Common Practice Problems Electrical Part 1 (21May11)

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

[email protected]

26th May [email protected]

CONTENT2

System Design Wiring Design Protection Installation

Objectives of this Workshop

1. Tariff selection

2. Generator sizing

3. Earthing system

4. PE cable

selection

5. Mains cable

sizing

6. Al vs Cu cables

7. RCD selection

8. Motor starting

9. Standards

10.Malaysian

wiring code

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‟.

OBJECTIVES OF WORKSHOP3


This Workshop Module List 5 Common Problems !

1. System design failures

2. Wiring design failures

3. Protection design failures

4. Installation & Other issues

OBJECTIVES OF WORKSHOP4


This workshop list 10 common design failures.

Failures are grouped as follows:


Se lect ing Tar i f f St r uct ure1

At des ign planning stage theM&E engineer i s required to:


SYSTEM DESIGN – CORRECT TARIFF SELECTION6

Load and electricity capacity

calculations

Substation Space

Planning

Advice Client on Tariff Selection

Formal application

to TNB.

1

TNB Supply Handbook 2007



1

Voltage Leve l Se lect ion



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


switch room)

500kVA –

25000kVA


switch room

Above 25000kVA 132kV

275kV

Directly fed thro’ TNB 132/275kV system

(132/275kV switching stations)

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

C1 (MV General Commercial)

MD consumption

kWh consumption

RM/kW

sen/kWh

24.6

29.6

C2 (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

D (LV Industrial)

Up to 200kWh

>200kWh

sen/kWh

sen/kWh

34.2

36.6

1


Tariff Unit Rates

E1 (MV General Industrial)

MD consumption

kWh consumption

RM/kW

sen/kWh

24.6

28.0

E2 (MV peak/off peak Industrial)

MD consumption



RM/kW

sen/kWh

sen/kWh

30.8

29.6

18.2

E3 (HV peak/off peak Industrial)

MD consumption



RM/kW

sen/kWh

sen/kWh

29.6

28.6

16.8

T h e s e l e c t i o n o f v o l ta g e c o n n e c t i o n a n d t a r i f fs e l e c t i o n h a v e s o m e r e l at i o n . H o we v e r at“bo rder l ine cases”, s o m e d e v i a t i o n s a r e a l l o w e d .


1

S e l e c t i n g t a r i f f h a s c o m m e r c i a l c o n s e q u e n c e



LV ConnectionTariff B/D

LV or MV

connection

<1000kVA

1000kVA –1500kVA

>2000kVA

MV Connection

Tariff C/E

Pattern of

Consumption,

Load Profile

GeneralTariff C1/E1

peak/off peakTariff C2/E2

1

Cost differencebetween tariff E1(higher) & D (lower) at80% load diversity

Cost differencebetween tariff E1(higher) & D (lower) at60% load diversity

1

1Monthly chargedifference betweentariff E1 (higher) andE2 (lower) for 3 shifts

At only 2 shiftscharges under tariffE1 may be lowerthan tariff E2.


C o m m o n f a i l u r e b y i n e x p e r i e n c e d p r a c t i t i o n e r ( i n c l u d i n g c o n t r a c t o r s w h o a d v i s e c l i e n t s ) ?


1

N o s t u d y i s d o n e t o d e t e r m i n e t h e c o r r e c t t a r i f fs t r u c t u r e d u r i n g p l a n n i n g s t a g e .

T h e w r o n g t a r i f f s t r u c t u r e i n c a s e o f M D a r o u n d7 5 0 k VA t o 1 . 5 k VA i s p r o p o s e d .

T h e w r o n g t a r i f f s t r u c t u r e i n c a s e o f s e l e c t i o nb e t w e e n p e a k / o f f p e a k o r g e n e r a l t a r i f f s t r u c t u r ei s p r o p o s e d .


C o m m o n f a i l u r e b y i n e x p e r i e n c e d p r a c t i t i o n e r s ( i n c l u d i n g c o n t r a c t o r s w h o a d v i s e d c l i e n t s ) ?


1

P r e 2 0 0 6 Tar iff Structure

At less than 1MVA LV tar iff is lower than MV tar iff.

