10 Common Practice Problems Electrical Part 1...
25 Common Problems for M&E Engineers Armada Hotel , Petaling Jaya 26th May 2011
10 Common Electrical Design Problems – Part I Ir. Looi Hip Peu Hon Sec, ACEM (2010/11) B.Eng (Hons) (Electrical) P.Eng (5226), Jurutera Gas
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CONTENT
2
Objectives of this Workshop
System Design
Wiring Design
Protection
Installation
1. Tariff selection
3. Earthing system
7. RCD selection
9. Standards
2. Generator sizing
4. PE cable selection
8. Motor starting
10. Malaysian wiring code
5. Mains cable sizing 6. Al vs Cu cables
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26th May 2011
OBJECTIVES OF WORKSHOP
3
What are the Objectives of this Workshop? In the Malaysian context, Electrical engineers are required to design to MS standards and standards approved. This responsibility requires: 1. Understanding design principles 2. Understanding Technical Standards 3. Awareness of latest Standards 4. Staying „current‟ in knowledge base The above protects the consultant engineer from claims of „negligence‟.
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26th May 2011
OBJECTIVES OF WORKSHOP
4
This Workshop Module List 5 Common Problems ! This workshop list 10 common design failures. Failures are grouped as follows: 1. System design failures 2. Wiring design failures 3. Protection design failures 4. Installation & Other issues
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S e l e c t i n g Ta r i f f S t r u c t u re
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SYSTEM DESIGN – CORRECT TARIFF SELECTION
At d e s i gn p l a n n i n g sta ge t h e M & E e n g i n e e r i s r e q u i re d t o :
Load and electricity capacity calculations
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Substation Space Planning
Advice Client on Tariff Selection
Formal application to TNB.
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SYSTEM DESIGN – CORRECT TARIFF SELECTION
1
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TNB Supply Handbook 2007
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SYSTEM DESIGN – CORRECT TARIFF SELECTION
Vo l ta ge L e ve l S e l e c t i o n
1 MD ranges
Supply Voltage
Typical Supply Scheme
Up to 12kVA
230V
Overhead service mains from LV mains
12kVA – 100kVA
400V
3 phase o/h or u/g LV existing service mains
100kVA – 1000kVA 400V
Direct cable service from LV board in substations (single or double chamber)
1000kVA – 5000kVA
11kV
Directly fed thro’ TNB 11kV system (11kV switch room
1000kVA – 10000kVA
22kV
Directly fed thro’ TNB 11kV system (22kV switch room)
500kVA – 25000kVA
33kV
Directly fed thro’ TNB 11kV system (33kV switch room
Above 25000kVA
132kV 275kV
Directly fed thro’ TNB 132/275kV system (132/275kV switching stations)
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26th May 2011
SYSTEM DESIGN – CORRECT TARIFF SELECTION
9 1 Tariff
Unit
Rates
A
(Domestic)
sen/kWh
21.8 to 46 sen
B
(LV Commercial) Up to 200kWh >200kWh
sen/kWh sen/kWh
38.0 40.8
(MV General Commercial) MD consumption kWh consumption
RM/kW sen/kWh
24.6 29.6
(MV peak/off peak Commercial) MD consumption kWh consumption (peak hours) kWh consumption (off peak hours)
RM/kW sen/kWh sen/kWh
36.6 29.6 18.2
(LV Industrial) Up to 200kWh >200kWh
sen/kWh sen/kWh
34.2 36.6
C1
C2
D
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26th May 2011
SYSTEM DESIGN – CORRECT TARIFF SELECTION
10 1
Tariff
E1
E2
E3
Unit
Rates
(MV General Industrial) MD consumption kWh consumption
RM/kW sen/kWh
24.6 28.0
(MV peak/off peak Industrial) MD consumption kWh consumption (peak hours) kWh consumption (off peak hours)
RM/kW sen/kWh sen/kWh
30.8 29.6 18.2
(HV peak/off peak Industrial) MD consumption kWh consumption (peak hours) kWh consumption (off peak hours)
RM/kW sen/kWh sen/kWh
29.6 28.6 16.8
The selection of voltage connection and tariff selection have some relation. However at “ b o r d e r l i n e c a s e s ”, s o m e d e v i a t i o n s a r e a l l o w e d .
