as listed; c the cables from each transformer to its LV circuit breaker comprise 5 metres of singlecore conductors; c between each incoming-circuit CBM and each outgoing-circuit CBP there is 1 metre of busbar; c the switchgear is installed in a floormounted enclosed switchboard, in an ambient-air temperature of 30 °C. Moreover, this table shows selected circuit breakers of M-G manufacture recommended for main and principal circuit breakers in each case. number and kVA ratings of 20/0.4 kV transformers 2 x 400 3 x 400 2 x 630 3 x 630 2 x 800 3 x 800 2 x 1000 3 x 1000 2 x 1250 3 x 1250 2 x 1600 3 x 1600 2 x 2000 3 x 2000

minimum S.C. breaking capacity of main CBs (Icu)* kA 14 27 22 43 24 48 27 54 31 62 36 72 39 77

main circuit breakers (CBM) total discrimination with out going-circuit breakers (CBP) M08 N1/C 801 N ST M08 N1/C 801 N ST M10N1/CM1250/C 1001 N M10H1/CM1250/C 1001 N M12N1/CM1250/C 1251 N M12H1/CM1250/C 1251 N M16N1/CM1600 M16H2/CM1600 M20N1/CM2000 M20H1/CM2000 M25N1/CM2500 M20H2/CM2500H M32H1/CM3200 M32H2/CM3200H

minimum S.C. breaking cap. of principal CBs (Icu)* kA 27 40 42 64 48 71 54 80 60 91 70 105 75 112

rated current In of principal circuit breaker (CPB) 250 A NS 250 N NS 250 H NS 250 H NS 250 H NS 250 H NS 250 L NS 250 H NS 250 L NS 250 H NS 250 L NS 250 H NS 250 L NS 250 L NS 250 L

table H2-43: maximum values of short-circuit current to be interrupted by main and principal circuit breakers (CBM and CBP respectively), for several transformers in parallel. * or Ics in countries where this alternative is practised.

Example: (figure H2-44) c circuit breaker selection for CBM duty: In for an 800 kVA transformer = 1.126 A (at 410 V, i.e. no-load voltage) Icu (minimum) = 48 kA (from table H2-43), the CBM indicated in the table is a Compact C1251 N (Icu = 50 kA) (by Merlin Gerin) or its equivalent; c circuit breaker selection for CBP duty: The s.c. breaking capacity (Icu) required for these circuit breakers is given in the table (H2-43) as 71 kA. A recommended choice for the three outgoing circuits 1, 2 and 3 would be current-limiting circuit breakers types NS 400 L, NS 100 L and NS 250 L respectively (by MG) or their equivalents. The Icu rating in each case = 150 kA. These circuit breakers provide the advantages of: v absolute discrimination with the upstream (CBM) breakers, v exploitation of the "cascading" technique, with its attendant economy for all downstream components.

3 Tr 800 kVA 20 kV/400V CBM

CBP1

400 A

CBP3

CBP2

100 A

200 A

fig H2-44: transformers in parallel.

the protection of circuits - the switchgear - H2-25

4. circuit breakers (continued)

H2 4.4 selection of a circuit breaker (continued) short-circuit fault-current levels at any point in an installation may be obtained from tables.

cascading: a particular solution to problems of CBs insufficiently rated for S.C. breaking duty.

associating fuses with CBs avoids the need for a fuse in the neutral, except in particular circumstances on some IT systems.

H2-26 - the protection of circuits - the switchgear

Choice of outgoing-circuit CBs and final-circuit CBs c use of table H1-40 From this table, the value of 3-phase shortcircuit current can be determined rapidly for any point in the installation, knowing: v the value of short-circuit current at a point upstream of that intended for the CB concerned; v the length, c.s.a., and the composition of the conductors between the two points. A circuit breaker rated for a short-circuit breaking capacity exceeding the tabulated value may then be selected. c detailed calculation of the short-circuit current level In order to calculate more precisely the shortcircuit current, notably, when the short-circuit current-breaking capacity of a CB is slightly less than that derived from the table, it is necessary to use the method indicated in chapter H1 clause 4. c two-pole circuit breakers (for phase and neutral) with one protected pole only These CBs are generally provided with an overcurrent protective device on the phase pole only, and may be used in TT, TN-S and IT schemes. In an IT scheme, however, the following conditions must be respected: v condition (c) of table H1-65 for the protection of the neutral conductor against overcurrent in the case of a double fault; v short-circuit current-breaking rating: A 2-pole phase-neutral CB must, by convention, be capable of breaking on one pole (at the phase-to-phase voltage) the current of a double fault equal to 15% of the 3-phase short-circuit current at the point of its installation, if that current is i 10 kA; or 25% of the 3-phase short-circuit current if it exceeds 10 kA; v protection against indirect contact: this protection is provided according to the rules for IT schemes, as described in chapter G sub-clause 6.2. c insufficient short-circuit currentbreaking rating In low-voltage distribution systems it sometimes happens, especially in heavy-duty networks, that the Isc calculated exceeds the Icu rating of the CBs available for installation, or system changes upstream result in lowerlevel CB ratings being exceeded. v solution 1: check whether or not appropriate CBs upstream of the CBs affected are of the current-limiting type, allowing the principle of cascading (described in sub-clause 4.5) to be applied;

v solution 2: install a range of CBs having a higher rating. This solution is economically interesting only where one or two CBs are affected; v solution 3: associate current-limiting fuses (gG or aM) with the CBs concerned, on the upstream side. This arrangement must, however, respect the following rules: - the fuse rating must be appropriate - no fuse in the neutral conductor, except in certain IT installations where a double fault produces a current in the neutral which exceeds the short-circuit breaking rating of the CB. In this case, the blowing of the neutral fuse must cause the CB to trip on all phases.

H2 4.5 coordination between circuit breakers Preliminary note on the essential function of current limiting circuit breakers Low-voltage current-limiting CBs exploit the resistance of the short-circuit current arc in the CB to limit the value of current. An improved method of achieving currentlevel limitation is to associate a separate current-limiting module (in series) with a standard CB. A contact bar (per phase) in the module bridges two (specially-designed heavy-duty) contacts, the contact pressure of which is accurately maintained by springs. Other rigidly-fixed conductors are arranged in series with, and close to the contact bar, such that when current is passed through the ensemble, the electromagnetic force tends to move the contact bar to open its contacts. This occurs at relatively low values of shortcircuit current, which then passes through the arcs formed at each contact. The resistance of the arcs is comparable with system impedances at low voltage, so that the current is correspondingly restricted.

the technique of "cascading" uses the properties of current-limiting circuit breakers to permit the installation of all downstream switchgear, cables and other circuit components of significantly lower performance than would otherwise be necessary, thereby simplifying and reducing the cost of an installation.

in general, laboratory tests are necessary to ensure that the conditions of exploitation required by national standards are met and compatible switchgear combinations must be provided by the manufacturer.

Furthermore, the higher the current, the more the repulsive force on the bar and the greater the arc resistance as its path lengthens, i.e. the current magnitude is (to some extent) self-regulating. The circuit breaker is easily able to break the resulting low value of current, particularly since the power factor of the fault-current loop is increased by the resistive impedance of the arcs. When used in a cascading scheme as described below, the tripping of the limiting CB main contacts is briefly delayed, to allow downstream high-speed circuit breakers to clear the (limited) current, i.e. the currentlimiter CB remains closed. The contact bar in the limiter module resets under the influence of its pressure springs when the flow of short-circuit current ceases. Failure of downstream CBs to trip will result in the tripping of the current-limiting CB, after its brief time delay.

cascading Definition of the cascading technique By limiting the peak value of short-circuit current passing through it, a current-limiting CB permits the use, in all circuits downstream of its location, of switchgear and circuit components having much lower short-circuit breaking capacities, and thermal and electromechanical withstand capabilities than would otherwise be the case. Reduced physical size and lower performance requirements lead to substantial economies and to the simplification of installation work. It may be noted that, while a current-limiting circuit breaker has the effect on downstream circuits of (apparently) increasing the source impedance during short-circuit conditions, it has no such effect at any other time; for example, during the starting of a large motor (where a low source impedance is highly desirable). A new range of Compact* current-limiting circuit breakers with powerful limiting performances (namely: NS 100, NS 160, NS 250 and NS 400) is particularly interesting. Conditions of exploitation Most national standards permit use of the cascading technique, on condition that the amount of energy "let through" by the limiting CB is less than that which all downstream CBs and components are able to withstand without damage. In practice this can only be verified for CBs by tests performed in a laboratory. Such tests are carried out by manufacturers who provide the information in the form of tables, so that users can confidently design a cascading scheme based on the combination of circuit breaker types recommended. By way of an example, table H2-45 indicates the possibilities of cascading circuit breaker types* C 60 and NC 100 when installed downstream of current-limiting CBs NS 250 N, H or L for a 230/400 V or 240/415 V 3-phase installation. * Merlin Gerin products

the protection of circuits - the switchgear - H2-27

4. circuit breakers (continued)

H2 4.5 coordination between circuit breakers (continued) Advantages of cascading The limitation of current benefits all downstream circuits that are controlled by the current-limiting CB concerned. The principle is not restrictive, i.e. currentlimiting CBs can be installed at any point in an installation where the downstream circuits would otherwise be inadequately rated. The result is: c simplified short-circuit current calculations; c simplification, i.e. a wider choice of downstream switchgear and appliances; c the use of lighter-duty switchgear and appliances, with consequently lower cost; c economy of space requirements, since light-duty equipment is generally less voluminous.

Short-circuit breaking capacity of the upstream (limiter) CBs kA r.m.s. 150 NS250L 100 70 NS250H 36 NS250N 25 22 Short-circuit breaking capacity of the downstream CBs (benefiting from the cascading technique) kA r.m.s. 150 NC100LH NC100LMA 100 NC100LS 70 NC100LS NC100L NC100LH NC100LMA 50 NC100L 40 C60L i 40 C60L i 40 30 C60H C60N C60N C60L C60H C60H C60L C60L (50 to 63) (50 to 63) NC100H NC100H 25 C60N NC100H 20 C60a C60a 15 C60a tables H2-45: example of cascading possibilities on a 230/400 V or 240/415 V 3-phase installation.

discrimination may be absolute or partial, and based on the principles of current levels, or time-delays, or a combination of both. A more recent development is based on the principles of logic. A (patented) system by Merlin Gerin exploits the advantages of both current-limitation and discrimination.

H2-28 - the protection of circuits - the switchgear

discriminative tripping (selectivity) Discrimination is achieved by automatic protective devices if a fault condition, occurring at any point in the installation, is cleared by the protective device located immediately upstream of the fault, while all other protective devices remain unaffected (figure H2-46). Discrimination between circuit breakers A and B is absolute if the maximum value of shortcircuit-current on circuit B does not exceed the short-circuit trip setting of circuit breaker A. For this condition, B only will trip (figure H2-47). Discrimination is partial if the maximum possible short-circuit current on circuit B exceeds the short-circuit trip-current setting of circuit breaker A. For this maximum condition, both A and B will trip (figure H2-48).

IscA A

IscB B

absolute discrimination

Icc IccB

IrB

partial discrimination B only open A and B opens IrB

Ic

IccB

fig. H2-46: absolute and partial discrimination.

Icc

H2 t

t

B

A

B

A

Isc downstream of B Ir B

Ir A

Icc B Irm A

I

Ir B

Ir A

Irm A Isc B

B only opens

1. discrimination based on current levels. This method is realized by setting successive relay tripping thresholds at stepped levels, from downstream relays (lower settings) towards the source (higher settings).

fig. H2-48: partial discrimination between CBs A and B.

Discrimination is absolute or partial, according to the particular conditions, as noted in the above examples.

t

B

In the two-level arrangement shown, upstream circuit breaker A is delayed sufficiently to ensure absolute discrimination with B (for example: Masterpact electronic).

I

A and B open

fig. H2-47: absolute discrimination between CBs A and B.

A

I

Irm A Isc B

Irm B

2. discrimination based on stepped time delays. This method is implemented by adjusting the time-delayed tripping units, such that downstream relays have the shortest operating times, with progressively longer delays towards the source.

IscA

A t B A ∆t B Isc B

3. discrimination based on a combination of methods 1 and 2. A mechanical time-delay added to a currentlevel scheme can improve the overall discrimination performance.

Discrimination is absolute if Isc B < Irm A (instantaneous). The upstream CB has two high-speed magnetic tripping thresholds: - Irm A (delayed) or a SD* electronic timer - Irm A (instantaneous) standard (Compact type SA)

t

B

chambers of the CBs. The heated-air pressure level depends on the energy level of the arc, as described in the following pages (figures H2-54 and H2-55).

A Isc B

* short-delay.

4. discrimination based on arc-energy levels (Merlin Gerin patent) In the range of short-circuit currents, this system provides absolute discrimination between two circuit breakers passing the same fault current. This is achieved by using current-limiting CBs and initiating CB tripping by pressure-sensitive detectors in the arcing

I

Irm A delayed

Irm A instantaneo us

I

t

conventional instantaneous magnetic-trip characteristic pressure operated magnetic-trip characteristic

Irm B

Irm A

Isc

table H2-49: summary of methods and components used in order to achieve discriminative tripping.

the protection of circuits - the switchgear - H2-29

4. circuit breakers (continued)

H2 4.5 coordination between circuit breakers (continued) current-level discrimination is achieved with stepped current-level settings of the instantaneous magnetic-trip elements.

Current-level discrimination Current-level discrimination is achieved with circuits breakers, preferably limiters, and stepped current-level settings of the instantaneous magnetic-trip elements. c the downstream circuit breaker is not a current-limiter. The discrimination may be absolute or partial for a short-circuit fault downstream of B, as previously noted in 1, above. Absolute discrimination in this situation is practically impossible because Isc A z Isc B, so that both circuit breakers will generally trip in unison. In this case discrimination is partial, and limited to the Irm of the upstream circuit breaker. c the downstream circuit breaker is a current limiter. Improvement in discriminative tripping can be obtained by using a current limiter in a downstream location, e.g. for circuit breaker B. For a short-circuit downstream of B, the limited level of peak current IB would operate the (suitably adjusted) magnetic trip unit of B, but would be insufficient to cause circuit breaker A to trip. Note: All LV breakers (considered here) have some inherent degree of current limitation, even those that are not classified as currentlimiters. This accounts for the curved characteristic shown for the standard circuit breaker A in figure H2-50. Careful calculation and testing is necessary, however, to ensure satisfactory performance of this arrangement. c the upstream circuit breaker is highspeed with a short-delay (SD) feature. These circuit breakers are fitted with trip units which include a non-adjustable mechanical short-time-delay feature. The delay is sufficient to ensure absolute discrimination with any downstream high-speed CB at any value of s.c. current up to Irms (figure H2-51).

Example: circuit breaker A: Compact NS250 N fitted with a trip unit which includes a SD feature. Ir = 250 A, magnetic trip set at 2,000 A circuit breaker B: Compact NS100N Ir = 100 A The Merlin Gerin distribution catalogue indicates a discrimination limit of 3,000 A (an improvement over the limit of 2,500 A obtained when using a standard tripping unit).

I peak A current limitation curve for circuit breaker (see note) B

fault upstream of B fault downstream of B

I Isc prospective (rms)

Isc

fig. H2-50: downstream limiting circuit breaker B. t A (compact S) B

only B opens

A and B open Irm A Irm S delayed instantaneous

I

fig. H2-51: use of a "selective" circuit breaker upstream.

discrimination based on time-delayed tripping uses CBs referred to as "selective" (in certain countries). Application of these CBs is relatively simple and consists in delaying the instant of tripping of the several series-connected circuit breakers in a stepped time sequence.

H2-30 - the protection of circuits - the switchgear

Time-based discrimination This technique requires: c the introduction of "timers" into the tripping mechanisms of CBs; c CBs with adequate thermal and mechanical withstand capabilities at the elevated current levels and time delays envisaged. Two circuit breakers A and B in series (i.e. passing the same current) are discriminative if the current-breaking period of downstream breaker B is less than the non-tripping time of circuit breaker A. Discrimination at several levels An example of a practical scheme with (MG) circuit breakers Masterpact (electronic protection devices). These CBs can be equipped with adjustable timers which allow 4 time-step selections, such as: c the delay corresponding to a given step is greater than the total current breaking time of the next lower step;

c the delay corresponding to the first step is greater than the total current-breaking time of a high-speed CB (type Compact for example) or of fuses (figure H2-52). t A non tripping time of A

B

current-breaking time for B

only B open Ir B

Isc B

Isc I

fig. H2-52: discrimination by time delay.

H2 discrimination schemes based on logic techniques are possible, using CBs equipped with electronic tripping units designed for the purpose (Compact, Masterpact by MG) and interconnected with pilot wires.

recently-introduced circuit breakers such as Merlin Gerin type NS, use the principle of arc-energy levels to obtain discrimination.

Discrimination logic This discrimination system requires CBs equipped with electronic tripping units, designed for this application, together with interconnecting pilot wires for data exchange between the CBs. With 2 levels A and B (figure H2-53), circuit breaker A is set to trip instantaneously, unless the relay of circuit breaker B sends a signal to confirm that the fault is downstream of B. This signal causes the tripping unit of A to be delayed, thereby ensuring back-up protection in the event that B fails to clear the fault, and so on… This system (patented by Merlin Gerin) also allows rapid localization of the fault. Limitation and discrimination by exploitation of arc energy The technique of "arc-energy discrimination" (Merlin Gerin patent) is applied on circuits having a short-circuit current level u 25 In and ensures absolute selectivity between two CBs carrying the same short-circuit current. Discrimination requires that the energy allowed to pass by the downstream CB (B) is less than that which will cause the upstream CB (A) to trip (fig. H2-54 (a)). Operation principle Both CBs are current limiters, so that the electromagnetic forces due to a short-circuit downstream of CB (B) will cause the currentlimiting arcing contacts of both CBs to open simultaneously. The fault current will be very strongly limited by the resistance of the two series arcs. The intense heat of the current arc in each CB causes a rapid expansion of the air in the confined space of the arcing chambers, thereby producing a correspondingly rapid pressure rise. Above a certain level of current, the pressure rise can be reliably detected and used to initiate instantaneous tripping. Discrimination principle If both CBs include a pressure tripping device suitably regulated, then absolute discrimination between two CBs of different current ratings can be achieved by setting CB (B) to trip at a lower pressure level than that of CB (A) (fig. H2-54). If a short-circuit occurs downstream of CB (A) but upstream of CB (B), then the arc resistance of CB (A) only will limit the current. The resulting current will therefore be significantly greater than that occurring for a short-circuit downstream of CB (B) (where the two arcs in series cause a very strong limitation, as previously mentioned). The larger current through CB (A) will produce a correspondingly greater pressure, which will be sufficient to operate its pressure-sensitive tripping unit (diagrams (b) and (c) of fig. H2-54). As can be seen from figure H2-49 (4), the larger the short-circuit current, the faster the CB will trip. Discrimination is assured with this particular switchgear if: c the ratio of rated currents of the two CBs u 2.5; c the ratio of the two trip-unit current ratings is > 1.6, as shown (typically) in figure H2-55.

A

pilot wires

B

fig. H2-53: discrimination logic.

CB (A) Compact NS

(a) CB (B) Compact NS

Isc = 50 kA I Isc (prospective) CB (A) only CB (A) and CB (B) in series

(b)

Isc (limited) t Pressure in arcing chamber CB (A) setting

(c) CB (B) setting

t

fig. H2-54: arc-energy discrimination principles.

CB (A)

CB (B)

NS250N TM260D

NS100N TM100D

fig. H2-55: ratio of rated currents of CBs and of tripping units, must comply with limits stated in the text, to ensure discrimination.

For overcurrent conditions less than those of short-circuits i 25 In, the conventional protection schemes are employed, as previously described in this chapter. the protection of circuits - the switchgear - H2-31

4. circuit breakers (continued)

H2 4.6 discrimination HV/LV in a consumer's substation In general the transformer in a consumer's substation is protected by HV fuses, suitably rated to match the transformer, in accordance with the principles laid down in IEC 787 and IEC 420, by following the advice of the fuse manufacturer. The basic requirement is that a HV fuse will not operate for LV faults occurring downstream of the transformer LV circuit breaker, so that the tripping characteristic curve of the latter must be to the left of that of the HV fuse pre-arcing curve. This requirement generally fixes the maximum settings for the LV circuit breaker protection: c maximum short-circuit current-level setting of the magnetic tripping element; c maximum time-delay allowable for the short-circuit current tripping element. See also Chapter C sub-clause 3.2.7, and Appendix C1, for further details.

63 A

full-load current 1760 A 3-phase short-circuit current level 31.4 kA

1250 kVA 20 kV / 400 V

Visucompact CM 2000 set at 1800 A

fig. H2-56: example. c short-circuit level at HV terminals of transformer: 250 MVA; c transformer HL/LV: 1,250 kVA 20/0.4 kV; c HV fuses: 63 A (table C 11); c cabling, transformer - LV circuit breaker: 10 metres single-core cables; c LV circuit breaker: Visucompact CM 2000 set at 1,800 A (Ir). What is the maximum short-circuit trip current setting and its maximum time delay allowable? The curves of figure H2-57 show that discrimination is assured if the short-time delay tripping unit of the CB is set at: c a level i 6 Ir = 10.8 kA; c a time-delay setting of step O or A. A general policy for HV fuse/LV circuit breaker discrimination, adopted in some countries, which is based on standardized manufacturing tolerance limits, is mentioned in chapter C sub-clause 3.2.7, and illustrated in figure C-21. Where a transformer is controlled and protected on the high-voltage side by a circuit breaker, it is usual to install separate CT- and/ or VT- operated relays, which energize a shunt-trip coil of the circuit breaker. Discrimination can be achieved, together with high-speed tripping for faults on the transformer, by using the methods described in chapter C sub-clause 3.2.

H2-32 - the protection of circuits - the switchgear

t (ms) 1000

CM 2000 set at 1800 A

minimum pre-arcing curve for 63 A HV fuses (current referred to the secondary side of the transformer)

200 100

10 1 Ir

4 Ir

6 Ir 8 Ir

220 1

step C step B step A

50

step 0

0,01

1800 A Ir

10 kA

Isc maxi 31,4 kA

fig. H2-57: curves of HV fuses and LV circuit breaker.

