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  • i

    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

  • ii

    List of Appendices

    Appendix A Tables Used From BS 7671

    Appendix B Busduct Technical Data

  • Page 1 of 9

    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.

  • Page 2 of 9

    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%.

    Example Calculation 2

    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

  • Page 3 of 9

    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.

  • Page 4 of 9

    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.

    4. The conductor length has tolerance of

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

    Example Calculation 3

    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

  • Page 5 of 9

    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

    Correcting CPC temperature to average temperature during fault,

    Where

    and