IET Lecture - LV Circuit Design September 2014

Cable Sizing For Safe Power

System Based on IEC Standards

I r . H . P. L o o i ( m e k t r i c o n @ g m a i l . c o m )

B . E n g ( H o n s ) , F I E M , J u r u t e r a G a s

Cable Sizing Fundamentals and

E l e c t r i c a l LV S t a n d a r d

A r m a d a H o t e l , P e t a l i n g J a y a

2 3 r d A u g u s t 2 0 1 4

P a r t 1 – C i r c u i t D e s i g n

‘Cable sizing is a fundamental skill set required of the design engineer.

Unfortunately the theory and empirical basis on which cable sizing rest on is not

well understood amongst practicing engineers. Such gaps in knowledge may have

implication in the design of wiring systems particularly on the safety of wiring

systems (particularly thermal safety).

This Presentation provides an introduction to basic concepts in sizing of cables in

electrical (LV) system based on the IEC standards. The presentation is based on

the handbook published and is in the following logical procession:

2 SYNOPSIS

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1. Introduction

2. Scope

3. General Design Procedure (design road map)

4. Earthing system

5. Cable types & installation method

6. Circuit configuration

7. Conductor sizing

3 THE SPONSOR – WIRING HANDBOOK

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‘The International Copper Association Ltd. (ICA) is a non-profit

organization promoting the use of copper worldwide . ICA increases

awareness and usage of copper by communicating its unique attributes as a

sustainable element an essential to life, science and technology, and a higher

standard of living. ICA Southeast Asia established by ICA in 1992 acts as a

strategic and networking base for regional copper and associated industries’

associations.

This handbook is distributed FREE courtesy of the sponsor

5 INTRODUCTION – ROAD MAP TO IEC60364

Load Estimation

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L.V. Wiring Design Circuit

Configuration

Installation Method

Cable Selection

Protection Device

Earthing

Equipment Selection

Verification

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8 VOLTAGE STANDARD

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Malaysian Standard Voltages

It is a misconception that Malaysia Standard Voltage is at 415V/240V !

As of 1st January 2008, Malaysia Standard Voltage is declared at

400V/230V

Refer ST link for the official notification.

Rationalisation of voltage is to bring Malaysia in line with other

member countries of the IEC. Singapore and the UK has

already converted to 400V/230V from the old 415V/240V.

Other IEC member countries which have yet to convert are

Germany / China (380V/220V) and India (415V/240V).

Practitioners must therefore design power distribution system

to 400V/230V.

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Malaysian Standard Voltages at 400V/230V +10%, -6%

Major reasons for rationalising to 400V/230V

Standardisation among IEC countries. Multiple certification due to different

voltage levels is avoided or minimised.

Most motors and fluorescent light fittings works most efficiently within the

voltage band of 210V to 220V. Thus reducing to 220V will

Save energy

Prolong life-span of electrical equipment.

Electrical power distribution system should be designed to 400V/230V !

Change all reference in specifications to 400V/230V.

Specify transformers at nominal 11kV/420V (not 11kV/433V).

Check cable sizing as 400V/230V incur about 4% higher current.

9 VOLTAGE STANDARD

10 VOLTAGE STANDARD

Version 3 Published in 2013!

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11 THE DESIGN WORK FLOW

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13 EARTHING SYSTEM

60364 - Part 1; 312 – 2 Letters definition:

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14 EARTHING SYSTEM

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Network / Operation Criteria System Chosen / Preferred

Service Criteria Competent Maintenance

Continuity of service critical Available IT System preferred

Continuity of service critical Not consistently available No satisfactory solution IT system preferred as discriminitive trippings are easier to implement and damages less with respect to TN system

Continuity of service NOT critical Available TN-S system preferred (rapidrepairs and extension easily performed.

Continuity of service NOT critical Not available No satisfactory solution. IT system preferred.

Fire hazard critical Available IT system and use of 5mA RCD or TT system preferred.