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SYSTEM DESIGN – CORRECT TARIFF SELECTION
1
Selecting tariff has commercial consequence LV Connection Tariff B/D
2000kVA
peak/off peak Tariff C2/E2
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26th May 2011
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Cost difference between tariff E1 (higher) & D (lower) at 80% load diversity
Cost difference between tariff E1 (higher) & D (lower) at 60% load diversity
1
Monthly charge difference between tariff E1 (higher) and E2 (lower) for 3 shifts
At only 2 shifts charges under tariff E1 may be lower than tariff E2.
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SYSTEM DESIGN – CORRECT TARIFF SELECTION
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Common failure by inexperienced practitioner (including contractors who advise clients)?
No study is done to determine the correct tariff structure during planning stage. The wrong tariff structure in case of MD around 7 5 0 k VA t o 1 . 5 k V A i s p r o p o s e d . The wrong tariff structure in case of selection between p eak/o ff p eak or gen eral tar iff structu re is proposed.
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SYSTEM DESIGN – CORRECT TARIFF SELECTION
1
Common failure by inexperienced practitioners (including contractors who advised clients)?
P r e 2 0 0 6 Tar iff Str ucture At less th an 1M VA LV tar iff is lo wer th an M V tar iff.
For lo ad > 1M VA and load di ver sit y > 7 5% M V tar i ff str u ctu r e is ch eap er. Ho we ver a d eta il l oad an a l ys i s need to b e exe cuted to d eter m ine th e b est tar i f f str u ctu re . Th ey ex i s t a c l e a r d e l i n e a t i o n b e t w e e n C 2 / E 2 (peak/off peak tariffs) and C1/E1 (general tariffs) when at least 3 shifts are run. However this 3 shifts must be translated into high kWh consumption during off-peak hours. Study need to be done to ascertain at planning stage.
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SYSTEM DESIGN – CORRECT TARIFF SELECTION
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Monthly Charges
Monthly Charges
1
MD kW
Monthly Charges
Monthly Charges
MD kW
kWh consumption
Pre 2006 Tariff
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kWh consumption
Post 2006 Tariff 26th May 2011
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SYSTEM DESIGN – CORRECT TARIFF SELECTION
Conclusion 1. A clear delineation between LV and MV tariff is presented in the post 2006 tariff. The consultant if possible should opt for LV tariff taking into consideration availability and capacity of existing LV network and substations. Planners can also look at the possibility of „connection at MV‟ but „metering at LV‟ since new rules are being drafted by TNB to force connection at MV even at 500kVA. 2. The post 2006 tariff structure is much less supportive of „peak/off peak‟ demand-management. This require a more stringent analysis of load demand and the economics of running 3-shifts. 3. Current TNB supply agreement specify that tariff opted must stay for 5 years before changing tariff in agreement.
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S e l e c t i n g G e n e ra to r C a p a c i t y
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SELECTING GENERATOR CAPACITY
19 2
G e n e ra to r s e t s a r e m a n d a to r y i n t h e fo l l o w i n g i n s t a n c e s : Law / By Law
Conditions
Standby Generator
1 UBBL Section 243
Fire Lift to be provided for Standby generator buildings where last occupied required as 2ndary floor is 18.5m above fire supply for fire lifts. appliance access level.
2 UBBL Sections 226, 231, 232, 244, 247 etc and 10 Schedule
Sprinkler, Wet Riser, Pressurised Hydrants, Hose Reel system(s) to be provided in accordance with occupancy hazard class and built-up area in accordance with Tenth Schedule of UBBL.
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Standby generator only required if the 2ndary supply to services pumps are NOT diesel engine.
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SELECTING GENERATOR CAPACITY
20 2
U B B L Te n t h S c h e d u l e ; Secondary supply for Fire Services Typical Scheme
Standby services for fire pump is electric. Mains standby generator provide 2ndary supply to standby electric pump. Mains Standby Generator has to be sized to cater for fire pump starting surge
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SELECTING GENERATOR CAPACITY
21 2
U B B L Te n t h S c h e d u l e ; Secondary supply for Fire Services Alternative Case 1 Standby services for fire pump diesel engine. Mains standby generator not required if UBBL 2 4 3 d o n o t a p p l y.