I

1. general

H1 1.1 methodology and definitions component parts of an electric circuit and its protection are determined such, that all normal and abnormal operating constraints are satisfied.

methodology Following a preliminary analysis of the power requirements of the installation, as decribed in Chapter B Clause 4, a study of cabling* and its electrical protection is undertaken, starting at the origin of the installation, through the intermediate stages to the final circuits. The cabling and its protection at each level must satisfy several conditions at the same time, in order to ensure a safe and reliable installation, e.g. it must: c carry the permanent full load current, and normal short-time overcurrents, c not cause voltage drops likely to result in an inferior performance of certain loads, for example: an excessively long acceleration period when starting a motor, etc. Moreover, the protective devices (circuit breakers or fuses) must: c protect the cabling and busbars for all levels of overcurrent, up to and including short-circuit currents,

c ensure protection of persons against indirect contact hazards, particularly in TN- and IT- earthed systems, where the length of circuits may limit the magnitude of short-circuit currents, thereby delaying automatic disconnection (it may be remembered that TT- earthed installations are obligatorily protected at the origin by a RCD, generally rated at 500 mA). The cross-sectional areas of conductors are determined by the general method described in Sub-clause 1.2 of this Chapter. Apart from this method some national standards may prescribe a minimum cross-sectional area to be observed for reasons of mechanical endurance. Particular loads (as noted in Chapter J) require that the cable supplying them be oversized, and that the protection of the circuit be likewise modified. * the term "cabling" in this chapter, covers all insulated conductors, including multi-core and single-core cables and insulated wires drawn into conduits, etc.

kVA to be supplied

short-circuit MVA at the origin of the circuit

maximum load current

short-circuit current

upstream or downstream network

IB

Isc

rated current of protective device (C.B. or fuses) In

choice of C.B. or fuses

choice of protective device

conditions of installation

short-circuit current-breaking rating of C.B. or fuses I scb

cross-sectional area of conductors of the circuit

verification of thermal withstand requirements

verification of the maximum voltage drop

IT or TN scheme

verification of the maximum length of the circuit TT scheme determination of the cross-sectional area of the conductors

confirmation of the cross-sectional area of the cabling, and the choice of its electrical protection

table H1-1: logigram for the selection of cable size and protective-device rating for a given circuit.

the protection of circuits - the switchgear - H1-1

1. general (continued)

H1 1.1 methodology and definitions (continued) definitions Maximum load current: IB c at the final circuits level, this current corresponds to the rated kVA of the load. In the case of motor-starting, or other loads which take an initially-high current, particularly where frequent starting is concerned (e.g. lift motors, resistance-type spot welding, and so on) the cumulative thermal effects of the overcurrents must be taken into account. Both cables and thermaltype relays are affected; c at all upstream circuit levels this current corresponds to the kVA to be supplied, which takes account of the factors of simultaneity (diversity) and utilization, ks and ku respectively, as shown in figure H1-2. Maximum permissible current: IZ This is the maximum value of current that the cabling for the circuit can carry indefinitely, without reducing its normal life expectancy. The current depends, for a given crosssectional area of conductors, on several parameters: c constitution of the cable and cable-way (Cu or Alu conductors; PVC or EPR etc. insulation; number of active conductors); c ambient temperature; c method of installation; c influence of neighbouring circuits.

main distribution board

combined factors of simultaneity (or diversity) and utilization ks x ku = 0.69

IB = 290 x 0.69 = 200 A

sub-distribution board

80 A

60 A

100 A

IB = 50 A

M

normal load motor current 50 A

fig. H1-2: calculation of maximum load current IB.

overcurrents An overcurrent occurs each time the value of current exceeds the maximum load current IB for the load concerned. This current must be cut off with a rapidity that depends upon its magnitude, if permanent damage to the cabling (and appliance if the overcurrent is due to a defective load component) is to be avoided. Overcurrents of relatively short duration can however, occur in normal operation; two types of overcurrent are distinguished: Overloads These overcurrents can occur in healthy electric circuits, for example, due to a number of small short-duration loads which occasionally occur co-incidentally; motorstarting loads, and so on. If either of these conditions persists however beyond a given period (depending on protective-relay settings or fuse ratings) the circuit will be automatically cut off.

H1-2 - the protection of circuits - the switchgear

Short-circuit currents These currents result from the failure of insulation between live conductors or/and between live conductors and earth (on systems having low-impedance-earthed neutrals) in any combination, viz: c 3 phases short-circuited (and to neutral and/or earth, or not); c 2 phases short-circuited (and to neutral and/or earth, or not); c 1 phase short-circuited to neutral (and/or to earth).

H1 1.2 overcurrent protection principles A protective device is provided at the origin of the circuit concerned. c acting to cut-off the current in a time shorter than that given by the I2t characteristic of the circuit cabling; c but allowing the maximum load current IB to flow indefinitely. The characteristics of insulated conductors when carrying short-circuit currents can, for periods up to 5 seconds following short-circuit initiation, be determined approximately by the formula: Is2 x t = k2 x S2 which shows that the allowable heat generated is proportional to the cross-sectional-area of the condutor squared. Where: t: duration of short-circuit current (seconds); S: c.s.a. of insulated conductor (mm2); Is: short-circuit current (A r.m.s.); k: insulated conductor constant (values of k2 are given in table H1-54). For a given insulated conductor, the maximum permissible current varies according to the environment. For instance, for a high ambient temperature (θa1 > θa2), IZ1 is less than IZ2 (fig. H1-5). θ means "temperature". Note: Isc means 3-phase short-circuit current. IscB means rated 3-ph. short-circuit breaking current of the circuit breaker. Ir (or Irth)* means regulated "nominal" current level; e.g. a 50 A nominal circuit breaker can be regulated to have a protective range, i.e. a conventional overcurrent tripping level (see figure H1-6) similar to that of a 30 A circuit breaker.

t maximum load current

I2t cable characteristic

circuit-breaker tripping curve

temporary overload

IB Ir Iz

ISCB PdC

I

fig. H1-3: circuit protection by circuit breaker. t I2t cable characteristic

fuse curve

temporary overload

IB

Ir cIz Iz

I

fig. H1-4: circuit protection by fuses. t

1

2

* both designations are commonly used in different standards.

θa1 > θa2

5s I2t = k2S2

Iz1 < Iz2

I

fig. H1-5: I2t characteristic of an insulated conductor at two different ambient temperatures.

the protection of circuits - the switchgear - H1-3

1. general (continued)

H1 1.3 practical values for a protection scheme The following methods are based on rules laid down in the IEC standards, and are

representative of the practices in many countries.

loads

circuit cabling

Iz x

a lo d cu n rre t IB

m ax i

1. 45

um im

m u cu m rre pe nt rm Iz iss i

ax

bl e

m IB In zone a

ISC

1.45 Iz

Iz I2

ISCB zone c

n its om re in gu al la cu te rr d en cu t rre In nt or Ir co nv en tri tio p na cu l o rre ve nt rc I2 urr en

t

zone b

g t ui atin irc ng r c i t or ak sh t bre h n p 3- urre -c ult

fa

protective device

fig. H1-6: current levels for determining circuit breaker or fuse characteristics.

IB i In i Iz I2 i 1,45 Iz ISCB u ISC

zone a zone b zone c

general rules A protective device (circuit breaker or fuse) functions correctly if: c its nominal current or its setting current In is greater than the maximum load current IB but less than the maximum permissible current IZ for the circuit, i.e. IB i In i IZ corresponding to zone "a" in figure H1-6; c its tripping current I2 "conventional" setting is less than 1.45 IZ which corresponds to zone "b" in figure H1-6.

The "conventional" setting tripping time may be 1 hour or 2 hours according to local standards and the actual value selected for I2. For fuses, I2 is the current (denoted If) which will operate the fuse in the conventional time; c its 3-phase short-circuit fault-current breaking rating is greater than the 3-phase short-circuit current existing at its point of installation. This corresponds to zone "c" in figure H1-6.

criteria for a circuit breaker: IB i In (or Ir) i Iz and, rated short-circuit breaking current ISCB u ISC the 3-ph. short-circuit current level at the point of CB installation.

applications

criteria for fuses: IB i In i IZ k3 and, the rated short-circuit current breaking capacity of the fuse ISCF u ISC the 3-ph. short-circuit current level at the point of fuse installation.

Protection by fuses The condition I2 i 1.45 IZ must also be taken into account, where I2 is the fusing (meltinglevel) current, equal to k2 x In (k2 ranges from 1.6 to 1.9) according to the particular fuse concerned. A further factor k3 has been introduced (in the national standards from which these notes have been abstracted) such that I2 i 1.45 IZ will be valid if In i IZ/k3.

For fuses type gl: In i 10 A k3 = 1.31 10 A < In i 25 A k3 = 1.21 In > 25 A k3 = 1.10 Moreover, the short-circuit current breaking capacity of the fuse ISCF must exceed the level of 3-phase short-circuit current at the point of installation of the fuse(s).

Association of different protective devices The use of protective devices which have fault-current ratings lower than the fault level existing at their point of installation are permitted by IEC and many national standards in the following conditions: c there exists upstream, another protective device which has the necessary short-circuit rating, and c the amount of energy allowed to pass through the upstream device is less than that which the downstream device and all

associated cabling and appliances can withstand without damage. In pratice this arrangement is generally exploited in: c the association of circuit breakers/fuses; c the technique known as "cascading" in which the strong current-limiting performance of certain circuit breakers effectively reduces the severity of downstream short-circuits. Possible combinations which have been tested in laboratories are indicated in certain manufacturers catalogues.

H1-4 - the protection of circuits - the switchgear

Protection by circuit breaker By virtue of its high level of precision the current I2 is always less than 1.45 In (or 1.45 Ir) so that the condition, that I2 i 1.45 IZ (as noted in the "general rules" above) will always be respected.

Particular case: if the circuit breaker itself does not protect against overloads, it is necessary to ensure that, at a time of lowest value of short-circuit current, the overcurrent device protecting the circuit will operate correctly. This particular case is examined in Sub-clause 5.1.

H1 1.4 location of protective devices a protective device is, in general, required at the origin of each circuit.

general rule A protective device is necessary at the origin of each circuit where a reduction of permissible maximum current level occurs.

P

P2

P3

50 mm2

possible alternative locations in certain circumstances The protective device may be placed part way along the circuit: c if AB is not in proximity to combustible material, and c if no socket-outlets or branch connections are taken from AB. Three cases may be useful in practice. Consider case (1) in the diagram c AB i 3 metres, and c AB has been installed to reduce to a practical minimum the risk of a short-circuit (wires in heavy steel conduit for example). Consider case (2) c the upstream device P1 protects the length AB against short-circuits in accordance with Sub-clause H1-5.1. Consider case (3) c the overload device (S) is located adjacent to the load. This arrangement is convenient for motor circuits. The device (S) constitutes the control (start/stop) and overload protection of the motor while (SC) is: either a circuit breaker (designed for motor protection) or fuses type aM, c the short-circuit protection (SC) located at the origin of the circuit conforms with the principles of Sub-clause H1-5.1.

P4

10 mm2

25 mm2

P1 A short-circuit sc protective device

sin ϕ. The difference between EI sin ϕ' and VI sin ϕ gives the kvar per phase absorbed by XL. It can be shown that this kvar value is equal to I2XL (which is analogous to the I2R activepower (kW) losses due to the series resistance of power lines, etc.). From the I2XL formula it is very simple to deduce the kvar absorbed at any load value for a given transformer, as follows: If per-unit values are used (instead of percentage values) direct multiplication of I and XL can be carried out. XL

E source

V load

E V ϕ

ϕ'

I sin ϕ

I sin ϕ'

fig. E21: reactive power absorption by series inductance. Example: A 630 kVA transformer with a short-circuit reactance voltage of 4% is fully loaded. What is its reactive-power (kvar) loss? 4% = 0.04 pu Ipu = 1 loss = I2XL = 12 x 0.04 = 0.04 pu kvar where 1 pu = 630 kVA

IXL

represented by the elementary diagram of figure E22. All reactance values are referred to the secondary side of the transformer, where the shunt branch represents the magnetizing-current path. The magnetizing current remains practically constant (at about 1.8 % of full-load current) from no load to full load, in normal circumstances, i.e. with a constant primary voltage, so that a shunt capacitor of fixed value can be installed at the HV or LV side, to compensate for the reactive energy absorbed. perfect transformer primary winding

secondary winding

leakage reactance magnetizing reactance

fig. E22: transformer reactances (per phase). The 3-phase kvar losses are 630 x 0.04 = 25.2 kvar (or, quite simply, 4% of 630 kVA). At half load i.e. I = 0.5 pu the losses will be 0.52 x 0.04 = 0.01 pu = 630 x 0.01 = 6.3 kvar and so on... This example, and the vector diagram of figure E21 show that: c the power factor at the primary side of a loaded transformer is different (normally lower) than that at the secondary side (due to the absorption of vars), c that full-load kvar losses due to leakage reactance are equal to the transformer percentage reactance (4% reactance means a kvar loss equal to 4% of the kVA rating of the transformer), c that kvar losses due to leakage reactance vary according to the current (or kVA loading) squared. To determine the total kvar losses of a transformer the constant magnetizing-current circuit losses (approx. 1.8% of the transformer kVA rating) must be added to the foregoing "series" losses. Table E24 shows the no-load and full-load kvar losses for typical distribution transformers. In principle, series inductances can be compensated by fixed series capacitors (as is commonly the case for long HV transmission lines). This arrangement is operationally difficult, however, so that, at the voltage levels covered by this guide, shunt compensation is always applied. In the case of HV metering, it is sufficient to raise the power factor to a point where the transformer plus load reactive-power consumption is below the level at which a billing charge is made. This level depends on the tariff, but often corresponds to a tan ϕ value of 0.31 (cos ϕ of 0.955). As a matter of interest, the kvar losses in a transformer can be completely compensated by adjusting the capacitor bank to give the load a (slightly) leading power factor. In such a case, all of the kvar of the transformer is being supplied from the capacitor bank, while the input to the HV side of the transformer is at unity power factor, as shown in figure E23.

power factor improvement - E15

6. compensation at the terminals of a transformer (continued)

E 6.2 compensation of reactive energy absorbed by the transformer (continued) E (input voltage) IXL

I ϕ

V (load voltage) I0 compensation current

load current

fig. E23: overcompensation of load to completely compensate transformer reactive-power losses. In practical terms, therefore, compensation for transformer-absorbed kvar is included in the capacitors primarily intended for powerfactor correction of the load, either globally, partially, or in the individual mode. Unlike most other kvar-absorbing items, the transformer absorption (i.e. the part due to the leakage reactance) changes significantly with variations of load level, so that, if individual compensation is applied to the transformer, then an average level of loading will have to be assumed. Fortunately, this kvar consumption generally forms only a relatively small part of the total reactive power of an installation, and so mismatching of compensation at times of load change is not likely to be a problem. Table E24 indicates typical kvar loss values for the magnetizing circuit (“no-load kvar” columns), as well as for the total losses at full load, for a standard range of distribution transformers supplied at 20 kV (which include the losses due to the leakage reactance). rated power kVA 50 100 160 250 315 400 500 630 800 1 000 1 250 1 600 2 000 2 500

reactive power (kvar) to be compensated oil immersed type cast resin type no load full load no load full load 1.5 2.9 2.5 5.9 2.5 8.2 3.7 9.6 3.7 12.9 5.3 14.7 5.0 19.5 6.3 18.3 5.7 24.0 7.6 22.9 6.0 29.4 9.5 28.7 7.5 36.8 11.3 35.7 8.2 45.2 20 66.8 10.4 57.5 24.0 82.6 12.0 71.0 27.5 100.8 15.0 88.8 32.0 125.9 19.2 113.9 38.0 155.3 22.0 140.6 45.0 191.5 30.0 178.2

table E24: reactive power consumption of distribution transformers with 20 kV primary windings. Note: for a 630 kVA transformer, the range of kvar losses extends from 11.3 at no load to 35.7 kvar at full load. These values correspond closely to those given in the worked example above.

E16 - power factor improvement

7. compensation at the terminals of an induction motor

E 7.1 connection of a capacitor bank and protection settings individual motor compensation is recommended where the motor power (kVA) is large with respect to the declared power of the installation.

general precautions Because of the small kW consumption, the power factor of a motor is very low at no-load or on light load. The reactive current of the motor remains practically constant at all loads, so that a number of unloaded motors constitute a consumption of reactive power which is generally detrimental to an installation, for reasons explained in preceding sections. Two good general rules therefore are that unloaded motors should be switched off, and motors should not be oversized (since they will then be lightly loaded).

connection The bank of capacitors should be connected directly to the terminals of the motor.

special motors It is recommended that special motors (stepping, plugging, inching, reversing motors, etc.) should not be compensated.

effect on protection settings After applying compensation to a motor, the current to the motor-capacitor combination will be lower than before, assuming the same motor-driven load conditions. This is because a significant part of the reactive component of the motor current is being supplied from the capacitor, as shown in figure E25. Where the overcurrent protection devices of the motor are located upstream of the motorcapacitor connection (and this will always be the case for terminal-connected capacitors), the overcurrent relay settings must be reduced in the ratio: cos ϕ before compensation cos ϕ after compensation For motors compensated in accordance with the kvar values indicated in Table E28 (maximum values recommended for avoidance of self-excitation of standard induction motors, as discussed in sub-clause 7.2), the above-mentioned ratio will have a value similar to that indicated for the corresponding motor speed in Table E26. before compensation

speed in R.P.M. 750 1000 1500 3000

reduction factor 0.88 0.90 0.91 0.93

table E26: reduction factor for overcurrent protection after compensation.

after compensation transformer

power made available active power C

M

motor

M

reactive power supplied by capacitor

fig. E25: before compensation, the transformer supplies all the reactive power; after compensation, the capacitor supplies a large part of the reactive power.

power factor improvement - E17

7. compensation at the terminals of an induction motor (continued)

E 7.2 how self-excitation of an induction motor can be avoided when a capacitor bank is connected to the terminals of an induction motor, it is important to check that the size of the bank is less than that at which self-excitation can occur.

When a motor is driving a high-inertia load, the motor will continue to rotate (unless deliberately braked) after the motor supply has been switched off. The "magnetic inertia" of the rotor circuit means that an emf will be generated in the stator windings for a short period after switching off, and would normally reduce to zero after 1 or 2 cycles, in the case of an uncompensated motor. Compensation capacitors however, constitute a 3-phase "wattless" load for this decaying emf, which causes capacitive currents to flow through the stator windings. These stator currents will produce a rotating magnetic field in the rotor which acts exactly along the same axis and in the same direction as that of the decaying magnetic field. The rotor flux consequently increases; the stator currents increase; and the voltage at the terminals of the motor increases; sometimes to dangerously-high levels. This phenomenon is known as self-excitation and is one reason why a.c. generators are not normally operated at leading power factors, i.e. there is a tendency to spontaneously (and uncontrollably) selfexcite.

Notes 1. The characteristics of a motor being driven by the inertia of the load are not rigorously identical to its no-load characteristics. This assumption, however, is sufficiently accurate for practical purposes. 2. With the motor acting as a generator, the currents circulating are largely reactive, so that the braking (retarding) effect on the motor is mainly due only to the load represented by the cooling fan in the motor. 3. The (almost 90° lagging) current taken from the supply in normal circumstances by the unloaded motor, and the (almost 90° leading) current supplied to the capacitors by the motor acting as a generator, both have the same phase relationship to the terminal voltage. It is for this reason that the two characteristics may be superimposed on the graph. In order to avoid self-excitation as described above, the kvar rating of the capacitor bank must be limited to the following maximum value: Qc i 0,9 Io Un e where Io = the no-load current of the motor and Un = phase-tophase nominal voltage of the motor in kV. Table E28 gives appropriate values of Qc corresponding to this criterion. Example A 75 kW, 3 000 Rpm, 400 V, 3-phase motor may have a capacitor bank no larger than 17 kvar according to Table E28. The table values are, in general, too small to adequately compensate the motor to the level of cos ϕ normally required. Additional compensation can, however, be applied to the system, for example an overall bank, installed for global compensation of a number of smaller appliances. High-inertia motors and/or loads In any installation where high-inertia motordriven loads exist, the circuit breakers or contactors controlling such motors should, in the event of total loss of power supply, be rapidly tripped. If this precaution is not taken, then selfexcitation to very high voltages is likely to occur, since all other banks of capacitors in the installation will effectively be in parallel with those of the high-inertia motors.

fig. E27: connection of the capacitor bank to the motor. The protection scheme for these motors should therefore include an overvoltage tripping relay, together with reverse-power checking contacts (the motor will feed power to the rest of the installation, until the stored inertial energy is dissipated). An undervoltage relay would not be suitable because the voltage is not only maintained, but will increase, immediately following the loss of power supply.

E18 - power factor improvement

E If the capacitor bank associated with a highinertia motor is larger than that recommended in Table E28, then it should be separately controlled by a circuit breaker or contactor, which trips in unison with the main motorcontrolling circuit breaker or contactor, as shown in figure E27. Closing of the main contactor is commonly subject to the capacitor contactor being previously closed. 3-phase motors 230/400 V nominal kvar to be installed power speed of rotation (RPM) kW hp 3000 1500 1000 750 22 30 6 8 9 10 30 40 7.5 10 11 12.5 37 50 9 11 12.5 16 45 60 11 13 14 17 55 75 13 17 18 21 75 100 17 22 25 28 90 125 20 25 27 30 110 150 24 29 33 37 132 180 31 36 38 43 160 218 35 41 44 52 200 274 43 47 53 61 250 340 52 57 63 71 280 380 57 63 70 79 355 482 67 76 86 98 400 544 78 82 97 106 450 610 87 93 107 117 table E28: maximum kvar of P.F. correction applicable to motor terminals without risk of self-excitation. Note Exact sizing of capacitor unit for a particular motor is only possible when the "no-load current" or "no-load magnetising" kvar is known.

power factor improvement - E19

8. example of an installation before and after power-factor correction

E installation before P.F. correction

installation after P.F. correction

*→→→ kVA=kW+kvar

→→→ kVA=kW+kvar

kVA kW

kvar

630 kVA

c kvarh are billed heavily above the declared level, c apparent power kVA is significantly greater than the kW demand, c the corresponding excess current causes losses (kWh) which are billed, c the installation must be over-dimensioned. * the arrows denote vector quantities.

