Grounding Fundamentals Course Presentation

Grounding Fundamentals Instructor: Allan Bozek, P.Eng. www EngWorks ca www.EngWorks.ca

1. 1 5 EICCEUs

Introduction • •

•

• •

•

Introductions Please introduce yourself – name, job title and experience Sign-in sheet circulated, everyone please sign in and return Emergency response requirements Please turn off all cell phones or turn to silent mode Washrooms and Breaks

www.EngWorks.ca

Grounding Fundamentals

2

Safety Topic Static Electricityy and Refuelling g

www.EngWorks.ca

Grounding Fundamentals

3

Safety Topic Static Electricityy and Refuelling g 

Some statistics  Petroleum Equipment Institute reports 175 fires since 1992  50% of the accidents occurred when the refueler returned to their vehicle  Women account for 75% of all static ignition fires



Safety Guidelines when refueling    



Turn off engine D 't smoke Don't k Never re-enter your vehicle while refueling. Do not overfill or top off your tank

If a fire starts  Do not remove the nozzle from the vehicle or try to stop the flow of g gasoline. Immediately y leave the area and call for help

www.EngWorks.ca

Grounding Fundamentals

4

Learning Objectives 1. To understand why we ground 2 To describe the difference between grounding and 2. bonding 3. To apply the safety requirements as defined by the Canadian Electrical Code and the IEEE as they relate to grounding 4. To select the appropriate systems grounding scheme for an industrial facility  

Sizing of components How it impacts the overall design of a facility

www.EngWorks.ca

Grounding Fundamentals

5

Learning Objectives 5. To implement a static electricity control and lightning g gp protection system y 6. To avoid the problems typically associated with the grounding g g of sensitive electronic systems y  

Ground loops Methods of noise mitigation

7. To design a ground grid for a high voltage industrial substation 

Concept of ground potential rise and touch and step potential

www.EngWorks.ca

Grounding Fundamentals

6

Agenda Overview 1. History of Grounding 2 System grounding 2.  Generator and UPS systems grounding

3. 3 4. 5 5. 6.

Equipment bonding Static Protection Lightning Protection Grounding of Electronic and Instrumentation Systems 7. Station Ground Grid Systems Design 8 Tutorial 8. T t i l www.EngWorks.ca

Grounding Fundamentals

7

Introduction Section 1

Edison's Pearl Street Generation Station 

www.EngWorks.ca

Pearl Street Generation station was initially constructed in 1882 to provide DC current for lighting systems in New York's financial district

Grounding Fundamentals

9

Edison’s Floating Approach to DC Systems y 



Original design used an earth ground for DC lighting systems Several incidents associated with “stray currents” forced Edison to revise his plan currents  One dead horse  Workers W k nearby b the th generating ti station t ti could ld ffeell the current Believed the there was a “devil devil in the wire wire”

www.EngWorks.ca

Grounding Fundamentals

10

Edison’s Floating Approach to DC Systems y Current Flow + G Gen -

Intended Return Path

L

L

Unintended Return Path

www.EngWorks.ca

Grounding Fundamentals

11

Human Sensitivity to Electricity 

Physiological y g Reaction to Electric Current  Range from minor muscular contraction to ventricular fibrillation  Function of body y weight g  Current magnitude  Current duration

H Human b body d can b be considered id d a 1000Ω resistor i t www.EngWorks.ca

Grounding Fundamentals

12

Human Sensitivity to Electricity Human Response

Direct Current (ma)

Alternating g Current (ma)

Men

Women

Men

Women

1

0.6

0.4

0.3

“Let Go” Threshold

6.2

3.5

1.1

0.7

Shock – Not Painful

9

6

1.8

1.2

Painful Shock – Muscular Control Loss

62

41

9

6

Severe Shock – Breathing Difficult

90

60

23

15

Slight Sensation on Hand

www.EngWorks.ca

Grounding Fundamentals

13

Edison's’ Isolated 3 Wire System 

Edison later adopted a 3wire system y that did not rely on a earth path for return

Positive

+ 100V

G1

 Allowed two circuits to be run with three wires +  Circuit was isolated from ground 100V G2  All currents within the circuit could be measured and accounted for

www.EngWorks.ca

Grounding Fundamentals

L

L Neutral

L

200V

L

Negative

14

Shock Current Path 



A shock current path requires q two p points

Single point of contact

 One point for the current to enter and the second to exit  Voltage difference is required for current to flo flow An isolated system under normal operating conditions insures a single point of contact

www.EngWorks.ca

L

G1 N Neutral l

Isolated Ground S t System No Shock Current Exists

Grounding Fundamentals

15

Shock Current Path 

Under fault conditions, an isolated system ground creates a shock hazard

Single point of contact t t

Accidental A id t l Ground

L

G1 Neutral

Alternate circuit path leads to shock hazard

www.EngWorks.ca

Grounding Fundamentals

16

Ground Fault Detection An isolated system cannot detect the presence of a ground fault

Circuit protection cannot detect the accidental ground

Accidental Ground

Fuse



L

G1 Neutral

www.EngWorks.ca

Grounding Fundamentals

17

System Overvoltage and Surges 

An isolated system cannot dissipate a high voltage surge  Usually results in equipment damage

Equipment insulation is stressed as the high voltage surge finds its way to ground

www.EngWorks.ca

Fuse

Lightning Strike

L

G1 Neutral

Grounding Fundamentals

18

The Intentional Grounding of Circuits 

Elihu Thompson  Founder of Thompson-Houston Industries  Later merged with Edison General and became General Electric

 Author of over 700 patents 

Advocated AC systems should be intentionally earthed  Proposed as a safeguard against a breakdown in insulation of a primary circuit conductor  Proposal created a large amount of controversyy

www.EngWorks.ca

Grounding Fundamentals

19

Grounding Premise



An intentionally grounded circuit provides a circuit path back to the source in the event of an p accidental ground Allows the circuit p protective devices to function preventing the circuit from becoming a safety hazard

Low Impedance path to source allows fuse to operate

www.EngWorks.ca

Accidental Ground

Fuse



L

G1 Neutral

Grounding Fundamentals

20

History of Grounding 

Practice of earthing the secondary (neutral) conductor was banned by the New York Board of Fire Underwriters  Speculation p that Thomas Edison was behind the scenes with his patented 3 wire un-grounded circuit



AIEE (Precursor to the IEEE) recommended that low voltage AC systems be grounded where a reliable ground connection could be secured  Advocated a solid connection without a fuse on the neutral t l wire i

www.EngWorks.ca

Grounding Fundamentals

21

History of Grounding 

NFPA later resolved that grounding the secondary circuit was the only way of absolutely insuring the safety f t off the th circuit i it  The debate continued from 1903 – 1913 when it was passed into law  Secondaries of all circuits 550V or less must be grounded  Recommended that all circuits 300V or less be grounded



Original rule has not been changed in substance since the original 1913 rule in the NEC  Section 10 of the CEC Part 1 also adheres to the fundamental premise of the rule

www.EngWorks.ca

Grounding Fundamentals

22

Canadian Electrical Code - Part 1 CSA C22.1-06 

Minimum safety standards for installation and maintenance of electrical equipment  Compliance will ensure a safe installation



Section 10 deals specifically with grounding and bonding  Significant re-write in 2006  Minor updates p in 2009

www.EngWorks.ca

Grounding Fundamentals

23

Canadian Electrical Code - Part 1 CSA C22.1-06 

Scope and object: Rules 10-000 and Rule 10-002  Protect life from the danger g of shock  Limit the voltage on a circuit  Facilitate operation of protective devices



System and circuit grounding: Rules 10-100 to 10-116  All circuits must be grounded with the exception of: Electric Arc furnaces Cranes installed in Class III locations Isolated systems in patient care areas Circuits less than 50V

www.EngWorks.ca

Grounding Fundamentals

24

CEC Handbook 





Provides background information and commentary on the th Rules R l off th the Canadian C di Electrical Code, Part I Intended to provide a clearer understanding of the safety requirements of the Code I Incorporates t information i f ti on:  Rational  Intent  Field Considerations

www.EngWorks.ca

Grounding Fundamentals

25

IEEE Standard 142 ((Green Book)) 

Recommended practices and methods associated with grounding     



Systems grounding Equipment grounding and bonding Static and lightning protection Grounding electrode design Grounding of electronic equipment

Applies to industrial and commercial power systems  Utility grounding methods are not covered

 www.EngWorks.ca

Recommended Purchase Grounding Fundamentals

26

Commonly Used Grounding Terms and Definitions Neutral Conductor

Neutral Point

Neutral Ground Device Grounding Conductor Grounding Electrode www.EngWorks.ca

Metallic Enclosure Bonding Conductor

Stray Current

Earth Grounding Fundamentals

27

Earth 

Conducting body of varying resistance  Earthing – A connection to earth Interchangeable with the term ground

Earth www.EngWorks.ca

Grounding Fundamentals

28

Ground 

A conducting connection by which an electrical circuit is connected to earth  Grounding Electrode – a conductor buried in earth and used for collecting or dissipating ground current to earth  Grounding g Conductor – conductor used to connect the service equipment to a ground electrode

Grounding G g Conductor Grounding Electrode www.EngWorks.ca

Grounding Fundamentals

29

Bonding 

Low impedance path created by joining all noncurrent-carrying metal parts to ensure electrical continuity ti it  Bonding Conductor – conductor that connects the noncurrent carrying parts of electrical equipment, raceways or enclosures

B di Conductor Bonding C d t (Equipment ground conductor)

www.EngWorks.ca

Grounding Fundamentals

30

Neutral Point 

The point of a symmetrical system which is normally at zero voltage g  Neutral Conductor – a system conductor, other than a phase conductor that provides a return path for current to the source Neutral Point

Neutral Ground Device

www.EngWorks.ca

Neutral Conductor

Grounding Fundamentals

31

Definitions 

Ground Fault Current – ground current resulting from anyy p phase-conductor-to-earth fault  Normal – brief flow of current that occurs until the protective device opens  Abnormal – continuous flow of current from a phase conductor to ground  Often referred to as the Zero Sequence Current



Neutral grounding devices include grounding resistors, grounding transformers, ground-fault ground fault neutralizers, reactors, capacitors, or a combination of these components

www.EngWorks.ca

Grounding Fundamentals

32

Ground Fault Current

Neutral Ground Device

Metallic Enclosure Intended G Ground dF Fault lt Current Path Ground Fault

Earth www.EngWorks.ca

Grounding Fundamentals

33

Stray Current 

The uncontrolled flow of current over and through the earth  results in undesired safety and system performance characteristics

Stray Current Earth www.EngWorks.ca

Grounding Fundamentals

34

Systems Grounding Section 2

Purpose of a Systems Ground “System grounding, or the intentional connection off a phase h or neutral t l conductor d t to t earth, th is i for f the purpose of controlling the voltage to earth, or ground ground, within predictable limits” limits IEEE 142 Green Book 

Most system faults are ground fault related

www.EngWorks.ca

Grounding Fundamentals

36

Systems Ground 

A systems ground will:  Control the voltage g to g ground to p prevent stressing g equipment insulation  Allow the operation of ground fault detection protection d i devices  Reduce the risk of fire and shock hazard to persons who might come in contact with live conductors  In some cases provide service continuity  Allow the ground fault to be isolated and repaired at a convenient time ti

www.EngWorks.ca

Grounding Fundamentals

37

Concept of a System Ground 

A grounding system consists of all interconnected grounding connections in a specific power system and is isolated from adjacent; grounding systems through a high impedance  Isolation occurs via an ungrounded transformer winding connection

www.EngWorks.ca

Systems Ground 1

Y

Systems Ground 2

Y Y

Y

Systems Ground 3

Y

M

Grounding Fundamentals

PP

M

M

Systems Ground 4

38

Transformer Winding Connections 

∆ (delta) Connections  Isolates the p power system y from ground  Important is creating “zones of protection”



Y (wye) Connections  Y point provides a neutral point for managing ground faults  Opportunity for multiple voltages

www.EngWorks.ca

Grounding Fundamentals

39

System Grounding Classifications

Ungrounded

Resistance Ground

www.EngWorks.ca

Solid Ground

Reactance Ground

Grounding Fundamentals

40

System Grounding Classifications Systems Grounding Ungrounded

Grounded

Impedance Grounded Resistance Low Resistance

www.EngWorks.ca

High Resistance

Solid Grounded Reactance Reactance

Tuned Reactance

Grounding Fundamentals

41

Ungrounded 

Historically was used on power systems where a high g level of p process continuity y was required q  Exists in many process facilities designed prior to 1980



Advantages g  Single ground fault does not does not allow current to flow  Allowed for a controlled shutdown for fault repairs

 Eliminates the need for f elaborate protection schemes  Grounding system cost is minimized A N G

B C

www.EngWorks.ca

Grounding Fundamentals

42

Ungrounded 

Disadvantages  On a g ground faults,, the voltage g to g ground for the remaining g phases is elevated by 73%  Higher insulation rating required for system components

 Transient T i t overvoltages lt can be b a problem bl  Voltages up to 6X system voltage stresses insulation eventually leading to a second ground fault and subsequently a phase to phase fault

www.EngWorks.ca

Grounding Fundamentals

43

Ground Fault Voltage Shift Normal Operating Conditions

A

A

VAG

IA

IA

N

IB

VCG

B IB CA

CB

C

C CC

IC

VCN

IC

B A

VAN N

VAG

G

VAB

VCA VCG

VBN

If CA = CB = CC then IA+ IB + IC = 0 www.EngWorks.ca

VBG

Grounding Fundamentals

C

N G VBG VBC

B

44

Ground Fault Voltage Shift Ground Fault Phase C

A

IG VAG

A

IB

IA

IA

N

C G VCG=0

B IB

IG

CA

C

VBN

N

VAN

A

CB VCA

VCN

G

www.EngWorks.ca

B

VBG

IA + IB = IG

VAG

VAB

N

VCG=0

C G Grounding Fundamentals

VBC VBG

B

45

Intermittent Ground Faults  A IA

N B IB

IG G

CA

C

CB

Breakdown in insulation results in phase to phase fault IG = ISC

www.EngWorks.ca

Intermittent or restriking type yp g ground faults on isolated grounded systems can cause severe system overvoltages  Up to 6 or 8 times line to line voltage  Will eventually lead to an g in insulation failure resulting a phase to phase fault  Must be detected and corrected ASAP

Grounding Fundamentals

46

Ungrounded System Ground Fault Detection Scheme 10-106 Alternating-current systems (see Appendix B) ((2)) Wiring g systems y supplied pp by y an ungrounded g supply pp y shall be equipped with a suitable ground detection device to indicate the presence of a ground fault.