For load >1MVA and load diversity >75% MV tar i ffstructure is cheaper. However a deta i l load analys isneed to be executed to determine the best tar i f fstructure.

They exis 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( p e a k / o f f p e a k t a r i f f s ) a n d C 1 / E 1 ( g e n e r a l t a r i f f s )w h e n a t l e a s t 3 s h i f t s a r e r u n . H o w e v e r t h i s 3s h i f t s m u s t b e t r a n s l a t e d i n t o h i g h k W hc o n s u m p t i o n d u r i n g o f f - p e a k h o u r s . S t u d y n e e d t ob e d o n e t o a s c e r t a i n a t p l a n n i n g s t a g e .


SYSTEM DESIGN – CORRECT TARIFF SELECTION16M

onth

lyC

harg

es

MD kW

Month

lyC

harg

es

MD kW

Pre 2006 Tariff Post 2006 Tariff

Month

lyC

harg

es

kWh consumption

Month

lyC

harg

es

kWh consumption

1


Conc lus ion

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.


1


Se lect ing G enerator Capac i ty2


SELECTING GENERATOR CAPACITY19

G enerator s et s are mandatory in t he fo l lowing instances:

2

Law / By Law Conditions Standby Generator

1 UBBL Section

243

Fire Lift to be provided for

buildings where last occupied

floor is 18.5m above fire

appliance access level.

Standby generator

required as 2ndary

supply for fire lifts.

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.

Standby generator

only required if the

2ndary supply to

services pumps are

NOT diesel engine.



UB B L Tent h Schedule ;S e c o n d a r y s u p p l y f o r F i r e S e r v i c e s

2

T y p i c a l S c h e m e

S t a n d b y s e r v i c e sf o r f i r e p u m p i se l e c t r i c .

M a i n s s t a n d b yg e n e r a t o r p r o v i d e2 n d a r y s u p p l y t os t a n d b y e l e c t r i cp u m p .

M a i n s S t a n d b yG e n e r a t o r h a s t ob e s i z e d t o c a t e rf o r f i r e p u m ps t a r t i n g s u r g e




2

A l t e r n a t i v e C a s e 1

S t a n d b y s e r v i c e sf o r f i r e p u m pd i e s e l e n g i n e .

M a i n s s t a n d b yg e n e r a t o r n o tr e q u i r e d i f U B B L2 4 3 d o n o t a p p l y .




2

A l t e r n a t i v e C a s e 2

S t a n d b y s e r v i c e sf o r f i r e p u m p i sd i e s e l e n g i n e .

U B B L 2 4 3 ( f i r el i f t ) r e q u i r e sm a i n s s t a n d b yg e n e r a t o r.

M a i n s S t a n d b yG e n e r a t o r o n l yh a s t o b e s i z e d t oc a t e r f o r f i r e l i f ts t a r t i n g s u r g e



2 M a i n s S t a n d b y g e n e r a t o r n e e d t o

c a t e r f o r f i r e p u m p ( 5 0 H P )s t a r t i n g s u r g e .

M a i n s e s s e n t i a l s u p p l y s t e a d yl o a d 2 0 0 k W.

S t a n d b y G e n e r a t o r : 5 0 0 - 6 0 0 k V Ar e q u i r e d ,

O n l y 1 p u m p t o m a i n t a i n .

M a i n s S t a n d b y g e n e r a t o r n e e d t oo n l y t o c a t e r f o r f i r e l i f t s t a r t i n gs u r g e .

C a p a c i t y o f s t a n d b y g e n e r a t o r :2 5 0 k V A .

S e p a r a t e 5 0 H P d i e s e l e n g i n e f o rf i r e p u m p .

2 p u m p s t o m a i n t a i n .



2 G e n e r a t o r n o t s i z e d t o t a k e i n t o a c c o u n t

s t a r t i n g s u r g e o f l a r g e s t m o t o r i n s y s t e m .

G e n e r a to r S e l e c t i o n ; C o m m o n Fa i l u r e s

G e n e r a t o r n o t s i z e d t o t a k e i n t oa c c o u n t s t a r t i n g s u r g e o f l a r g e s tm o t o r i n s y s t e m .