Characteristics of the installation 500 kW cos ϕ = 0.75 c transformer is overloaded c the power demand is P 500 S= = = 665 kVA cos ϕ 0.75 S = apparent power

c the consumption of kvarh is v eliminated, or v reduced, according to the cos ϕ required, c the tariff penalties v for reactive energy where applicable v for the entire bill in some cases are eliminated, c the fixed charge based on kVA demand is adjusted to be close to the active power kW demand.

kVA kW

Characteristics of the installation 500 kW cos ϕ = 0.928 c transformer no longer overloaded c the power-demand is 539 kVA c there is 14% spare-transformer capacity available.

630 kVA

400 V

400 V

c the current flowing into the installation downstream of the circuit breaker is P I= = 960 A eU cos ϕ

c the current flowing into the installation through the circuit breaker is 778 A.

c losses in cables are calculated as a function of the current squared: (960)2 P=I2R

c the losses in the cables are (778)2 reduced to = 65% of the former value, (960)2 thereby economizing in kWh consumed.

cos ϕ = 0.75 c reactive energy is supplied through the transformer and via the installation wiring, c the transformer, circuit breaker, and cables must be over-dimensioned.

cos ϕ = 0.928 c reactive energy is supplied by the capacitor bank. kvar

Capacitor bank rating is 250 kvar in 5 automatically-controlled steps of 50 kvar.

cos ϕ = 0.75 workshop

cos ϕ = 0.928 workshop

Note: In fact, the cos ϕ of the workshop remains at 0.75 but cos ϕ for all the installation upstream of the capacitor bank to the transformer LV terminals is 0.928. As mentioned in Sub-clause 6.2, the cos ϕ at the HV side of the transformer will be slightly lower, * due to the reactive power losses in the transformer. * moreso in the pre-corrected case.

fig. E29: technical-economic comparison of an installation before and after power-factor correction.

E20 - power factor improvement

9. the effect of harmonics on the rating of a capacitor bank

E 9.1 problems arising from power-system harmonics Equipment which uses power electronics components (variable-speed motor controllers, thyristor-controlled rectifiers, etc.) have, in recent years, considerably increased the problems caused by harmonics in powersupply systems. Harmonics have existed from the earliest days of the industry and were (and still are) caused by the non-linear magnetizing impedances of transformers, reactors, fluorescent lamp ballasts, etc. Harmonics on symmetrical 3-phase power systems are generally* odd-numbered: 3rd, 5th, 7th, 9th..., and the magnitude decreases as the order of the harmonic increases. All of these features may be used in various ways to reduce specific harmonics to negligible values - total elimination is not possible. In this section, practical means of reducing the influence of harmonics are recommended, with particular reference to capacitor banks. Capacitors are especially sensitive to harmonic components of the supply voltage due to the fact that capacitive reactance decreases as the frequency increases. In practice, this means that a relatively small percentage of harmonic voltage can cause a significant current to flow in the capacitor circuit. The presence of harmonic components causes the (normally sinusoidal) wave form of voltage or current to be distorted; the

greater the harmonic content, the greater the degree of distortion . If the natural frequency of the capacitor bank/ power-system reactance combination is close to a particular harmonic, then partial resonance will occur, with amplified values of voltage and current at the harmonic frequency concerned. In this particular case, the elevated current will cause overheating of the capacitor, with degradation of the dielectric, which may result in its eventual failure. Several solutions to these problems are available, which aim basically at reducing the distortion of the supply-voltage wave form, between the equipment causing the distortion, and the bank of capacitors in question. This is generally accomplished by shunt connected harmonic filter and/or harmonic-suppression reactors.

countering the effects of harmonics

Harmonic distortion of the voltage wave frequently produces a "peaky" wave form, in which the peak value of the normal sinusoidal wave is increased. This possibility, together with other overvoltage conditions likely to occur when countering the effects of resonance, as described below, are taken into account by increasing the insulation level above that of "standard" capacitors. In many instances, these two counter measures are all that is necessary to achieve satisfactory operation.

* With the advent of power electronics devices, and associated non-linear components, even-numbered harmonics are now sometimes encountered.

9.2 possible solutions harmonics are taken into account mainly by oversizing capacitors and including harmonic-suppression reactors in series with them.

The presence of harmonics in the supply voltage results in abnormally high current levels through the capacitors. An allowance is made for this by designing for an r.m.s. value of current equal to 1.3 times the nominal rated current. All series elements, such as connections, fuses, switches, etc., associated with the capacitors are similarly oversized, between 1.3 to 1.5 times nominal rating.

countering the effects of resonance Capacitors are linear reactive devices, and consequently do not generate harmonics. The installation of capacitors in a power system (in which the impedances are predominantly inductive) can, however, result in total or partial resonance occurring at one of the harmonic frequencies. The harmonic order ho of the natural resonant frequency between the system inductance and the capacitor bank is given by SSC / Q where Ssc = the level of system short-circuit kVA at the point of connection of the capacitor Q = capacitor bank rating in kvar; and ho = the harmonic order of the natural frequency fo i.e. fo/50 for a 50 Hz system, or fo/60 for a 60 Hz system.

For example: SSC / Q may give a value for ho of 2.93 which shows that the natural frequency of the capacitor/system-inductance combination is close to the 3rd harmonic frequency of the system. From ho = fo/50 it can be seen that fo = 50 ho = 50 x 2.93 = 146.5 Hz The closer a natural frequency approaches one of the harmonics present on the system, the greater will be the (undesirable) effect. In the above example, strong resonant conditions with the 3rd harmonic component of a distorted wave would certainly occur.

power factor improvement - E21

9. the effect of harmonics on the rating of a capacitor bank (continued)

E 9.2 possible solutions

(continued)

countering the effects of resonance (continued) In such cases, steps are taken to change the natural frequency to a value which will not resonate with any of the harmonics known to be present. This is achieved by the addition of a harmonic-suppression inductor connected in series with the capacitor bank. On 50 Hz systems, these reactors are often adjusted to bring the resonant frequency of the combination, i.e. the capacitor bank + reactors to 190 Hz. The reactors are adjusted to 228 Hz for a 60 Hz system. These frequencies correspond to a value for ho of 3.8 for a 50 Hz system, i.e. approximately mid-way between the 3rd and 5th harmonics.

In this arrangement, the presence of the reactor increases the fundamental-frequency (50 Hz or 60 Hz) current by a small amount (7-8%) and therefore the voltage across the capacitor in the same proportion. This feature is taken into account, for example, by using capacitors which are designed for 440 V operation on 400 V systems.

9.3 choosing the optimum solution A choice is made from the following parameters: c Gh = the sum of the kVA ratings of all harmonic-generating devices (static converters, inverters, speed controllers, etc.) connected to the busbars from which the capacitor bank is supplied. If the ratings of some of these devices are quoted in kW only, assume an average power factor of 0.7 to obtain the kVA ratings. c Ssc = the 3-phase short-circuit level in kVA at the terminals of the capacitor bank. capacitors supplied at LV via transformer(s) c general rule valid for any size of transformer Ssc i Gh i Ssc Gh i Ssc 120 120 70 standard capacitors capacitor voltage rating increased by 10% (except 230 V units) c simplified rule if transformer(s) rating Sn i 2 MVA Gh i 0.15 Sn 0.15 Sn < Gh i 0.25 Sn standard capacitors capacitor voltage rating increased by 10% (except 230 V units)

c Sn = the sum of the kVA ratings of all transformers supplying (i.e. directly connected to) the system level of which the busbars form a part. If a number of transformers are operating in parallel, the removal from service of one or more, will significantly change the values of Ssc and Sn. From these parameters, a choice of capacitor specification which will ensure an acceptable level of operation with the system harmonic voltages and currents, can be made, by reference to the following table.

Gh > Ssc 70 capacitor voltage rating increased by 10% + harmonic-suppression reactor 0.25 Sn < Gh i 0.60 Sn capacitor voltage rating increased by 10% + harmonic suppression reactor

Gh > 0.60 Sn filters

table E30: choice of solutions for limiting harmonics associated with a LV capacitor bank.

E22 - power factor improvement

E examples Three cases are presented, showing (respectively) situations in which standard, overdimensioned, and overdimensioned plus harmonic-suppression-equipped capacitor banks should be installed. Example 1: 500 kVA transformer having 4% short-circuit voltage. Total rating of harmonic-generating devices Gh = 50 kVA 100 Ssc = 500 x = 12,500 kVA 4 Ssc 12,500 = = 104 120 120 Ssc Gh = 50 i 120 Solution: use standard capacitors. Example 2: 1,000 kVA transformer having 6% short-circuit voltage. Total rating of harmonic-generating devices Gh = 220 kVA 100 Ssc = 1,000 x = 16,667 kVA 6 Ssc 16,667 = = 139 120 120 Ssc 16,667 = = 238 70 70 Ssc Ssc Gh = 220 is between and 120 70 Solution: use overated (440 V) capacitors.

Example 3: 630 kVA transformer having 4% short-circuit voltage. Total rating of harmonic-generating devices Gh = 250 kVA 100 Ssc = 630 x = 15,750 kVA 4 Ssc 15,750 = = 225 70 70 Scc Gh = 250 > 70 Solution: use overated (440 V) capacitors and harmonic-suppression reactors.

9.4 possible effects of power-factor-correction capacitors on the power-supply system it is necessary to ensure that interaction between harmonicgenerating devices and P.F. correction capacitors, does not result in unacceptable levels of voltage and/or current wave-form distortion on the power-supply network.

Power-supply authorities generally impose a strict limit on the total-harmonic distortion (THD) permitted at the point of power supply to a consumer. The degree of distortion is measured as the ratio of the r.m.s. value of all harmonics present, with respect to the r.m.s. value of the fundamental frequency wave (50 or 60 Hz). For LV loads supplied through a transformer from a high-voltage service connection, this means that a maximum value of 4 or 5% for voltage THD is permissible at the LV terminals of the transformer.

If this value of THD is unattainable, then recourse has to be made to low-voltage L-C series filters. Such filters are shuntconnected, and are tuned to resonate at harmonic frequencies to which they present practically zero impedance. Filters connected in this way fortuitously have the added benefit of contributing to reactivepower compensation for the installation.

power factor improvement - E23

10. implementation of capacitor banks

E 10.1 capacitor elements technology The capacitors are dry-type units (i.e. are not impregnated by liquid dielectric) comprising metallized polypropylene self-healing film in the form of a two-film roll. They are protected by a high-quality system (overpressure disconnector used with an HPC fuse) which switches off the capacitor if an internal fault occurs. The protection scheme operates as follows: c a short-circuit through the dielectric will blow the fuse, c current levels greater than normal, but insufficient to blow the fuse sometimes occur, e.g. due to a microscopic flow in the dielectric film. Such "faults" often re-seal due to local heating caused by the leakage current, i.e. the units are said to be "self-healing"; v if the leakage current persists, the defect may develop into a short-circuit, and the fuse will blow, v gas produced by vaporizing of the metallisation at the faulty location will gradually build up a pressure within the plastic container, and will eventually operate a pressure-sensitive device to short-circuit the unit, thereby causing the fuse to blow. Capacitors are made of insulating material providing them with double insulation and avoiding the need for a ground connection.

fuse discharge resistor short-circuiting contacts overpressure device

fig. E31: cross-section of a capacitor element.

electrical characteristics standards operating rated voltage range rated frequency capacitance tolerance temperature maximum temperature range average temperature over 24 h average annual temperature minimum temperature insulation level permissible current overload permissible voltage overload current on 400 V - 50 Hz supply consumption on 230 V - 50 Hz supply

E24 - power factor improvement

IEC 831, NF C 54-104, VDE 0560 CSA standards, UL tests 400 V 50 Hz 0 to + 5% 55 °C 45 °C 35 °C -25 °C 50 Hz 1 mn withstand voltage: 6 kV 1.2/50 µs impulse withstand voltage: 25 kV standard range H range 30% 50% 10% 20% 2 A/kvar 2.2 A/kvar 3.5 A/kvar

E 10.2 choice of protection, control devices, and connecting cables due to the possible presence of harmonic currents and to manufacturing tolerances, components must be over-sized, and based on 1.5 times rated current.

component dimensions The choice of upstream cables and protection and control devices depends on the current loading. For capacitors, the current is a function of: c the applied voltage and its harmonics, c the capacitance value. The nominal current In a capacitor of kvar rating Q, supplied from a 3-phase system having a phase/phase voltage of Un kilo-volts, is given by: Q A In = 3 Un The permitted range of applied voltage at fundamental frequency, plus harmonic components, together with manufacturing tolerances of actual capacitance (for a declared nominal value) can result in a 50% increase above the calculated value of current.

Approximately 30% of this increase is due to the voltage increases, while a further 15% is due to the range of manufacturing tolerances, so that 1.3 x 1.15 = 1.5 In. All components carrying the capacitor current therefore, must be adequate to cover this "worst-case" condition, in an ambient temperature of 50 °C maximum. In the case where higher temperatures (than 50 °C) occur in enclosures, etc. derating of the components will be necessary.

protection At the instant of closing a switch to energize a capacitor, the current is limited only by the impedances of the network upstream of the capacitor, so that high peak values of current will occur for a brief period, rapidly falling to normal operating values. This transient overcurrent however, is generally a high-frequency phenomenon, which is superimposed on the 50 Hz (or 60 Hz) current wave. The first peak of transient high-frequency or (sometimes) unidirectional* current has the greatest magnitude. The maximum value attainable, when charging an initially uncharged capacitor, will occur if the closing switch contacts touch at the instant of peak power-supply voltage. For this condition, the maximum highfrequency peak current is given by: 2C A 3 Lo Where U = system phase-to-phase voltage in Volts C = capacitance of capacitor in Farads Lo = inductance of system impedance in Henrys (system resistance is ignored). The frequency fo of the transient current surge is given by: 1 fo = Hz 2Π LoC

Ip = U

c for a single capacitor bank, the upstream cables and transformers constitute the predominant part of Lo (the system inductance), c where a bank of capacitors is automatically switched in steps, however, those units which are already in service will initially discharge into an uncharged capacitor group at the instant of switching it into service. The transient in-rush current from the previouslycharged units will then amount to an initial peak of 2C n ( )A 3 L n +1 where L = the supply cable inductance in series with each capacitor n = the number of capacitor steps already energized before closure of the switch C = capacitance of each group forming 1 step (all steps are electrically identical). The frequency f'o of the current from the energized capacitors is given by 1 fo' = Hz 2Π LC

I' P = U

The total inrush current is the sum of the two infeeds, i.e. from the system and from the previously-charged bank. Generally, the frequencies of the two infeeds will not be equal.

* In general, peak unidirectional currents are lower than the first peaks of high-frequency currents.

power factor improvement - E25

10. implementation of capacitor banks (continued)

E 10.2 choice of protection, control devices, and connecting cables (continued) The peak value of this transient current must not exceed 100 times the rated current of the capacitors in one step of a multi-step bank (IEC 831-1). This maximum transient current peak occurs when the last step is energized. It is sometimes necessary to install small series inductors to achieve this limitation, in which case the manufacturer of the capacitors should be consulted. In order to avoid undesirable nuisance tripping of controlling circuit breakers at the instant of energizing a capacitor bank, the instantaneous elements of overcurrent tripping relays should be given a suitably high setting. Note: The short-circuit current-breaking rating of the circuit breaker must be adequate to match the short-circuit level existing at the point of connection of the capacitor bank.

cross-sectional area of conductors The current rating of cables, as previously noted, must be based on 1.5 times the nominal current rating for the capacitor bank concerned.

voltage transients High-frequency voltage transients accompany the high-frequency current transients, the maximum voltage transient peak never (in the absence of steady-state harmonics) exceeds twice the peak value of rated voltage, when switching an uncharged capacitor into service. In the case of a capacitor being already charged at the instant of switch closure, however, the voltage transient can attain a maximum value approaching 3 times the normal rated peak value. This maximum condition occurs only if: c the existing voltage at the capacitor is equal to the peak value of rated voltage, and c the switch contacts close at the instant of peak supply voltage, and c the polarity of the power-supply voltage is opposite to that of the charged capacitor. In such a situation, the current transient will be at its maximum possible value, viz: twice that of its maximum when closing on to an initially uncharged capacitor, as previously noted.

E26 - power factor improvement

Section H1-2 of chapter H facilitates the selection of suitable cables, or other types of conductor, as a function of their characteristics, method of installation, and ambient temperature, etc.

For any other values of voltage and polarity on the pre-charged capacitor, the transient peaks of voltage and current will be less than those mentioned above. In the particular case of peak rated voltage on the capacitor having the same polarity as that of the supply voltage, and closing the switch at the instant of supply-voltage peak, there would be no voltage or current transients. Where automatic switching of stepped banks of capacitors is considered, therefore, care must be taken to ensure that a section of capacitors about to be energized is fully discharged. The discharge delay time may be shortened, if necessary, by using discharge resistors of a lower resistance value.

11. elementary harmonic filters

E For this appendix, the more commonlyoccurring odd-numbered harmonics are shown in the diagrams. Before the advent of power-electronics, even-numbered harmonics were rarely encountered, so that the 100 Hz (on 50 Hz systems) separating one harmonic frequency from the next made the task of filters (despite manufacturing tolerances, and impedance changes with temperature, etc.) relatively straightforward, so that satisfactory results were achieved (and still are in all but exceptional cases) by the methods described below. If it is required to eliminate (almost) a harmonic voltage existing across two points A and B in a network, a series-connected LCR circuit (figure AE3-1(a)) tuned to resonate at the harmonic frequency concerned, will constitute a virtual shortcircuit to the current of that harmonic frequency, thereby reducing VAB(h) to practically zero. The same procedure can be adopted for any number harmonic frequencies known to be present, the individual filters being connected in parallel across the points A-B (figure AE3-1(b)). (a)

A C L

f=

1 2 π v LC

R B (b)

A

protected network and power source

harmonic source

An exact analysis of the combination is not simple, since each filter is affected by those in parallel with it, as well as by the powersystem source reactance shunting the filter bank (shown dotted in figure AE3-3). When all factors have been taken into account, including a degree of damping caused by the load impedance, the response of the filter bank in terms of its impedance at different frequencies is shown in figure AE3-2. IZI Ω

fh/f50 1

5

7

11

13 (harmonic order)

fig. AE3-2. It will be seen that, at each harmonic frequency for which a filter has been provided, the impedance is very low, while at intermediate frequencies, high-impedance values occur. Care should be taken to ensure that frequencies corresponding to the lowimpedance point are not close to control frequencies (such as those of ripple-control schemes used by many power companies for remote control of power-network devices). Otherwise, the control signals will be virtually short-circuited. Harmonic-producing equipment must create the harmonic e.m.f.s. and resulting currents in order to function correctly. The role of a filter bank, as described, is to allow a free flow of harmonic currents to circulate between the harmonic source and the filter bank, while practically eliminating these currents and voltages from the rest of the network.

B

fig. AE3-1.

Appendix E3 - 1

E A

Zh a

loads

Vh harmonic source 5

7

11

13

filter bank

fig. AE3-3. In figure AE3-3, it will be seen that since the filters are practically short-circuits to harmonics, most of the harmonic voltage Vh will be dropped across the internal impedance Zh of the harmonic source and that small harmonic-current components only will pass through the power-system source impedance Xs and the loads (the latter having relatively high impedance). Since at fundamental frequency the capacitive reactance of each filter is much greater than its inductive reactance, most of the power-frequency voltage will appear across the capacitors, so that a useful contribution to any power-factor correction requirement is fortuitously available.

damped harmonic filters As noted in Chapter E, Sub-clause 9.1, the magnitude of harmonic emfs diminishes as the order of the harmonic increases. The filtering requirements are not, therefore, so critical for high-order harmonics as those necessary for lower-order harmonics. For that reason, the filter for the highest harmonic of a bank, such as that shown in figure AE3-1 (b) is often damped, by connecting a resistor in parallel with the reactor. The result is a filter which is less effective (but adequate) at its tuned frequency, while at all higher frequencies, the impedance will be low (inductive/resistive), approaching the value of the resistor only (figure AE3-4) as the frequency increases (i.e. it forms a "highpass" filter). Such a high-pass filter is commonly used for the highest-order harmonic filter (the 11th or 13th for example) of a bank, as shown dotted in figure AE3-3. IZI Ω R

L C r

R

Hz fo

fig. AE3-4: damped filter circuit and characteristic impedance/frequency curve. There are several variations of damped filters and many combinations of band-pass and undamped filters in service, according to particular requirements. In fact, the successful application of power electronics devices is largely due to the development of effective filtering techniques which are, however, beyond the scope of these brief notes.

2 - Appendix E3

B

X source

12. harmonic suppression reactor for a single (power factor correction) capacitor bank

E As shown in Appendix E2, the crux of the problem for capacitor banks is that a fraction of the total component of a given harmonic current can be magnified to dangerous levels in a parallel LCR circuit if that circuit resonates at the harmonic frequency concerned. By connecting a reactor L in series with the capacitor bank, the parallel resonant condition is moved away from the harmonic frequency towards a lower frequency, as shown in figure AE4-1 (b). In fact, the circuit now resonates at two different frequencies; the lower frequency is due to the parallel Ls//LCR combination, and the upper due to the series LCR circuit. C L

(a)

LS

R

IZI Ω

parallel resonance fP

(b)

stepped banks of capacitors Power-factor correction capacitor banks are frequently made up of a number of switched sections, so that the amount of compensation can be adjusted to suit the requirements of a changing load. If all switched steps have the same kvar rating, then the series resonant frequency of each step must be the same, i.e. the first step in service must fulfill the conditions of series resonance already mentioned and shown in figure AE4-1 (b). The addition of further identical steps in parallel will not affect the two resonant frequencies fp and fs. This is because, although the capacitance has increased n times (for n steps in service), the inductance has reduced to 1/n times its original value, so that the product LC, on which the series resonant frequency depends, remains constant. By similar reasoning, it follows that mixed steps of any kvar rating may be paralleled, providing that every step is tuned to the same series resonant frequency.

series resonance fS power source impedance

f (Hz) f1

fp

fS

range of unwanted harmonic frequencies

fig. AE4-1. It is sufficient that the two resonant frequencies be lower than those of the harmonics to be protected against, to ensure complete immunity from resonance. The reason for this is that for frequencies higher than the series-resonant frequency XL > XC so that the LCR branch behaves as an inductance + resistance series circuit. This branch being in parallel with LS, the powersystem source inductance, no resonant condition is possible. Furthermore, the addition of reactor L means that changes in the power-system source reactance will have much less influence (than formerly) on the parallel-resonant frequency, since L generally has a much greater value than LS (e.g. 2 to 9 times) and the parallelresonant frequency depends on L + LS. It may be noted that, although a harmonicsuppression reactor protects the capacitor bank against the problem of resonance with the source reactance, it does not reduce the amount of harmonic current which passes through the HV/LV transformer to the source. Such currents must be eliminated by shuntconnected series filters, as described in Appendix E3.