Ground Fault

L

L

L

0V

Light Dims Or Extinguishes

Ground fault Detection Scheme www.EngWorks.ca

Grounding Fundamentals

47

Solid Ground 

A solid grounded system is one in which the neutral points have been intentionally connected to earth ground with a conductor having no intentional impedance  Often referred to as effective grounding

A N G

B C

www.EngWorks.ca

Grounding Fundamentals

48

Solid Ground

Uniground System Used in Industrial Systems

www.EngWorks.ca

Multi-grounded System Used by Utilities in Rural Distribution Systems

Grounding Fundamentals

49

Solid Ground 

Advantages  Partially reduces the problem of transient over-voltages  Reduced R d d insulation i l ti llevell required i d

 Ground faults do not shift the system neutral  Simple ground relay schemes provide for circuit protection 

Disadvantages  Damage at the fault may be excessive  Arc flash hazard due to high ground fault current levels  Difficult to coordinate ground fault protection  Magnitude of the fault current is unknown

www.EngWorks.ca

Grounding Fundamentals

50

CEC Definition 



Effective Grounding - a path to ground from circuits, equipment, q p , or conductor enclosures that is permanent and continuous and has carrying capacity ample to conduct safely any currents liable to be imposed upon it CEC Rule 10-500 in Appendix B states that the complete fault path of the circuit conductor conductor, together with the bonding return return, should have an impedance that allows at least five times the current setting of the overcurrent device to flow when a fault of negligible impedance occurs

www.EngWorks.ca

Grounding Fundamentals

51

Solid Ground A N B C

O/C fuse may not clear arcing ground fault

IG G

www.EngWorks.ca

High Impedance Ground fault

Grounding Fundamentals

VBN

N

VAN

VCN

52

High Resistance Ground 

System is grounded through a high-impedance resistor  High-impedance grounding typically limits ground fault current to 25 A or less  Typically used on low voltage (600V or less) systems under 3000 Amps A N 2 - 25A

www.EngWorks.ca

G

B C

Grounding Fundamentals

53

High Resistance Ground Scheme

1000 KVA Xfmr 25kV – 600V 5.75% Z

AL

51G

Y

5 Amp NGR

NGR 5A

M 150HP Cooling Ground Fan

Fault www.EngWorks.ca

LP

U/H

45kVA

Lighting Panel

30kW Unit Heater

Grounding Fundamentals

Starter

75kVA

X2

HTP Heat Trace Panel

M

M

75HP Recycle Pump

25HP Injection u p Pump

X2

54

High Resistance Ground 

Advantages  Allows system y to operate p under a g ground fault condition  Reduces arc flash energy associated with a ground fault  Insures a ground fault of a known magnitude  Aids in protective relay coordination and limiting equipment damage

 Reduces transient ground fault overvoltages  Allows easy identification and isolation of the ground fault location 

Disadvantages  Neutral shift on ground fault

www.EngWorks.ca

Grounding Fundamentals

55

Low Resistance Ground 

System is grounded through a low-impedance resistor  Low-resistance grounding typically limits ground fault current to 400A or less for a short period of time (10 sec)  Typically used on medium and high voltage industrial power distribution systems A N 25 - 400A

www.EngWorks.ca

G

B C

Grounding Fundamentals

56

Low Resistance Ground 

Advantages  Allows p protective relay y devices to q quickly y clear a g ground fault  Limits damage to equipment and reduces overheating and mechanical h i l stress t on conductors d t



Disadvantages  Neutral Ne tral voltage oltage shift of limited d duration ration

www.EngWorks.ca

Grounding Fundamentals

57

Low Resistance Ground Scheme Trip Upstream Breaker 51 NGR 400A

Trip setting ~ 20% of NGR rating

Y

Trip Downstream Breaker

400A NGR 13.8kV

M

51 XFMR

SGR (Secondary Ground Resistor)

Y 5A NGR

600V

Alternate Arrangement M

www.EngWorks.ca

Grounding Fundamentals

M

58

LR Grounding Resistor Connection to Neutral Point on Transformer

Connection to ground

Resistors Current Transformer

51 NGR 400A

www.EngWorks.ca

Grounding Fundamentals

59

LR Grounding Transformer Ground resistor

51 XFMR

SGR (Secondary Ground Resistor)

Grounding Transformer

www.EngWorks.ca

Grounding Fundamentals

60

Alternate Grounding Schemes 

Corner-of-the-Delta System  Applicable pp to low-voltage g systems  Not widely used in industrial systems t



A

I B C

I

Delta One Phase Grounded at Midpoint  Applicable to single phase 120/240V loads

240V 120V

120V G

www.EngWorks.ca

Grounding Fundamentals

240V

61

Reactance Grounding 

Ground fault current is a function of the neutral reactance  Typically results in higher ground fault currents than a resistance grounded system  25 5 – 60% of o three t ee phase p ase fault au t current

 Primarily used by Utilities on multi grounded systems on multi-grounded systems above 5kV  Seldom used in industrial plant applications www.EngWorks.ca

Grounding Fundamentals

N

51 Reactor

IG

62

Resonant Grounding 

Tuning reactor is used to ground the neutral point to ground  Reactor is tuned to match the system capacitance

N

 Results in a very low value of ground fault current  75% of line to ground faults are selfextinguishing

 C Complex l controls t l are required i d tto constantly match the reactance to the system capacitance  Primarily Pi il used d on overhead h d and d transmission lines above 15kV  Rarely used in industrial applications

www.EngWorks.ca

51 Reactor Ground Fault Neutralizer

Grounding Fundamentals

IG

63

Grounding System Comparison Ung grounded

Solid Ground

Low Resistance

High Resistance

Immunity to transient overvoltages

Worst

Good

Good

Best

Arc Fault Damage Protection

Worst

Poor

Better

Best

Safety to Personnel

Worst

Better

Good

Best

Service Reliability

Worst

Good

Better

Best

Continued operation after initial ground fault

Better

Poor

Poor

Best

Not Possible

Good

Better

Best

Condition

Ground fault locating

www.EngWorks.ca

Grounding Fundamentals

64

Ground Fault Sensing

www.EngWorks.ca

Grounding Fundamentals

65

Ground Fault Sensing 

Ground Return  Ground fault current is measured in the neutral to ground connection  Applicable only at a source transformer or 51G generator g  Often used for ground fault alarm sensing on LV di t ib ti systems distribution t

www.EngWorks.ca

Grounding Fundamentals

Phase A Neutral Phase B Phase C

66

Ground Fault Sensing 

Zero Sequence Relay  Measures zero sequence or ground currents by sensing the magnetic fields surrounding th phase the h and d neutral t l conductors

Phase A Neutral Phase B Phase C

Should cancel under normal conditions

 Often used in motor protection and feeder breaker relays

www.EngWorks.ca

Grounding Fundamentals

51G

67

Ground Fault Sensing 

Differential  Phase current and neutral current values are measured and ground fault current is calculated l l t d as th the diff difference  Used in applications where current transformers are required for phase overcurrent relays  High Hi h accuracy iin d detecting t ti ground faults

www.EngWorks.ca

Grounding Fundamentals

Phase A Neutral Phase B Phase C

51G

68

High Resistance Ground Detection Scheme

NGR 5A

Pulsing reading on phase indicates ground fault

AL

Y Pulsing Resistor

Clamp On CT

45kVA M

150HP Cooling Ground Fan

Fault www.EngWorks.ca

1000 KVA Xfmr 25kV – 600V 5.75% Z

5 Amp NGR

LP

U/H

51G

Lighting Panel

30kW Unit Heater

Grounding Fundamentals

Starter

75kVA

X2

HTP Heat Trace Panel

M

M

75HP Recycle Pump

25HP Injection u p Pump

X2

69

High Resistance Ground fault Detection System y

www.EngWorks.ca

Grounding Fundamentals

70

Ground Fault Relay Settings 

Alarm only on continuous rated ground resistor applications pp  Alarm setting at 80% of maximum current level allowed by ground resistor Above system charging current level



Trip on short time duty ground resistor applications  High resistance ground applications Trip at 80% of maximum current level allowed by resistor

 Low resistance g ground applications pp Trip at 20% of maximum current level allowed by ground resistor

www.EngWorks.ca

Grounding Fundamentals

71

Low Resistance Ground Detection Scheme Trip p Upstream p Breaker 51G

NGR 400A

Trip setting ~ 20% of NGR rating

Y Trip Downstream Breaker

400A NGR

Trip

ZCT

51G

13.8kV Trip

Trip ZCT

51G

Trip ZCT

ZCT - Zero Sequence CTs

www.EngWorks.ca

Grounding Fundamentals

51G

Trip ZCT 51G

ZCT

51G M

M

72

GF Relay Time Coordination Curves 



Settings for ground-fault relays can be determined during the relayy coordination studyy GF curves are plotted on the coordination diagrams  Set parameters include ti time and d currentt llevell Ground Fault coordination curves

www.EngWorks.ca

Grounding Fundamentals

73

CEC Requirements Associated with Systems Grounding

www.EngWorks.ca

Grounding Fundamentals

74

CEC Code Requirements 10-1102 – Installation of Neutral Grounding Devices 1) Neutral grounding devices can only be installed on systems where line to neutral loads are not served No single phase loads from a resistance grounded system

2) S Systems t with ith voltages lt > 5kV shall h ll b be d de-energized i d on detection of a ground fault a)) Electrical systems y operating p g at 5 kV or less are p permitted to remain energized if the ground fault current is controlled at 10A or less i.

Audible alarm is required

www.EngWorks.ca

Grounding Fundamentals

75

CEC Code Requirements 3) Where line-to-neutral loads are served, the system must be de de-energized energized on occurrence of a: 1) Ground fault 2) Grounded neutral on the load side of the NGR 3) Break in the continuity of the conductor connecting the NGR to ground Apparent conflict between subsection 1) and subsection 3)

www.EngWorks.ca

Grounding Fundamentals

76

NGR with Isolated System Neutral A Trip main breaker

HT Ckt

HT Ckt

HT Ckt

HT Ckt

HT Ckt

HT Ckt

N HT Ckt

B C

51

NGR IG

www.EngWorks.ca

Rule 10-1102 requires the system to g on detection of be de-energized ground current

Grounding Fundamentals

77

Neutral Ground Devices 10-1104 NGRs must be approved for the application  CAN/CSA-C22.2 No. 0.4 – Bonding and Grounding of Electrical E i Equipment (P (Protective i G Grounding) di )  CAN/CSA-C22.2 No.14 – Industrial Control Equipment  CAN/CSA-C22.2 No. 94 – Special Purpose Enclosures 

Must be continuously rated where provisions are not made to interrupt the fault  Maximum temperature allowed is 375°C





Where not continuously rated, the time rating of the device y must be coordinated with the protective devices of the system Must have an insulation voltage equal to the line-to-neutral system voltage

www.EngWorks.ca

Grounding Fundamentals

78

Location of Grounding Devices 10-1106 All live parts must be enclosed  Must be placed in a location accessible to qualified personnel  Must be placed in a location where it can dissipate the heat under ground fault conditions  Warning g signs g must be p provided indicating g the system is impedance grounded and located at:  Transformer or generator, or both  Consumers service switchgear  Supply authorities metering equipment

www.EngWorks.ca

Grounding Fundamentals

79

Grounding Conductors 

System grounding conductors must be copper  Solid g grounded systems y sized as p per CEC Table 17  Based on the ampacity of the largest service conductor



No splicing is permitted

www.EngWorks.ca

Grounding Fundamentals

80

CEC Code Requirements

www.EngWorks.ca

Grounding Fundamentals

81

NGR Conductors 10-1108 conductors connecting the NGR to the Neutral point of the system p y must be identified as white or grey  Must not be grounded  Sized to conduct the rated current of the device  No less than #8 AWG

 Conductor connecting the NGR to the system ground electrode may be insulated green or bare  Made of copper pp

www.EngWorks.ca

Grounding Fundamentals

82

NGR Conductors

www.EngWorks.ca

Grounding Fundamentals

83

Bonding of Conduit Enclosing a Grounding g or Bonding g Conductor 

Magnetic effect of metal conduit can increase the impedance of the grounding circuit by a factor of 40!  Not an issue with PVC or aluminium conduits



Problem can be mitigated by bonding the grounding conductor to the metal conduit at both ends  Allow the metal conduit to carry a portion of the ground current  New CEC rule 10-806 makes this mandatory

www.EngWorks.ca

Grounding Fundamentals

84

Sizing and Specification of Neutral Ground Resistors

NGR Sizing Criteria 

NGRs are sized based on the following criteria  Charging current HRG - Maximum ground current must be greater than 3X the charging current for the system LRG – Charging current not a factor

 Temperature rise Based on how long the fault is allowed to persist – Continuous – Extended E t d d titime (1 minute) i t ) – 10 seconds

www.EngWorks.ca

Grounding Fundamentals

86

NGR Sizing Criteria RNGR =

51 NGR

IG

VLL √ G √3I

XCO RNGR ≤ 3 IG ≥ 3ICO WNGR = IG2RNGR RNGR = Resistor Size ((Ohms)) IG = Maximum Ground Current (Amps) ICO = System Charging Current (Amps) WNGR = Resistor Size (Watts)

www.EngWorks.ca

Grounding Fundamentals

87

NGR Sizing Criteria Secondaryy Ground Resistor

51 XFMR

SGR

RNGR RSGR = N2 VLN(Pri) N= VLN(Sec) ISGR = NIG KVA = PNGR = IGVLN(Pri)

RNGR = Equivalent Primary Resistance (Ohms) RNGR = Equivalent Primary Resistance (Ohms) IG = Maximum M i G Ground dC Currentt (Amps) (A ) ISGR = Maximum Ground Current (Amps) N = Turns ratio PNGR = Resistor Power Rating (Watts) www.EngWorks.ca

Grounding Fundamentals

88

Charging Current - Estimation 

Resistor must be sized to ensure that the ground fault current limit is g greater than the system's y total capacitance-to-ground charging current System Voltage

Charging Current (3ICO) Amps per 1000 kVA of System Capacity

480

0.1 – 2.0

600

0.1 – 2.0

2400

2.0 – 5.0

4160

2.0 – 5.0

13800

5.0 – 10.0

Typical Charging Currents based on Voltage Level www.EngWorks.ca

Grounding Fundamentals

89

Charging Current – More Detailed Analysis System Voltage 600V

4160V

13800V

Component Cable

Type

Typical Charging Current

3/C - 250 – 500MCM

0.15A/1000ft

3/C - #1 – 4/0AWG

0.02A/1000ft

Transformers

0.02A/MVA

Motors

0.01A/1000HP

Cable

3/C - 500–1000MCM Shielded

0.58A/1000ft

3/C – 1/0 – 350MCM Shielded

0.23/1000ft

Non Shielded

0.1A/1000ft

T Transformers f

0 05A/MVA 0.05A/MVA

Surge Suppressor.