Generator size = 150kVA


G e n e r a t o r w i l l s t a l l o n l i f ts t a r t i n g !

C o r r e c t s i z i n g o f G e n e r a t o r t o t a k ei n t o a c c o u n t l i f t s t a r t i n g s u r g e( a s s u m e 4 x r a t e d ) .



2 G e n e r a t o r n o t s i z e d t o t a k e i n t o a c c o u n t

s t a r t i n g s u r g e o f l a r g e s t m o t o r i n s y s t e m .

G e n e r a to r S e l e c t i o n ; C o m m o n Fa i l u r e s

G e n e r a t o r n o t s i z e d t o t a k e i n t oa c c o u n t s t a r t i n g s u r g e o f l a r g e s tm o t o r i n s y s t e m .

A s s u m e e s s e n t i a l s t e a d y l o a d= 1 5 0 k V A

F i r e p u m p = 7 5 H P & s t a r t i n g s u r g e5 . 5 x r a t e d


G e n e r a t o r w i l l s t a l l o n F i r eP u m p s t a r t i n g !

Generator size = 420kVA (min size)

C o r r e c t s i z i n g o f G e n e r a t o rt o t a k e i n t o a c c o u n t l i f ts t a r t i n g s u r g e .



2 Best Pract i ce

L a r g e I n s t a l l a t i o n w h e r e s t a n d b y g e n e r a t o r i sr e q u i r e d u n d e r U B B L 2 4 3 . U s e s e p a r a t e d i e s e le n g i n e f o r s t a n d b y f i r e p u m p .

I n a l l c a s e s i z e g e n e r a t o r t o c a t e r f o r s t a r t i n gs u r g e o f l a r g e s t m o t o r :

G e n e r a t o r C a p a c i t y =

( S t e a d y s t a t e o f E s s e n t i a l L o a d ) – ( L e s s l a r g e s tm o t o r ) + ( S t a r t i n g s u r g e o f l a r g e s t m o t o r )


Ear th System3


WIRING DESIGN – EARTHING SYSTEM28

M a ny E l e c t r i c a l E n g i n e e r s a r e st i l lc o n f u s e d o n s p e c i f y i n g t y p e o f e a r t h i n gsy s t e m

W h a t a r e t h e t y p e s o f e a r t h i n g s y s t e m s u s e d i n M a l a y s i aw h i c h a r e i n a c c o r d a n c e w i t h M S , I E C o r B S w i r i n gs t a n d a r d s ?

3

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.

EARTHING FOR POWER DISTRIBUTION


British IEE or BS7671 has the same definitions.

3



3

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



3


Various sub system can co-exist within one system.



3

TT System Earth at both ends has 3P and N (4 cables)

TNS System Earth at source only, has 3P, N & P.E. (5 cables)



3



3

Malaysia

UK (parts)TN-C system in public distribution

TT system in public distribution



3


WIRING DESIGN – EARTH & P.E.36 3

Network/Operation Criteria System Chosen /Preferred

Continuity of service critical and

maintenance service available

IT chosen

Continuity of service critical and

consistent maintenance service NOT

available

No satisfactory solution, TT

preferred as discrimination tripping

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;

competent maintenance service NOT

available.

No satisfactory solution, TT system

preferred.

Fire hazard critical and maintenance

services available.

IT and use of 5mA RCD

OR TT system



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


WIRING DESIGN – EARTH & P.E.38

C o m m o n Fa i l u r e s

N o c l e a r u n d e r s t a n d i n g o f E a r t h i n g s y s t e m

I n c a m p u s t y p e e n v i r o n m e n t , T T s y s t e m b e t w e e ns w i t c h b o a r d s a r e u s e d ( o n l y 4 c a b l e s a n d w i t h o u t P E )h o w e v e r p r o p e r e a r t h i n g a t b o t h e n d s a r e n o ti n s t a l l e d .