Appendix E4 - 1

1. low-voltage public distribution networks

D 1.1 low-voltage consumers the most-common LV supplies are within the range 120 V single phase to 240/415 V 3-phase 4-wires. Loads up to 250 kVA can be supplied at LV, but power-supply organizations generally propose a HV service at load levels for which their LV networks are marginally adequate. An international voltage standard for 3-phase 4-wire LV systems is recommended by the IEC to be 230/400 V.*

the adjoining table is extracted from the document "World Electricity Supplies", fourth edition.

table D1: survey of electricity supplies in various countries around the world. Bracketed letters relate to the circuit diagrams at the end of the table, while bracketed numbers refer to the notes which follow the diagrams.

Low-voltage consumers are, by definition, those consumers whose loads can be satisfactorily supplied from the low-voltage system in their locality. The voltage of the local LV network may be 120/208 V or 240/415 V, i.e. the lower or upper extremes of the most common 3-phase levels in general use, or at some intermediate level, as shown in table D1. An international voltage standard for 3-phase 4-wire LV systems is recommended by the IEC to be 230/400 V.* country

Loads up to 250 kVA can be supplied at LV, but power-supply organizations generally propose a HV service at load levels for which their LV networks are marginally adequate.

Australia

frequency & tolerance Hz & % 50 ± 0.1

Western Algeria

50 50 ± 1.5

250/440 (A) 127/220 (E) 220 (L) (1)

(9) 220/380 (A) 127/220 (A)

Argentina

50 ± 1.0

225 (L) (1) 220 (L) (1)

225/390 (A) 220/380 (A) 220 (L)

Brazil

60

220 (L) (1) 127 (L) (1)

220/380 (A) 127/220 (A)

Belgium

50 ± 3

220/380 (A) 127/220 (A) 220 (F)

220/380 (A) 127/220 (A) 220 (F)

Bolivia Cambodia

50 ± 1 50

Canada

60 ± 0.02

115/230 (H) 120/208 (A) 120 (L) 120/240 (K)

115/230 (H) 220/380 (A) 120/208 (A) 347/600 (A) 480 (F) 240 (F) 120/240 (K) 120/208 (A)

Chile China Colombia

50 50 60 ± 1

Costa Rica

60

220 (L) (1) 220 (L) (1) 120/240 (G) 120 (L) 120 (L) (1)

Czechoslovakia

50 ± 0.1

230/380 (A) 220 (L)

220/380 (A) (1) 220/380 (A) 120/240 (G) 120 (L) 120/240 (K) 120 (L) (1) 220/380 (A) 220 (L)

Denmark

50 ± 0.4

220/380 (A) 220 (L)

220/380 (A) 220 (L)

Egypt (AR)

50 ± 1

220/380 (A) 220 (L)

220/380 (A) 220 (L)

Finland

50 ± 0.1

220 (L) (1)

220/380 (A)

France

50 ± 1

230/400 (A) 220/380 (A) 220 (L)

230/400 (A) 220/380 (A) 220/380 (D)

127/220 (E) 127 (L) 220/380 (A) 220 (L)

380 (B)

Germany Ex-DRG

50 ± 0.3

Ex-FRG

50 ± 0.3

Greece

50 ± 1

Hong Kong (and Kowloon)

50 ± 2

domestic

commercial

industrial

240/415 (A) (E) 240 (L)

240/415 (A) 250/440 (A) 440 (N) (6)

22 kV 11 kV 6.6 kV 240/415 (A) 250/440 (A) (9) 10 kV 5.5 kV 6.6 kV 220/380 (A) 13.2 kV 6.88 kV 225/390 (A) 220/380 (A) 13.8 kV 11.2 kV 220/380 (A) 127/220 (A) 15 kV 6 kV 220/380 (A) 127/220 (A) 220 (F) 115/230 (H) (3) 220/380 (A) (3) 120/208 (A) 7.2/12.5 kV 347/600 (A) 120/208 600 (F) 480 (F) 240 (F) 220/380 (A) (3) 220/380 (A) (3) 13.2 kV 120/240 (G) 120/240 (G) (3)

220/380 (A) 220 (L)

220/380 (A) 220 (L) 127/220 (A) 127 (L) 220 (L) (1)

220/380 (A) 220 (L)

200/346 (A) 200 (L) (1)

11 kV 200/346 (A) 220/380 (A) 200 (L)

6.6 kV 220/380 (A)

22 kV 15 kV 6 kV 3 kV 220/380 (A) 30 kV 10 kV 220/380 (A) 11 kV 6.6 kV 220/380 (A) 380/660 (A) 500 (B) 220/380 (A) (D)

low-voltage tolerance % ±6

± 6 (10) + 5 and + 10

± 10 (9)

+ 5 (day) ± 10 (night)

±5 (9) ±4 - 8.3

(9) ±7 ± 10 (9) ± 10

± 10 + 10 ± 10 ± 10

20 kV 15 kV 230/400 380 (B) 220/380 (A) (D) 20 kV 10 kV 220/380 (A) 10 kV 6 kV 380/660 (A) 220/380 (A) 22 kV 20 kV 15 kV 6.6 kV 220/380 (A) 11 kV 200/346 (A) 220/380 (A) (3)

± 10

±5

±5

±6

* IEC 38 (1983).

low-voltage service connections - D1

1. low-voltage public distribution networks (continued)

D 1.1 low-voltage consumers (continued) country

Hungary

frequency & tolerance Hz & % 50 ± 2

Iceland

50 ± 0.1

220/380 (A) 220 (L)

220/380 (A) 220 (L)

India (4) Bombay

50 ± 1

250/440 (A) 230 (L)

250/440 (A) 230 (L)

11 kW 250/440 (A)

+4

New Delhi

50 ± 3

230/400 (A) 230 (L)

230/400 (A) 230 (L)

11 kV 230/400 (A)

+6

Romakrishnapuram (2)

230/400 (A) 230 (L) 230/460 (P) 127/220 (A)

+6

220 (L) (1)

220/400 (A) 230 (L) 230/460 (P) 220/380 (A) 127/220 (A) 220/380 (A)

22 kV and 11 kV (9) (9) 220/380 (A) (3)

Iran

50 ± 3 25 d.c. 50 ± 1 -2 50 ± 5

Iraq

50

220 (L) (1)

220/380 (A)

Ireland (Northern)

50 ± 0.4

230 (L) (1) 220 (L) (1)

230/400 (A) 220/380 (A)

Ireland (Republic of) Israel

50

220 (L) (1)

220/380 (A)

50 ± 0.2

230/400 (A) 230 (L)

230/400 (A) 230 (L)

Italy

50 ± 0.4

220/380 (A) 127/220 (E) 220 (L)

220/380 (A) 127/220 (E)

Japan (East) (4)

50 ± 0.2 (5)

100/200 (K) 100 (L)

100/200 (H) (K)

Japan (West) (4)

60 ± 0.1 (5)

105/210 (K) 100/200 (K) 100 (L)

105/210 (H) (K) 100/200 (K) 100 (L)

Korea (North)

60 + 0 -5

220 (L)

220/380 (A)

22 kV 6.6 kV 105/210 (H) 100/200 (H) 220/380 (A)

Korea (South) Kuwait Luxembourg

60 50 50 ± 0.5

Malaysia

50 ± 1.0

100 (L) 240 (L) (1) 220/380 (A) 127/220 (A) 120/208 (A) 240 (L) (1)

100/200 (K) 240/415 (A) 220/380 (A) 127/220 (A) 120/208 (A) 240/415 (A)

(9) 240/415 (A) (3) 20 kV 15 kV 5 kV 240/415 (A) (3)

Mexico

60 ± 0.2

127/220 (A) 220 (L) 120 (M)

127/220 (A) 220 (L) 120 (M)

Morocco

50

220/380 (A)

Netherlands

50 ± 0.4

127/220 (A) 115/200 (A) 220/300 (A) 220 (E) (L)

13.8 kV 13.2 kV 277/480 (A) 127/220 (B) 220/380 (A) (3)

New Zealand

50 ± 1.5

230/400 (A) (E) 230 (L) 240 (L)

Nigeria

50 ± 1

230 (L) (1) 220 (L) (1)

240/415 (A) (E) 230/400 (A) (E) 230 (L) 240 (L) 230/400 (A) 220/380 (A)

Norway

50 ± 0.2

230 (B)

220/380 (A) 230 (B)

Pakistan

50

230 (L) (1)

Philippines

60 ± 0.16

110/220 (K)

230/400 (A) 230 (L) 13.8 kV 4.16 kV 2.4 kV 110/220 (H)

Manila

60 ± 5

240/120 (H) (K) 240/120 (H)

240/120 (H) (K) 240/120 (H)

Peru

60

225 (B) (M)

225 (B) (M)

Indonesia

table D1: survey of electricity supplies in various countries around the world. Bracketed letters relate to the circuit diagrams at the end of the table, while bracketed numbers refer to the notes which follow the diagrams (continued). D2 - low-voltage service connections

domestic

commercial

industrial

220/380 (A) 220 (L)

220/380 (A)

20 kV 220 (L) 10 kV 220/380 (A) 220/380 (A) (3)

220/380 (A)

20 kV 11 kV 231/400 (A) 220/380 (A) 11 kV 6.6 kV 3 kV 220/380 (A) 230/400 (A) (3) 220/380 (A) 10 kV 220/380 (A) 22 kV 12.6 kV 6.3 kV 230/400 (A) 20 kV 15 kV 10 kV 220/380 (A) 220 (C) 6.6 kV 100/200 (H) 200 (G) (J)

10 kV 3 kV 220/380 (A) 11 kV 230/400 (A) 240/415 (A) 440 (N) (6) 15 kV 11 kV 230/400 (A) 220/380 (A) 20 kV 10 kV 5 kV 220/380 (A) 230 (B) 230/400 (A) (3)

low-voltage tolerance % + 5 - 10

(9)

+5 + 15

+5

+6 (9) +6

± 5 (urban) ± 10 (rural)

± 10

± 10

+ 6.8 - 13.6 (10) (9) (9) ± 5 and ± 10 +5 - 10 ±6

(9) ±6 ±5

±5

± 10

(9)

13.8 kV 4.16 kV 2.4 kV 440 V (B) 110/220 (H)

±5

20 kV 6.24 kV 3.6 kV 240/120 (H) 10 kV 6 kV 225 (B)

±5

(9)

D 1.1 low-voltage consumers (continued) country

table D1: survey of electricity supplies in various countries around the world. Bracketed letters relate to the circuit diagrams at the end of the table, while bracketed numbers refer to the notes which follow the diagrams (continued).

Poland

frequency & tolerance Hz & % 50 ± 1

domestic

commercial

industrial

220 (L) (1)

220/380 (A)

15 kV 5 kV 220/380 (A) 220 (L) 220/380 (L)

15 kV 6 kV 220/380 (A) 15 kV 5 kV 220/380 (A)

low-voltage tolerance % ±5

Portugal

50 ± 1

220/380 (A) 220 (L)

Rumania

50 ± 1

220 (L) (1)

Saudi Arabia

60 ± 0.5

127/220 (A)

Singapore

50 ± 0.5

230/400 (A) 230 (L)

Spain

50 ± 3

South Africa

50 ± 2.5 25 (8)

Sweden

50 ± 0.2

220/380 (A) (E) 220 (L) 127/220 (A) (E) 127 (L) 250/433 (A) (7) 230/400 (A) (7) 220/380 (A) 220 (L) 230/400 (A) (7) 220/380 (A) 220/380 (A) 220 (L)

Syria

50

220 (L) (1) 115 (L) (1)

Taiwan

60 ± 4

Tunisia

50 + 2

220/380 (A) 220 (L) 110/220 (K) 110 (L) 220/380 (A) 220 (L)

220/380 (A) 220 (L)

Turkey

50 ± 2

220 (L) (1)

220/380 (A)

United Kingdom

50 ± 1

240 (L) (1)

240/415 (A)

U.S.A. (4) Charlotte (North Carolina)

60 ± 0.06

120/240 (K) 120/208 (A)

265/460 (A) 120/240 (K) 120/208 (A)

14.4 kV 7.2 kV 2.4 kV 575 (F) 460 (F) 240 (F) 265/460 (A) 120/240 (K) 120/208 (A)

+ 5 - 2.5

Detroit (Michigan)

60 ± 0.2

120/240 (K) 120/208 (A)

480 (F) 120/240 (H) 120/208 (A)

13.2 kV 4.8 kV 4.16 kV 480 (F) 120/240 (H) 120/208 (A)

+ 4 - 6.6

Los Angeles (California)

60 ± 0.2

120/240 (K)

4.8 kV 120/240 (G)

4.8 kV 120/240 (G)

±5

Miami (Florida)

60 ± 0.3

120/240 (K) 120/208 (A)

120/240 (K) 120/240 (H) 120/208 (A)

13.2 kV 2.4 kV 480/277 (A) 120/240 (H)

±5

New York (New York)

60

120/240 (K) 120/208 (A)

120/240 (K) 120/208 (A) 240 (F)

12.47 kV 4.16 kV 277/480 (A) 480 (F)

(9)

Pittsburgh (Pennsylvania)

60 ± 0.03

120/240 (K)

265/460 (A) 120/240 (K) 120/208 (A) 460 (F) 230 (F)

13.2 kV 11.5 kV 2.4 kV 265/460 (A) 120/208 (A) 460 (F) 230 (F)

± 5 (lighting) ± 10 (power)

Portland (Oregon)

60

120/240 (K)

227/480 (A) 120/240 (K) 120/208 (A) 480 (F) 240 (F)

19.9 kV 12 kV 7.2 kV 2.4 kV 277/480 (A) 120/208 (A) 480 (F) 240 (F)

(9)

±5

20 kV 10 kV 6 kV 220/380 (A) 13.8 kV 220/380 (A) 22 kV 6.6 kV 230/400 (A) 15 kV 11 kV 220/380 (A)

±5

11 kV 6.6 kV 3.3 kV 250/433 (A) (7) 220/380 (A)

11 kV 6.6 kV 3.3 kV 500 (B)

±5

220/380 (A) 220 (L)

20 kV 10 kV 6 kV 220/380 (A) 220/380 (A) (3) 115/200 (A)

± 10

127/220 (A) 220/380 (A) 6.6 kV 230/400 (A) 220/380 (A) 127/220 (A)

220/380 (A) 220 (L) 115/200 (A) 115 (L) 220/380 (A) 110/220 (H)

22.8 kV 11.4 kV 220/380 (A) 220 (H) 15 kV 10 kV 220/380 (A) 15 kV 6.3 kV 220/380 (A) 22 kV 11 kV 6.6 kV 3.3 kV 240/415 (A)

±5 ±3 ±7

(9)

± 5 and ± 10

+ 10 ± 10 ±6

low-voltage service connections - D3

1. low-voltage public distribution networks (continued)

D 1.1 low-voltage consumers (continued) country

San Francisco (California)

frequency & tolerance Hz & % 60 ± 0.08

domestic

commercial

industrial

120/240 (K)

277/480 (A) 120/240 (K)

20.8 kV 12 kV 4.16 kV 277/480 (A) 120/240 (G) 12.47 kV 7.2 kV 4.8 kV 4.16 kV 480 (F) 277/480 (A) 120/208 (A) 220/380 (A) (3)

Toledo (Ohio)

60 ± 0.08

120/240 (K) 120/208 (A)

277/480 (C) 120/240 (H) 120/208 (K)

USSR (former)

50

220/380 (A) 220 (L)

Viet-Nam

50 ± 0.1

Yugoslavia

50

220/380 (A) 220 (L) 127/220 (A) 127 (L) 220 (L) (1) 120 (L) (1) 220/380 (A) 220 (L)

220/380 (A) 120/208 (A) 220/380 (A) 220 (L)

15 kV 220/380 (A) 10 kV 6.6 kV 220/380 (A)

low-voltage tolerance % ±5

±5

(9)

± 10 (9)

table D1: survey of electricity supplies in various countries around the world. Bracketed letters relate to the circuit diagrams at the end of the table, while bracketed numbers refer to the notes which follow the diagrams.

D4 - low-voltage service connections

D circuit diagrams*

(a) three-phase star; four-wire: earthed neutral

(b) three-phase star: three-wire

(d) three-phase star; four-wire: non-earthed neutral

(e) two-phase star; three-wire: earthed neutral

(g) three-phase delta; four-wire: earthed mid point of one phase

(k) single-phase; three-wire: earthed mid point

V

(c) three-phase star; three-wire: earthed neutral

(f) three-phase delta: three-wire

(h) three-phase open delta; four-wire: earthed mid point of one phase

(l) single-phase; two-wire: earthed end of phase

(j) three-phase open delta: earthed junction of phases

(m) single-phase; two-wire: unearthed

kV

(n) single-wire: earthed return (swer)

(p) d.c.: three-wire: unearthed

* Windings (A) (B) (C) (D) and (F) may be transformer-secondary windings or alternator stator windings.

Notes (1) The supply to each house is normally single-phase using one line and one neutral conductor of systems (A) or (G). (2) Frequencies below 50 Hz and d.c. supplies are used in limited areas only. The examples given show the diversity of possibilities existing. (3) Information on higher voltage supplies to factories is not available. (4) More than one area of country is given to illustrate the differences which exist.

(5) Frequency is 50 Hz (eastern area) and 60 Hz (western area). The dividing line from north to south passes through Shizuoka on Honshu Island. (6) Some remote areas are supplied via a Single Wire Earthed Return (SWER) system. (7) A few towns only have this supply. (8) Refers to isolated mining districts. (9) Information not available. (10) Observed values.

low-voltage service connections - D5

1. low-voltage public distribution networks (continued)

D 1.1 low-voltage consumers (continued) residential and commercial consumers The function of a LV "mains" distributor (underground cable or overhead line) is to provide service connections to a number of consumers along its route. The current-rating requirements of distributors are estimated from the number of consumers to be connected and an average demand per consumer. The two principal limiting parameters of a distributor are: c the maximum current which it is capable of carrying indefinitely, and c the maximum length of cable which, when carrying its maximum current, will not exceed the statutory voltage-drop limit. system 120 V 1-phase 2-wire 120/240 V 1-phase 3-wire 120/208 V 3-phase 4-wire 220/380 V 3-phase 4-wire 230/400 V 3-phase 4-wire 240/415 V 3-phase 4-wire table D2.

These constraints mean that the magnitude of loads which power-supply organizations are willing to connect to their LV distribution mains, is necessarily restricted. For the range of LV systems mentioned in the second paragraph of this sub-clause (1.1) viz: 120 V single phase to 240/415 V 3-phase, typical maximum permitted loads connected to a LV distributor might* be:

assumed max. permitted current per consumer service 60 A 60 A 60 A 120 A 120 A 120 A

kVA 7.2 14.4 22 80 83 86

* The table D2 values shown are indicative only, being (arbitrarily) based on 60 A maximum service currents for the first three systems, since smaller voltage drops are allowed at these lower voltages, for a given percentage statutory limit. The second group of systems is (again, arbitrarily) based on a maximum permitted service current of 120 A.

Practices vary considerably from one powersupply organization to another, and no "standardized" values can be given. Factors to be considered include: c the size of an existing distributor to which the new load is to be connected, c the total load already connected to the distributor, c the location along the distributor of the proposed new load, i.e. close to the substation, or near the remote end of the distributor, etc.

In short, each case must be examined individually. The load levels listed above are adequate for all normal domestic consumers, and will be sufficient for the installations of many administrative, commercial and similar buildings.

medium-size and small industrial consumers (with dedicated LV lines direct from a public-supply HV/LV substation)

For these reasons, dedicated supply lines at LV are generally applied (at 220/380 V to 240/415 V) to a load range of 80 kVA to 250 kVA. Consumers normally supplied at low voltage include: c domestic dwellings, c shops and commercial buildings, c small factories, workshops and filling stations, c restaurants, c farms, etc.

Medium and small industrial consumers can also be satisfactorily supplied at low-voltage. For loads which exceed the maximum permitted limit for a service from a distributor, a dedicated cable can usually be provided from the LV distribution fuse- (or switch-) board from which the mains distributors emanate, in the power-supply authority substation. In principle, the upper load limit which can be supplied by this means is restricted only by the available spare transformer capacity in the substation. In practice, however: c large loads (e.g. > 300 kVA) require correspondingly large cables, so that, unless the load centre is close to the substation, this method can be economically unfavourable, c many power-supply organizations prefer to supply loads exceeding 200 kVA (this figure varies with different suppliers) at high voltage.

D6 - low-voltage service connections

D 1.2. LV distribution networks in cities and large towns, standardsized LV distribution cables form a network through link boxes. Some links are removed, so that each (fused) distributor leaving a substation forms a branched openended radial system, as shown in figure D3.

In European countries the standard 3-phase 4-wire distribution voltage levels are 220/380 V, 230/400 V, or 240/415 V. Many countries are currently converting their LV systems to the latest IEC standard of 230/400 V nominal (IEC 38-1983). The target date for completion is the year 2003. Medium to large-sized towns and cities have underground cable distribution systems. HV/LV distribution substations, mutually spaced at approximately 500-600 metres, are typically equipped with: c a 3-or 4-way HV switchboard, often made up of incoming and outgoing load-break switches forming part of a ring main, and one or two HV circuit breakers or combined fuse/ load-break switches for the transformer circuits, c one or two 1.000 kVA HV/LV transformers, c one or two (coupled) 6-or 8-way LV 3-phase 4-wire distribution fuse boards, or moulded-case circuit breaker boards, control and protect outgoing 4-core distribution cables, generally referred to as “distributors". The output from a transformer is connected to the LV busbars via a load-break switch, or simply through isolating links.