1.35A per Set

Motors

0.1A/1000HP

Cable

3/C - 600–1000MCM Shielded

0.65A/1000ft

3/C – 250 – 350MCM Shielded

0.75/1000ft

3/C - #1 – 4/0AWG Shielded

0.65A/1000ft

Transformers

0.05A/MVA

Surge Suppressor

2 25A per Set 2.25A

Motors

0.15A/1000HP

www.EngWorks.ca

Grounding Fundamentals

90

Charging Current Calculation Example p ROT → IG ≥ 3ICO IG ≥ 3(4.78A) 3(4 78A) = 14.34A 14 34A Qty

Total Ch i Charging Current

Transformer

0.05A /MVA

17.5

500MCM Cable

0.58A /1000ft

4200 ft 2.43A

250MCM Cable

0.23A /1000ft

600 ft

Surge Suppressor

1.35A /Set

1

T t l Total www.EngWorks.ca

Y

12 MVA

10A NGR

0.875A

4160V 1500ft 500MCM

Charge C Current t

Surge Suppressor

1500ft 500MCM

Component

15A 5 NGR G more oe appropriate size

0.13A Y

2MVA

Y

2A NGR

1.35A

4 78A 4.78A

2MVA

600V

Y

1200ft 500MCM

600ft 250MCM 1 5MVA 1.5MVA

Surge Suppressor M

3000HP M

Grounding Fundamentals

M

91

Charging Current Test Procedure 

A IA



N B IB CA

CB

C CC

IC



A Ammeter 0-10A 0 10A

 Ammeter will indication charging current (3ICO) 

G

www.EngWorks.ca

Connect an ammeter to ground g a resistance,, switch and through a fuse Increase the resistance to maximum level and close the di disconnect t Slowly reduce the resistance to zero All three phases should be g used measured and the average as the system charging current

Grounding Fundamentals

92

Cable Insulation Ratings on Resistance Grounded Systems y 

Low Voltage Systems (≤ 600V)  100% % insulation rating g acceptable p for all applications pp  Refer to Standata CEC 12



Medium Voltage g Systems y ((IEEE Recommendations))  100% insulation level required where clearing time will not exceed one minute  133% insulation level required where clearing time will not exceed one hour  173% insulation level required where clearing time exceeds one hour

www.EngWorks.ca

Grounding Fundamentals

93

NGR Ratings 

Based on the criteria defined in IEEE 32 - Standard Requirements, Terminology, and Test Procedure for Neutral Grounding Devices  Current Current through the device during a ground fault condition

 Voltage V = IR at 25ºC May need to be de-rated at elevations above 1000m

 Frequency Circuit Voltage of System

 Service NEMA Type 1 for Indoor Applications NEMA Type 3 for Outdoor Applications www.EngWorks.ca

Grounding Fundamentals

94

NGR Ratings 

Basic Impulse Insulation Level System Insulation Class

www.EngWorks.ca

Class

BIL

1.2kV 2 5kV 2.5kV 5kV 8 7kV 8.7kV 15kV 23kV

45 60 75 95 100 150 Grounding Fundamentals

95

NGR Ratings 

Time Rating and Permissible Temperature Rise under fault conditions Permissible Temperature Rise (Rise Above 30ºC Ambient)

Time Rating Ten Seconds (Short Time) (NGRs

760ºC 760 C

used with Protective Relay)

One Minute (Short Time)

760ºC

Ten Minutes (Short Time) (seldom

610ºC

specified)

Extended Time

(GF

610ºC 610 C

allowed to persist > 10min)

Steady State (Continuous)

385ºC*

*CSA permissible rise is 375ºC over 40ºC Ambient www.EngWorks.ca

Grounding Fundamentals

96

NGR Monitoring

Broken Spot Weld

NGR Thermal Failure Broken Resistor Wire www.EngWorks.ca

Grounding Fundamentals

97

NGR Monitor 

The NGR monitor measures changes in NGR resistance, current in the neutral, and neutral-toground voltage g g  Anomalies are detected and an alarm or trip signal is activated

www.EngWorks.ca

Grounding Fundamentals

98

NGR Sizing Tutorial

NGR Sizing Tutorial 

Modular Substation incorporating  5 kV Switchgear and MCCs  600 V Switchgear and MCCs



Grounding system consists of:  Power Distribution System Ground  5kV L Low resistance i t ground d system t  600V High resistance ground system



Objective  Size the grounding resistors for the 5kV LRG system and the 600V HRG system  Assume 1.5A charging current for the 600V System  Assume 8A charging current for the 5kV System

www.EngWorks.ca

Grounding Fundamentals

100

Substation Single Line

LRG NGR

Y 5kV

M

HRG NGRY

M

M

600V ~=

M

M

=

~

UPS PP

www.EngWorks.ca

Grounding Fundamentals

101

NGR Sizing Tutorial Answers

Sizing the NGRs RNGR =

51 NGR

IG

VLL √ G √3I

XCO RNGR ≤ 3 IG ≥ 3ICO WNGR = IG2RNGR RNGR = Resistor Size ((Ohms)) IG = Maximum Ground Current (Amps) ICO = System Charging Current (Amps) WNGR = Resistor Size (Watts)

www.EngWorks.ca

Grounding Fundamentals

103

HRG Sizing ROT → IG ≥ 3ICO

ICO = 1.5A

IG ≥ 4.5 → Choose 5A as the HRG Current Rating RNGR = RNGR =

VLL √3IG 600V √3 x 5A

RNGR = 69.3Ω

www.EngWorks.ca

WNGR = IG2RNGR WNGR = 5A2 x 69.3Ω WNGR = 1733watts

Grounding Fundamentals

104

LRG Sizing ROT → IG-Trip Setting ≥ 3ICO to avoid nuisance tripping

ICO = 8A

Trip T i setting i is i roughly hl 20% off the h LRG resistor i size i IG-Trip Setting ≥ 24A to avoid nuisance tripping IG ≥ 120A → Choose Ch 125A as the h LRG Current C Rating R i RNGR = RNGR =

VLL √ G √3I 4160V

√3 x 125A

RNGR = 19.2Ω www.EngWorks.ca

WNGR = IG2RNGR WNGR = 125A2 x 19.2Ω WNGR = 300kW Grounding Fundamentals

105

System Grounding Application Summary 

Solid Systems Ground  Industrial applications pp 208V or less  Commercial Applications 600V or less



High g Resistance Ground ((5-15A))  Industrial applications 600V or less  CEC allows HRG to be used on applications up to 5kV



Low Resistance Ground (100 – 400A)  Industrial applications 5kV – 34.5kV  Ground G d ffaultlt protection t ti provided id d by b Z Zero Sequence S CTs CT on individual equipment items  GF relays set to trip at 10 -20% of maximum ground fault current

www.EngWorks.ca

Grounding Fundamentals

106

Obtaining a Systems Neutral Application of Grounding Transformers

Obtaining a Systems Neutral 

Often there are cases where a systems neutral point must be established for the purposes of:

Y

 Servicing line to neutral loads  Establishing a systems ground d point i t tto ground d th the system through a HRG, LRG or solid ground connection ti

13.8kV

M

 Example: Conversion of a isolated ground system to a high resistance ground system www.EngWorks.ca

Grounding Fundamentals

108

Grounding Transformers 

Grounding transformers are the standard means of obtaining a systems neutral  Provide a low impedance path for ground fault currents



Zig-Zag transformer  Often Oft referred f d to t as a grounding di transformer t f  Specialized transformer with no secondary winding



Wye-delta Wye delta transformer configuration  Delta winding is left unconnected

www.EngWorks.ca

Grounding Fundamentals

109

Grounding Transformer Schemes A B C

I

I G

Zig Zag Transformer

www.EngWorks.ca

I G

Wye-Delta Transformer

Grounding Fundamentals

110

Transformer Connection 

The grounding transformer is connected to the main bus and serves as the return path for any unbalanced or ground fault currents

Y

 A NGR is then connected to th neutral the t l point i t off th the grounding transformer establishing a connection to ground d

13.8kV

M

LRG www.EngWorks.ca

Grounding Fundamentals

111

Specifying a Grounding Transformer 

Parameters for specifying a grounding transformer  Primaryy Voltage g  BIL (Basic Impulse Level) rating  Defined by IEEE standards (refer to IEEE 141 Red book)

 Transformer impedance  Typically very high (up to 100%) to minimize magnetizing current flows

 Continuous neutral current rating  Applicable to four wire application

 Fault F l current and dd duration i  If a LRG scheme of limited duration is used, ( typically 10 – 60 seconds) the grounding transformer does not need a continuous duty rating www.EngWorks.ca

Grounding Fundamentals

112

G Grounding di off Generators G t Section 3

Generator Grounding 

Generators differ from transformers in several ways  Less able to withstand the heating g and mechanical effects of a short circuit  Will have a higher initial ground fault current than three phase h ground d currentt  Can develop third harmonic voltages and currents  Less able to withstand voltage surges

www.EngWorks.ca

Grounding Fundamentals

114

Objective of Generator Neutral Grounding 







Minimize the damage associated with internal ground faults Limit mechanical stresses in the generator for external ground faults Limit temporary and transient overvoltages on the generator t insulation i l ti system t Provide a means of system ground fault protection

www.EngWorks.ca

Grounding Fundamentals

115

Systems Ground Incorporating Generation System Ground #1 NGR

Y 5kV

System Ground #2

M G

Y

G

600V Normal Bus

System Ground #3

M

M

G

600V Emergency Bus

www.EngWorks.ca

Grounding Fundamentals

116

Generator Ground Fault IGF 400A NGR 2 X IGF IGF

400A NGR

Stator Ground Fault near Generator terminals Breaker Closed

Initial ground fault current results in 2 X 400A flowing into fault

www.EngWorks.ca

Grounding Fundamentals

117

Generator Ground Fault IGF 400A NGR IGF 400A NGR Stator Ground Fault near Generator terminals Breaker Open

Upon breaker trip, ground fault current continues to flow due to the residual magnetism and inertia of the machine www.EngWorks.ca

Grounding Fundamentals

118

Ground Fault Magnitude 

 

Magnitude of a ground fault is determined by the impedance p of the g generator or transformer winding g Maximum ground fault will occur on the system bus Maximum theoretical ground fault current in the generator will occur at the generator terminals  Closer the stator fault is to the generator terminals, the higher the fault



Resulting damage is a function of current and time

www.EngWorks.ca

Grounding Fundamentals

119

Solid System and Generator Ground



NOT RECOMMENDED  Results in very high ground fault currents resulting in g extensive damage  Risk of abnormal third-harmonic currents when more than one generator is connected in parallel  Increased magnetic core losses in both generator and transformer

www.EngWorks.ca

Grounding Fundamentals

120

Low Voltage Emergency Generator Scheme

Bonding Conductor

Normal Bus

Emergency Bus Gnd

www.EngWorks.ca

Grounding Fundamentals

Gnd

121

Single Unparalleled Generator Grounding Solid Grounded with Neutral Equipment Ground conductor 3 pole Transfer Switch

Normal Bus

Neutral Conductor Zero Sequence CT is bypassed resulting in false ti trip www.EngWorks.ca

51G N

Neutral Connected To ground

Solid Neutral

Emergency Bus Gnd

Grounding Fundamentals

N

Gnd

122

Single Unparalleled Generator Grounding Solid Grounded with Neutral 

Connection of the neutral to ground at the generator can cause problems p  Allows stray current to flow between the neutral and the ground conductors  Allow zero sequence (ground fault current) to flow in the neutral causing nuisance tripping of the main breaker  Prevent ground fault relays from detecting a ground fault



A neutral should not be connected to ground on the load side of a service disconnect

www.EngWorks.ca

Grounding Fundamentals

123

Option 1 – Switch Neutral Equipment Ground conductor 4 Pole Transfer Switch

Normal Bus

Neutral Conductor

GFP N

www.EngWorks.ca

Neutral Connected To ground Neutral Switched With load conductors

Emergency Bus Gnd

Grounding Fundamentals

N

Gnd

124

Option 2 – Connect Generator Neutral with Transformer Neutral Equipment Ground conductor 3 pole Transfer Switch

Normal Bus

Neutral Conductor Zero Sequence CT read full neutral current value l www.EngWorks.ca

51G N

Generator G t Neutral connected to transformer neutral in transfer switch

Emergency Bus Gnd

Grounding Fundamentals

N

Gnd

125

Additional References 

IEEE 446 Orange Book  Provides application pp information for the system grounding and transfer switching of standby generators 600V or less

www.EngWorks.ca

Grounding Fundamentals

126

Single Unparalleled Generator Grounding High g Resistance Grounded HRG Bonding Conductor

HRG

Normal Bus

Emergency Bus Gnd

www.EngWorks.ca

Grounding Fundamentals

Gnd

127

HRG Source and Generator Grounding HRG HRG



Advantages  Ground fault current limited to a veryy low value



Disadvantage  Selective tripping on downstream breakers is not practical

www.EngWorks.ca

Grounding Fundamentals

128

LRG Source and Generator Grounding LRG LRG



Advantages  Allows selective tripping of downstream feeders



Disadvantage  Damage can occur to the generator from high ground fault currents  Variations in fault current can cause relay coordination problems

www.EngWorks.ca

Grounding Fundamentals

129

LRG Source and HRG Generator Grounding g HRG LRG



Advantages  Allows selective tripping of downstream feeders  Reduced R d d level l l off fault f lt currentt to t the th generators t minimizing i i i i d damage



Disadvantage  System is high resistance grounded when the generator is operating alone – makes selective tripping impossible

www.EngWorks.ca

Grounding Fundamentals

130

Source and Generator Grounded with Artificial Neutral

LRG



Advantages  Allows selective tripping of downstream feeders  Allows All either ith source or generator t to t provide id power



Disadvantage  Damage can occur to the generator from restriking and intermittent ground faults

www.EngWorks.ca

Grounding Fundamentals

131

LRG Source and Hybrid LRG/HRG Generator Grounding g LRG LRG



HRG

Ground fault will cause generator t breaker b k to t trip and open LRG circuit

Advantages  Allows selective tripping of downstream feeders  Allows generator to operate without the source transformer energized



Disadvantage  Additional complexity in the grounding and relaying system

www.EngWorks.ca

Grounding Fundamentals

132

Unit Connected Generator Grounding HRG

LRG LRG



Advantages  Allows selective tripping of downstream feeders  Allows generator to operate without the source transformer energized



Disadvantage  Cost of the additional transformer

www.EngWorks.ca

Grounding Fundamentals

133

E i Equipment tB Bonding di Section 4

System Grounding 



Grounding and bonding have distinct meanings within the context of the CEC Grounding refers to a conductive path direct to the grounding g g electrode  Low impedance path to ground  Conductors are sized to carry the expected fault current  Insure the operation of protective devices in the circuit should a fault occur

www.EngWorks.ca

Grounding Fundamentals

135

Equipment Bonding 

Refers to the interconnection and connection to earth of all normallyy non-current carrying y g metal p parts  Insures that all metal parts remain at ground potential  Reduces the shock hazard to personnel  Provides a low impedance return path for ground currents  Allows the circuit protection device to operate

