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Main switch short circuit capacity.

wessex
Some guidance please.

Three phase TN-S supply from a private 800 kVA transformer. Distribution via a 240mm2 4 core SWA approx 20m in length from a transformer mounted feed pillar and protected by 315A fuses. At the intake position 100A BS 1361 type II fuses are fitted. I was looking to install a REC 4 main switch down stream from which would be three phase distribution by 10kA rated MCB's. My question is about the suitability of the main switch and its short circuit capacity at it's point of installation with regard to prospective fault current at this point which will be greater than 16kA or do the BS 1361 afford adequate protection for the switch ? Your thoughts please.
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  • Former Community Member
    0 Former Community Member
    Hi Mike


    It might help to think of the RMS value of current being the value of current that can be safely broken by the device (so for example a rated 50kA 3 second switchboard that can and will open safely the design RMS value)


    Then think of the peak current as being the safe making current (if you were unlucky enough to close into a fault) - so a board that may be rated at 100kA peak


    The relationship used for all practicable purposes is a factor of ranging from 1.5 through to 2.5 for design conditions. Alternatively you may use a value of 2 x square root of the RMS value squared (the so called doubling effect) if you are in the typical range of transformer fed LV switchboards at around 50kA prospective levels (eg a parallel pair of 1.5MVA Transformers feeding into A and B switchboards with bus sections closed)


    Note the 2.5 factor is only used for very large fault levels - there is a range of multipliers depending on fault levels - these range from say 1.5 for faults less than 5kA to a factor of 2 for faults at say 50kA(as above), to a factor of 2.5 for faults exceeding 80kA - these are further defined in switchgear protection standards such as BS EN 60947-1 (the so called "n" factor)


    Regards


    OMS
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  • Former Community Member
    0 Former Community Member
    Hi Mike


    It might help to think of the RMS value of current being the value of current that can be safely broken by the device (so for example a rated 50kA 3 second switchboard that can and will open safely the design RMS value)


    Then think of the peak current as being the safe making current (if you were unlucky enough to close into a fault) - so a board that may be rated at 100kA peak


    The relationship used for all practicable purposes is a factor of ranging from 1.5 through to 2.5 for design conditions. Alternatively you may use a value of 2 x square root of the RMS value squared (the so called doubling effect) if you are in the typical range of transformer fed LV switchboards at around 50kA prospective levels (eg a parallel pair of 1.5MVA Transformers feeding into A and B switchboards with bus sections closed)


    Note the 2.5 factor is only used for very large fault levels - there is a range of multipliers depending on fault levels - these range from say 1.5 for faults less than 5kA to a factor of 2 for faults at say 50kA(as above), to a factor of 2.5 for faults exceeding 80kA - these are further defined in switchgear protection standards such as BS EN 60947-1 (the so called "n" factor)


    Regards


    OMS
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