In densely-loaded areas, a standard size of distributor is laid to form a network, with (generally) one cable along each pavement and 4-way link boxes located in manholes at street corners, where two cables cross. Recent trends are towards weather-proof cabinets above ground level, either against a wall, or where possible, flush-mounted in the wall. Links are inserted in such a way that distributors form radial circuits from the substation with open-ended branches (see figure D3). Where a link box unites a distributor from one substation with that from a neighbouring substation, the phase links are omitted or replaced by fuses, but the neutral link remains in place. This arrangement provides a very flexible system in which a complete substation can be taken out of service, while the area normally supplied from it is fed from link boxes of the surrounding substations. Moreover, short lengths of distributor (between two link boxes) can be isolated for fault-location and repair.

4-way link box

HV/LV substation

service cable

phase links removed

fig. D3: showing one of several ways in which a LV distribution network may be arranged for radial branched-distributor operation, by removing (phase) links. low-voltage service connections - D7

1. low-voltage public distribution networks (continued)

D 1.2 LV distribution networks (continued) in less-densely loaded urban areas a more-economic system of tapered radial distribution is commonly used, in which conductors of reduced size are installed as the distance from a substation increases.

Where the load density requires it, the substations are more closely spaced, and transformers up to 1.500 kVA are sometimes necessary. Other forms of urban LV network, based on free-standing LV distribution pillars, placed above ground at strategic points in the network, are widely used in areas of lower load density. This scheme exploits the principle of tapered radial distributors in which the distribution cable conductor size is reduced as the number of consumers downstream diminish with distance from the substation. In this scheme a number of large-sectioned LV radial feeders from the distribution board in the substation each supply the busbars of a distribution pillar, from which smaller distributors supply consumers immediately surrounding the pillar. Distribution in market towns, villages and rural areas generally has, for many years, been based on bare copper conductors supported on wooden, concrete or steel poles, and supplied from pole-mounted or ground-mounted transformers.

improved methods using insulated twisted conductors to form a polemounted aerial cable are now standard practice in many countries.

In recent years, LV insulated conductors, twisted to form a two-core or 4-core selfsupporting cable for overhead use, have been developed, and are considered to be safer and visually more acceptable than bare copper lines. This is particularly so when the conductors are fixed to walls (e.g. under-eaves wiring) where they are hardly noticeable. As a matter of interest, similar principles have been applied at higher voltages, and selfsupporting “bundled” insulated conductors for HV overhead installations are now available for operation at 24 kV. Where more than one substation supplies a village, arrangements are made at poles on which the LV lines from different substations meet, to interconnect corresponding phases at times of emergency. The neutral conductors are permanently connected.

in Europe, each public-supply distribution substation is able to supply at LV an area corresponding to a radius of approximately 300 metres from the substation. North and Central American systems of distribution consist of a HV network from which numerous (small) HV/LV transformers each supply one or several consumers, by direct service cable (or line) from the transformer location.

In Europe, each public-supply distribution substation is able to supply at LV an area corresponding to a radius of approximately 300 metres from the substation. North and Central American practice differs fundamentally from that in Europe, in that LV networks are practically nonexistent, and 3-phase supplies to domestic premises in residential areas are rare. The distribution is effectively carried out at high voltage in a way, which again differs from standard European practices. The HV system is, in fact, a 3-phase 4-wire system from which single-phase distributors (phase and neutral conductors) supply numerous single-phase transformers, the secondary windings of which are centre-tapped to produce 120/240 V single-phase 3-wire supplies. The central conductors provide the LV neutrals, which, together with the HV neutral conductors, are solidly earthed at intervals along their lengths. Each HV/LV transformer normally supplies one or several premises directly from the transformer position by radial service cable(s) or by overhead line(s).

D8 - low-voltage service connections

D Many other systems exist in these countries, but the one described appears to be the most common. Figure D4 shows the main features of the two systems. for primary voltages > 72.5 kV (see note) primary winding may be : – delta with on-load – earthed star tap changer – earthed zigzag depending on the country concerned

}

13.8 kV / 2.4-4.16 kV N 1

2

3

each HV/LV transformer shown represents many similar units tertiary delta normally (not always) used if the primary winding is not delta

2 3 N 2.4 kV / 120-240 V 1 ph - 3 wire distribution transformer

1 ph HV / 230 V service transformer to isolated consumer(s) (rural supplies)

}

HV (1)

Ph

N 1

1 N

N

resistor replaced by a Petersen coil on O/H line systems in some countries

HV (2)

N 2

2 N

1 2 3 N main 3 ph and neutral HV distributor

3 ph HV / 230/400 V 4-wire distribution transformer

N

N 1 2 3 LV distribution network (1): 132 kV for example (2): 11 kV for example

fig. D4: widely-used American and European-type systems. Note: at primary voltages greater than 72.5 kV in bulk-supply substations, it is common practice in some European countries to use an earthed-star primary winding and a delta secondary winding. The neutral point on the secondary side is then provided by a zigzag earthing reactor, the star point of which is connected to earth through a resistor. Frequently, the earthing reactor has a secondary winding to provide LV 3-phase supplies for the substation. It is then referred to as an “earthing transformer”.

low-voltage service connections - D9

1. low-voltage public distribution networks (continued)

D 1.3 the consumer-service connection service components and metering equipment were formerly installed inside a consumer's building. The modern tendency is to locate these items outside in a weatherproof cabinet.

In the past, an underground cable service or the wall-mounted insulated conductors from an overhead line service, invariably terminated inside the consumer's premises, where the cable-end sealing box, the supplyauthority fuses (inaccessible to the consumer) and meters were installed. A more recent trend is (as far as possible) to locate these service components in a weatherproof housing outside the building. The supply-authority/consumer interface is often at the outgoing terminals of the meter(s) or, in some cases, at the outgoing terminals of the installation main circuit breaker (depending on local practices) to which connection is made by supply-authority personnel, following a satisfactory test and inspection of the installation. A typical arrangement is shown in figure D5.

fig. D5: typical service arrangement for TT-earthed systems. A = Service cable tee-joint F = Supply authority fuses C = Metering equipment S = Isolating link DB = Installation main circuit breaker

LV consumers are normally supplied according to the TN or TT system, as described in chapters F and G. The installation main circuit breaker for a TT supply must include a residualcurrent earth-leakage protective device. For a TN service, overcurrent protection by circuit breaker or switch-fuse is required.

D10 - low-voltage service connections

A MCCB which incorporates a sensitive residual-current earth-fault protective feature is mandatory at the origin of any LV installation forming part of a TT earthing system. The reason for this feature and related leakage-current tripping levels are discussed in Clause 3 of Chapter G. A further reason for this MCCB is that the consumer cannot exceed his (contractual) declared maximum load, since the overload trip setting, which is sealed by the supply authority, will cut off supply above the declared value. Closing and tripping of the MCCB is freely available to the consumer, so that if the MCCB is inadvertently tripped on overload, or due to an appliance fault, supplies can be quickly restored following correction of the anomaly.

In view of the inconvenience to both the meter reader and consumer, the location of meters is nowadays generally outside the premises, either: c in a free-standing pillar-type housing as shown in figures D6 and D7, c in a space inside a building, but with cable termination and supply authority’s fuses located in a flush-mounted weatherproof cabinet accessible from the public way, as shown in figure D8.

D

fig. D6: typical rural-type installation. In this kind of installation it is often necessary to place the main installation circuit breaker some distance from the point of utilization, e.g. saw-mills, pumping stations, etc.

fig. D7: semi-urban installations (shopping precincts, etc.). The main installation CB is located in the consumer's premises in cases where it is set to trip if the declared kVA load demand is exceeded.

fig. D8: town centre installations. The service cable terminates in a flushmounted wall cabinet which contains the isolating fuse links, accessible from the public way. This method is preferred for esthetic reasons, when the consumer can provide a suitable metering and main-switch location.

low-voltage service connections - D11

1. low-voltage public distribution networks (continued)

D 1.3 the consumer-service connection (continued) c for private domestic consumers, the equipment shown in the cabinet in figure D5 is installed in a weatherproof cabinet mounted vertically on a metal frame in the front garden, or flush-mounted in the boundary wall, and accessible to authorized personnel from the pavement. Figure D9 shows the general arrangement, in which removable fuse links provide the means of isolation. Experiments are now well-advanced in the field of electronic metering; reading, and recording on magnetic cards is now possible, using information technology (IT) techniques, and it is confidently predicted that, in addition to remote reading and recording, the modification of tariff structures for a given meter will be possible from a central control location, in areas where it is economically justified.

supply authority/ consumer interface overhead line LV distributor service cable isolation by fuse links

installation

meter

meter cabinet

main installation circuit breaker

fig. D9: typical LV service arrangement for domestic consumers.

D12 - low-voltage service connections

D 1.4 quality of supply voltage The quality of the LV network supply voltage in its widest sense implies: c compliance with statutory limits of magnitude and frequency, c freedom from continual fluctuation within those limits, c uninterrupted power supply, except for scheduled maintenance shutdowns, or as a result of system faults or other emergencies, c preservation of a near-sinusoidal wave form. In this Sub-clause the maintenance of voltage magnitude only will be discussed, the remaining subjects are covered in Clause 2 of chapter F. In most countries, power-supply authorities have a statutory obligation to maintain the level of voltage at the service position of consumers within the limits of ± 5% (or in some cases ± 6% or more-see table D1) of the declared nominal value. Again, IEC and most national standards recommend that LV appliances be designed and tested to perform satisfactorily within the limits of ± 10% of nominal voltage. This leaves a margin, under the worst conditions (of minus 5% at the service position, for example) of 5% allowable voltage drop in the installation wiring. The voltage drops in a typical distribution system occur as follows: the voltage at the HV terminals of a HV/LV transformer is normally maintained within a ± 2% band by the action of automatic onload tapchangers of the transformers at bulk-supply substations, which feed the HV network from a higher-voltage subtransmission system.

an adequate level of voltage at the consumers supply-service terminals is essential for satisfactory operation of equipment and appliances. Practical values of current, and resulting voltage drops in a typical LV system, show the importance of maintaining a high Power Factor as a means of reducing voltage drop.

If the HV/LV transformer is in a location close to a bulk-supply substation, the ± 2% voltage band may be centred on a voltage level which is higher than the nominal HV value. For example, the voltage could be 20.5 kV ± 2% on a 20 kV system. In this case, the HV/LV distribution transformer should have its HV off-circuit tapping switch selected to the + 2.5% tap position. Conversely, at locations remote from bulksupply substations a value of 19.5 kV ± 2% is possible, in which case the off-circuit tapping switch should be selected to the - 5% position. The different levels of voltage in a system are normal, and depend on the system powerflow pattern. Moreover, these voltage differences are the reason for the term “nominal” when referring to the system voltage.

104% at no-load*, when nominal voltage is applied at HV, or is corrected by the tapping switch, as described above. This would result in a voltage band of 102% to 106% in the present case. A typical LV distribution transformer has a short-circuit reactance voltage of 5%. If it is assumed that its resistance voltage is one tenth of this value, then the voltage drop within the transformer when supplying full load at 0.8 power factor lagging, will be: V% drop = R% cos ø + X% sin ø = 0.5 x 0.8 + 5 x 0.6 = 0.4 + 3 = 3.4% The voltage band at the output terminals of the fully-loaded transformer will therefore be (102 - 3.4) = 98.6% to (106 - 3.4) = 102.6%. The maximum allowable voltage drop along a distributor is therefore 98.6 - 95 = 3.6%. This means, in practical terms, that a medium-sized 230/400 V 3-phase 4-wire distribution cable of 240 mm2 copper conductors would be able to supply a total load of 292 kVA at 0.8 PF lagging, distributed evenly over 306 metres of the distributor. Alternatively, the same load at the premises of a single consumer could be supplied at a distance of 153 metres from the transformer, for the same volt-drop, and so on... As a matter of interest, the maximum rating of the cable, based on calculations derived from IEC 287 (1982) is 290 kVA, and so the 3.6% voltage margin is not unduly restrictive, i.e. the cable can be fully loaded for distances normally required in LV distribution systems. Furthermore, 0.8 PF lagging is appropriate to industrial loads. In mixed semi-industrial areas 0.85 is a more common value, while 0.9 is generally used for calculations concerning residential areas, so that the volt-drop noted above may be considered as a “worst case” example. * Transformers designed for the 230/400 V IEC standard will have a no-load output of 420 V, i.e. 105% of the nominal voltage.

practical application With the HV/LV transformer correctly selected at its off-circuit tapping switch, an unloaded transformer output voltage will be held within a band of ± 2% of its no-load voltage output. To ensure that the transformer can maintain the necessary voltage level when fully loaded, the output voltage at no-load must be as high as possible without exceeding the upper + 5% limit (adopted for this example). In present-day practice, the winding ratios generally give an output voltage of about low-voltage service connections - D13

2. tariffs and metering

D tariffs and metering No attempt will be made in this guide to discuss particular tariffs, since there appears to be as many different tariff structures around the world as there are distribution authorities. Some tariffs are very complicated in detail but certain elements are basic to all of them and are aimed at encouraging consumers to manage their power consumption in a way which reduces the cost to the supply authority of generation, transmission and distribution. The two predominant ways in which the cost of supplying power to consumers can be reduced, are: c reduction of power losses in the generation, transmission and distribution of electrical energy. In principle the lowest losses in a power system are attained when all parts of the system operate at unity power factor, c reduction of the peak power demand, while increasing the demand at low-load periods, thereby exploiting the generating plant more fully, and minimizing plant redundancy.

reduction of losses Although the ideal condition noted in the first possibility mentioned above cannot be realized in practice, many tariff structures are based partly on kVA demand, as well as on kWh consumed. Since, for a given kW loading, the minimum value of kVA occurs at unity power factor, the consumer can minimize billing costs by taking steps to improve the power factor of the load (as discussed in Chapter E). The kVA demand generally used for tariff purposes is the maximum average kVA demand occurring during each billing period, and is based on average kVA demands, over fixed periods (generally 10, 30 or 60 minute periods) and selecting the highest of these values. The principle is described below in "principle of kVA maximum-demand metering".

reduction of peak power demand The second aim, i.e. that of reducing peak power demands, while increasing demand at low-load periods, has resulted in tariffs which offer substantial reduction in the cost of energy at: c certain hours during the 24-hour day, c certain periods of the year. The simplest example is that of a domestic consumer with a storage-type water heater (or storage-type space heater, etc.). The meter has two digital registers, one of which operates during the day and the other (switched over by a timing device) operates during the night. A contactor, operated by the same timing device, closes the circuit of the water heater, the consumption of which is then indicated on the register to which the cheaper rate applies. The heater can be switched on and off at any time during the day if required, but will then be metered at the normal rate. Large industrial consumers may have 3 or 4 rates which apply at different periods during a 24-hour interval, and a similar number for different periods of the year. In such schemes the ratio of cost per kWh during a period of peak demand for the year, and that for the lowest-load period of the year, may be as much as 10: 1. D14 - low-voltage service connections

meters It will be appreciated that high-quality instruments and devices are necessary to implement this kind of metering, when using classical electro-mechanical equipment. Recent developments in electronic metering and micro-processors, together with remote ripple-control* from a supply-authority control centre (to change peak-period timing throughout the year, etc.) are now operational, and facilitate considerably the application of the principles discussed. * Ripple control is a system of signalling in which a voicefrequency current (commonly at 175 Hz) is injected into the LV mains at appropriate substations. The signal is injected as coded impulses, and relays which are tuned to the signal frequency and which recognize the particular code will operate to initiate a required function. In this way, up to 960 discrete control signals are available.

D In most countries, certain tariffs, as noted above, are partly based on kVA demand, in addition to the kWh consumption, during the billing periods (often 3-monthly intervals). The maximum demand registered by the meter to be described, is, in fact, a maximum (i.e. the highest) average kVA demand

registered for succeeding periods during the billing interval. Figure D10 shows a typical kVA demand curve over a period of two hours divided into succeeding periods of 10 minutes. The meter measures the average value of kVA during each of these 10 minute periods.

maximum average value during the 2 hour interval average values for 10 minute periods

kVA

0

1

time

2 hrs

fig. D10: maximum average value of kVA over an interval of 2 hours.

principle of kVA maximumdemand metering A kVAh meter is similar in all essentials to a kWh meter but the current and voltage phase relationship has been modified so that it effectively measures kVAh (kilo-volt-amphours). Furthermore, instead of having a set of decade counter dials, as in the case of a conventional kWh meter, this instrument has a rotating pointer. When the pointer turns it is measuring kVAh and pushing a red indicator before it. At the end of 10 minutes the pointer will have moved part way round the dial (it is designed so that it can never complete one revolution in 10 minutes) and is then electrically reset to the zero position, to start another 10 minute period. The red indicator remains at the position reached by the measuring pointer, and that position, corresponds to the number of kVAh (kilo-volt-ampere-hours) taken by the load in 10 minutes. Instead of the dial being marked in kilo-VAhours at that point however it can be marked in units of average kVA. The following figures will clarify the matter. Supposing the point at which the red indicator reached corresponds to 5 kVAh. It is known that a varying amount of kVA of apparent power has been flowing for 10 minutes, i.e. 1/6 hour. If now, the 5 kVAh is divided by the number of hours, then the average kVA for the period is obtained. In this case the average kVA for the period will be: 1 5x = 5 x 6 = 30 kVA 1/6 Every point around the dial will be similarly marked i.e. the figure for average kVA will be 6 times greater than the kVAh value at any given point. Similar reasoning can be applied to any other reset-time interval.

At the end of the billing period, the red indicator will be at the maximum of all the average values occurring in the billing period. The red indicator will be reset to zero at the beginning of each billing period. Electro-mechanical meters of the kind described are rapidly being replaced by electronic instruments. The basic measuring principles on which these electronic meters depend however, are the same as those described above.

low-voltage service connections - D15

1. supply of power at high voltage

C At present there is no international agreement on precise limits to define “High” voltage. Voltage levels which are designated as “high” in some countries are referred to as “medium” in others. In this chapter, distribution networks which operate at voltages of 1,000 V or less are referred to as Low-Voltage systems, while systems of power distribution which require

one stage of stepdown voltage transformation, in order to feed into lowvoltage networks, will be referred to as HighVoltage systems. For economic and technical reasons the upper nominal voltage limit of high-voltage distribution systems, as defined above, seldom exceeds 36.5 kV.

1.1 power-supply characteristics of high voltage distribution networks the main features which characterize a power-supply system include: c the nominal voltage and related insulation levels, c the short-circuit current, c the rated normal current of items of plant and equipment, c the method of earthing. Note: All voltages and currents are r.m.s. values, unless otherwise stated. in this document, the word “nominal” voltage is used for the network and the word “rated” voltage is used for the equipment.

nominal voltage and related insulation levels The nominal voltage of a system or of an equipment is defined in IEC 38 as “the voltage by which a system or equipment is designated and to which certain operating characteristics are referred”. Closely related to the nominal voltage is the “highest voltage for equipment” which concerns the level of insulation at normal working frequency, and to which other characteristics may be referred in relevant equipment recommendations. The “highest voltage for equipment” is defined in IEC 38 as: “the maximum value of voltage for which the equipment may be used, that occurs under normal operating conditions at any time and at any point on the system. It excludes voltage transients, such as those due to system switching, and temporary voltage variations”. Notes: 1.- The highest voltage for equipment is indicated for nominal system voltages higher than 1,000 V only. It is understood that, particularly for certain nominal system voltages, normal operation of equipment cannot be ensured up to this highest voltage for equipment, having regard to voltage sensitive characteristics such as losses of capacitors, magnetizing current of transformers, etc. In such cases, the relevant recommendations must specify the limit to which the normal operation of this equipment can be ensured. 2.- It is understood that the equipment to be used in systems having nominal voltage not exceeding 1,000 V should be specified with reference to the nominal system voltage only, both for operation and for insulation. 3.- The definition for “highest voltage for equipment” given in IEC 38 is identical to that given in IEC 694 for “rated voltage”. IEC 694 concerns switchgear for nominal voltages exceeding 1,000 V.

HV/LV distribution substations - C1

1. supply of power at high voltage (continued)

C 1.1 power-supply characteristics of high voltage distribution networks (continued) The following Table C1, taken from IEC 38, lists the most-commonly used standard levels of high-voltage distribution, and relates the nominal voltages to corresponding standard values of “Highest Voltage for Equipment”. Two series of highest voltages for equipment are given below, one for 50 Hz and 60 Hz systems (Series I), the other for 60 Hz systems (Series II - North American practice). It is recommended that only one of these series should be used in any one country. It is also recommended that only one of the two series of nominal voltages given for Series I should be used in any one country. These systems are generally three-wire systems unless otherwise indicated. The values shown are voltages between phases. The values indicated in parentheses should be considered as non-preferred values. It is recommended that these values should not be used for new systems to be constructed in future. series I highest voltage for equipement (kV) 3.6 (1) 7.2 (1) 12 (17.5) 24 36 (3) 40.5 (3)

nominal system voltage (kV) 3.3 (1) 3 (1) 6.6 (1) 6 (1) 11 10 (15) 22 20 33 (3) 35 (3)

Notes: 1 - It is recommended that in any one country the ratio between two adjacent nominal voltages should be not less than two. 2 - In a normal system of Series I, the highest voltage and the lowest voltage do not differ by more than approximately ± 10% from the nominal voltage of the system. In a normal system of Series II, the highest voltage does not differ by more than + 5% and the lowest voltage by more than - 10% from the nominal voltage of the system.

series II highest voltage for equipment (kV) 4.40 (1) 13.2 (2) 13.97 (2) 14.52 (1) 26.4 (2) 36.5 (2) -

nominal system voltage (kV) 4.16 (1) 12.47 (2) 13.2 (2) 13.8 (1) 24.94 (2) 34.5 (2) -

1) These values should not be used for public distribution systems. 2) These systems are generally four-wire systems. 3) The unification of these values is under consideration.

table C1: relating nominal system voltages with corresponding rated system voltages (r.m.s. values). In order to ensure adequate protection of equipment against abnormally-high shortterm power-frequency overvoltages, and transient overvoltages caused by lightning, switching, and system fault conditions, etc. all HV equipment must be specified to have appropriate Rated Insulation Levels. Switchgear Table C2 shown below, is extracted from IEC 694 and lists standard values of “withstand” voltage requirements. The choice between List 1 and List 2 values of table C2 depends on the degree of exposure to lightning and switching overvoltages*, the type of neutral earthing, and the type of overvoltage protection devices, etc. (for further guidance reference should be made to IEC 71). * This means basically that List 1 generally applies to switchgear to be used on underground-cable systems while List 2 is chosen for switchgear to be used on overhead-line systems.