 Minimize the fire and explosion hazard  Reduce accumulated static charges

www.EngWorks.ca

Grounding Fundamentals

136

Ground Return Path through Earth Insufficient current to operate protection device Line

~

S

Metallic Enclosure Ground Fault

V Neutral

Short circuit must take high impedance path to source High Impedance Ground Path

www.EngWorks.ca

Grounding Fundamentals

137

Ground Return with Metallic Path High Current Operates Protection Device Line ~ Metallic Enclosure

S

Ground Fault

V Neutral

Low Impedance Path through Bonding Conductor High Impedance Ground Path

www.EngWorks.ca

Grounding Fundamentals

138

Bonding Fundamentals 

To reduce electrical shock exposure:  the impedance p of the bonding g conductor must be capable p of carrying the fault current  Must provide a lower impedance than all other parallel paths th



For fire protection:  M Must st be able to conduct cond ct the available a ailable gro ground nd fa fault lt ccurrent rrent without excessive temperature rise or arcing  Joints and connections are critical components



Overcurrent Protection Operation:  Provide a low impedance current path back to the source

www.EngWorks.ca

Grounding Fundamentals

139

Bonding – CEC Requirements 10-400 

All exposed non-current carrying metal parts of fixed equipment q p  Supplied by a conduit wiring system  Supplied by a wiring system that contains a bonding conductor  Located in a wet location  In a hazardous location  Operates at more than 150V to ground



Examples  Distribution equipment, motor and generator frames  Lighting fixtures housings

www.EngWorks.ca

Grounding Fundamentals

140

Bonding Methods  

Bonding conductor in a cable or raceway Rigid metal conduit  Bonding conductor is required if the conduit is in underground service or installed in concrete slabs



EMT conduit  Bonding conductor required if installed in concrete or masonry slabs





Sheath of a mineral insulated cable if manufactured of copper or aluminum CEC Not acceptable  Metal armor of liquid tight flex or cable assemblies  Conduit made of stainless steel

www.EngWorks.ca

Grounding Fundamentals

141

Bonding Methods - Effectiveness Cable or Conduit

DC Resistance Ω/1000ft

Voltage Drop V/1000A/100ft

1-1/4” Rigid Steel Conduit

0.0108

11

1-1/4” EMT

0.0205

22

0.435

436

1-1/4” Flexible Conduit 3/C St Steell A Armored dC Cable bl

55

3/C Steel Armored Cable with Ground Conductor

11

3/C Aluminum Armored Cable

0.286

3/C Aluminum Armored Cable with Ground Conductor www.EngWorks.ca

151 12

Grounding Fundamentals

142

Bonding Conductors 

Bonding conductors may be:  Be copper or other corrosion resistant material  Aluminium conductors are acceptable  May be insulated or bare  Insulated bonding g conductors shall be coloured g green

 

May be spliced or tapped as required If installed to supplementary bond a raceway:  Must be insulated  Must be run in the same raceway



M st be protected against mechanical inj Must injury r ifif:  Copper - Smaller than #6 AWG  Aluminum – Smaller than #4 AWG

www.EngWorks.ca

Grounding Fundamentals

143

Bonding Conductors Equipment q p and Raceways y

www.EngWorks.ca

Grounding Fundamentals

144

Bonding of Cable Trays 

Rule 12-2208 of the CEC requires that cable trays be bonded to ground g  If the metal supports for cable tray are in good contact with the grounded structural metal frame of a building, the tray shall h ll b be d deemed d tto b be b bonded d d tto ground d  If not in direct contact, a bonding conductor must be installed and the tray bonded to the conductor at intervals not exceeding 15m  Sized as per CEC table 16 based on the largest ungrounded conductor in the tray

 A bonding conductor may also be required in the cases that the tray supports single conductor cables of a three phase system www.EngWorks.ca

Grounding Fundamentals

145

Bonding of Single Conductor Cables 



Separate ground conductor required to bond the metallic equipment at either end Must follow the same routing as the phase conductors

www.EngWorks.ca

Grounding Fundamentals

146

Bonding Considerations 

Bonding connections require a clean surface  Paint must be removed from connection points



Connections between dissimilar metals should be avoided  Potential for deterioration of the connection due to galvanic action





Mechanical strength may often determine the size of conductor d t Electrical continuity of expansion joints  Cable tray connections

www.EngWorks.ca

Grounding Fundamentals

147

Equipotential Bonding 

Practice of bonding all exposed and extraneous conductive p parts ((Ref CEC 10-406))  Purpose is to ensure that under fault conditions, all conductive parts remain at the same potential



Applies to     

Metallic water and sewer piping G piping Gas HVAC ducting Exposed metal equipment and structures Raised computer floors

www.EngWorks.ca

Grounding Fundamentals

148

Equipotential Bonding 

CEC requires a minimum #6 AWG conductor

www.EngWorks.ca

Grounding Fundamentals

149

Bonding of Portable Equipment 

Non-current carrying metal parts of portable equipment q p must be bonded when:    

Equipment is used in a hazardous location Equipment is used in wet or damp locations Equipment operates at more than 150V to ground When the equipment is provided with a grounding means  Three Th prong plug l with ith ground d

www.EngWorks.ca

Grounding Fundamentals

150

Grounding of Portable Equipment 

Exceptions apply to double insulated equipment p products  Additional insulation barrier added to the electrical device  Will be marked with a double insulated symbol



Ground may omitted if a Class A ground fault circuit interrupter is used

www.EngWorks.ca

Grounding Fundamentals

151

GFCI Schematic





Designed to provide protection against electric shock from leakage current flowing to ground Provide supplementary pp yp protection but are not a substitute for insulation and grounding protection

www.EngWorks.ca

Grounding Fundamentals

152

Ground Fault Circuit Interrupters 

GFCI Class A  Primarily used for personnel protection  Typically trip at 5ma  Time to trip based on the formula T=



20 I

1.43

T in seconds I fault current between 4mA and 260 mA

GFCI Class B (Ground Fault Equipment Protectors)  Used for equipment protection  Heat trace circuits in hazardous locations

 30ma trip level

www.EngWorks.ca

Grounding Fundamentals

153

GFCI – Where Required    



Outdoor receptacles Wet locations Health care facilities Panels supplying power for buildings or projects under construction Heat trace systems

www.EngWorks.ca

Grounding Fundamentals

154

Static Grounding Section 5

Did the Cellphone Cause the Ignition?

www.EngWorks.ca

Grounding Fundamentals

156

Static Hazards in Industry 

Aviation Industry  Static charges are built up during flight and on the ground



Manufacturing  Paper and Printing  Power P and d conveyor b belts lt moving i over pulleys ll

 Paint operations  Transfer of fluids 

Coal, Flour and Grain Industry  Movement and accumulation of dust and particles



Petrochemical P h i lP Processing, i R Refining fi i and d Transportation  Movement of materials

www.EngWorks.ca

Grounding Fundamentals

157

Reasons for Static Grounding   



Reduce the risk of fires and explosions Improve process and quality control Reduce the operating costs associated with storing flammable materials Minimize the potential for damage to sensitive electronic equipment q p  Loss of electronic data





Comply p y with hazardous g goods transport p and storage g regulations Reduce the cost of insurance

www.EngWorks.ca

Grounding Fundamentals

158

Energy from a Static Discharge CH4/Air

Ignition Energy (mJ))

10

H2/Air

E = CV2 X 10-9

Typical range of spark discharge energy from a human body

Where C = Capacitance in pF V = Voltage in V E = Energy in mJ

1.0

Stoichiometric CH4/Air Mixture 0.274 mJ

0.1

Stoichiometric Air/H2 Mixture 0.017 mJ 0

www.EngWorks.ca

60 20 40 Fuel (% Volume)

80

Material

Dust Cloud

Dust Layer

Coal

60 mJ

560 mJ

Grain

30 mJ

-

Sulfur

15 mJ

1.6 mJ

Energy Required for Dust Ignition Grounding Fundamentals

159

Typical Values of Static Voltages and Capacitances p Equipment

Voltage

Object

Capacitance

Carpet Walk

12 kV

Human Being

200 pF

28.8 mJ

Fabric on Fabric

25 kV

Automobile

500 pF

312.5 mJ

Tank Truck

25 kV

Tank Truck

1000 pF

625 mJ

Tank Truck

25 kV

3.6m Tank with Insulated Lining

www.EngWorks.ca

100000 pF

Grounding Fundamentals

Energy

62,500 mJ

160

Static Charge Generation 

Static electricity is generated by the movement of dissimilar poor conducting materials in close contact Non conductive fluids or powders in motion are a frequent cause of static



Static charge increases as the velocity of movement is increased.  Anything which generates eddies, turbulence or discontinuities in flow Filters Changes in piping cross sectional area

www.EngWorks.ca

Grounding Fundamentals

161

Static Charge Generation 

Triboelectric Effect  contact electrification in which certain materials become electrically charged when coming into contact with another and are then separated

www.EngWorks.ca

Grounding Fundamentals

162

Electrostatic Charge Dissipation 

Electrostatic charges continually leak away from a charged g body y  Termed electrostatic dissipation  Determined by a materials conductivity  Measured in pS/m (picosiemens per meter) for petroleum products

 Electrostatic charges accumulate when they are generated at a higher rate than they are dissipated  Function of the relaxation time constant  Time required for a charge to dissipate to approximately 37% of its original i i l value l

www.EngWorks.ca

Grounding Fundamentals

163

Conductivity and Time Constants for Typical yp Materials Product

Conductivity (pS/m)

Relaxation Time (Seconds)

Benzene

0.005

>>100

Toluene

1

21

Gasoline

10 – 3000

0.006 - 1.8

0.5 – 50

0.36 - 36

50 - 1000

0 018 – 0.36 0.018 0 36

Crude Oil

> 1000

< 0.018

www.EngWorks.ca

Grounding Fundamentals

Diesel Fuel Oil

164

Static Discharge 

For an electrostatic charge to be a source of ignition, four conditions must be present: p  A means of generating an electrostatic charge  A means of accumulating an electrostatic charge capable of producing an incendiary spark  A spark gap  An ignitable vapor vapor-air air mixture in the spark gap

www.EngWorks.ca

Grounding Fundamentals

165

Static Charge Generation

www.EngWorks.ca

Grounding Fundamentals

166

Static Discharge Spark discharges occur between conductive objects that are at different voltages g  Brush discharges can occur between a grounded conductive object j and a charged g low conductivity y material  Incendive discharge is a discharge that has enough energy to cause ignition 

www.EngWorks.ca

Grounding Fundamentals

167

Industrial Materials Prone to Static Electricityy Nonconductive glass  Nonconductive conveyor belts  Rubber  Plastic resins  Dry gases  Paper  Petroleum fluids 

 Oil water mixtures

www.EngWorks.ca

Grounding Fundamentals

168

Sources of Static Electricity Dry materials handling equipment  Flammable liquid pumps and handling equipment 

Charge Separation in a Pipe

 Multiphase flow enhances charge generation

Liquid filling operations  Plastic piping systems  Conveyor Belts  Liquid motion in tanks 

www.EngWorks.ca

Grounding Fundamentals

169

Sources of Static Electricity

API 2003 www.EngWorks.ca

Grounding Fundamentals

170

Spark Promoters 

A spark promoter will provide the necessary conditions for a spark gap to occur  Loose floating conductive objects  Conductive downspouts  Gage tapes, thermometers or sample p containers lowered into a tank  “tank gauging rod, high-level sensor,, or other conductive device that projects into the cargo space of a tank truck” API 2003

www.EngWorks.ca

Grounding Fundamentals

171

Static Sparks in Kanses

www.EngWorks.ca

Grounding Fundamentals

172

Static Control Ignition hazards from static sparks can be eliminated by y controlling g the g generation or accumulation of static charges  Static removal involves recombining g separated p charges 

 Usually met by bonding all electrically conducting parts

www.EngWorks.ca

Grounding Fundamentals

173

Methods of Static Control 

Piping Systems  Keep fluid velocities low  Max 15 ft/sec



Filling Operations  Eliminate splash filling and free fall of materials  Reduce filling velocity to less than 3 ft/sec



Fluid Storage  Non Non-conductive conductive material storage containers are not allowed for NFPA Class I, Class II and Class III materials

www.EngWorks.ca

Grounding Fundamentals

174

Methods of Static Control 

Humidity Control  65% or higher will prevent static discharge



Antistatic treatments  Addition Additi off carbon b bl black k tto materials t i l



Use bonding and grounding to prevent build-up build up of potential differences between conductive parts  Small gauge conductors generally sufficient to prevent the b ild p of static build-up

www.EngWorks.ca

Grounding Fundamentals

175

Static Grounding

Vehicle Connected to Ground

Vehicle Bonded Together

Vehicle Bonded together and To Ground

www.EngWorks.ca

Grounding Fundamentals

176

Static Grounding

Drum Container Storage Scheme

www.EngWorks.ca

Grounding Fundamentals

177

Static Grounding

Bulk Fluid Transfer Operation

www.EngWorks.ca

Grounding Fundamentals

178

Static Grounding

Bonding connections should be less than 10Ω for static control www.EngWorks.ca

Grounding Fundamentals

179

Railcar Loading Bonding Scheme

www.EngWorks.ca

Grounding Fundamentals

180

API RP 2003 







www.EngWorks.ca

Provides guidance on how to protect against hydrocarbon ignition from static, lightning and stray current discharges Discusses how static charges are accumulated and how they can be safely dissipated Lightning protection for metallic tanks equipment and structures Identification and mitigation of stray currents resulting from fault currents and cathodic protection applications

Grounding Fundamentals

181

NFPA 77 



www.EngWorks.ca

Applies to the identification, assessment,, and control of static electricity for purposes of preventing fires and explosions Provides guidelines for controlling t lli static t ti electricity l t i it iin selected industrial applications

Grounding Fundamentals

182

Lightning Protection Section 6

Lightning Strikes

www.EngWorks.ca

Grounding Fundamentals

184

Lightning 

No such thing as a standard lightning strike  Highly g y complex p p phenomenon  Described by statistical means + + + + - - - - - -

+

Charge Separation in Cloud

High electric field causes ionization of air Corresponding charge + Induced in ground + Current flow in metallic pathways + + + + +

www.EngWorks.ca

Grounding Fundamentals

185

Lightning Strike Initiation

+ + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - Downward leader --Upwards Charge flows - leader meets - to ground + downwards leader - through Upward leader + structure + + + + + + + + + + + + + + + + - + -