W r o n g e a r t h i n g s y s t e m s e l e c t e d , e . g . m o s t e n g i n e e r sa r e N O T f a m i l i a r w i t h “ I T ” s y s t e m ( u n e a r t h e dn e u t r a l ) . H o w e v e r I T s y s t e m s h o u l d b e u s e d i n c r i t i c a ls e r v i c e s s u c h a s o p e r a t i n g t h e a t r e . C u r r e n t i n i t i a t i v e sa r e b e i n g u n d e r t a k e n t o d r a f t s t a n d a r d s a n d p r o m o t eu s e o f I T s y s t e m f o r c r i t i c a l s e r v i c e i n M a l a y s i a .

3


The Protect ive Ear th4



M a n y E l e c t r i c a l E n g i n e e r s a r e s t i l l c o n f u s e d o ns p e c i f y i n g t y p e o f e a r t h i n g s y s t e m a n d t h ea s s o c i a t e d p r o t e c t i v e e a r t h c o n d u c t o r s .

Ty p i c a l s p e c i f i c a t i o n s :

W i r i n g t o c o m p l y w i t h I E E w i r i n g r e g u l a t i o n s…

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 . L i g h t i n g w i r i n g u s i n g 2 x 1 . 5 m m ² - 1 C + E2 . S u b m a i n s w i r i n g u s i n g 4 x 5 0 m m ² - 1 C + E

4

W h a t i s t h i s “+ E” ?

TT System Earth at both ends has 3P and N (4 cables)

TNS System Earth at source only, has 3P, N & P.E. (5 cables)



B e f o r e 2 0 0 0

“ Ea r t h c on t i n u i t y ” c a b l e s a r e t y p i c a l l y :

C o p p e r t a p e

C a b l e a r m o u r i n g

Ev 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 .

A f t e r 2 0 0 0

T h i s i s N O TA C C E P TA B L E .

C a b l i n g s y s t e mw i l l b e d e e m e dn o t i n c o m p l i a n c ew i t h M S , I E C a n dI E E w i r i n g c o d e s .

4


What is a l lowable as P.E .?


Types of P.E. approved

Conductors in multi core cables

Insulated or bare conductors in a common enclosure with liveconductors

Fixed installed bare or insulated conductors

Metallic cables sheath, cable screen, cable armour, wirebraid,concentric conductors, metallic conduit subject to compliancewith 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

4



Cross sectional

areas of line

conductors

S

(mm²)

Minimum cross sectional area of the corresponding

protective conductor (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 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).

EARTHING ARRANGEMENT PART 5-54



EARTHING ARRANGEMENT PART 5-54

Generally P.E. must be half size phase conductor.

Circuit with phases <16mm² must have P.E. SAME size as phase

condutor.

If cable armouring and cable trays/trunking (steel) are to be

used as P.E., then:

1. Above 2 rules must be complied; and

2. Effective area of P.E. (different from copper) must be

corrected with K1 and K2 factors which will require larger

cross sectional area of P.E. for steel.

4



S p e c i f y S u b M a i n s :

1 . S u b M a i n s w i r i n g u s i n g 4 x 5 0 m m ² - 1 C + 1 x 2 5 m m ² - 1 C

2 . S u b m a i n s w i r i n g u s i n g 5 x 1 6 m m ² - 1 C

F i n a l s u b c i r c u i t :

1 . L i g h t i n g w i r i n g3 x 1 . 5 m m ² - 1 C

2 . F i n a l s u b c i r c u i tw i r i n g 5 x 1 0 m m ² - 1 C

COMPLY WITH MS, IEC AND BS WIRING CODES


Cor rect ly S i z ing Main Cables5

Image: http://www.leadercable.com.my/


WIRING DESIGN – MAINS CABLE SIZING47

5 TYPICAL CABLING SIZING FOR COST OPTIMISATION

Transformer Neutral Earth

MSB Main Earth Bar

Main Cables;1200A = 7x300mm² XLPE/PVC1600A = 11x400mm² XLPE/PVC2000A = 14x400mm² XLPE/PVC2000A = 7x630mm² XLPE/PVC3000A = 14x630mm² XLPE/PVC

TT Earthing System

FAIL Cable Size

http://upload.wikimedia.org/wikipedia/commons/8/80/Drehstromtransformater_im_Schnitt_Hochspannung.jpg