C2 - HV/LV distribution substations

C Based on current practice in most European and several other countries rated voltage U (r.m.s. value)

(kV) 3.6 7.2 12 17.5 24 36 52 72.5

rated lightning impulse withstand voltage (peak value) list 1 list 2 to earth, across the to earth, between poles isolating between poles and across distance and across open open switching switching device device (kV) (kV) (kV) 20 23 40 40 46 60 60 70 75 75 85 95 95 110 125 145 165 170 250 325

rated I min powerfrequency withstand voltage (r.m.s. value) across the to earth, across the isolating between poles isolating distance and across distance open switching device (kV) (kV) (kV) 46 10 12 70 20 23 85 28 32 110 38 45 145 50 60 195 70 80 290 95 110 375 140 160

Note: The withstand voltage values “across the isolating distance” are valid only for the switching devices where the clearance between open contacts is designed to meet safety requirements specified for disconnectors (isolators). table C2: switchgear rated insulation levels. It should be noted that, at the voltage levels in question, no switching overvoltage ratings are mentioned. This is because overvoltages due to switching transients are less severe at these voltage levels than those due to lightning. Transformers The two tables C3A and C3B shown below have been extracted from IEC 76-3, and refer to the current practices in countries other than those of North America (Series I) and to those of North America and some other countries (Series II). The significance of list 1 and list 2 in Series I is the same as that for the switchgear table, i.e. the choice depends on the degree of exposure to lightning, etc. highest voltage for equipment Um (r.m.s.) (kV) i 1.1 3.6 7.2 12 17.5 24 36 52 72.5

rated short duration power frequency withstand voltage (r.m.s.) (kV) 3 10 20 28 38 50 70 95 140

rated lightning impulse withstand voltage (peak) list 1 list 2 (kV) (kV) 20 40 40 60 60 75 75 95 95 125 145 170 250 325

table C3A: transformers rated insulation levels in series I (based on current practice other than in the United States of America and some other countries).

HV/LV distribution substations - C3

1. supply of power at high voltage (continued)

C 1.1 power-supply characteristics of high voltage distribution networks (continued) highest voltage for equipment Um (r.m.s.)

rated short duration power frequency withstand voltage (r.m.s.)

(kV) 4.40 13.20 13.97 14.52 26.4 36.5 72.5

(kV) 19

rated lightning impulse withstand voltage (peak) distribution other transformers transformers (kV) (kV) 60 75

34

95

}

110

50 70 140

150 200 350

table C3B: transformers rated insulation levels in series II (based on current practice in the United States of America and some other countries). Other components It is evident that the insulation performance of other HV components associated with these major items, e.g. porcelain or glass insulators, HV cables, instrument transformers, etc. must be compatible with that of the switchgear and transformers noted above. Test schedules for these items are given in appropriate IEC publications.

the national standards of any particular country are normally rationalized to include one or two levels only of voltage, current, and fault-levels, etc.

General note The IEC standards are intended for worldwide application and consequently embrace an extensive range of voltage and current levels. These reflect the diverse practices adopted in countries of different meteorologic, geographic and economic constraints. The national standards of any particular country are normally rationalized to include one or two levels only of voltage, current, and fault-levels, etc.

a circuit breaker (or fuse switch, over a limited voltage range) is the only form of switchgear capable of safely breaking the very high levels of current associated with short-circuit faults occurring on a power system.

short-circuit current A circuit breaker (or fuse switch, over a limited voltage range) is the only form of switchgear capable of safely breaking the very high levels of current associated with short-circuit faults occurring on a power system. Standard values of circuit breaker shortcircuit current-breaking capability are normally given in kilo-amps. These values refer to a 3-phase short-circuit condition, and are expressed as the average of the r.m.s. values of the a.c. component of current in each of the three phases. Short-circuit current-breaking ratings For circuit breakers in the rated voltage ranges being considered in this chapter, IEC 56 gives standard short-circuit currentbreaking ratings as follows. kV kA (r.m.s.)

3.6 10 16 25 40

7.2 8 12.5 16 25 40

12 8 12.5 16 25 40 50

17.5 8 12.5 16 25 40

24 8 12.5 16 25 40

36 8 12.5 16 25 40

52 8 12.5 20

table C4: standard short-circuit current-breaking ratings extracted from table X IEC 56.

C4 - HV/LV distribution substations

C Where the installation of a circuit breaker is electrically remote from a power source, it is only necessary to check that the power factor of the faulty circuit is not less than 0.07 and that the minimum operating time of protective relaying is not less than a half cycle of the power-supply frequency (i.e. 10 ms at 50 Hz). In the great majority of cases, these conditions will be satisfied in a conventional HV distribution network. In such circumstances, it is then only necessary to ensure that the IEC-rated shortcircuit current-breaking capability of the circuit breaker exceeds the r.m.s. value of 3-phase short-circuit current at the point of installation. Where circuit breakers are to be installed close to generating plant, the a.c. component of short-circuit current will diminish rapidly from its initial value (i.e. the a.c. decrement) and the power factor of the fault circuit may be less than 0.07. Such a case would need further investigation along the lines indicated in IEC 56, since the result could lead to an absence of current zeros for several initial cycles*. Maximum peak of current Another aspect of short-circuit current stresses that may be imposed on the component parts of a power system, concerns the maximum possible peak of current which can occur if a circuit breaker is closed on to a dead circuit which is shortcircuited. For such a possibility, circuit breakers have a short-circuit current-making rating, expressed in kA of peak current. The numerical value of this rating is 2.5 times the short-circuit current-breaking rating of the circuit breaker. Explanation The value 2.5 is derived as follows: shortcircuit current is normally highly inductive so that at least two of the phases will contain a transient d.c. component. In the worst possible case, the value of the d.c. component in one of the phases will be equal

to the peak value of the a.c. component, producing the so-called “doubling effect”. However, the d.c. transient diminishes rapidly from the instant of fault, while the peak current occurs a half cycle after that instant. Allowance is made for the diminution in the d.c. component by reducing the doubling factor (2) to a value of 1.8. In IEC 56 this reduction is based on an inductive d.c. time constant value which is representative of average HV distribution systems. The peak current value is therefore rIrms x 1.8 = 2.54 Irms which is rounded off for standardization purposes to 2.5 Irms. The form of the fully-offset short-circuit current is shown in figure C5, reproduced from IEC 56. Note: When a short-circuit (s.c.) occurs on a power system, all electric motors act for a very brief period (1-2 cycles) as generators, and feed current (typically 50 % - 80 % of the motorstarting current) into the fault. This is due to the collapsing magnetic flux in each motor and is generally significant only for the first power-frequency cycle from the moment of s.c. For the latter reason, apart from very exceptional cases, it is not necessary to take account of its effect on the s.c. currentbreaking rating of a circuit breaker (CB). It cannot be neglected however, in the case of s.c. current-making rating. If there are large concentrations of motors near the point of installation of a CB, the s.c. current-making level will be greater than 2.5 times the s.c. current-breaking level at the same location. In order to ensure an adequate s.c. currentmaking capacity, therefore, a CB having an oversized s.c. current-breaking capacity is necessary in such circumstances. * A "natural" current zero is essential for the correct functioning of a CB, unless especially designed for the purpose.

I A E D

A’

C IMC

B

IAC

IDC

D’

C’ X t

B’

E’

AA' = envelope of current-wave BB’ BX = normal zero line CC’ = displacement of current-wave zeroline at any instant DD’ = r.m.s. value of the a.c. component of current at any instant, measured from CC’ EE’ = instant of contact separation (initiation of the arc)

IMC = making current = (A-C) 1.8 where A and C are measured at t = 0 IAC = peak value of a.c. component of current at instant EE’ IAC = r.m.s. value of the a.c. component r of current at instant EE’ IDC = d.c. component of current at instant EE’ IDC x 100 = percentage value of the d.c. IAC component

fig. C5: determination of short-circuit making and breaking currents, and of percentage d.c. component. HV/LV distribution substations - C5

1. supply of power at high voltage (continued)

C 1.1 power-supply characteristics of high voltage distribution networks (continued) the short-circuit current level of a HV distribution system is frequently limited by design techniques to a predetermined maximum value typically in the range of 12.5 kA to 25 kA. All HV equipments connected to the system must be capable of withstanding, without damage, the thermal and mechanical stresses of the maximum short-circuit current for 1 second, or in particular cases (depending on equipment specifications) for 3 seconds.

In such a case, a CB having a s.c. currentbreaking rating sufficiently high to ensure an adequate s.c. current-making performance must be installed. The short-circuit current level of a HV distribution system is frequently limited by design techniques to a pre-determined maximum value typically in the range of 12.5 kA to 25 kA. All HV equipments connected to the system must be capable of withstanding, without damage, the thermal and mechanical stresses of the maximum short-circuit current for 1 second, or in particular cases (depending on equipment specifications) for 3 seconds.

the most common normal current rating for general-purpose HV distribution switchgear is 400 A.

Rated normal current The rated normal current is defined as “the r.m.s. value of the current which can be carried continuously at rated frequency with a temperature rise not exceeding that specified by the relevant product standard”. The rated normal current requirements for switchgear are decided at the substation design stage. The most common normal current rating for general-purpose HV distribution switchgear is 400 A. In industrial areas and high-load-density urban districts, circuits rated at 630 A are sometimes required, while at bulk-supply substations which feed into HV networks, 800 A; 1,250 A; 1,600 A; 2,500 A and 4,000 A circuit breakers are listed in IEC 56 as standard ratings for incoming-transformer circuits, bus-section and bus-coupler CBs, etc. At HV/LV substations which include one (or more) transformer(s) with a normal primary current of less than 45 A, a HV switch associated with a set of 3 fuses (or a combination switch-fuse) is generally used to control and protect the transformer, as a more economic alternative to a CB. There are no IEC-recommended normalcurrent rating tables for the combination in these cases. The actual rating will be given by the switch-fuse manufacturer, according to the fuse characteristics, and details of the transformer, such as: c normal current at HV, c permissible overcurrent and its duration, c max. peak and duration of the transformer energization inrush magnetizing current, c off-circuit tapping-switch position, etc. as shown in the example given in Appendix A of IEC 420, and summarized in Appendix C1 of this guide.

C6 - HV/LV distribution substations

The IEC recommends that the normal-current rating value, assigned to the combination by the manufacturer, be one of the “R10” series of (ISO) preferred numbers, viz: 10, 12.5, 16, 20, 25, 31.5, 40, 50, 63, 80 with multiples (or sub-multiples) of 10 as required. In such a scheme, the load-break switch must be suitably rated to trip automatically, e.g. by relays, at low fault-current levels which must cover (by an appropriate margin) the rated minimum breaking current of the HV fuses. In this way, high values of fault current which are beyond the breaking capability of the load-break switch will be cleared by the fuses, while low fault-current values, that cannot be correctly broken by the fuses, will be cleared by the relay-operated load-break switch. Appendix C1 gives further information on this arrangement, as applied to HV switch-fuse combination units.

C Influence of the ambient temperature and altitude on the rated current Normal-current ratings are assigned to all current-carrying electrical appliances, and upper limits are decided by the acceptable temperature rise caused by the I2R (watts) dissipated in the conductors, (where I = r.m.s. current in amperes and R = the resistance of the conductor in ohms), together with the heat produced by magnetic-hysteresis and eddy-current losses in motors, transformers, etc. and dielectric losses in cables and capacitors, where appropriate. The temperature rise above the ambient temperature will depend mainly on the rate at which the heat is removed. For example, large currents can be passed through electric motor windings without causing them to overheat, simply because a cooling fan fixed to the shaft of the motor removes the heat at the same rate as it is produced, and so the temperature reaches a stable value below that which could damage the insulation and

earth faults on high-voltage systems can produce dangerous voltage levels on LV installations. LV consumers (and substation operating personnel) can be safeguarded against this danger by: c restricting the magnitude of HV earth-fault currents, c reducing the substation earthing resistance to the lowest possible value, c creating equipotential conditions at the substation and at the consumer's installation.

result in a burnt-out motor. Oil- and/or air-cooled transformers are among the most widely known examples of such “forced-cooling” techniques. The normal-current values recommended by IEC are based on ambient-air temperatures common to temperate climates at altitudes not exceeding 1,000 metres, so that items which depend on natural cooling by radiation and air-convection will overheat if operated at rated normal current in a tropical climate and/ or at altitudes exceeding 1,000 metres. In such cases, the equipment has to be derated, i.e. be assigned a lower value of normalcurrent rating according to IEC 76-2. In the case of force-cooled transformers it is generally sufficient to provide sun shields, and increase the oil-cooling radiator surfaces, the amount of cooling oil, the power of the circulating-oil pumps, and the size of the aircirculating fans, to maintain the original IEC rating.

earthing connections Earth faults on high-voltage systems can produce dangerous voltage levels on LV installations. LV consumers (and substation operating personnel) can be safeguarded against this danger by: c restricting the magnitude of HV earth-fault currents, c reducing the substation earthing resistance to the lowest possible value, c creating equipotential conditions at the substation and at the consumer's installation. Earthing and equipment-bonding earth connections require careful consideration, particularly regarding safety of the LV consumer during the occurrence of a shortcircuit to earth on the HV system. Earth electrodes In general, it is preferable, where physically possible, to separate the electrode provided for earthing exposed conductive parts of HV equipment from the electrode intended for earthing the LV neutral conductor. This is commonly practised in rural systems where the LV neutral-conductor earth electrode is installed at one or two spans of LV distribution line away from the substation. In most cases, the limited space available in urban substations precludes this practice, i.e. there is no possibility of separating a HV electrode sufficiently from a LV electrode to avoid the transference of (possibly dangerous) voltages to the LV system. Earth-fault current Earth-fault current levels at high voltage are generally (unless deliberately restricted) comparable to those of a 3-phase shortcircuit. Such currents passing through an earth electrode will raise its voltage to a high value with respect to “remote earth” (the earth surrounding the electrode will be raised to a high potential; “remote earth” is at zero potential). For example, 10,000 A of earth-fault current passing through an electrode with an (unusually low) resistance of 0.5 ohms will raise its voltage to 5,000 V. Providing that all exposed metal in the substation is “bonded” (connected together) and then connected to the earth electrode,

and the electrode is in the form of (or is connected to) a grid of conductors under the floor of the substation, then there is no danger to personnel, since this arrangement forms an equipotential “cage” in which all conductive material, including personnel, is raised to the same potential. Transferred potential A danger exists however from the problem known as Transferred Potential. It will be seen in figure C6 that the neutral point of the LV winding of the HV/LV transformer is also connected to the common substation earth electrode, so that the neutral conductor, the LV phase windings and all phase conductors are also raised to the electrode potential. Low-voltage distribution cables leaving the substation will transfer this potential to consumers installations. It may be noted that there will be no LV insulation failure between phases or from phase to neutral since they are all at the same potential. It is probable, however, that the insulation between phase and earth of a cable or some part of an installation would fail. Solutions A first step in minimizing the obvious dangers of transferred potentials is to reduce the magnitude of HV earth-fault currents. This is commonly achieved by earthing the HV system through resistors or reactors at the star points of selected transformers*, located at bulk-supply substations. A relatively high transferred potential cannot be entirely avoided by this means, however, and so the following strategy has been adopted in some countries. The equipotential earthing installation at a consumer's premises represents a remote earth, i.e. at zero potential. However, if this earthing installation were to be connected by a low-impedance conductor to the earthelectrode at the substation, then the equipotential conditions existing in the substation would also exist at the consumer's installation. * the others being unearthed. A particular case of earth-fault current limitation, namely, by means of a Petersen coil, is discussed at the end of Sub-clause 3.2.

HV/LV distribution substations - C7

1. supply of power at high voltage (continued)

C 1.1 power-supply characteristics of high voltage distribution networks (continued) HV

LV 1 2 3 N

fault If consumer If

V=IfRs

Rs

fig. C6: transferred potential. Low-impedance interconnection This low-impedance interconnection is achieved simply by connecting the neutral conductor to the consumer's equipotential installation, and the result is recognized as the TN system of earthing (IEC 364-3) as shown in diagram A of figure C7. The TN system is generally associated with a Protective Multiple Earthing (PME) scheme, in which the neutral conductor is earthed at intervals along its length (every 3rd or 4th pole on a LV overhead-line distributor) and at each consumer's service position. It can be seen that the network of neutral conductors radiating from a substation, each of which is earthed at regular intervals, constitutes, together with the substation earthing, a very effective low-resistance earth electrode. The combination of restricted earth-fault currents, equipotential installations and lowresistance substation earthing, results in greatly reduced levels of overvoltage and limited stressing of phase-to-earth insulation during the type of HV earth-fault situation described above. Limitation of the HV earth-fault current and earth resistance of the substation Another widely-used system of earthing is shown in diagram C of figure C7. It will be seen that in the TT system, the consumer's earthing installation (being isolated from that of the substation) constitutes a remote earth. This means that, although the transferred potential will not stress the phase-to-phase insulation of the consumer's equipment, the phase-to-earth insulation of all three phases will be subjected to overvoltage. The strategy in this case, is to: c restrict the value of HV earth-fault currents, as previously discussed, c reduce the resistance of the substation earth electrode, such that the standard value of 5-second withstand-voltage-to-earth for LV equipment and appliances will not be exceeded.

C8 - HV/LV distribution substations

C HV

HV

LV

LV 1

2

2

3

3

N

N

RS

A LV

HV

B cases C and D

LV

1

1

2

2

3

3

N

N

RS

Where Uw = the rated normal-frequency withstand voltage for low-voltage equipment at consumer installations Uo = phase to neutral voltage at consumer's installations Im = maximum value of HV earth-fault current

D HV

LV

cases E and F

LV

1

1

2

2

3

3

N

N

IT(S)

TT(S) RN

Uw - Uo Im

RS

C

RS

Rs i

IT(N)

TT(N)

HV

No particular resistance value for Rs is imposed in these cases.

IT(R)

TN(R)

HV

cases A and B

1

E

RS

RN

Rs i

Uws - U Im

Where Uws = the normal-frequency withstand voltage for low-voltage equipments in the substation (since the exposed conductive parts of these equipments are earthed via Rs) U = phase to neutral voltage at the substation for the TT(s) system, but the phase-tophase voltage for the IT(s) system Im = maximum value of HV earth-fault current

F

In cases E and F the LV protective conductors (bonding exposed conductive parts) in the substation are earthed via the substation earth electrode, and it is therefore the substation LV equipment (only) that could be subjected to overvoltage. Notes: (R) signifies that the HV and LV exposed conductive parts at the substation and those at the consumer's installations, together with the LV neutral point of the transformer, are all earthed via the substation electrode system. (N) signifies that the HV and LV exposed conductive parts at the substation, together with the LV neutral point of the transformer are earthed via the substation electrode system. (S) signifies that the LV neutral point of the transformer is separately earthed outside of the area of influence of the substation earth electrode. Uw and Uws are commonly given the (IEC 644) value 1.5 Uo + 750 V, where Uo is the nominal phase-to-neutral voltage of the LV system concerned. fig. C7: maximum earthing resistance Rs at a HV/LV substation to ensure safety during a short-circuit to earth fault on the high-voltage equipment for different systems of earthing.

HV/LV distribution substations - C9

1. supply of power at high voltage (continued)

C 1.1 power-supply characteristics of high voltage distribution networks (continued) Practical values adopted by one national electrical power-supply authority, on its 20 kV distribution systems, are as follows: c maximum earth-fault current on overheadline distribution systems, or mixed (O/H line and U/G cable) systems, is 300 A, c maximum earth-fault current on underground systems is 1,000 A. The formula required to determine the maximum value of earthing resistance Rs at the substation, to ensure that the LV withstand voltage will not be exceeded, is: Uw - Uo Rs = in ohms Im (see cases C and D in figure C7). Where Uw = the lowest standard value (in volts) of short-term (5 s) withstand voltage for the consumer's installation and appliances = 1.5 Uo + 750 V (IEC 644 (1991)) Uo = phase to neutral voltage (in volts) at the consumer's LV service position Im = maximum earth-fault current on the HV system (in amps). A third form of system earthing referred to as the “IT” system in IEC 364 is commonly used where continuity of supply is essential, e.g. in hospitals, continuous-process manufacturing, etc. The principle depends on taking a supply from an unearthed source, usually a transformer, the secondary winding of which is unearthed, or earthed through a high impedance (u 1,000 ohms). In these cases, an insulation failure to earth in the low-voltage circuits supplied from the secondary windings will result in zero or negligible fault-current flow, which can be allowed to persist until it is convenient to shut-down the affected circuit to carry out repair work. Diagrams B, D and F of figure C7 show IT systems in which resistors (of approximately 1,000 ohms) are included in the neutralearthing lead. If however, these resistors were removed, so that the system is unearthed, the following notes apply. Diagram B. All phase wires and the neutral conductor are “floating” with respect to earth, to which they are “connected” via the (normally very high) insulation resistances and (very small) capacitances between the live conductors and earthed metal (conduits, etc.).

C10 - HV/LV distribution substations

Assuming perfect insulation, all LV phase and neutral conductors will be raised by electrostatic induction to a potential approaching that of the equipotential conductors. In practice, it is more likely, because of the numerous earth-leakage paths of all live conductors in a number of installations acting in parallel, that the system will behave similarly to the case where a neutral earthing resistor is present, i.e. all conductors will be raised to the potential of the substation earth. In these cases, the overvoltage stresses on the LV insulation are small or non-existent. Diagrams D and F. In these cases, the high potential of the substation (S/S) earthing system acts on the isolated LV phase and neutral conductors: c through the capacitance between the LV windings of the transformer and the transformer tank, c through capacitance between the equipotential conductors in the S/S and the cores of LV distribution cables leaving the S/S, c through current leakage paths in the insulation, in each case. At positions outside the area of influence of the S/S earthing, system capacitances exist between the conductors and earth at zero potential (capacitances between cores are irrelevant - all cores being raised to the same potential). The result is essentially a capacitive voltage divider, where each “capacitor” is shunted by (leakage path) resistances. In general, LV cable and installation wiring capacitances to earth are much larger, and the insulation resistances to earth are much smaller than those of the corresponding parameters at the S/S, so that most of the voltage stresses appear at the substation between the transformer tank and the LV winding. The rise in potential at consumers’ installations is not likely therefore to be a problem where the HV earth-fault current level is restricted as previously mentioned. All IT-earthed transformers, whether the neutral point is isolated or earthed through a high impedance, are routinely provided with an overvoltage limiting device which will automatically connect the neutral point directly to earth if an overvoltage condition approaches the insulation-withstand level of the LV system. In addition to the possibilities mentioned above, several other ways in which these overvoltages can occur are described in Clause 3.1.