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SELECTING GENERATOR CAPACITY
22 2
U B B L Te n t h S c h e d u l e ; Secondary supply for Fire Services Alternative Case 2
Standby services for fire pump is diesel engine. UBBL 243 (fire lift) requires mains standby g e n e r a t o r.
Mains Standby Generator only has to be sized to cater for fire lift starting surge
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SELECTING GENERATOR CAPACITY 2
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Mains Standby generator need to cater for fire pump (50HP) starting surge.
Mains essential l o a d 2 0 0 k W.
Standby Generator: required,
Only 1 pump to maintain.
Mains Standby generator need to only to cater for fire lift starting surge.
Capacity 2 5 0 k VA .
Separate 50HP fire pump.
2 pumps to maintain.
of
supply
standby diesel
steady
5 0 0 - 6 0 0 k VA
generator: engine
for
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SELECTING GENERATOR CAPACITY
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G en erator Sel ec t i o n; Co mmo n Fai l u res Generator not sized to take into account starting surge of largest motor in system. Ge nerator not s ized to ta ke into account starting surge of largest motor in system.
Generator size = 150kVA Generator starting!
will
stall
on
lift
Generator size = 250kVA C orre ct s iz ing of Ge nerator to take into account lift starting surge (assume 4x rated).
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26th May 2011
SELECTING GENERATOR CAPACITY
25 2
G en erator Sel ec t i o n; Co mmo n Fai l u res Generator not sized to take into account starting surge of largest motor in system. Ge nerator not s ized to take into account starting surge of largest motor in system. Assume = 1 5 0 k VA
essential
steady
load
Fire pump= 75HP & starting surge 5.5 x rated
Generator size = 250kVA
Generator size = 420kVA (min size)
Generator will stall on Fire Pump starting!
Correct sizing of Generator to take into account lift starting surge.
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26th May 2011
SELECTING GENERATOR CAPACITY
26 2
B e s t P ra c t i c e
Large Installation where standby generator is required under UBBL 243. Use separate diesel engine for standby fire pump. In all case size generator to cater for starting surge of largest motor:
Generator Capacity = (Steady state of Essential Load) – (Less largest motor) + (Starting surge of largest motor)
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26th May 2011
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Ea r t h Sys t e m
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3 WIRING DESIGN – EARTHING SYSTEM
Many Electrical Engineers are still confused on specifying type of earthing system What are the types of earthing systems used in Malaysia which are in accordance with MS, IEC or BS wiring standards?
EARTHING FOR POWER DISTRIBUTION
DEFINITIONS – TT, IT, TN-C & TN-S SYSTEMS
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3 WIRING DESIGN – EARTHING SYSTEM
EARTHING FOR POWER DISTRIBUTION
DEFINITIONS – TT, IT, TN-C & TN-S SYSTEMS MS/IEC60364 earthing – 2 letters:
(a) 1st letter Transformer neutral: (i) “T” for connected
(ii) “I” for isolated (b) 2nd letter; Frame earth: (i) “T” for directly connected to earth (ii) “N” for connected to the neutral at the origin of installation. British IEE or BS7671 has the same definitions.
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3 WIRING DESIGN – EARTHING SYSTEM DEFINITIONS – TN-C, TN-S, TN-CS SYSTEMS The TN System can also be sub divided into sub system by the addition of a 3rd letter: TN-C; N & PE are common (PEN) TN-S; N & PE are separate TN-C-S; TN-C occurs up -stream & TN-S downstream
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3 WIRING DESIGN – EARTHING SYSTEM
DEFINITIONS – TT, IT, TN-C & TN-S SYSTEMS
Various sub system can co-exist within one system.
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3 WIRING DESIGN – EARTHING SYSTEM TT System TNS System
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Earth at both ends has 3P and N (4 cables) Earth at source only, has 3P, N & P.E. (5 cables)
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3 WIRING DESIGN – EARTHING SYSTEM
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3 WIRING DESIGN – EARTHING SYSTEM
TT system in public distribution
Malaysia
TN-C system in public distribution
UK (parts)
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3 WIRING DESIGN – EARTHING SYSTEM
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WIRING DESIGN – EARTH & P.E.
Network/Operation Criteria Continuity of service critical maintenance service available
System Chosen /Preferred and
IT chosen
Continuity of service critical and No satisfactory solution, TT consistent maintenance service NOT preferred as discrimination tripping available is easier to implement and damages lesser with respect to TN. Continuity of service NOT critical; competent maintenance services available.