5 COMPLIANCE WITH MS, IEC AND BS WIRING CODES.

Current Carrying capacities Annex A – a road map

Cable in air, ambient temp < 30degCCable in ground, ambient temp < 20degC

Temp correction factorTable A52.14 & A52.15

Soil thermal resistivity < 2.5 K-m/Wcorrection factor soil t-rTable A52.15

Groups of Cables types A to D in table 52.1 Table A52.2 to A52.7

Table A52.17 to A52.19Group reduction factor

Groups of Cables types E to F in table 52.1 Table A52.8 to A52.13

Table A52.20 to A52.21Group reduction factor

Groups in conduits, trays, ductingsF = 1/ SQRT(n)


5

5



5 BASED ON TABLE B52.10 (CURRENT RATING) AND

TABLE B52.17 (GROUP REDUCTION FACTOR)

Rating Cable Size No of

cables per

phase

Rating of

individual cables

(trefoil) B52.10

Group

reduction

factor B52.17

Effective rating of

cable system

1200A 7x300mm² 2 561 Amps 0.88 987 – Fail

1200A 11x300mm² 3 561 Amps 0.82 1380A – Pass

1200A 7x400mm² 2 656 Amps 0.88 1154A – Fail

1600A 11x400mm² 3 656 Amps 0.82 1613A – Pass

1600A 14x300mm² 4 561 Amps 0.77 1727A – Pass

2000A 14x500mm² 4 749 Amps 0.77 2306A – Pass

3000A 14x500mm² 4 749 Amps 0.77 2306A – Fail

3000A 14x630mm² 4 905 Amps 0.77 2787A – Fail

3000A 18x630mm² 5 905 Amps 0.75 3393A – Pass

MethodF5 (Trefoil)

5


Rating Cable Size No of

cables per

phase

Rating of

individual cables

(flat) B52.10

Group

reduction

factor B52.17

Effective rating of

cable system

1200A 7x300mm² 2 629 Amps 0.88 1107 – Fail

1200A 11x300mm² 3 629 Amps 0.82 1547 – Pass

1200A 7x400mm² 2 754 Amps 0.88 1327A – Pass

1600A 11x400mm² 3 754 Amps 0.82 1854A – Pass

1600A 14x300mm² 4 629 Amps 0.77 1937A – Pass

2000A 14x500mm² 4 868 Amps 0.77 2673A – Pass

3000A 14x500mm² 4 868 Amps 0.77 2637A – Fail

3000A 14x630mm² 4 1005 Amps 0.77 3095A – Pass

BASED ON TABLE B52.10 (CURRENT RATING) AND

TABLE B52.17 (GROUP REDUCTION FACTOR)

Method F4 (Flat)


A f t e r C a b l e S i z i n g e n s u r e t h a t t h e s e l e c t e dm e t h o d o f i n s t a l l a t i o n i s a d h e r e d a s c u r r e n tr a t i n g d e p e n d s o n i n s t a l l a t i o n m e t h o d .


5

Rating Cable Size Cables

/phase

Individual cables

(trefoil) B52.10

Group reduction

B52.17

Effective rating of

cable system

3000A 18x630mm² 5 905 Amps 0.75 3393A – Pass

Rating Cable Size Cables

/phase

Individual cables

(trefoil) B52.10

Group reduction

B52.17

Effective rating of

cable system

3000A 14x630mm² 4 1005 Amps 0.77 3095A – Pass

1 cable diameter distance

Neutral

Neutral


C o m m o n p r o b l e m s w i t h M a i n s C a b l e S i z i n g

Cable not sized based on MS/IEC method. Usually groupreduction factor NOT applied.

After sizing cables, the cable rating is based oninstallation method (flat or trefoil). Installation methodDO NOT follow recommended installation method atdesign. Cable heating occurs.

Best Practice


5

1. Never run cables >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.

25 Common Problems

for M&E Engineers

Armada Hotel , Petaling Jaya

26th May 2011

Ir. Looi Hip Peu | [email protected]

Common Practice Problems Electrical Part 1 (21May11)

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