+

www.EngWorks.ca

Grounding Fundamentals

186

Lightning Discharge 

Direct effects of lightning

Lightning Stroke

 Heat energy and large mechanical h i l fforces  Direct ignition of flammable materials

Indirect effects of lightning    

Incendive sparks Electromagnetic pulse Earth current transients Bounded charges

Subsequent kA

98%

95%

4

80%

50%

20 46 4.6

5% 90

12

Typical Lightning Current Value

C Current



First negative kA

Cumulative Frequency

Time Typical Lightning Discharge www.EngWorks.ca

Grounding Fundamentals

187

Difference between Lightning and High Voltage Electricity Factor

Lightning

High Voltage

Energy Level

25 kA typical, millions of volts

Usually much lower

Time of Exposure

Brief, instantaneous

Prolonged

y Pathway

Flashover,, orifice

Deep, p, internal

Burns

Superficial and minor

Deep with major injury j y

Cardiac

Primary & secondary arrest, asystole

Fillibration

www.EngWorks.ca

Grounding Fundamentals

188

Incidence of Lightning 

Lighting varies with    

Terrain Altitude Latitude Time of the year

Number of flashes per square kilometre per year

www.EngWorks.ca

Grounding Fundamentals

189

Lightning Protection 

Lightning strikes cannot be stopped but their energy can be diverted in a controlled manner  Strike frequency goes up with the square of the height above the average terrain  Damage g is caused by y the lightning g g energy gy taking g a random – high g impedance path to ground



3 components to a lightning protection system  Air terminal or electrode the intercepts the surge  Low impedance conductor system to ground  Ground electrode to dissipate the energy



If all equipment within an elevated potential area is bonded together, the potential for damage is minimized

www.EngWorks.ca

Grounding Fundamentals

190

Inherent Grounding 

Inherent grounding  Metallic equipment, tanks and structures in direct contact with the ground do not require additional grounding if:  The thickness of tanks, vessels and process equipment is greater than 5mm and are capable of withstanding a direct lightning strike without damage  Indirect contact with the ground (self grounded)  Sealed to prevent the escape of liquids, vapours or gas





Mostt petrochemical M t h i l ffacilities iliti are iinherently h tl grounded d d and d require no additional lightning protection Equipment that may require special consideration  Open floating roof tanks  Tank farms incorporating a containment liner

www.EngWorks.ca

Grounding Fundamentals

191

Bounded Charge Dissipation 

Bounded Charges  Occurs when a storm cell induces an electrical charge on everything beneath it  Consideration with open floating roof tanks

--------------Teflon seal isolates + roof from tank + + + + +

Bounded Charge

++++++++++++++ Flammable Product

Floating Roof tank + + + + + +

+++++++++++++++++++++++++++++ www.EngWorks.ca

Grounding Fundamentals

192

Bounded Charge Dissipation

-

Bounded Charge

++++++++++++++ Flammable Product

Floating Roof tank -

Incendive discharge to ground

--------------------------------------------------- -

www.EngWorks.ca

Grounding Fundamentals

193

Protection against Lightning 

Floating Roof Lightning Protection  Floating roof cable connection  Grounding Shunts (Not Recommended)

-------Cable connection to floating roof

Bounded charge Dissipated with Lightning strike Grounding shunt

Flammable Product

www.EngWorks.ca

Grounding Fundamentals

194

Methods of Lightning Protection 

Conventional air terminal  Provides a low impedance path to ground

+ + + + - - - - - --

+

 Lightning rods (sometimes called Franklin rods)  Conducting masts  Overhead wires

RA = 0.84 x

h0.6

x

RA

I0.74

RA = Attractive Att ti radius di iin meters t h = height of lightning mast in meters I = Peak lightning current in kA

www.EngWorks.ca

RA

Grounding Fundamentals

+

+

+ + + +

+

195

Dissipative Array System (DAS) 

Claim of the technology is to dissipate a charge before a lightning strike occurs  No scientific proof that this in fact occurs  Renamed the Charge Transfer System (CTS) technology in recent years  Still considered ineffective

www.EngWorks.ca

Grounding Fundamentals

196

Early Stream Emission Air Terminals 

Consist of lightning rods incorporating a device that triggers the early initiation of a lightning strike  Effectiveness is also questioned

www.EngWorks.ca

Grounding Fundamentals

197

Lightning Surge Protection 

Transient Overvoltages can damage electrical equipment  Result in insulation breakdown and eventual failure



Mitigated g by y  Surge arrestors  Equipment insulation standards

Lightning Strike EMF Travelling Wave

Surge Voltage Wave

Line www.EngWorks.ca

Grounding Fundamentals

198

Surge Protection

Diminished Surge Voltage Wave 35kV

Transient Voltage Surge 70kV

13 8kV 13.8kV 25kV

Surge Arrestor

Is Surge Voltage is Surge suppressor Induced on secondary reduces surge g voltage g winding by capacitive to below BIL of transformer coupling effect

www.EngWorks.ca

Grounding Fundamentals

199

Equipment Insulation Voltage Withstand Requirements q 



Basic Impulse Level (BIL) is used to describe the insulation class of electrical equipment q p Based on the voltage rating of the equipment  Based on specified p crest value kV  Specified in the various equipment standards

Surge Voltage and Current Wave www.EngWorks.ca

Grounding Fundamentals

200

Surge Arrestors 



Surge arrestor must have a high resistance i t under d normall conditions diti and a very low resistance under surge conditions Metal oxide arrestor is the industry standard  Consist of a series connection of zinc oxide elements

www.EngWorks.ca

Grounding Fundamentals

201

Surge Arrestors 

Class of Surge Arrestors (IEEE Std C62.11)     

Station Class Intermediate Class Distribution Class – Heavy duty Distribution Class – Normal duty y Secondary

600V Secondary Surge Arrestor www.EngWorks.ca

Distribution Class Surge Arrestor

Grounding Fundamentals

202

Surge Arrestor Installation Considerations 









Should be mounted as close as possible to the transformer bushings Arrestor must be coordinated with the BIL of the equipment it is protecting A dedicated “down down lead” lead conductor to ground required for each arrestor Down lead conductor should be mechanicallyy and thermallyy capable of handling the surge voltage to ground Down lead should be as short as possible with no changes in direction  Minimum radius of 200mm  No bends greater than 90º

www.EngWorks.ca

Grounding Fundamentals

203

Lightning Arrestors – CEC Requirements 10-1000 Lightning Arrestors on Secondary Services 1) Grounding conductor shall be as short (and straight) as possible 2) The lightning arrestor grounding conductor may connected to the: a) b) c) d)

Grounded service conductor Common grounding conductor Service equipment grounding conductor Separate grounding conductor

Common ground conductor Grounded service conductor

Service i equipment i grounding di conductor d www.EngWorks.ca

Grounding Fundamentals

204

Lightning References 





NFPA 780 “Standard for the Installation of Lightning g g Protection Systems” provides detailed guidance on the design of lightning protection systems” API 2003 “Protection against I iti Ignitions A Arising i i outt off St Static, ti Lightning and Stray Currents” IEC 61024 “P “Protection t ti off St Structures t Against Lightning – Part 1”

www.EngWorks.ca

Grounding Fundamentals

205

NFPA 780 



Installation standard for lightning protection systems for building structures and facilities handling flammable vapors p g gases and liquids q Does not apply to electric generating, transmission and d di distribution t ib ti systems t

www.EngWorks.ca

Grounding Fundamentals

206

Electronic Equipment Grounding

Section 7

History 

Grounding principles for communication systems were developed to meet the operational characteristics h t i ti off th the equipment i t  Early telegraph systems used a two wire circuit path  Later systems used the earth return as the signal path

Morse Landline Telegraph System www.EngWorks.ca

Grounding Fundamentals

208

History 

Earth return offered several advantages  Iron wire was used for telegraph conductors  The use of an “earth” ground doubled the distance a circuit could be run

 Eliminated one wire from the circuit 

Problems endured:  Quality of the signal was effected by weather  Leakage current to ground during wet weather  Resistance of the return ground path varied with soil conditions  Presence of “foreign” voltages

www.EngWorks.ca

Grounding Fundamentals

209

History 

Early development of the telephone system also relied on the th solid lid DC reference f ground as the return path  lines were p particularly y noisy, y, picking up electrical noise from power lines, adjacent telephone lines, telegraph g p lines, streetcars, and machinery



The grounded system was later replaced with a system employing two wires per telephone line eliminating most of the noise

www.EngWorks.ca

Grounding Fundamentals

210

Electronic Equipment Grounding Terms 

Signal Common Grounding  referred to as the “DC Signal Common”  Zero reference system for data lines  Very sensitive to transient voltages



DC Power Supply Reference Ground Bus  -ve terminal on a DC power Supply



Equipment Ground Bus  Used for equipment chassis bonding  Often referred to as the safety ground bus



Variety of other terms used (depending on manufacturer)  AC Safety Ground, Computer Reference Ground, DC Signal Common, Earth Common, DC Ground Bus

www.EngWorks.ca

Grounding Fundamentals

211

Electronic Systems Grounding 

Most electronic computer systems employ a DC reference ground  Required for logic circuits



Problems occur when the DC reference ground is tied to the AC safety ground  With the logic circuits referenced to the equipment chassis ground, any small amount of chassis potential caused by current flow in the grounding of the device could cause reference error in the equipment.

www.EngWorks.ca

Grounding Fundamentals

212

Noise in Sensitive Circuits 

Errors result when the noise is greater than the actual signal g  Results in parity check errors  signal is ignored if check fails

30-50V L i Logic Signal Noise does not impact signal Noise cause parity check errors 3-5V Logic Signal

www.EngWorks.ca

Grounding Fundamentals

213

Circuit Noise Sensitivity Measurement 

Signal to Noise Ratio



 Measure of the interference in a communications circuit  Measured in dB SNR = 10log S dB N Bit Error Rate  Measure of the number of bits received to those in error Bit E Error Rate

10-66 10-7 10-88 10-9

0 www.EngWorks.ca

10

20

30

40

50

60

SNR (dB)

Grounding Fundamentals

214

Categories of Noise 

Traverse Mode Noise  A disturbance that appears pp between two active conductors in an electrical system  Measurable between two line conductors or from line to neutral t l  Originates from within the power system A B

V

C N G

www.EngWorks.ca

Grounding Fundamentals

215

Categories of Noise 

Common Mode Noise  Appears pp simultaneously y in each active conductor  The term "common" refers to the fact that identical noise appears on both the active and neutral wires  Generally involves the ground conductor A B C

V

N G

www.EngWorks.ca

Grounding Fundamentals

216

Typical Problems Associated with Electronic Systems y Grounding g Electronic Equipment Symptoms

Electrical Condition

Temporary or chronic data hang-ups Slow data transfers, multiple retries I/O Damage

Different signal reference levels I d Induced d currents t on cable bl

Intermittent lock-ups Corrupted Signals I/O damage

Transient voltages and currents

Random data errors Slow transfer in analog circuits

Stray currents and common mode noise in equipment gro nding cond grounding conductor ctor

www.EngWorks.ca

Grounding Fundamentals

217

Electronic Equipment Grounding 

Computers require a “quiet” ground where no voltage transients or electromagnetic g noise occurs  Stabilize input voltage levels  Act as a zero voltage reference point for circuits





Led to the practice of installing an “Isolated Ground” system specifically for electronic equipment This practice was in direct conflict with the CEC which requires that all grounding systems be i t interconnected t d  CEC is concerned with safety – not with performance

www.EngWorks.ca

Grounding Fundamentals

218

Principles of Noise Mitigation 

For noise to be a problem  Requires q a noise source of sufficient magnitude g  Some means of communicating the unwanted noise to the electronic circuit  Galvanic coupling  Electrostatic / Capacitive coupling  Magnetic g or Inductive coupling p g



Solving the problem involves either reducing the amplitude of the noise voltage or effectively isolating the circuit from the noise source

www.EngWorks.ca

Grounding Fundamentals

219

Source of Electrical Noise 

Motor starts  High g current in-rush is impressed p on the communications circuit



Fluorescent lighting  High frequency noise associated with the ballast operation



Switching power supplies or VFD systems  High frequency noise associated with switching power supplies



High Hi h voltage lt surges d due tto lilightning ht i strikes t ik and d electrical faults

www.EngWorks.ca

Grounding Fundamentals

220

Galvanic Coupling 

Occurs when two circuits share a common conductor  Examples: Telephone circuits that used the same common return t path th as DC ttram lilines iin the early days



Easily solved by separating the circuits by using separate return conductors

www.EngWorks.ca

Grounding Fundamentals

221

Electrostatic/Capacitive Coupling 



Form of coupling that is proportional p p to the capacitance between the noise source and the signal wires Function of:  Distance from the noise source to the signal wires  Length g of the signal g wires  Strength of the noise voltage  Frequency of the noise voltage

www.EngWorks.ca

R1 I1 C1 I2 R2 Noise source

Grounding Fundamentals

I3

RL

C2 E

dV dT

222

Electrostatic/Capacitive Coupling 

Mitigation     

Shielding g of the signal g wires Separating the source from the noise Reducing the amplitude of the noise voltage Reducing the frequency of the noise voltage Twisting of the signal wires

www.EngWorks.ca

Grounding Fundamentals

223

Shielding 

Conductor shield provides a lower impedance path for the noise current to flow I2 R1

Copper braid (85% coverage) provides a noise reduction ratio of 100:1 Aluminum Mylar tape with drain wiren provides a noise reduction ratio of 6000:1

C3

I3 I5

I4

RL

C4

R2 I1 C1

C3 and C4 are 1/100 C1 and C2

www.EngWorks.ca

Noise source E

Grounding Fundamentals

C2 dV dT 224

Shield Grounding 



Shielding on instrumentation and communication circuits eliminates the electrostatic induction into wires carrying low signal voltages Shielding g method may y be  Braided copper wire  Metalized foil, with a copper drain wire  Metal conduit (if steel conduit, this also serves as a magnetic shield)



Shields must be grounded  One end only for frequencies up to 1 Mhz  Two or more locations for frequencies > 1 Mhz

www.EngWorks.ca

Grounding Fundamentals

225

Magnetic or Inductive Coupling  Depends of the rate of change and the mutual inductance between the source of noise and the signal g wires  Influenced by:  Magnitude of the noise current  Frequency of the noise current  Area A enclosed l db by the h signal i l wires  Distance between the noise sources and d th the wires i www.EngWorks.ca

Grounding Fundamentals

226

Magnetic or Inductive Coupling 

Mitigation  Twist the signal g conductors  This results in lower noise due to the smaller area for each loop. This means less magnetic flux to cut through the loop and consequently a lower induced noise voltage  Noise voltage that is induced in each loop tends to cancel out the noise voltages from the next sequential loop

