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Electrical Calculation Report

Content

1 INTRODUCTION......................................................................................................................................... 1

2 CABLE SIZING AND VOLTAGE DROP........................................................................................................... 1

2.1 CABLE AMPACITY CHECK ........................................................................................................................... 1

2.2 VOLTAGE DROP CHECK ............................................................................................................................. 2

2.3 SHORT CIRCUIT WITHSTAND CHECK ............................................................................................................. 3

3 BOARD CPC SIZES AND DISCONNECTION TIMES CALCULATION ................................................................. 3

3.1 DETERMINATION OF DISCONNECTION TIME.................................................................................................... 3

3.2 DETERMINATION OF CPC SIZE .................................................................................................................... 6

4 BOARD SHORT CIRCUIT FAULT CALCULATION ........................................................................................... 7

4.1 DETERMINATION OF MAXIMUM PROSPECTIVE SHORT CIRCUIT CURRENT ............................................................... 7

4.2 DETERMINATION OF MAXIMUM DISCONNECTION TIME .................................................................................... 7

4.3 DETERMINATION OF ENERGY LET THROUGH TO CABLE ...................................................................................... 7

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List of Appendices

Appendix A Tables Used From BS 7671

Appendix B Busduct Technical Data

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1 Introduction This electrical calculation report presents the calculation method of electrical services in KL Sentral Station. This

report is based on standards IEC 60287-1, BS 7671, and IEC 60947-2.

2 Cable Sizing and Voltage Drop Cable are sized based on 3 calculation checking which are

1. Cable Ampacity

2. Total voltage drop

3. Short circuit withstand capacity

And the following assumption are made

1. The conductor length has tolerance of

2.1 Cable Ampacity Check

Cables are sized according to cable types and design current, based on BS 7671: 2008. According to IEE

Wiring Regulation, two conditions to be applicable;

Condition 1.

Where,

is design current

is circuit breaker current rating

is cable current carrying capacity for continuous service

e.g Nominal current rule

The nominal current of the fuse/ MCB must be less than the current rating of the cable it is protecting, but higher than the current that it will carry continuously. For example, a 32-amp MCB is suitable for a current of 30 amps in a 35-amp cable circuit.

Condition 2.

Where,

is tripping current of a circuit breaker for protection against overload.

e.g Tripping rule

A current of 1.45 times the nominal current must cause the device to trip in less than 1 hour. All modern devices

meet this requirement except re-wirable fuses. This is why re-wirable fuses are discouraged. These fuses normally require about twice the nominal current to blow them in one hour.

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Example Calculation 1

For Way-1 (to DB-C-N-61) of MSB-1A, (based on DB maximum demand estimation) and .

Cable type used is multicore XLPE, armoured and LSOH sheath.

1. Calculate Derated Current Rating, . Assuming no simultaneous overload occurs on a group of cables,

is chosen based on the higher value of the following equations

and

Where

is coefficient related to ambient temperature at cable installation

is coefficient related to thermal insulation applied to cable installation

is coefficient for type of protective device or installation condition.

is coefficient taking consideration the thermal effect for cable grouping.

Hence,

due to ambient of based on Table 4B1

due to cable in free air

due to cable not installed in a buried duct and not protected by semi-enclosed fuse

due to cable installed in single layer flat touching. Maximum 3 circuits per cable tray and 3

cable trays are grouped.

Hence

2. Refer to Table 4E4A, cable size can be used because .

2.2 Voltage Drop Check

Based on Appendix 12 of BS7671, Table 12A, for private installation, allowable voltage drop from source to

equipment is 8% for general load and 6% for lighting load. To allow small size of final circuit cables for ease of

installation, voltage drop from source to sub-main distribution is set to be maximum 4.5%.


To continue from Example Calculation 1, voltage drop at DB-C-N-61 is calculated by the following steps

1. Calculate maximum voltage drop, from Transformer to MSB-1A

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Where,

= voltage drop per ampere per metre for the cable

= length in metre

considering contingency

From catalogue, Cu busbar has the following voltage drop per meter at rated current and 0.8 pf

Assuming the busbar is fully loaded,

2. Calculate cable voltage drop, from MSB-1A to DB-C-N-61

Where

From table 4E4B,

Hence,

So

3. Total voltage drop is calculated

which is lower than requirement of . So cable size

valid.

2.3 Short Circuit Withstand Check

Refer Section 4.3.

3 Board CPC Sizes and Disconnection Times Calculation

3.1 Determination of Disconnection Time

BS 7671:2008 regulates that for a TN-S system, at nominal supply voltage of , the disconnection time for

any fault type at any point of installation, the maximum disconnection time must not exceed for final circuit

not exceeding 32 A. For sub-main installation or final circuit more than 32 A, a disconnection time not exceeding

to be achieved.

For final circuit, line to earth fault will be the fault of lowest value. To ensure the disconnection time is achieved,

RCCB is proposed at all final circuit.

For the sub-main installation, the line to earth fault will be the fault of lowest value as well. Thus, to ensure

disconnection time criterion is met the minimum earth fault current to be determined and associated

disconnection time to be checked. If the maximum disconnection time couldnt be achieved, Earth Leakage Relay

to be introduced.