Special feature, very long networks Assumed available TT preferred

Special feature, standby power supply Assumed available TT preferred

Special feature, load sensitive to high currents (e.g. motors)

Assumed available TT preferred; IT can be acceptable

Special feature, low natural insulation (furnace) OR very large HF filters (computers)

Assumed available TN-S preferred

Special feature, control and monitoring systems Assumed available IT for continuity of services; TT for enhanced equipotential

15 EARTHING SYSTEM

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16 EARTHING SYSTEM – SIZING OF P.E. (TT)

60364-5-54, 312 :

For TT system, PE need for

exceed

25mm² (copper)

35mm² (aluminium)

As TT system effectively DO

NOT have PE cables within the

distribution system, this may

presumably mean the earth

electrode or frame earth system.

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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).

17 EARTHING SYSTEM – SIZING OF P.E. (TNS)

60364-5-54, 543 For TN-S system (method 1):

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S = PE size (mm²); I = prospective fault current rms; t = operating time of

protection device; k = factor based on conductor material and insulation.

60364-5-54, 543 For TN-S system where the protective device afford

disconnecting time NOT exceeding 5 seconds, the following formula may apply:

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Case :

Calcu lat ing

I ² t for PE:

From the above, we note that for up 15kA rating table 54.3 sizing for PE cable

will be acceptable in terms of “energy-let-through” for both gG fuses and MCB

class C and B types.

4x16mm² PVC +

1x16mm² PVC (PE)

30A

CB

SCI=10kA

70°C PVC cables k= 115 SC I= 3 kA 5 kA 6 kA 10 kA 15 kA 25 kA

30A MCB class C, trip time t= 0.1 s 0.01 s 0.01 s 0.01 s 0.01 s 0.01 s

S min size of PE cable (mm²) 8.25 4.3 5.2 8.7 13.0 21.7

32A gG Fuse, trip time t= 0.01 s 0.01 s 0.01 s 0.01 s 0.01 s 0.01 s

S min size of PE cable (mm²) 2.61 4.3 5.2 8.7 13.0 21.7

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21 CABLE TYPES

Cable types and installation methods have implication for cable sizing and

Ampere capacity.

IEC60364-5-52 : Cable Selection can be summarised:

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22 CABLE TYPES – ASSESSING CONDITIONS

Part 5-51 defines environmental classes for use in defining

installation and equipment selection criteria:

A A 2

Number – severity index

2nd alphabet –

Specific conditions

A = Environment

B = Utilisation

C = Building

Specific conditions of environment usage etc are coded.

Later prescription in standards specify technical

conditions concerning conditions code.

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External influences Selection considerations

AA4, and AA5 Ambient temp.

-5ºC to +40ºC

Normal cable insulation PVC, or XLPE

AD5 to AD7 Presence of water Cable jointing glands to be IPX5 and better,

extruded inner sheath for cables for AD7

(submerged)

AE5 and AE6 Dusty atmosphere Cable jointing glands to be IP6X, bare conductor

on insulator not recommended.

AG3 Mechanical stress Underground cables, mechanical protection of

cables, sheathing of cables, armouring of cables,

cable in ducts

AH2 Vibrations Connections to motor; termination lugs, cable

slack to allow for vibration

AM1 Harmonics Mitigations for harmonics; Annex D in 60364-5-52

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External influences Selection considerations

AN3 Solar radiation Cable sheath or insulation which are immune

against UV radiation (PVC sheath or insulation not

permitted).

AP3 and AP4 Seismic effect Cable tray preferred over conduit, slack in cable

run etc.

BD2 and BD4 Emergency

evacuation;

difficult exit

Low smoke (LSZH) cables

BE2 Fire risk Fire rated cables for essential services; LSZH

cables

CA2 Presence of

combustibles

Fire rated cables for essential services; LSZH

cables

CA2 and BE2 Combustibles and

explosion risk

Design to IECEx standard IEC 60079 series.