Inductive coupling is reduced by ratios varying from 14:1 for a fourinch lay to 141:1 for one-inch lay

www.EngWorks.ca

Grounding Fundamentals

227

Magnetic or Inductive Coupling 

Mitigation  Enclose the signal g wires with a magnetic g shield  The magnetic flux generated from the noise currents induces small eddy currents in the magnetic shield which then create an opposing magnetic flux Ø1 to the original flux Ø2  Galvanized steel conduit is an effective magnetic shield Placing parallel (untwisted) wires into a steel conduit will provide a noise reduction of approximately 22:1

www.EngWorks.ca

Grounding Fundamentals

228

Physical Segregation 

Separate the noise sources from the noise sensitive equipment q p  Cable spacing based on susceptibility levels defined by IEEE 518  Level 1 – High: Analog signals less than 50V and digital signals less than 15V  Level 2 – Medium: analog signals greater than 50V  Level 3 – Low: Switching signals greater than 50V  Level 4 – Power: Voltages g 0 – 1000V;; Currents 20–800A

www.EngWorks.ca

Grounding Fundamentals

229

Physical Segregation Level Level

Separation

1

2

2 – 30mm 30

1

3

3 – 160mm

1

4

4 – 670mm

Cables Contained in Separate Trays

Level Level Separation 1 2 2 – 30mm 1 3 3 – 110mm 1 4 4 – 460mm One Cables in Conduit and the other In Trayy

Level Level Separation 1 2 2 – 30mm 1 3 3 – 80mm 1 4 4 – 310mm Both Cables in Conduits www.EngWorks.ca

Grounding Fundamentals

230

Electrical Segregation

Y

Y

Y Shielded Isolation Transformer

M

M

M

M

UPS M

M

~

Y

Sensitive Loads

Worst Case Sensitive Loads are subject to voltage fluctuations caused by motor loads

=

=

~

Better Best

www.EngWorks.ca

Grounding Fundamentals

231

Separately Derived AC Power Distribution System y using g an Isolation Transformer 

Isolates power to the control system from the rest of the AC distribution system y  Provides good line regulation and transient filtering



Transformers should be of a shielded design g  Provide superior noise isolation using the same concepts used for shielded cables





Input power to the transformers should be sourced from the highest line voltage available K factor f t transformers t f should h ld be b considered id d if th the control system load employs a large number of switching power supplies

www.EngWorks.ca

Grounding Fundamentals

232

Isolation Transformer Grounding

www.EngWorks.ca

Grounding Fundamentals

233

Separately Derived AC Power Distribution System y using g an UPS 



Provides a continuous power supply to the control system y in the event of a p power interruption p Protects the control system from power system surges g  Isolation transformers cannot prevent surge events from being transmitted to the load without additional surge protection



Provides a “conditioned” AC power supply to the control system  Completely disconnects the control system power supply from the source providing superior isolation from power system transients and noise

www.EngWorks.ca

Grounding Fundamentals

234

1 Phase UPS

www.EngWorks.ca

Grounding Fundamentals

235

3 phase UPS

www.EngWorks.ca

Grounding Fundamentals

236

UPS Grounding



UPS configuration with common source for UPS and bypass yp circuit  Does not meet the definition of a separately derived circuit  Common mode noise attenuation may be a problem

www.EngWorks.ca

Grounding Fundamentals

237

UPS Grounding



Addition of a bypass transformer meets the definition of a separately derived source  Improved common mode noise attenuation  Neutrals in UPS and bypass transformer are connected  Power distribution center must be within 15m of the UPS

www.EngWorks.ca

Grounding Fundamentals

238

UPS Grounding



Best configuration for common mode noise attenuation  No restriction on distances  Allows more flexibility in UPS voltages

www.EngWorks.ca

Grounding Fundamentals

239

Multiple UPS Grounding Scheme

www.EngWorks.ca

Grounding Fundamentals

240

Ground Loops 

Occurs when there is more than one ground connection path between two pieces of equipment  ground current may take more than one path to return to the grounding electrode  form the equivalent of a loop antenna which very efficiently picks up interference currents  Conductor lead resistance transform the currents into voltage fluctuations



Consequences  Ground reference in the system is no longer a stable potential  Signals ride on the noise  Noise becomes part of the program signal



Example  Audible 60hz noise in your stereo system

www.EngWorks.ca

Grounding Fundamentals

241

Ground Loop U it A Unit Internal Connection

+ Output -

Unit B

Communication Cable

+

Input -

Power Ground

Power Ground

1A Current Flowing 0.1V

0.2V Low Resistance 0.1Ω

Stray Current in Ground Causes Current to Flow in communication conductors

www.EngWorks.ca

Grounding Fundamentals

242

Ground Loop – Motor Start Unit A Internal Connection

+ Output -

Communication Cable

Unit B +

Input -

Power Ground

From Electrical Sources

Power Ground

M Motor Start

Motor frame bonded To ground www.EngWorks.ca

Grounding Fundamentals

243

Ground Loop Mitigation 

Add one or more separate grounds  Not N t CEC code d compliant li t Unit A Internal Connection

+ Output -

Communication Cable

Unit B +

Input -

Power Ground

C Current t Fl Flowing i Mi Minimized i i d High Resistance

www.EngWorks.ca

Separate p Instrumentation Ground

Grounding Fundamentals

Motor frame bonded 244 To ground

Ground Loop Mitigation 

Interrupting the continuity of the grounding conductor  Shielded communication cables

Unit A + Output -

Communication Cable

Unit B +

Input -

Power Ground

www.EngWorks.ca

Interrupt ground path here

Power Ground

Grounding Fundamentals

245

Ground Loop Mitigation •

Control the path of the ground current • Use an insulated ground receptacle Unit A + Output Power Ground

Unit B

Communication Cable

+

Input -

Isolated Instrument ground

Power Ground

Insulated ground conductor Single point ground

www.EngWorks.ca

Grounding Fundamentals

246

Isolated Ground Receptacle 



Helps to limit electrical noise introduced into a circuit via the grounding conductor Establishes a dedicated ground path connected to ground at one point only

Conventional Receptacle www.EngWorks.ca

Isolated Ground Receptacle Grounding Fundamentals

247

Isolated Ground Receptacle

Power Transformer

Branch B h Ci Circuit it Panelboard Junction Box

NEMA IG#5-15R2 Isolated Ground Receptacle

Metal Device Box

System Ground

www.EngWorks.ca

Insulated Isolated Ground wire

Conduit or Cable

Bare Bonding Conductor or Conduit

Grounding Fundamentals

248

Concept of a Single Point Ground System 



Poor or faulty grounds are the most common causes of control system y faults The best way of insuring the performance and reliabilityy of a control system y is to employ p y a single g point ground network system  Consists of an organized system of ground wiring that t terminates i t in i a single, i l d dedicated di t d point i t on th the plant l t ground d grid  Provides a clean reference for control signals

www.EngWorks.ca

Grounding Fundamentals

249

Single Point Ground System

www.EngWorks.ca

Grounding Fundamentals

250

Single Point Ground System Multiple Enclosures

www.EngWorks.ca

Grounding Fundamentals

251

Instrument Tri-Ground System Main Transformer

Ground C d t Conductor NGR

Lightning Arrestor Connection to system ground may be temporarily disconnected to isolate ground loop

System Ground AC Ground

Instrument Tri Ground www.EngWorks.ca

Grounding Fundamentals

252

Intrinsically Safe Circuit Grounding H Hazardous d A Area

Field Device IS Apparatus

Non Hazardous Area IIntrinsic i i Safe Barrier

Controll C System Interface

Associated pp Apparatus Interconnecting Wiring System

www.EngWorks.ca

Grounding Fundamentals

253

Intrinsic Safety – Simple Field Devices Non Hazardous

Hazardous Location Th Thermocouple l

Controller Simple Device

Internal Fault Controller

Internal Fault Controller

Explosion IS Barrier

IS Ground www.EngWorks.ca

Grounding Fundamentals

Device is Considered Safe under Fault C di i Conditions 254

Intrinsic Safe Barrier Circuit Protects Zener from Destruction Limits input current

Limits the output currentt

Hazardous Area

Safe Area Fuse

Control System Interface

Zener Z Diodes

Limits the output voltage lt www.EngWorks.ca

Current Limiting Resistor

Field Device

IS Ground

Grounding Fundamentals

255

Intrinsic Safety - Grounding

Incorrect Ground Scheme

www.EngWorks.ca

Correct Ground Scheme

Grounding Fundamentals

256

Intrinsic Safety - Grounding   

Extremely important for the safe operation of an IS wiring systems M t be Must b visibly i ibl id identified, tifi d secure and d accessible ibl Must be capable of carrying the maximum fault current  #12 AWG minimum conductor size

 

Total resistance must not exceed 1Ω Must be insulated from f ground in all places except at the point of connection to the ground electrode  Duplicate up ca e g ground ou d co conductors duc o s required equ ed



Aluminium must not be used as a ground conductor material  Potential for electrolytic corrosion

www.EngWorks.ca

Grounding Fundamentals

257

IEEE Standard 1100 ((Emerald Book)) 

Recommended engineering principles and p practices for p power and g grounding g sensitive electronic equipment  Provides consensus in an area where conflicting information has prevailed  Excellent reference that describes the many challenges associated with grounding electronic equipment  Power related noise control  Signal related noise control

www.EngWorks.ca

Grounding Fundamentals

258

Station Electrode Design Section 8

Ground Grid Design

www.EngWorks.ca

Grounding Fundamentals

260

Ground Grid Design Fundamentals 

In the event of a fault or transient phenomena ((lightning g g or switching g transients)) the g ground g grid must  Ensure personnel safety  Protect equipment against damage



Design Considerations  Grid must be able to withstand the maximum ground current without damage  Limit the ground potential rise between two points to a safe value

www.EngWorks.ca

Grounding Fundamentals

261

CEC Code Requirements 

Section 10 “Grounding and Bonding” addresses grounding g g electrodes for facilities operating p g at less than 750V to ground  Requirements are minimal



Section 36 “High Voltage Installations” addresses the grounding of facilities operating at more than 750V to ground d  Requirements are in addition to those defined in Section 10  More substantial in nature and therefore require a deeper understanding of ground electrode theory

www.EngWorks.ca

Grounding Fundamentals

262

CEC Section 10 Requirements 

CEC Section 10 does not specify a minimum ground resistance for a g g grounding g electrode but specifies the acceptable methods of obtaining a grounding electrode 

NEC specifies a ground resistance of 25Ω or less

10-700 Grounding G Electrodes shall consist off a) Manufactured grounding electrodes b) Field Fi ld assembled bl d grounding di electrodes l t d c) In-situ grounding electrodes

www.EngWorks.ca

Grounding Fundamentals

263

Manufactured Ground Electrode 

Must be certified to CSA C22.2 No.41 “Grounding and Bonding g Equipment” q p  Rods must be driven to their full length and separated by a minimum of 3m  Connected by a bonding conductor sized by Table 17

R d El Rod Electrode t d www.EngWorks.ca

Plate Electrode Grounding Fundamentals

264

Field Assembled Ground Electrode 

Min 6m bare copper conductor buried or encased in concrete

 conductor must be encased within the bottom 50 mm of a concrete foundation footing, with the footing  in direct contact with the earth,, at not less than 600 mm below finished grade Field Assembled Ground Electrode www.EngWorks.ca

Grounding Fundamentals

265

In-situ Ground Electrode  



Copper water pipe Metal reinforcement of concrete slabs, concrete pilings, and concrete foundations Iron pilings, when they are in significant contact with earth 600 mm or more below finished grade

In-situ Grounding Electrode www.EngWorks.ca

Grounding Fundamentals

266

10-700 Grounding Electrodes (5) Where local conditions such as rock or permafrost prevent a rod or g p grounding gp plate from being g installed at the required burial depth, a lesser depth shall be permitted

Horizontal Ground Rod Installation www.EngWorks.ca

Grounding Fundamentals

267

Other Section 10 Requirements 



Lightning rod systems must be connected to ground using g a separate p g grounding g electrode that is not used as the grounding electrode for any other system Where a facilityy incorporates p more than one g ground electrode for lightning, communication or other systems  Must be separated by a minimum of 2m  Bonded together by a minimum #6AWG conductor

www.EngWorks.ca

Grounding Fundamentals

268

CEC Section 36 Requirements 36-302 Station ground electrode Every outdoor station shall be grounded by means of a station ground electrode that shall meet the requirements of Rule 36-304 and shall a)

consist of a minimum of four driven ground rods not less than 3 m long and 19.0 mm in diameter spaced at least the rod length apart and, where practicable, located adjacent to the equipment to be grounded;

b) have the ground rods interconnected by ground grid conductors not less than No. 2/0 AWG bare copper buried to a maximum depth of 600 mm below the rough station grade and a minimum depth of 150 mm below the finished station grade; and c) have the station ground grid conductors in Item (b) connected to all non-current-carrying non current carrying metal parts of equipment and structures www.EngWorks.ca

Grounding Fundamentals

269

Distribution Utility Standard Ground Electrode Design g

www.EngWorks.ca

Grounding Fundamentals

270

Distribution Utility Standard Ground Electrode Design g

www.EngWorks.ca

Grounding Fundamentals

271

CEC Requirements for Station Ground Resistance 36-304 Station ground resistance (see Appendix B) 1) The maximum permissible resistance of the station ground electrode shall be determined by the maximum available g ground fault current injected j into the ground by the station ground electrode or by the maximum fault current in the station, and the ground d resistance i t shall h ll b be such h th thatt under d allll soilil conditions that exist in practice (e.g., wet, dry, and frozen conditions) …..

www.EngWorks.ca

Grounding Fundamentals

272

CEC Section 36 – Ground Potential Rise 36-304 Station ground resistance ….the the maximum ground fault current conditions shall limit the potential rise of all parts of the station ground g g grid to 5000V 2) In addition to subrule (1), the touch and step voltage at the edge, within, and around the station grounding electrode…..shall not exceed the tolerable values specified in Table 52

www.EngWorks.ca

Grounding Fundamentals

273

CEC Section 36 – Tolerable Touch and Step p Voltages g

www.EngWorks.ca

Grounding Fundamentals

274

IEEE Standard 80 

IEEE Standard 80  Defines the safe limits for touch and step potentials  Provides guidance on the design of ground systems for outdoor substations  Primarily Pi il used db by utilities tiliti for grounding on high g substations voltage

www.EngWorks.ca

Grounding Fundamentals

275

Ground Potential Rise (GPR) 

Ground potential rise is a function of the current g injected j into the earth and the soil magnitude resistivity  Measured with respect to a remote point  May vary from a few meters to several hundred meters away



5000V criteria specified in the CEC is based on the maximum GPR communication circuits are d i designed d tto h handle dl

GPR = IG X Rg

IG = Maximum Grid Current Rg = Grid Resistance www.EngWorks.ca

Grounding Fundamentals

276

Ground Potential Rise 3 wire i ttransmission i i with ith no Metallic return path