TN-S chosen (rapid repairs and extension easily performed).
Continuity of service NOT critical; No satisfactory solution, TT system competent maintenance service NOT preferred. available. Fire hazard critical and maintenance services available.
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IT and use of 5mA RCD OR TT system 26th May 2011
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WIRING DESIGN – EARTH & P.E.
Network/Operation Criteria
System Chosen /Preferred
Special features; very long networks
TT preferred
Special features; Standby Power Supply
TT preferred
Special features; Load sensitive to high load currents (e.g. motors)
TT preferred OR IT can be acceptable
Special features; Low natural insulation (furnace) OR very large HF filters (computers).
TN-S preferred.
Special features; control and monitoring systems
IT for continuity of service OR TT for enhanced equipotentiality
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WIRING DESIGN – EARTH & P.E.
Co mmo n Fai l u res No clear understanding of Earthing system In campus type environment, TT system between switch boards are used (only 4 cables and without PE) however proper earthing at both ends are not installed. Wro n g e art h in g syste m s e le cted , e . g . mo st en gine ers are N OT fam iliar wit h “ IT ” system (un ea rth ed neutral) . However IT system should be used in critical ser vices such as operating theatre. Current initiatives are being undertaken to draft standards and promote use of IT system for critical service in Malaysia.
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26th May 2011
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T h e P r o t e c t i ve Ea r t h
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WIRING DESIGN – EARTH & P.E.
Many Electrical Engineers are still confused on specifying type of earthing system and the associated protective earth conductors.
Ty p i c a l s p e c i f i c at i o n s : Wiring to comply with IEE wiring regulations…
Ty p i c a l d e s c r i p t i o n i n p r i c e d e s c r i p t i o n 1. Lighting wiring using 2x1.5mm²-1C + E 2. Sub mains wiring using 4x50mm²-1C + E
What is this “+ E” ? TT System TNS System
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Earth at both ends has 3P and N (4 cables) Earth at source only, has 3P, N & P.E. (5 cables) 26th May 2011
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WIRING DESIGN – EARTH & P.E.
After 2000 This is A C C E P TA B L E .
Before 2000 “Earth continuity ” cables are typically:
NOT
Cabling system will be deemed not in compliance with MS, IEC and IEE wiring codes.
C o p p e r t a p e C a b l e a r m o u r i n g E v e n c o n t i n u i t y o f c a b l e t r u n k i n g / t r a y s .
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WIRING DESIGN – EARTH & P.E.
W h a t i s a l l o wa b l e a s P. E . ? Types of P.E. approved
Conductors in multi core cables Insulated or bare conductors in a common enclosure with live conductors Fixed installed bare or insulated conductors
Metallic cables sheath, cable screen, cable armour, wirebraid, concentric conductors, metallic conduit subject to compliance with 543.2.2. In China, Italy, UK etc cable trays and ladders can be used as P.E. Types of P.E. not approved [543.2.3] Water pipes
Support wire …others
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WIRING DESIGN – EARTH & P.E.
EARTHING ARRANGEMENT PART 5-54 Cross sectional areas of line conductors S (mm²)
If the protective conductor is of the same material as the line conductor
If the protective conductor is not of the same material as the line conductor
S < 16
S
K1/K2 x S
16 < S < 35
16 a
K1/K2 x 16
S > 35
Minimum cross sectional area of the corresponding protective conductor (mm²)
S/2
a
K1/K2 x S/2
Where K1 is the value of k for the line conductor, selected from table A54.1 or from the tables of IEC50364-4-43 according to the resistance of the conductor and insulation. K2 is the value of k for the conductor selected from tables A 54.2 to A54.6 as applicable a
for a PEN conductor the reduction of the cross section area is permitted only in accordance with the rules for sizing of the neutral conductor (see IEC 60364-5-52).
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WIRING DESIGN – EARTH & P.E.
EARTHING ARRANGEMENT PART 5-54 Generally P.E. must be half size phase conductor.
Circuit with phases 1200A more than 100m 2. Ensure installation method match designed method.
3. Using many cables per phase has diminishing returns. Use bus ducts for >1200A.
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26th May 2011
25 Common Problems for M&E Engineers Armada Hotel , Petaling Jaya 26th May 2011
Ir. Looi Hip Peu |
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