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25 CABLES FOR FIRE & SMOKE CONDITIONS

Flame retardant cables do not promote or propagate the spread of fire.

Flame retardant cable installation is specified under conditions of

emergency evacuation (BD2 and BD4 external influences). These

conditions of emergency evacuation also apply in building spaces

designated as fire escape routes or emergency evacuation routes under

“life safety” standards for building design.

The ‘degree’ of flame retardant required will depend on the designed

‘evacuation-time’ of the escape routes. The test for compliance to flame

retardant properties are defined in the IEC 60332 series:

IEC 60332-1 & 60332-2; flame propagation test on

single cable is the most basic flame retardant test

(whilst Part 1 specify 1kW flame for general insulated

cables, Part 2 specify ‘diffuse’ flame for single

insulated small cables);

IEC 60332-3; specify more stringent flame

propagation test on bunched cables.

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Low Smoke, Zero Halogen (LSZH) cables reduces smoke with zero

halogen emission. Classes of cables listed as Low Smoke Flame

Retardant (LSFR) or Low Smoke Zero Halogen (LSZH) are defined by the

following test standards:

The test for Low Smoke is the smoke density test of IEC 61034;

The test for halogen emission is specified in IEC 60754-1; and

The test for degree of acidity of is specified in IEC 60754-2.

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Fire Resistant (FR) cables can maintain circuit integrity even in the

presence of a fire. The test for fire resistive behaviour is specified in IEC

60331 fire integrity tests. FR cables are mandatory where essential

services (e.g. fire fighting services, fire lifts etc) require ‘circuit survivability’

even during a fire emergency).

1. LSFR, LSZH sheath

2. Binder tape

3. Filler, non-hygroscopic

4. Insulation, XLPE-FR

5. Mica tape (fire resistant barrier)

6. Conductor, solid or stranded.

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30 CABLE TYPES

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31 CABLE TYPES – COMMON DEFECTS

Conductors and cables Bare

conductors

Insulated

conductors

Sheathed cables

(including armoured and

mineral insulated)

Multi core Single core

Meth

od o

f In

sta

llation

Without fixings NA

Clipped direct

Conduit systems

Cable trunking systems (including

skirting trunking, flush floor

trunking)

1

Cable ducting systems

Cable ladder, cable tray,

cable brackets

On insulators NA NA

Support wire

– permitted – not permitted NA – not applicable or not normally

used in practice

Table 5A – Cable installation method and cable types from IEC 60364-5-52

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3. Conduit embedded in concrete are of the wrong grade (conduit not properly graded

will crack when laid in concrete).

4. No proper junction boxes and/or terminal boxes.

5. Trunking and/or conduit ‘overloaded’.

6. Splicing/jointing of cables without proper terminal box.

Some Common Defects

1. PVC conduit system are

substandard and are NOT

certified to electrical grade.

2. PVC conduit and/or cables

laid in condition where

continuous sunlight occurs

(PVC will deteriorate in the

presence of UV light).

PVC products should NOT be

installed exposed to sunlight.

No proper evaluation on fire-

rating of cables in complying

with fire properties.

32 CABLE TYPES – COMMON DEFECTS

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34 CIRCUIT CONFIGURATION – EARTHING

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35 CIRCUIT CONFIGURATION

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Cable Sizing For Safe Power System Based

o n I E C S t a n d a r d s

I r . H . P. L o o i ( m e k t r i c o n @ g m a i l . c o m )

B . E n g ( H o n s ) , F I E M , J u r u t e r a G a s

Cable Sizing Fundamentals and

E l e c t r i c a l LV S t a n d a r d

A r m a d a H o t e l , P e t a l i n g J a y a

2 3 r d A u g u s t 2 0 1 4

P a r t 1 – C i r c u i t D e s i g n

Intermission-Continue to Part 2,,,

IET Lecture - LV Circuit Design September 2014

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Transcript of IET Lecture - LV Circuit Design September 2014