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To calculate minimum prospective earth fault current, the following assumption is made

1. The incoming supply external impedance is zero.

2. Cable skin effect and proximity effect factor for CPC resistance is neglected.

3. Fault occurring during the conductors are fully loaded at rated temperature and the fault causes the

conductors to be heated until average temperature during conductor fault carrying period and the

heating process is adiabatic.


After the is obtained, the disconnection time to be determined to comply with BS 7671 regulations.


Continuing from Example Calculation 2, can be calculated by using the following formula

Where is taken at worst scenario condition.

is transformer internal impedance

is busduct impedance

is XLPE cable impedance

is CPC impedance

1. Calculate

2. Calculate

From catalogue (refer Appendix B), at the characteristic of 5000A busduct are the following

From M&WS, chosen busduct temperature rise, must not exceed at any condition. So since

ambient is , the busduct resistance at can be used. Hence

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3. Calculate

Since cable 2 is less than , reactance of cable can be neglected.

From Table 4E4B, the characteristic of armoured XLPE cables at is the following

Adjusting conductor temperature to average temperature during fault by using the following formula

Where is average conductor temperature during fault and is temperature the is derived

and . Hence

So,

4. Calculate

From earth terminal to DB-C-N-61 enclosure, insulated CPC proposed based on table 54.7 of

BS 7671. The impedance of this CPC is . From earth terminal near DB-C-N-61 to earth terminal in

PER, BCEW is used which has impedance of . From the earth terminal to star point of

transformer, conductor is used which has impedance . The calculation of and

are the followings

insulated CPC at has the following characteristic from Table 4D1A

Correcting CPC temperature to average temperature during fault,

and Hence

Assuming length of CPC1 from DB enclosure to earth terminal is 3 m and neglecting reactance of

CPC1,

From catalogue, at the characteristic of BCEW is the following


Where

and Hence

So

Calculate reactance of CPC,

(

)

Where and

Hence

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So

From catalogue, at the characteristic of BCEW is the following


Where

and Hence

So assuming wiring from earth terminal to star point is length,

Calculate reactance of CPC,

(

)

Where and

Hence

So

and

5. So

So

Note 1: The calculated represent worst case scenario where the return path loop will only go through one CPC.

In practicality, the would be higher as the earth fault loop will utilise cable armouring and perhaps cable

containment. These factors will reduce the overall earth fault impedance hence increasing earth fault current.

Typically the instantaneous tripping current, of an MCCB is . Chosen so

Instantaneous tripping time of an MCCB would be maximum . Hence during earth fault, the protection

provided would comply with BS 7671 requirement of minimum of disconnection time during fault for sub-main

installation.

3.2 Determination of CPC Size

To comply BS 7671:2008 regulation, the chosen CPC size must able to carry the maximum earth fault for the

duration of fault. To achieve this, the CPC are size according to Table 54.7 of BS7671.


Continuing from Example Calculation 3, the chosen CPC size is . Based on table 54.7, for phase cable

size under , same size of CPC to be used. For way to DB-C-N-61, phase cable is so CPC size of

is justified.

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4 Board Short Circuit Fault Calculation The comply with BS 7671:2008 regulations, the maximum short circuit fault is to be determined to ensure the

electrical installation is properly sized.

To calculate prospective short circuit fault current, the following assumption is made

1. The incoming supply external impedance is zero.

2. Fault occurring during the conductors are fully loaded at rated temperature and the fault causes the

conductors to be heated until average temperature during conductor fault carrying period and the

heating process is adiabatic.


4.1 Determination of Maximum Prospective Short Circuit Current

Maximum prospective short circuit current (3-phase fault) for way 1 (to DB-C-N-61) is calculated. The location of

maximum short circuit current is determined to be at the terminal of outgoing MCCB of way 1


Sample calculation is made on DB-C-N-61 just like previous examples.

To calculate maximum prospective short circuit fault current, by the following equation

Where

1. Calculate

2. Calculate

3. So

So

4.2 Determination of Maximum Disconnection Time

To check the disconnection time is under , the minimum prospective short circuit current to be

calculated. But since from Example Calculation 3 the is already caused the MCCB to trip under , the

which is will be of higher value than in turn will cause MCCB to trip under as well.

4.3 Determination of Energy Let Through to Cable

From energy let through characteristics of chosen MCCB, the cable short time withstand capacity is verified.


Continuing from Example Calculation 5, the is superimposed on below energy let through, characteristic of

63 A MCCB.

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Hence

cable chosen energy let through limit is

where

so

Hence the cable chosen can carry .

Since the MCCB chosen has current limiting characteristic, the actual maximum peak short circuit, can be

obtained by superimposing with the following graph.

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The from above is .

Appendix A

Table 4B1

Table 4E4A

Table 4E4B

Table 12A

Table 54.7

Appendix B

Busduct Technical Data

1 Introduction2 Cable Sizing and Voltage Drop2.1 Cable Ampacity CheckExample Calculation 1

2.2 Voltage Drop CheckExample Calculation 2

2.3 Short Circuit Withstand Check

3 Board CPC Sizes and Disconnection Times Calculation3.1 Determination of Disconnection TimeExample Calculation 3

3.2 Determination of CPC SizeExample Calculation 4

4 Board Short Circuit Fault Calculation4.1 Determination of Maximum Prospective Short Circuit CurrentExample Calculation 5

4.2 Determination of Maximum Disconnection Time4.3 Determination of Energy Let Through to CableExample Calculation 6

Appendix AAppendix B

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