C This kind of earth-fault is very rare, and when it does occur is quickly detected and cleared by the automatic tripping of a circuit breaker in a properly designed and constructed installation. Safety in situations of elevated potentials depends entirely on the provision of properly arranged equipotential areas, the basis of which is generally in the form of a widemeshed grid of interconnected bare copper conductors connected to vertically-driven copper-clad* steel rods. The equipotential criterion to be respected is that which is mentioned in Chapter G dealing with protection against electric shock by indirect contact, namely: that the potential between any two exposed metal parts which can be touched simultaneously by any parts of the body must never, under any circumstances, exceed 50 V in dry conditions, or 25 V in wet conditions. Special care should be taken at the boundaries of equipotential areas to avoid steep potential gradients on the surface of the ground which give rise to dangerous “step potentials”. This question is closely related to the safe earthing of boundary fences and is further discussed in Sub-clause 3.1 and in Appendix C2. * Copper is cathodic to most other metals and therefore resists corrosion.

1.2 different HV service connections According to the type of high-voltage network, the following supply arrangements are commonly adopted.

single-line service The substation is supplied by a single circuit tee-off from a HV distributor (cable or line). In general, the HV service is connected into a panel containing a load-break/isolating switch with series protective fuses and earthing switches, as shown in figure C8. In some countries a pole-mounted transformer with no HV switchgear or fuses (at the pole) constitutes the “substation”. Up to transformer ratings of 160 kVA this type of HV service is very common in rural areas. Protection and switching devices are remote from the transformer, and generally control a main overhead-line, from which a number of these elementary service lines are tapped.

overhead line

fig. C8: single-line service.

HV/LV distribution substations - C11

1. supply of power at high voltage (continued)

C 1.2 different HV service connections (continued) ring-main principle Ring-main units (RMU) are normally connected to form a HV ring main* or interconnector-distributor*, such that the RMU busbars carry the full ring-main or interconnector current (figure C9). The RMU consists of three compartments, integrated to form a single assembly, viz: c 2 incoming compartments, each containing a load-break/isolating switch and a circuit earthing switch, c 1 outgoing and general protection compartment, containing a load-break switch and HV fuses, or a combined load-break/fuse switch, or a circuit breaker and isolating switch, together with a circuit-earthing switch in each case. All load-break switches and earthing switches are fully rated for short-circuit current-making duty. This arrangement provides the user with a two-source supply, thereby reducing considerably any interruption of service due to system faults or operational manœuvres by the supply authority, etc. The main application for RMUs is in publicsupply HV underground-cable networks in urban areas. * A ring main is a continuous distributor in the form of a closed loop, which originates and terminates on one set of busbars. Each end of the loop is controlled by its own circuit breaker. In order to improve operational flexibility the busbars are often divided into two sections by a normallyclosed bus-section circuit breaker, and each end of the ring is connected to a different section. An interconnector is a continuous untapped feeder connecting the busbars of two substations. Each end of the interconnector is usually controlled by a circuit beaker. An interconnector-distributor is an interconnector which supplies one or more distribution substations along its length.

underground cable ring main

fig. C9: ring-main service.

parallel feeders Where a HV supply connection to two lines or cables originating from the same busbar of a substation is possible, a similar HV switchboard to that of a RMU is commonly used (figure C10). The main operational difference between this arrangement and that of a RMU is that the two incoming panels are mutually interlocked, such that one incoming switch only can be closed at a time, i.e. its closure prevents the closure of the other. On the loss of power supply, the closed incoming switch must be opened and the (formerly open) switch can then be closed. The sequence may be carried out manually or automatically. This type of switchboard is used particularly in networks of high-load density and in rapidly-expanding urban areas supplied by HV underground cable systems.

C12 - HV/LV distribution substations

paralleled underground-cable distributors

fig. C10: duplicated supply service.

C 1.3 some operational aspects of HV distribution networks overhead lines High winds, ice formation, etc., can cause the conductors of overhead lines to touch each other, thereby causing a momentary (i.e. not permanent) short-circuit fault. Insulation failure due to broken ceramic or glass insulators, caused by air-borne debris; careless use of shot-guns, etc., or again, heavily polluted insulator surfaces, can result in a short-circuit to earth. Many of these faults are self-clearing. For example, in dry conditions, broken insulators can very often remain in service undetected, but are likely to flashover to earth (e.g. to a metal supporting structure) during a rainstorm. Moreover, polluted surfaces generally cause a flashover to earth only in damp conditions. The passage of fault current almost invariably takes the form of an electric arc, the intense heat of which dries the current path, and to some extent, re-establishes its insulating properties. In the meantime, protective devices have usually operated to clear the fault, i.e. fuses have blown or a circuit breaker has tripped. Experience has shown that in the large majority of cases, restoration of supply by replacing fuses or by re-closing a circuit breaker will be successful. For this reason it has been possible to considerably improve the continuity of service on HV overhead-line distribution networks by the application of automatic circuit breaker reclosing schemes at the origin of the circuits concerned. These automatic schemes permit a number of reclosing operations if a first attempt fails, with adjustable time delays between successive attempts (to allow de-ionization of the air at the fault) before a final lock-out of the circuit breaker occurs, after all (generally three) attempts fail.

Other improvements in service continuity are achieved by the use of remotely-controlled section switches and by automatic isolating switches which operate in conjunction with an auto-reclosing circuit breaker. This last scheme is exemplified by the final sequence shown in figure C11, where the isolating switch is referred to as IACT* (voltage-drop-operated outdoor switch). The principle is as follows: If, after two reclosing attempts, the circuit breaker trips, the fault is assumed to be permanent, and, while the distributor is dead, the IACT opens to isolate a section of the network, before the third (and final) reclosure takes place. There are then two possibilities: 1) the fault is on the section which is isolated by IACT, and supply is restored to those consumers connected to the remaining section, or 2) the fault is on the section upstream of IACT and the circuit breaker will trip and lock out. The IACT scheme, therefore, provides the possibility of restoration of supplies to some consumers in the event of a permanent fault. While these measures have greatly improved the reliability of supplies from HV overhead line systems, the consumers must, where considered necessary, make their own arrangements to counter the effects of momentary interruptions to supply (between reclosures), for example: c uninterruptible standby emergency power, c lighting that requires no cooling down before re-striking. (See Chapter F section 2) * Interrupteur Aérien à ouverture dans le Creux de Tension (used by EDF, the French supply authority).

1-cycle RR + 1SR O1

If

RR

SR

O2

O3

In 15 to 30s

Io fault

permanent fault 0.3s

0.4s

2-cycle 2SR a-fault on main distributor If

O1

RR

SR2 O4

SR1 O3

O2

In 15 to 30s

15 to 30s

Io fault 0.3s

0.4s

permanent fault 0.45s

0.4s

b-fault on section supplied through IACT O1

If

RR

SR1 O3

O2

In

SR2 15 to 30s

Io

15 to 30s

fault

opening of IACT 0.3s

0.4s

0.4s

O = circuit breaker opening / RR = rapid reclosing / SR = slow reclosing / In = normal load current / If = fault current / I0 = zero current

fig. C11: automatic reclosing cycles of a circuit breaker controlling a radial HV distributor. HV/LV distribution substations - C13

1. supply of power at high voltage (continued)

C 1.3 some operational aspects of HV distribution networks (continued) underground cable networks Faults on underground cable networks are sometimes the result of careless workmanship by cable jointers or by cablelaying contractors, etc., but are more commonly due to damage from tools such as pick-axes, pneumatic drills and trench excavating machines, and so on, used by other utilities. Insulation failures sometimes occur in cableterminating boxes due to overvoltage, particularly at points in a HV system where an overhead line is connected to an underground cable. The overvoltage in such a case is generally of atmospheric origin, and electromagnetic-wave reflection effects at the joint box (where the natural impedance of the circuit changes abruptly) can result in overstressing of the cable-box insulation to the point of failure. Overvoltage protection devices, such as lightning arresters, are frequently installed at these locations. Faults occurring in cable networks are less frequent than those on overhead (O/H) line systems, but are almost invariably permanent faults, which require more time for localization and repair than those on O/H lines. Where a cable fault occurs on a ring main, supplies can be quickly restored to all consumers when the faulty section of cable has been determined. If, however, the fault occurs on a radial distributor, the delay in locating the fault and carrying out repair work can amount to several hours, and will affect all consumers downstream of the fault position. In any case, if supply continuity is essential on all, or part of, an installation, a standby source must be provided. Standby power equipment is described in Chapter F section 2.1.

centralized remote control, based on SCADA (Supervisory Control And Data Acquisition) systems and recent developments in IT (Information Technology) techniques, is becoming more and more common in countries in which the complexity of highlyinterconnected systems justifies the expenditure.

C14 - HV/LV distribution substations

remote control of HV networks Remote control of HV circuit breakers and switchgear, and tapchangers, etc. from a central control room is possible, while similar control facilities are also available from the console of a mobile control centre.

2. consumers HV substations

C Large consumers of electricity are invariably supplied at HV. On LV systems operating at 120/208 V (3-phase 4-wires), a load of 50 kVA might be considered to be “large”, while on a 240/415 V 3-phase system a “large” consumer could have a load in excess of 100 kVA. Both systems of LV distribution are common in many parts of the world. As a matter of interest, the IEC recommends a “world” standard of 230/400 V for 3-phase 4-wire systems. This is a compromise level and will allow existing systems which operate at 220/380 V and at 240/415 V, or close to these values, to comply with the proposed standard simply by adjusting the off-circuit tapping switches of standard distribution transformers. The distance over which the load has to be transmitted is a further factor in considering an HV or LV service. Services to small but isolated rural consumers are obvious examples. The decision of a HV or LV supply will depend on local circumstances and considerations such as those mentioned above, and will generally be imposed by the power-supply authority for the district concerned. When a decision to supply power at HV has been made, there are two widely-followed methods of proceeding.

(1) The power-supplier constructs a standard substation close to the consumer’s premises, but the HV/LV transformer(s) is (are) located in transformer chamber(s) inside the premises, close to the load centre. (2) The consumer constructs and equips his own substation on his own premises, to which the power supplier makes the HV connection. In method (1) the power supplier owns the substation, the cable(s) to the transformer(s), the transformer(s) and the transformer chamber(s), to which he has unrestricted access. The transformer chamber(s) is (are) constructed by the consumer (to plans and regulations provided by the supplier) and include plinths, oil drains, fire walls and ceilings, ventilation, lighting, and earthing systems, all to be approved by the supply authority. The tariff structure will cover an agreed part of the expenditure required to provide the service. Whichever procedure is followed, the same principles apply in the conception and realization of the project. The following notes refer to procedure (2).

2.1 procedures for the establishment of a new substation the consumer must provide certain data to the power-supply organization at the earliest stage of the project.

preliminary information

the power-supply organization must give specific information to the prospective consumer.

project studies

Before any negotiations or discussions can be initiated with the supply authorities, the following basic elements must be established: c maximum anticipated power (kVA) demand Determination of this parameter is described in Chapter B, and must take into account the possibility of future additional load requirements. Factors to evaluate at this stage are: v the utilization factor (ku), v the simultaneity factor (ks), c layout plans and elevations showing location of proposed substation Plans should indicate clearly the means of access to the proposed substation, with

From the information provided by the consumer, the power-supplier must indicate: c the type of power supply proposed and define: v the kind of power-supply system: overheadline or underground-cable network, v service connection details: single-line service, ring-main installation, or parallel feeders, etc., v power (kVA) limit and fault current level.

dimensions of possible restrictions, e.g. entrances corridors and ceiling height, together with possible load (weight) bearing limits, and so on, keeping in mind that: v the power-supply personnel must have free and unrestricted access to the HV equipment in the substation at all times, v only qualified and authorized consumer’s personnel are allowed access to the substation. c degree of supply continuity required The consumer must estimate the consequences of a supply failure in terms of its duration, v loss of production, v safety of personnel and equipment.

c the nominal voltage and rated voltage (Highest voltage for equipment) Existing or future, depending on the development of the system. c metering details which define: v the cost of connection to the power network, v tariff details (consumption and standing charges).

HV/LV distribution substations - C15

2. consumers HV substations (continued)

C 2.1 procedures for the establishment of a new substation (continued) the power-supply organization must give official approval of the equipment to be installed in the substation, and of proposed methods of installation.

implementation

after testing and checking of the installation by an independent test authority, a certificate is granted which permits the substation to be put into service.

commissioning

C16 - HV/LV distribution substations

Before any installation work is started, the official agreement of the power-supplier must be obtained. The request for approval must include the following information, largely based on the preliminary exchanges noted above: c location of the proposed substation, c one-line diagram of power circuits and connections, together with earthing-circuit proposals,

Commissioning tests must be successfully completed before authority is given to energize the installation from the powersupply system. The verification tests include the following: c measurement of earth-electrode resistances, c continuity of all equipotential earth-and safety bonding conductors, c inspection and testing of all HV components, c insulation checks of HV equipment, c dielectric strength test of transformer oil (and switchgear oil if appropriate), c inspection and testing of the LV installation in the substation, c checks on all interlocks (mechanical key and electrical) and on all automatic sequences, c checks on correct protective-relay operation and settings. It is also imperative to check that all equipment is provided, such that any properly executed operational manœuvre can be carried out in complete safety. On receipt of the certificate of conformity: c personnel of the power-supply authority will energize the HV equipment and check for correct operation of the metering, c the installation contractor is responsible for testing and connection of the LV installation.

c full details of electrical equipment to be installed, including performance characteristics, c layout of equipment and provision for metering components, c arrangements for power-factor improvement if eventually required, c arrangements provided for emergency standby power plant (HV or LV) if eventually required.

When finally the substation is operational: c the substation and all equipment belongs to the consumer, c the power-supply authority has operational control over all HV switchgear in the substation, e.g. the two incoming load-break switches and the transformer HV switch (or CB) in the case of a RMU, together with all associated HV earthing switches, c the power-supply personnel has unrestricted access to the HV equipment, c the consumer has independent control of the HV switch (or CB) of the transformer(s) only, c the consumer is responsible for the maintenance of all substation equipment, and must request the power-supply authority to isolate and earth the switchgear to allow maintenance work to proceed. The power supplier must issue a signed permit-to-work to the consumers maintenance personnel, together with keys of locked-off isolators, etc. at which the isolation has been carried out.

3. substation protection schemes

C The subject of protection in the electricalpower industry is vast: it covers all aspects of safety for personnel, and protection against damage or destruction of property, plant, and equipment. These different aspects of protection can be broadly classified according to the following objectives: c protection of personnel and animals against the dangers of overvoltages and electric shock, fire, explosions, and toxic gases, etc., c protection of the plant, equipment and components of a power system against the stresses of short-circuit faults, atmospheric surges (lightning) and power-system instability (loss of synchronism) etc., c protection of personnel and plant from the dangers of incorrect power-system operation, by the use of electrical and mechanical interlocking. All classes of switchgear (including, for example, tap-position selector switches on transformers, and so on...) have well-defined operating limits. This means that the order in which the different kinds of switching device can be safely closed or opened is vitally important. Interlocking keys and analogous electrical control circuits are frequently used to ensure strict compliance with correct operating sequences. It is beyond the scope of a guide to describe in full technical detail the numerous schemes of protection available to power-systems engineers, but it is hoped that the following sections will prove to be useful through a discussion of general principles. While some of the protective devices mentioned are of universal application, descriptions generally will be confined to those in common use on HV and LV systems only, as defined in Sub-clause 1.1 of this Chapter. Where some technical explanation is necessary to simplify an understanding of the text, reference is made to a related Appendix.

3.1 protection against electric shocks and overvoltages protection against electric shocks and overvoltages is closely related to the achievement of efficient (low resistance) earthing and effective application of the principles of equipotential environments.

protection against electric shocks Protective measures against electric shock are based on two common dangers: c contact with an active conductor, i.e. which is alive with respect to earth in normal circumstances. This is referred to as a “direct contact” hazard, c contact with a conductive part of an apparatus which is normally dead, but which has become alive due to insulation failure in the apparatus. This is referred to as an “indirect contact” hazard. It may be noted that a third type of shock hazard can exist in the proximity of HV or LV (or mixed) earth electrodes which are passing earth-fault currents. This hazard is due to potential gradients on the surface of the ground and is referred to as a “step-voltage” hazard; shock current enters one foot and leaves by the other foot, and is particular dangerous for four-legged animals. A variation of this danger, known as a “touch voltage” hazard can occur, for instance, where an earthed metal fence is situated in an area in which potential gradients exist. Touching the fence would cause shock current to pass through the hand and both feet (Appendix C2).

Animals with a relatively long front-to-hind legs span are particularly sensitive to stepvoltage hazards and cattle have been killed by the potential gradients caused by a low voltage (240/415 V) neutral earth electrode of insufficiently low resistance. Potential gradients on the surface of the ground can be reduced to safe values by measures such as those shown in Appendix C2. Potential-gradient problems of the kind mentioned above are not normally encountered in electrical installations of buildings, providing that equipotential conductors properly bond all exposed metal parts of equipment and all extraneous metal (i.e. not part of an electrical apparatus or the installation - for example structural steelwork, etc.) to the protective-earthing conductor.

HV/LV distribution substations - C17

3. substation protection schemes (continued)

C 3.1 protection against electric shocks and overvoltages (continued) Direct-contact protection The main form of protection against direct contact hazards is to contain all live parts in housings of insulating material, by placing out of reach (behind insulated barriers or at the top of poles) or by means of obstacles. Where insulated live parts are housed in a metal envelope, for example transformers, electric motors and many domestic appliances, the metal envelope is connected to the installation protective earthing system. For LV appliances this is achieved through the third pin of a 3-pin plug and socket. Total or even partial failure of insulation to the metal, can (depending on the ratio of the resistance of the leakage path through the insulation, to the resistance from the metal envelope to earth) raise the voltage of the envelope to a dangerous level.

in the case of a HV fault to a metallic enclosure, it may not be possible to limit the touch voltage to the safe value of 50 V*. The solution is to create an equipotent-situation as described in Sub-clause 1.1 "Earthing connections".

Indirect-contact protection A person touching the metal envelope of an apparatus of which the insulation is faulty, as described above, is said to be making an indirect contact. An indirect contact is characterized by the fact that a current path to earth exists (through the protective earthing (PE) conductor) in parallel with the shock current through the person concerned. c case of fault on L.V. system. Extensive tests have shown that, providing the potential of the metal envelope is not greater than 50 V* with respect to earth, or to any conductive material within reaching distance, no danger exists. c indirect-contact hazard in the case of a HV fault. If the insulation failure in an apparatus is between a HV conductor and the metal envelope, it is not generally possible to limit the rise of voltage of the envelope to 50 V or less, simply by reducing the earthing resistance to a low value. The solution in this case is to create an equipotential situation, as described in Sub-clause 1.1 “Earthing connections”. * in dry locations, 25 V in wet locations (bathrooms, etc.).

C18 - HV/LV distribution substations

protection against overvoltages The situation mentioned immediately above, describing an indirect-contact hazard resulting from faulty HV insulation, is one of a number of ways in which an abnormal overvoltage condition can occur. Methods of eliminating danger to personnel in such a case are described in Sub-clause 1.1. Other situations which can cause overvoltages to occur on HV and LV systems include: c surges of atmospheric origin, c a short-circuit earth fault on an unearthed (or high-impedance earthed) 3-phase system, c ferro-resonance, c energization of capacitor banks, c circuit breaker opening or fuse melting to break short-circuit current. Overvoltages created by the causes listed above can be divided according to characteristies such as: c duration: permanent, temporary, transient, c frequency: industrial frequency, harmonics of industrial frequency, high frequency, unidirectional surges. Overvoltages of atmospheric origin Protection against this kind of danger must be provided when a substation is supplied directly from an overhead-line system. The most common protective device used at present is a non-linear resistor-type of lightning arrester, which is connected (one for each phase) between a phase conductor and the substation earthing system, as close to the point of entry into the substation as possible. For consumers' substations, this protection is achieved by: c lightning arresters (one per phase conductor, which are sometimes connected in series with a device for automatic tripping of a circuit breaker) (see Chapter L) and/or by c the reduction of the substation-earthing resistance to the lowest possible value to avoid (as far as possible) a breakdown of LV insulation due to the rise in potential of the earthing system when discharging the surge current. Where it is advisable to protect a substation against direct strokes, lightning-discharge electrodes (Franklin type) and shield wires should be installed and connected to the substation earthing system. It may be noted that, at the voltage levels being considered (i 35 kV), switching surges are generally less severe than lightning surges, and so devices which are suitable for satisfactory lightning protection are adequate to protect against overvoltages due to switching surges.

C Earth faults on IT-earthed systems In normal conditions the phase conductors of a 3-phase IT system are all approximately at phase volts with respect to earth. The exact values depend on the capacitance and insulation resistance of each conductor to earth. On an unfaulted system these parameters are sensibly equal in all three phases so that the vector relationship of phase voltages will be as shown below in figure C12, and the neutral point of the transformer secondary winding will be at approximately zero volts with respect to earth. A short-circuit to earth on one phase will change the values of phase conductor voltages with respect to earth; the phase-tophase voltage values and their phase displacement relationships however, will remain unchanged. This latter feature,

together with the fact that current passing through the earth-fault path will be too small to constitute a hazard, are the reasons for adopting the IT system, where supply continuity must be maintained even in “firstfault” conditions. With one phase short-circuited to earth, and the neutral point of the transformer isolated: c the neutral point will rise to phase volts above earth, c the faulty phase conductor will be at zero volts with respect to earth, c the other two phases will rise to etimes the phase voltage, with respect to earth. As indicated above, this is a 50 Hz (or 60 Hz) stable (i.e. permanent) condition, and transformers, cabling and all appliances must be suitably insulated with respect to earth, when used on IT systems.

unearthed secondary winding of power transformer 1 insulation resistance

conductor capacitance

N

2

3

fault current normally restricted to several milli-amps depending on the size of the installation

V1

V1 3 I(C+R)

I(C+R)1

I(C+R)2

√3 I(C+R)1

√3 I(C+R)2 VNE

300 > 300 > 300 -

minimum calorific power (MJ/kg) 48 34 - 37 27 - 28 12

table C31: categories of dielectric fluids. National standards exist which define the conditions for the installation of liquid-filled transformers. No equivalent IEC standard has yet been established. The national standard is aimed at ensuring the safety of persons and property and recommends, notably, the minimum measures to be taken against the risk of fire. The main precautions to observe are indicated in Table C32. c for liquid dielectrics of class L3 there are no special measures to be taken, c for dielectrics of classes 01 and K1 the measures indicated are applicable only if there are more than 25 litres of dielectric liquid in the transformer, c for dielectrics of classes K2 and K3 the measures indicated are applicable only if there are more than 50 litres of dielectric liquid in the transformer.