G

∆

A

∆

N

N G

B C

Generator Transformer

Ground Fault

IG

NGR

EG

GPR

E th Earth Rg Ground Path

www.EngWorks.ca

Grounding Fundamentals

277

System Ground Fault Return Path Non Metallic conduit With no bonding conductor

Y

0.1Ω 500V

Ig=5kA Ground Return Path

Lack of bonding conductor forces ground fault return path through the earth creating personnel hazard www.EngWorks.ca

Grounding Fundamentals

278

System Ground Fault Return Path Metallic conduit with bonding conductor Low Impedance Ground Return Path

Y Ig=5kA

High Impedance Ground Return Path

Bonding conductor provides low impedance path to source: Stray current is minimized with improved safety www.EngWorks.ca

Grounding Fundamentals

279

Current GF Path with Local Source

Y

Y

Multiple low impedance ground paths limit the ground potential rise within th station the t ti www.EngWorks.ca

Grounding Fundamentals

280

Current GF Path with Remote Source

Overhead ground wire current path

Y

Y

Stray Current Paths

Multiple high impedance ground current paths back to source www.EngWorks.ca

Grounding Fundamentals

281

Ground Potential Rise (GPR) 

Grid system must limit the ground potential rise (GPR) between two points to a safe value  GPR can cause hazardous voltage in the form of Step & Touch Potentials  May occur in location remote to the actual fault

 Safe values of GPR, Touch and Step Voltages are determined by the human tolerance to shock currents  Function of current magnitude, g , duration and frequency q y

GPR = IG X Rg IG= Maximum Grid Current Rg = Grid Resistance

www.EngWorks.ca

Grounding Fundamentals

282

Step Voltage ESTEP

Potential Rise above remote earth during short circuit R1

IF

ESTEP

IF RF RK

R2 R0

RF

RK

R1 www.EngWorks.ca

R2

R0 Grounding Fundamentals

283

Touch Voltage ETOUCH

Potential Rise above remote earth during short circuit

IF

ETOUCH

IF

RF

R1

RK

RF/2 RK R0 RF/2

R1 www.EngWorks.ca

R0 Grounding Fundamentals

284

Touch and Step Potential IEEE 80

ρ = resistivity of earth beneath surface ρs = surface material resistivity (Ω . m) hs = thickness of surface material in m www.EngWorks.ca

Grounding Fundamentals

285

Relationship between GPR, Touch and Step p Potentials

Y

Es

Et

Emesh

GPR

Remote Earth

www.EngWorks.ca

Grounding Fundamentals

286

Mesh Voltage 

Defined as the maximum surface voltage g p potential difference between a grid conductor and and a point between two grid conductors  Th Theoretical ti l maximum i touch voltage found within a ground grid

www.EngWorks.ca

Grounding Fundamentals

287

Touch and Step Potential  1 1.

2.

Two ways of making a grounding system safe Minimize the touch and step voltages that may appear at any point within the substation and around its perimeter p Increase the tolerable touch and step voltages by placing a high resistivity material over rough grade  



Asphalt Crushed rock

Both methods are typically used together

www.EngWorks.ca

Grounding Fundamentals

288

Ground Surface Potential Gradients

High ground resistance increases step potential

10kA

Ground Rod

R1 Re1

Gn nd Surface Potential P

30kV

R = R1+Re1 = 3 ohm 0 Volts Infinite Earth

8kV Step Potential 0

www.EngWorks.ca

Distance from Rod Grounding Fundamentals

289

Ground Surface Potential Gradients Multiple ground rods reduce the ground surface potential low resistance surface reduces step potential

10kA

Gn nd Surface Potential P

R1+Re1 = 3 ohms

R1 4kA

R2

0 ohms

Re1

6kA

Re2

R2+Re2 = 2 ohms 0 Volts Infinite Earth

12kV 0

www.EngWorks.ca

Infinite Earth (0V) Grounding Fundamentals

290

Symmetrical Grid Current 





The current that causes the ground potential rise in a grid is from a remote source g Only a portion of the current is responsible for the ground p g potential rise Multiple return paths include  Overhead ground conductors  Cable shields



The current flowing into the ground that is responsible for the GPR is adjusted by a split factor to incorporate the effect of the multiple paths

www.EngWorks.ca

Grounding Fundamentals

291

Symmetrical Grid Current Ig = Sf x If Ig = RMS Symmetrical Grid Current Sf = Split factor (Current Division Factor) If = RMS value of the symmetrical ground fault current Sf may be estimated using computer programs or by graphical analysis  Typically ranges between 10 – 70% of If  Refer to IEEE 80 for more information www.EngWorks.ca

Grounding Fundamentals

292

Split Factor Split factor accounts for the multiple current paths that will occur in a fault situation Overhead ground wire current path

Y

Y

If

Ig = Sf x If

www.EngWorks.ca

Grounding Fundamentals

293

Split Factor Graphical Estimate IEEE 80 Annex C Curve most likely to be used for a single circuit customer owned substation

28%

The symmetrical grid current (Ig ) would be approximately 28% of the total fault current for a substation with a 2.5Ω grid resistance

2.5Ω Substation www.EngWorks.ca

Grounding Fundamentals

294

Grid Current for Design IG = Cp x Df x Ig IG = Maximum Grid Current Cp = Estimated growth factor during station life span Cp = 1 ffor zero growth g Df = Decrement factor for the duration of the fault Ig = RMS value off the symmetrical y ground g fault f current

www.EngWorks.ca

Grounding Fundamentals

295

Decrement Factor Decrement factor accounts for the total asymmetric fault current flowing between the grounding system and the surrounding earth Df Fault Duration tf Sec 0.008 01 0.1 0.25 0.5

Subtransient S b i T Transient i Network N k Network (0-5 cycles) (5-30 cycles) www.EngWorks.ca

Cycles 0.5 6 15 30

1.65 1 25 1.25 1.1 1.0

S Steady State S Network (>30 ( 30 cycles))

Industrial Power System Protection and Control

296 296

Design Information Provided by Utility Current values to be used in the design of the station ground d grid id

www.EngWorks.ca

Grounding Fundamentals

297

Ground Resistance  

The “WildCard” in the grounding design Ground grid resistance varies with soil conditions and may change over time  Changing Ch i water t ttable bl Resistivity of the ground will change in drying or drought conditions

 Chemical content of soil Presence of salts decrease resistivity

 Frozen ground or permafrost conditions Consideration in all Canadian g grounding g situations www.EngWorks.ca

Grounding Fundamentals

298

Soil Resistivity (Ω . cm) = 100 x (Ω . M) Medium

Resistivity (Ω . cm) Minimum

Surface Soil, Loam

100

Clay, Shale, Gumbo

300

Sand and Gravel Limestone

Average

Maximum 5,000

4,000

20,000

5 000 5,000

10 000 10,000

500

400,000

Granite, basalt

1,000,000

Low Hills, Rich Soil

3,000

Medium hills, Medium Soil

20,000

St Steep Hills, Hill R Rocky k S Soilil

50 000 50,000

Sandy, dry coastal country

30,000

50,000

500,000

Freshwater Lake

10,000

20,000

20,000,000

Sea water

2,000

10,000

20,000

www.EngWorks.ca

Grounding Fundamentals

299

Effects of Moisture on Resistivity Moisture Content 0 2.5 5 10 15 20 30

www.EngWorks.ca

Resistivity Ω Ω-cm cm Top Soil Sandy Loam > 109 > 109 250,000 150,000 165,000 43,000 53,000 18,500 19,000 10,500 12,000 6,300 6,400 4,200

Grounding Fundamentals

300

Effects of Temperature on Resistivity for Sandyy Loam

www.EngWorks.ca

Temperature ºC 20

Resistivity Ω cm Ω-cm 7,200

10

9,900

0 (Water)

13,800

0 (Ice)

30,000

-5

79,000

-15

330,000 Grounding Fundamentals

301

Seasonal Variation in Earth Resistance

19mm Rod in Stony Clay Soil Curve 1 – 1m below surface C Curve 2 – 3m 3 b below l surface f

Moisture and temperature p is more stable at g greater depths p below the surface

www.EngWorks.ca

Grounding Fundamentals

302

Effect of Chemicals on Earth Resistivity for Sandyy Loam Effect of Salt on Resistivity of Soil ((Moisture 15% Temp p 17ºC)) Added Salt % by Weight of Moisture

Resistivity (Ω-cm)

0

Effect of Temperature on Resistivity of Soil with Salt (20% Moisture 5% Salt) Temperature C ºC

Resistivity (Ω-cm)

10,700

20

110

0.1

1,800

10

142

1.0

460

0

190

5

190

-5 5

312

10

130

-13

1,440

20

100

www.EngWorks.ca

Grounding Fundamentals

303

Electrode Resistance

ρ(Ω.cm) Ω Rg (rod) = 335 cm Applies to 3m ground rod and is accurate within 15% www.EngWorks.ca

Grounding Fundamentals

304

Multiple Ground Rod Resistance 

Resistance of a grounding system of 2-24 rods placed on rod length p g apart p will p provide a g grounding g resistance divided by the number of rods multiplied by the factor F taken from Table 14 IEEE Std 142

www.EngWorks.ca

Grounding Fundamentals

305

Alternate Formulas for Ground Rod Resistance Contact resistance of one ground rod ρ 4L - 1 R= Ln X 2πL a ρ = Soil resistivity in Ω Ω-cm cm L = rod length in cm a = rod diameter in cm

Ground Rod Separation D = 2.2 X L

Contact resistance of multiple ground rods Rn =

R n

X 2 – e-0.17(n - 1)

n = number of ground rods

www.EngWorks.ca

Grounding Fundamentals

306

Smoking Ground Rod 

Current loading capacity of a ground rod is a factor  Current p passing g through g an electrode will have a direct impact on the temperature and moisture conditions immediately surrounding the ground rod  Must M t be b checked h k d I=

34,800 X d X L

√ ρXt

I = Current loading gp per foot of rod length g d = rod diameter in meters L = Length in meters ρ = ohm meter t = seconds (3 (3.0 0 seconds is the value recommended by IEEE) www.EngWorks.ca

Grounding Fundamentals

307

Ground Rod Resistance to Earth Tutorial

Ground Rod Resistance to Earth Tutorial 1. Determine the resistance to earth for a ground rod system y consisting g of Qty y 4 – 10 foot long g 5/8” (16mm) ground rods spaced at 10’ intervals and interconnected connected together and placed in clay soil 2. Calculate the resistance to earth for a ground rod system t consisting i ti off Qty Qt 4 – 20 ffoott 5/8” (16mm) (16 ) ground rods interconnected together and placed in clay soil 3. Calculate the current loading capacity of the system

www.EngWorks.ca

Grounding Fundamentals

309

Ground Rod Resistance to Earth Tutorial Answers

4 - 10’ Ground Rods ρ(Ω.cm) Ω Rg (rod) = 335 cm Rg (rod) =

Average ρ for clay soil = 4000 Ω·cm

4000 Ω.cm = 11.94 Ω 335 cm

For 4 ground rods 11.94Ω 11 94Ω Rg (4 rods) = 4

X 1.36 = 4Ω

IEEE 142 Table 14

www.EngWorks.ca

Grounding Fundamentals

311

4 – 20’ Ground Rods Contact resistance of one ground rod ρ = Soil resistivity in Ω-cm ρ 4L - 1 L = rod length in cm R= Ln X 2πL a a = rod diameter in cm 4000 4(610) - 1 R= Ln X = 6.649Ω 1.6 2π(610) Contact resistance of multiple ground rods Rn = Rn = www.EngWorks.ca

R n

X 2 – e-0.17(n - 1)

6.649 4

X

n = number of ground rods

2 – e-0.17(4 - 1) = 2.32Ω

Grounding Fundamentals

312

Current Carrying Capacity of Ground Rod System y I=

34,800 X d X L

√ ρXt

I = Current loading of the ground rod system d = rod diameter in meters L = Length in meters ρ = ohm meter t = seconds (3.0 seconds typical value)

34,800 34 800 X 0.016 0 016 X 3.048 3 048 X 4 = 620 Amps 4 – 10ft Ground Rods I = 40 X 3

√

4 – 20ft Ground Rods I =

www.EngWorks.ca

34,800 X 0.016 X 6.1 X 4

√ 40 X 3 Grounding Fundamentals

= 1240 Amps

313

Primary and Auxiliary Ground Electrodes 

Primary Ground Electrode  Installed specifically p y for g grounding gp purposes p  Ground rods  Interconnecting wire mesh



A ili Auxiliary G Ground d El Electrode d  Installed for purposes other than grounding  Typically T picall ha have e limited ccurrent rrent carr carrying ing capacit capacity  Examples  Steel building gp piles  Steel reinforced concrete foundations  Rebar grounding

www.EngWorks.ca

Grounding Fundamentals

314

UFER Ground 

First used by the US Army to ground a series of bomb storage vaults in the vicinity of Flagstaff, Arizona  Dry desert conditions made for a very poor ground electrode system  Herbert Ufer developed an alternate electrode system based on using the steel rebar used to reinforce concrete



Concrete is inherently alkaline and hydroscopic (absorbent) in nature  The high pH provides a supply of ions to conduct current  soil around concrete becomes “doped” by the concrete

 has an effective resistance of 3000 Ω-cm 

Bases for the development of concrete encased electrodes

www.EngWorks.ca

Grounding Fundamentals

315

Concrete Encased Electrodes 

Can be used as part of an effective low resistance grounding system  Will typically lower the overall resistance of the ground



Very cost effective!  Adds very little cost to the installation  Reduces the amount of buried conductor required for an installation  Aids in reducing the amount of construction re-work  Buried B i d ground d conductors d t are a popular l it item tto dig di up d during i ttrenching hi operations



The 2006 NEC requires that rebar encased in concrete be incorporated into the system ground  No equivalent CEC requirement

www.EngWorks.ca

Grounding Fundamentals

316

Methods of Connecting Rebar to Building Steel 

Option 1 – Connect structural steel and rebar using ground wire  Requires electrical trade to be on on-site site during pouring of foundations

Bolted Connection to Steel C Copper Wi Wire

Ground Well

Grounding Compression p Connection or Cadweld

To ground grid

www.EngWorks.ca

Grounding Fundamentals

317

Methods of Connecting Rebar to Building Steel 

Option 2 - Tie the vertical rebar to anchor bolts and the steel columns are g grounded through g the bolts and nuts

Rebar welded To anchor bolts

Ground Well

To ground grid

www.EngWorks.ca

Grounding Fundamentals

318

Rebar Grounding Installation Considerations 

  



Rebar must be bare or zinc coated Minimum length – 6m Minimum diameter 13mm I t ll d iin a minimum Installed i i off 50mm of concrete

Surge Current Conductivity of Rebar in Concrete Rebar Diameter (in.)

