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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
    I think you are mixing up peak and rms values


    The transformer could deliver a maximum RMS fault current of 24kA


    Applying that to a 100A Fuse, suggests to me that you could not possibly exceed 9kA RMS


    You have a switch capable of 16kA (making and breaking)


    Not seeing the problem


    If it gives some comfort, a typical DNO distribution TX is normally 800kVA - if that 100A fuse was at the cut out of a house, would you think twice about the REC isolator at 16kA


    Regards


    OMS
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  • Former Community Member
    0 Former Community Member
    I think you are mixing up peak and rms values


    The transformer could deliver a maximum RMS fault current of 24kA


    Applying that to a 100A Fuse, suggests to me that you could not possibly exceed 9kA RMS


    You have a switch capable of 16kA (making and breaking)


    Not seeing the problem


    If it gives some comfort, a typical DNO distribution TX is normally 800kVA - if that 100A fuse was at the cut out of a house, would you think twice about the REC isolator at 16kA


    Regards


    OMS
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