HV/LV distribution substations - C41

4. the consumer substation with LV metering (continued)

C 4.4 choice of HV/LV transformer (continued) class of dielectric fluid

O1 K1

K2 K3 L3

no. of litres above which measures must be taken 25

locations chamber or enclosed area reserved to qualified and authorized personnel, and separated from any other building by a distance D D>8m 4m XC. In these calculations XC = -j 1 pu impedance, and XL = j 10 pu. 1 90° I1 = = 0.1 0° = 0.1 + j 0 10 90° 1 -30° I2 = = 0.1 -120° = -0.05 - j 0.0866 10 90° 1 210° I3 = = 1 300° = 0.5 - j 0.866 1 -90° IN = 0.55 - j 0.953 = 1.1 -60°

calculation of ZNE ZNE is the parallel combination of XL1, XL2 and Xc. XL1 in parallel with XL2 = j 5 j 5 in parallel with XC3 = j 5 x (-j 1) 5 = = -j 1.25 j5-j1 j4 ZNE = - j 1.25 pu ohms.

calculation of VNE

source

1.1 -60° x 1.25 -90° = 1.375 -150° or 1.375 pu 210° V1

complete vector diagram

not to scale

N V3 VNE

I2

Other values shown in the vector diagram are easily obtained from the above calculations. ➀, ≠ and ➂ are the power-supply terminals. I1

1 VNE = 1.375 pu

V2

I1 + I2

2.066 pu IC3 N

I1 + I2 + I3 = IN

fig. AC3-1: calculation of VNE - circuit and vector diagrams. The procedure is as follows: c compute the current IN in an imaginary neutral of negligible impedance (i.e. ZN = 0) by summating the individual phase currents, as shown in figure AC3-1. According to Thévenin's theorem IN also equals VNE where ZNE + Z VNE is the voltage between N and E when no neutral connection exists, ZNE is the impedance of the network measured between terminals N and E when no neutral connection exists, ZN is the impedance of the neutral conductor (zero in the present case). This means that VNE = IN ZNE

V 3 E = 0.375 pu

1

not to scale 1 2

3 E

2.066 pu IL1

IL2 IL1 + IL2 = 0.375 pu

fig. AC3-2: vector diagram for the resonant condition.

Appendix C3 - 1

1. domestic and similar premises

L Electrical installations for inhabited premises need a high standard of safety and reliability.

1.1 general related standards Most countries have national regulations andor standards governing the rules to be strictly observed in the design and realization of electrical installations for domestic and similar premises. The relevant international standard is the IEC publication 364.

the power distribution authority connects the LV neutral point of its HV/LV distribution transformer to earth. All LV installations must be protected therefore by RCDs (for TT and IT schemes of earthing) or by shortcircuit protective devices for TN schemes. All exposed conductive parts must be bonded together and connected to earth, either directly to an electrode at the premises (TT or IT schemes) or by means of the neutral conductor (TN schemes)*.

the power network The vast majority of power-distribution authorities connect the low-voltage neutral point of their HV/LV distribution transformers to earth. The protection of persons against electric shock therefore depends, in such cases, on the principles discussed in Chapter F, Clause 4, and Chapter G, all Clauses. The measures required depend on whether the TT, TN or IT scheme of earthing is adopted, as treated in detail in Chapter G. RCDs are essential for TT- and IT-earthed installations, but high-speed overcurrent devices (MCBs or fuses) are commonly used to clear earth faults on TN-earthed schemes. However, in particular instances (e.g. circuits feeding socket-outlets), RCDs are strongly recommended on TN installations, as being the only sure means of protection against shock when very long flexible leads of small c.s.a. are supplied from a socket. See also Clause 3 concerning special installations. * for TN-C and TN-S schemes refer to Chapter G, Sub-clause 5.1.

domestic and similar premises and special locations - L1

1. domestic and similar premises (continued)

L 1.2 distribution-board components

control and distribution board

enclosure

incoming-supply circuit breaker service connection lightning arrester lightning protection

overcurrent protection and isolation

cartridge fuses

MCB phase + neutral

protection against direct and indirect contact, and protection against fire

differential MCB

differential load switch

remote control bistable switch

THP

CDSc

energy management

IHPc fig. L1-1: presentation of realizable functions on a consumer unit.

L2 - domestic and similar premises and special locations

Isis

L the quality of electrical equipment used in inhabited premises is commonly ensured by a mark of conformity situated on the front of each item.

Distribution boards (generally one only in domestic premises) usually include the meter(s) and in some cases (notably where the supply authorities impose a TT-earthing system and/or tariff conditions which limit the maximum permitted current consumption) an incoming-supply differential circuit breaker, which includes an overcurrent trip. This circuit breaker is freely accessible to the consumer. On installations which are TN-earthed, the supply authorities usually protect the installation simply by means of sealed fuse cut-outs immediately upstream of the meter(s). See figure L1-2. The consumer has no access to these fuses.

meter

fuses or circuit breaker depending on earthing system

tableau de répartition

fig. L1-2: components of a control and distribution board.

the incoming-supply circuit breaker The consumer is permitted to operate this CB if necessary (e.g. to reclose it if the current consumption had exceeded the authorized limit; to open it in case of emergency or for isolation purposes...). The differential trip generally has a 500 mA setting to provide indirect-contact protection (and a measure of fire protection) for the whole installation. Current ratings for these circuit breakers are commonly: c 15 - 90 A two-poles, c 10 - 60 A four-poles.

fig. L1-3: incoming-supply circuit breaker.

the control and distribution board (consumer unit) This board comprises: c a control panel for mounting (where appropriate) the incoming-supply circuit breaker and other control auxiliaries, as required, c a distribution panel for housing 1, 2 or 3 rows (of 24 multi 9 units) or similar MCBs or fuse units, etc., c installation accessories for fixing conductors, and rails for mounting MCBs, fuse bases, etc. neutral busbar and earthing bar, and so on..., c service-cable ducts or conduits, surface mounted or in cable chases embedded in the wall. Note: to facilitate future modifications to the installation, it is recommended to keep all relevant documents (photos, diagrams, characteristics, etc.) in a suitable location close to the distribution board. The board should be installed at a height such that the operating handles, indicating dials (of meters) etc. are between 1 metre and 1.80 metres from the floor (1.30 metres in situations where handicapped persons or persons of advanced age are concerned).

fig. L1-4: control and distribution board.

lightning arresters Where the keraunic level of a locality exceeds 25, and the supply is taken from an overhead line, the installation of a lightning arrester at the service position of a LV installation is prescribed in many national standards and is strongly recommended for installations which include sensitive (e.g. electronic) equipment. These devices must automatically disconnect themselves from the installation in case of failure or be protected by a RCD of

appropriate sensitivity, according to the resistance of the earthing electrode for the installation. In the case of domestic installations the use of a 500 mA differential incoming-supply circuit breaker type S (i.e. slightly time-delayed) will provide effective earth-leakage protection, while, at the same time, will not trip unnecessarily each time a lightning arrester discharges the current (of an overvoltage-surge) to earth.

domestic and similar premises and special locations - L3

1. domestic and similar premises (continued)

L 1.2 distribution-board components (continued) if, in TT schemes, the resistance of the installation earth electrode exceeds 100 ohms, then one or more 30 mA RCDs must be installed to take over the function of the differential device in the incoming-supply circuit breaker.

resistance value of the earth electrode (of a TT scheme) In the case where the resistance-to-earth is very high and exceeds the value Rt = 50 V = 100 ohms 500 mA one or several RCDs of appropriate sensitivity, i.e. 30 mA, should be used in place of the differential device of the incoming-supply circuit breaker.

1.3. protection of persons where public power-supply systems and consumers' installations form a TT-earthed scheme, the governing standards impose the use of RCDs to ensure the protection of persons.

On TT-earthed systems the protection of persons is ensured by the following measures: c protection against indirect-contact hazards by RCDs of medium sensitivity (300 or 500 mA) at the origin of the installation (incorporated in the incoming-supply circuit breaker, or on the incoming feed to the distribution board). This measure is associated with a consumer-installed earth electrode to which must be connected the protective-earth (PE) conductors from the exposed conductive parts of all class I insulated appliances and equipment, as well as those from the earthing pins of all socket outlets.

c where the CB at the origin of an installation has no RCD protection (see figure L1-7) the protection of persons shall be ensured by class II level of insulation on all circuits upstream of the first RCDs. In the case where the distribution board is metallic, care shall be taken that all live parts are double insulated (supplementary clearances or insulation, use of covers, etc.) and wiring reliably fixed, c obligatory protection by sensitive (30 mA) RCDs of socket-outlet circuits, and circuits feeding bathrooms, laundry rooms, and so on (for details of this latter obligation, refer to the table in Clause 3 of this Chapter).

incoming-supply circuit breaker with instantaneous differential relay In this case: c an insulation fault to earth could result in the shutdown of the entire installation, c where a lightning arrester is installed, its operation (i.e. discharging a voltage surge to earth) could appear to an RCD as an earth fault, with a consequent shutdown of the installation. Recommendation of suitable Merlin Gerin components c incoming-supply circuit breaker with 500 mA differential, and c RCD of type DDR-HS 30 mA (for example, differential circuit breaker 1P + N type Déclic Vigi) on the circuits supplying socket-outlets, c RCD of type DDR-HS 30 mA (for example, differential load switch type ID’clic) on circuits to bathrooms, shower rooms, laundry rooms, etc.) (lighting, heating, socket-outlets).

L4 - domestic and similar premises and special locations

DB 500 mA

30 mA

diverse circuits

socket-outlets circuit

30 mA

bathroom and/or shower room

fig. L1-5: installation with incomingsupply circuit breaker having instantaneous differential protection.

L incoming-supply circuit breaker type S with retarded differential relay This type of CB affords protection against insulation faults to earth, but by virtue of a short time delay, provides a measure of discrimination with downstream instantaneous RCDs. Tripping of the incoming-supply CB and its consequences (on deep freezers, for example) is thereby made less probable in the event of lightning, or other causes of voltage surges. The discharge of voltage-surge current to earth, through the lightning arrester, will leave the type S circuit breaker unaffected. Recommendation of suitable Merlin Gerin components c incoming-supply circuit breaker with 500 mA differential, type S, and c RCD of type DDR-HS 30 mA (for example, differential circuit breaker 1P + N type DéclicVigi) on the circuits supplying socket-outlets, c RCD of type DDR-HS 30 mA (for example, differential load switch, type ID’clic) on circuits to bathrooms, shower rooms, etc. (lighting, heating, socket-outlets), c RCD of type DDR-HS 30 mA (for example, differential circuit breaker 1P + N, type DéclicVigi) on circuits supplying washing machines and dish-washing machines.

DB type S 500 mA

30 mA

30 mA

diverse high-risk location socketcircuits (laundry room) outlets circuit

30 mA bathroom and/or shower room

fig. L1-6: installation with incomingsupply circuit breaker having short time delay differential protection, type S.

incoming-supply circuit breaker without differential protection In this case the protection of persons must be ensured by: c class II level of insulation up to the downstream terminals of the RCDs, c all outgoing circuits from the distribution board must be protected by 30 mA or 300 mA RCDs according to the type of circuit concerned as discussed in Chapter G, Clause 4, c where a voltage-surge arrester is installed upstream of the distribution board (to protect sensitive electronic equipment such as microprocessors, video-cassette recorders, TV sets, electronic cash registers, etc.) it is imperative that the device automatically disconnects itself from the installation following a rare (but always possible) failure. Some devices employ replaceable fusing elements; the recommended method however as shown in figure L1-7, is to use a RCD. Recommendation of suitable Merlin Gerin components Figure L1-7 refers. 1. Incoming-supply circuit breaker without differential protection. 2. Automatic disconnection device (if a lightning arrester is installed). 3. RCD of type DDR-HS 30 mA (for example, differential circuit breaker 1P + N type Déclic Vigi) on each circuit supplying one or more socket-outlets. 4. RCD of type DDR-HS 30 mA (for example, differential load switch type ID’clic) on circuits to bathrooms and shower rooms (lighting, heating and socket-outlets) or a 30 mA differential circuit breaker per circuit. 5. RCD of type DDR-HS 300 mA (for example, differential load switch) on all the other circuits.

6. Safety and tripping discrimination are improved by the protection of circuits by means of 30 mA RCDs (for example, differential circuit breaker type Déclic-Vigi 1P + N) on a circuit supplying an apparatus which involves large quantities of water.

1

2

class II insulation

5

6

300 mA

diverse circuits

30 mA

3

30 mA

4

30 mA

high-risk circuit socket- bathroom (dish-washing outlets and/or machine) circuit shower room

fig. L1-7: installation with incomingsupply circuit breaker having no differential protection.

domestic and similar premises and special locations - L5

1. domestic and similar premises (continued)

L 1.4 circuits the distribution and division of circuits provides comfort, and facilitates rapid location of faults.

subdivision National standards commonly recommend the sub-division of circuits according to the number of utilization categories in the installation concerned (see figure L1-8): c at least 1 circuit for lighting. Each circuit supplying a maximum of 8 lighting points, c at least 1 circuit for socket-outlets rated 10/16 A. Each circuit supplying a maximum of 8 sockets. The sockets may be single or double units (a double unit is made up of two 10/16 A sockets mounted on a common base in an embedded box, identical to that of a single unit), c 1 circuit for each appliance such as a water heater, washing machine, dish-washing machine, cooker, refrigerator, etc. Recommended numbers of 10/16 A (or similar) socket-outlets and fixed lighting points, according to the use for which the various rooms of a dwelling are intended, are indicated in the following table. room function living room bedroom, lounge, bureau, dining room kitchen bathroom, shower room entrance hall, box room WC, storage space laundry room

socketoutlets

lighting heating washing cooking machine apparatus

fig. L1-8: circuit division according to utilization.

minimum number of fixed lighting points 1 1

minimum number of 10/16 A socket-outlet 5 3

2 2 1 1 -

4 (1) 1 or 2 1 1

table L1-9: recommended minimum number of lighting and power points in domestic premises. (1) of which 2 above the working surface and 1 for a specialized circuit: in addition an independent socket-outlet of 16 A or 20 A for a cooker and a junction box or socket-outlet for a 32 A specialized circuit.

L6 - domestic and similar premises and special locations

L the inclusion of a protective conductor in all circuits is required by IEC and most national standards.

protective conductors IEC and most national standards require that each circuit includes a protective conductor. This practice is strongly recommended where class I insulated appliances and equipment are installed, which is the general case. The protective conductors must connect the earthing-pin contact in each socket-outlet, and the earthing terminals in class I equipment, to the main earthing terminal at the origin of the installation. Furthermore, 10/16 A (or similarly sized) socket-outlets must be provided with shuttered contact orifices.^

cross-sectional-area (c.s.a.) of conductors The c.s.a. of conductors and the rated current of the associated protective device depend on the current magnitude of the circuit, the ambient temperature, the kind of installation, and the influence of neighbouring circuits (refer to Chapter H1). Moreover, the conductors for the phase wires, the neutral and the protective conductors of a given circuit must all be of equal c.s.a. (assuming the same material for the conductors concerned, i.e. all copper or all aluminium). Table L1-11 indicates the c.s.a. required for commonly-used appliances. Protective devices 1 phase + N in 2 x 9 mm spaces comply with requirements for isolation, and for marking of circuit current rating and conductor sizes.

fig. L1-10: circuit breaker 1 phase + N 2 x 9 mm spaces Déclic 32.

type of circuit single-phase 230 V 1 ph + N or 1 ph + N + E fixed lighting

c.s.a. of the conductors 1.5 mm2 (2.5 mm2)

maximum power

protective device

2300 W

circuit breaker fuse

16 A 10 A

10/16 A socket-outlets

2.5 mm2 (4 mm2)

4600 W

circuit breaker fuse

25 A 20 A

2.5 mm2 (4 mm2)

4600 W

circuit breaker fuse

25 A 20 A

dish-washing machine

2.5 mm2 (4 mm2)

4600 W

circuit breaker fuse

25 A 20 A

clothes-washing machine

2.5 mm2 (4 mm2)

4600W

circuit breaker fuse

25 A 20 A

cooker or hot plates (1)

6 mm2 (10 mm2)

7300 W

circuit breaker fuse

40 A 32 A

electric space heater

1.5 mm2 (2.5 mm2)

2300 W

circuit breaker fuse

16 A 10 A

individual-load circuits water heater

table L1-11: c.s.a. of conductors and current rating of the protective devices in domestic installations (the c.s.a. of aluminium conductors are shown in brackets). (1) in a 230/400 V 3-phase circuit, the c.s.a. is 4 mm2 for copper or 6 mm2 for aluminium, and protection is provided by a 32 A circuit breaker or by 25 A fuses.

domestic and similar premises and special locations - L7

1. domestic and similar premises (continued)

L 1.5 protection against overvoltages and lightning the relevance of protective devices c disturbances Three types of disturbance often occur on electrical-power networks: v lightning and atmospheric electrical phenomena in general, with its direct and indirect consequences. The direct effects, which are fairly infrequent, concern its impact on overhead transmission and distribution lines. The indirect effects are more common and occur at lower energy levels. Such indirect phenomena are characterized by a powerful induction effect on the lines and/or by an increase of local earth potential, v operational overvoltages are transient, and are caused by abrupt changes in the circuit, such as the opening/closing of circuit breakers, load-break switches, contactors, etc., v overvoltages at normal system frequency can occur in many ways, and will do so, for example, if a neutral connection is broken on a 3-phase system, if the load is unbalanced, c the kind of installation to be protected. It is necessary to know in some detail the characteristics of the items to be protected, in order to select the most appropriate form of protection. The choice of protective device(s) depends on two factors: v the sensitivity: the ability of the equipment concerned to withstand an overvoltage condition i.e. its magnitude and duration, v the cost: which comprises the purchase price and the operational costs (possible losses, maintenance, etc.).

choice of a lightning arrester It depends on: c the level of the disturbance, c the cost, as noted above, c the connection to a LV electrical-power network, a telephone system, a buildingcontrol system bus, or to any other network, c the kind of installation-earthing scheme (see chapter F) (figure L1-12).

L8 - domestic and similar premises and special locations

L installation rules Three principal rules must be respected: c it is imperative that the three lengths of cable used for the installation of the arrester (see figure L1-13) each be less than 50 cm, i.e.: v the live conductors connected to the isolating switch, v from the isolating switch to the lightning arrester, v from the lightning arrester to the main distribution board (MDB) earth bar (not to be confused with the main protective-earth (PE) conductor or the main earth terminal for the installation). The MDB earth bar must evidently be located in the same cabinet as the lightning arrester, c it is necessary to use an isolating switch of a type recommended by the manufacturer of the lightning arrester c in the interests of a good continuity of supply it is recommended that the circuit breaker be of the time-delayed or selective type. The guide "Lightning protection" treats these rules quantitively and includes information which allows the user to dimension an arrester according to his needs.

domestic and similar premises and special locations - L9

2. bathrooms and showers

L Bathrooms and shower rooms are areas of high risk, because of the very low resistance of the human body when wet or immersed in water. Precautions to be taken are therefore correspondingly rigorous, and the regulations are more severe than those for most other locations. The relevant IEC standards are 364-7-701, 479 and 669-1. Precautions to observe are based on three aspects: c the definition of zones, numbered 0, 1, 2 and 3 in which the placement (or exclusion) of any electrical device is strictly limited or forbidden, and, where permitted, the electrical and mechanical protection is prescribed, c the establishment of an equipotential bond between all exposed and extraneous metal parts in the zones concerned, c the strict adherence to the requirements prescribed for each particular zone, as tabled in Clause L3.

2.1 classification of zones Sub-clause 701.32 of IEC 364-7-701 defines the zones 0, 1, 2 and 3, as shown in the following diagrams: plan views

zone 1 * zone 2

zone 1 * zone 2

zone 3

zone 0 0.60 m

zone 0 2.40 m

2.40 m 0.60 m

vertical cross-section zone 1

zone 2

zone 3

2.25 m zone 1

zone 0

zone 1

0.60 m

2.40 m

* zone 1 is above the bath as shown in the vertical cross-section.

fig. L2-1: zones 0, 1, 2, 3 in proximity to a bath-tub.

L10 - domestic and similar premises and special locations

zone 3

L zone 0 zone 1

zone 2

zone 3

0.60 m

2.40 m

zone 0 zone 1

zone 2

zone 3 2.40 m

0.60 m

zone 1

zone 2

zone 3

2.25 m zone 1 zone 0 0.60 m

2.40 m

fig. L2-2: zones 0, 1, 2, 3 in proximity of a shower with basin. fixed shower head (1)

fixed shower head (1)

0.60 m zone 1 0.60 m

0.60 m zone 1 0.60 m

zone 2

zone 2 2.40 m

2.40 m zone 3

zone 1

zone 3

zone 2

zone 3

2.25 m 0.60 m

permanent wall

fig. L2-3: zones 0, 1, 2, 3 in proximity of a shower without basin. (1) When the shower head is at the end of a flexible tube, the vertical central axis of a zone passes through the fixed end of the flexible tube.

0.60 m prefabricated shower cabinet 0.60 m

fig. L2-4: no switch or socket-outlet is permitted within 60 cm of the door opening of a shower cabinet.

domestic and similar premises and special locations - L11

2. bathrooms and showers (continued)

L 2.1 classification of zones (continued) classes of external influences

classes of external influences zone 3

AD 3 BB 2 BC 3

AD 3 BB 2 BC 3

dressing cubicles (zone 2)

AD 3 BB 3 BC 3 AD 7 BB 3 BC 3

AD 3 WC BB 2 BC 3 shower cabinets (zone 1)

fig. L2-5: individual showers with dressing cubicles. classes of external influences h