Surge Ampere per Foot

0 375 0.375

3400

 Preferably y located near the bottom of the foundation

0.5

4500

Concrete must be in direct contact with earth

0.625

5500

0.75

6400

10 1.0

8150

www.EngWorks.ca

Grounding Fundamentals

319

Ground Conductors 

For a given application, ground rods are more effective than g ground g grid conductors  Ground rods will penetrate the frost level injecting current into unfrozen ground



Basic Requirements for the selection of a Ground Grid conductor  H Have sufficient ffi i t conductivity d ti it  Resist fusing and mechanical deterioration under fault conditions  Be mechanically reliable and rugged  Resist corrosion

www.EngWorks.ca

Grounding Fundamentals

320

Ground Grid Conductor Options 

Copper    



Highest g conductivity y Highest cost Subject to theft Commercial hard drawn specification most often used

Copper-clad steel  Good option where theft is a problem



Aluminum  Not recommended as is subject to corrosion



Steel  Poor conductivity limits use

www.EngWorks.ca

Grounding Fundamentals

321

Sizing of Grid Conductors 

Based on the design ground fault current and the fault duration time

Akcmil = I · Kf √tc Akcmil = area of conductor in kcmil I = Fault current in kA tc = current duration in seconds (IEEE recommends 3.0 seconds) Kf = constant based on the material (Refer to table 2 IEEE 80)

www.EngWorks.ca

Grounding Fundamentals

322

Material Constants ((Excerpt p from Table 2 IEEE 80))

Material

Conductivity

Kf

Copper, Annealed Soft Drawn

100%

7.00

C Copper, C Commercial i lH Hard dD Drawn

97%

7 06 7.06

Copper Clad Steel Wire

40%

10.45

Aluminum 6201 Alloy

52.5%

12.47

Steel 1020

10.8%

15.95

Stainless Steel 304

2.4%

30.05

www.EngWorks.ca

Grounding Fundamentals

323

Grid Conductor Sizing Example  



Fault current is estimated at 6kA Commercial hard drawn copper selected as the grid conductor What size of grid conductor is appropriate to handle the maximum fault current for 3.0 seconds? Akcmil = I · Kf √tc Akcmil = 6 · 7.06 √3.0 Akcmil = 73.37kcmil → #1 AWG

www.EngWorks.ca

Grounding Fundamentals

324

Grid Conductor Sizing Table Current Carrying Capacity (kA) kcmil

AWG

pp HD Copper

Copper pp Clad Steel Wire

Steel 1020

500

-

40

27.62

18.1

250

-

20 4 20.4

13 81 13.81

9 05 9.05

212

4/0

17.34

11.71

7.67

133

2/0

10.88

7.35

4.81

83.7

#1

6.84

4.62

3

66.4

#2

5.43

3.67

2.4

Based on a 3.0 Second Fault Duration

www.EngWorks.ca

Grounding Fundamentals

325

Grounding Connections 





All grounding connections must be selected to withstand the short circuit forces and heating effects associated with an extended groung fault Resist the effects of corrosion High pullout resistance Time-current curves for ground grid conductors and connectors

www.EngWorks.ca

Grounding Fundamentals

326

Typical Connections found within a Grid Design g

Conductor to Ground Rod

Conductor to Conductor www.EngWorks.ca

Substation F Fence to t Grid G id Conductor Grounding Fundamentals

Equipment to G id C Grid Conductor d t 327

Cadweld Connections  Thermite welding process is used to fuse the connection  Suitable for high current applications  Will meet the requirements of IEEE 837 ““IEEE Standard for Qualifying Permanent Connections Used in Substation Grounding Grounding”

Cadweld Connection

www.EngWorks.ca

Grounding Fundamentals

328

Grounding Connections 

Compression Connections  Acceptable p alternative to Cadweld connections in substation applications

Compression Tool www.EngWorks.ca

Grounding Fundamentals

329

Corrosion Considerations 

A basic corrosion cell consists of the following:  Anode – An electrode losing g metal  Cathode – An electrode gaining metal  Electrolyte – chemicals in solution in contact with the anode and cathode 0.78V  Connecting conductor -

V

+

IRON Anode

Earth

COPPER Cathode

G l Galvanic i C Cellll www.EngWorks.ca

Grounding Fundamentals

330

Corrosion Considerations 

When iron or steel is connected to copper with a low impedance p conductor,, it corrodes  Rate of corrosion is dependent on the current flow  Each ampere-year of current flow will result in 20lbs of steel being lost μ –5 μA 5A + A

IRON IRON Anode

www.EngWorks.ca

COPPER Earth

Cathode

Soil Corrosivity Ohm-Cm Corrosivity Etouch = 734V  Go to step 8 and Calculate Mesh Voltage www.EngWorks.ca

Grounding Fundamentals

393

Step 8A Calculate Mesh Voltage 

Calculate the MESH voltage for the grid design Em =

ρ · Km· Ki · IG LM

=

60 · Km· Ki · 400 LM

EM = Mesh Voltage ρ = Soil resistivityy in Ω·m Km = Geometrical correction factor for grids of varying dimension Ki= Correction factor for grid geometry IG = Maximum grid current LM = Effective length of grid conductors and ground rods in m

www.EngWorks.ca

Grounding Fundamentals

394

Km = Geometrical correction factor 1

D2 Km = · ln l 16 · h · d 2·π

+

D+2·h2 8·D·d

-

h 4·d

+

Kii

8 · lln π(2 · n – 1)

Kh

For grids with ground rods along the perimeter and thoughout the grid area: Kii = 1

Kh =

1+ h √

ho

ho = 1m (grid reference depth) =

1 + 0.45 = 1.2 √

1

D = Spacing between parallel conductors in m = 3.5m h = Depth of ground grid conductors in m = 0.45m d = Diameter of grid conductor in m = 0.0093m n = Effective number of parallel conductors in a given grid

www.EngWorks.ca

Grounding Fundamentals

395

Km = Geometrical correction factor LC = is the total length of the conductor in the horizontal grid in m = 120m Lp = is the peripheral length of the grid in m = 48m A = Area of Grid = 140m2

n = na · n b · n c · n d na =

nb =

2 · LC Lp

=

Lp

2 · 120 48 =

=5

48 = 1.01 1 01 4 · √140

4 · √A nc = 1 for square and rectangular grids nd = 1 for square, rectangular and L – shaped grids

n = 5 · 1.01 · 1 · 1 = 5.05 Ki= Correction C ti factor f t for f grid id geometry t Ki = 0.644 + 0.148 · n = 0.644 + 0.148 · 5.05 = 1.39 www.EngWorks.ca

Grounding Fundamentals

396

LM = Length of Grid Conductors and Ground Rods For grids with ground rods in the corners, as well as along the perimeter and throughout the grid

Lr LM = LC + 1.55 + 1.22 √Lx2 · Ly2

• LR

Lr = is the length of each ground rod in meters Lx = is the maximum length of the grid in the x direction in m Ly = is the maximum length of the grid in the y direction in m LC = is i the th length l th off conductors d t in i the th grid id in i m LR = is the length of rods in the grid in m

LM = 120 + 1.55 + 1.22

www.EngWorks.ca

3 √102 · 142

Grounding Fundamentals

• 39 = 181.46

397

Mesh Voltage 1

3.52 Km = · ln + 16 · 0.45 · 0.0065 2·π

3.5 + 2 · 0.45 2 8 · 3.5 · 0.0065

0.45 4 · 0.0065

-

1

8 + · ln π(2 · 5.05 – 1) 12 1.2

Km = 0.70 0 70 Ki = 1.39 LM = 121.57 121 57 Em =

www.EngWorks.ca

ρ · Km· Ki · IG LM

=

60 · 0.7· 1.39 · 400 181.46

Grounding Fundamentals

= 128.7V

398

Step 8B Calculate Step Voltage Es =

ρ · Ks· Ki · IG LS

Es = Step Voltage ρ = Soil resistivity = 60 Ω·m Ki= Correction ffactor for f grid g geometry g y = 1.39 IG = Maximum grid current = 400A LS = Effective buried conductor length m Ks = Spacing factor for step voltage

LS = 0.75 · LC + 0.85 · LR LC = is the total length of the conductor conducto in the horizontal ho i ontal grid g id in m = 1200 LR = is the total length of all ground rods = 39m

LS = 0.75 0 75 · 120 + 0.85 0 85 · 39 = 123.15 123 15

www.EngWorks.ca

Grounding Fundamentals

399

Step 8B Calculate Step Voltage KS =

1 π

1 1 1 n-2 1 – 0.5 + + 2·h D+h D

D = Spacing between parallel conductors in m = 3.5 h = Depth of ground grid conductors in m = 0.45 d = Diameter of grid conductor in m = 0.0065 n = Effective number of parallel conductors in a given grid = 5.05

1 KS = π

Es =

www.EngWorks.ca

1 1 1 5 05 2 5.05-2 + + 1 – 0.5 2 · 0.45 3.5 + 0.45 3.5

ρ · Ks· Ki · IG LS

=

= 0.513

60 · 0.513 · 1.39 · 400 123.15

Grounding Fundamentals

= 139V

400

Step 9 Em < Etouch? Emesh = 128.7V < 734V = Etouch YES go to step 10

www.EngWorks.ca

Grounding Fundamentals

401

Step 10 Es < Estep? Es = 139V < 2446V = Estep YES go to Detailed Design

www.EngWorks.ca

Grounding Fundamentals

402

Detailed Design 2/0 AWG Insulated Gnd Wire

3.3m

3.3m

600V MCC DCS Eqpt Gnd Bus Isolated Inst. Gnd Bus

600V Swgr 5A NGR

33m groundd rods d (16 in total)

600V XFMR

UPS

5kV MCC

Ground rods with Inspection well

5 kV Swgr

5kV XFMR Surge Arrestor

Intergrid Grounding Conductors Bare 2/0 AWG

125A NGR 2/0AWG to Xo Terminal

Gnd Conductor as short as possible www.EngWorks.ca

Grounding Fundamentals

403

Substation Ground System 

Instrumentation Ground  Designed to insure that all components of the control system operate at the same potential Eliminate potential ground loops Isolate system noise on the ground system Allow ground to be accessible for disconnect to assist in isolating ground loops

www.EngWorks.ca

Grounding Fundamentals

404

Control System Grounding Scheme SWGR

UPS

UPS PNL

N

Main Transformer

Ground Conductor Lightning A Arrestor t

Instrument Panel Bonding Conductors

Bonding Conductor

NGR

System Ground

Isolated Ground Bus

CP

CP

CP

Insulated Instrument Ground conductors

www.EngWorks.ca

Grounding Fundamentals

405

Computer Analysis GPR < Touch Summer Conditions

www.EngWorks.ca

Grounding Fundamentals

406

Computer Analysis GPR < Touch Summer Conditions

www.EngWorks.ca

Grounding Fundamentals

407

Computer Analysis GPR < Touch Winter Conditions

www.EngWorks.ca

Grounding Fundamentals

408

Computer Analysis IEEE 80

www.EngWorks.ca

Grounding Fundamentals

409

Computer Analysis IEEE 80

www.EngWorks.ca

Grounding Fundamentals

410

Computer Analysis Optimized

www.EngWorks.ca

Grounding Fundamentals

411

Computer Analysis Optimized

www.EngWorks.ca

Grounding Fundamentals

412

Summary and Wrap up

Section 10

Learning Objectives Review 1. To understand why we ground  Protect life from the danger of shock  Limit the voltage on a circuit  Facilitate operation of protective devices

www.EngWorks.ca

Fuse

Low Impedance path to source allows fuse to operate

Accidental Ground

L

G1 Neutral

Grounding Fundamentals

414

Learning Objectives Review 2. To describe the difference between grounding and bonding g  System grounding refers to the intentional connection of a phase or neutral conductor to earth for the purpose of controlling t lli th the voltage lt tto ground, d within ithi predictable di t bl lilimits it  Bonding or equipment grounding refers to the interconnection and connection to earth of all normally non-current carrying metal parts  Insures that all metal parts remain at ground potential  Reduces the shock hazard to personnel  Provides a low impedance return path for ground currents – Allows the circuit protection device to operate

 Minimize the fire and explosion hazard www.EngWorks.ca

Grounding Fundamentals

415

Learning Objectives Review 3. To apply the safety requirements as defined by the Canadian Electrical Code and the IEEE as theyy relate to grounding 

CEC Section 10   



Defines when a system should be grounded and when equipment should be bonded Describes the acceptable methods for grounding and bonding and stipulates the size of grounding and bonding conductors Defines what an acceptable grounding electrode shall be

CEC Section 36 

Describes the grounding and bonding requirements for high voltage substations – –

www.EngWorks.ca

GPR < 5000\v Touch and Step potential as per table 52 Grounding Fundamentals

416

Learning Objectives Review 3. To apply the safety requirements as defined by the Canadian Electrical Code and the IEEE as theyy relate to grounding 

IEEE 142 (Green Book) 



IEEE 1100 (Emerald Book) 



Recommended Practice for the Grounding of Industrial and Commercial Power Systems Recommended Practice for Powering and Grounding Electronic Equipment

IEEE 80  

Guide for Safety in AC Substation Grounding Primarily concerned with outdoor AC substations

www.EngWorks.ca

Grounding Fundamentals

417

Learning Objectives 4. To select the appropriate systems grounding scheme for an industrial facilityy Ungrounded

Solid Ground

Low Resistance

High Resistance

Immunity to transient overvoltages

Worst

Good

Good

Best

Arc Fault Damage Protection

Worst

Poor

Better

Best

Safety to Personnel

Worst

Better

Good

Best

Service Reliability

Worst

Good

Better

Best

Continued operation after initial ground fault

Better

Poor

Poor

Best

Not Possible

G d Good

B tt Better

B t Best

Condition

Ground fault locating g

www.EngWorks.ca

Grounding Fundamentals

418

Learning Objectives 5. To implement a static electricity control and lightning g gp protection system y  Static Control  Bond together and to ground

 Lightning Protection  Lightning strikes cannot be stopped but their energy can be diverted in a controlled manner  Requires a low impedance path to ground

www.EngWorks.ca

Grounding Fundamentals

419

Learning Objectives 6. To avoid the problems typically associated with the grounding g g of sensitive electronic systems y  

Ground loops - use the single point grounding concept Methods of Noise Mitigation    

Physical Separation Electrical Segregation Harmonic Filtering Shielding or screening of noise sources

www.EngWorks.ca

Grounding Fundamentals

420

Learning Objectives 7. To design a ground grid for a high voltage industrial substation  Limit the ground potential rise between two points to a safe value  Limit the touch and step potentials to a safe value  Must be able to withstand the maximum ground current without damage  Important part of a safe and reliable electrical systems design

www.EngWorks.ca

Grounding Fundamentals

421