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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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    As a matter of interest and looking BS7671 Characteristics for the fuse, that for a 0.1 disconnection time that the fault clears at 1450 A which I take it is true up to 33kA.



    The fuse will certainly be much, much, faster than 0.1s at higher fault currents - if anything the energy let-though (I²t) of fuses declines (or at worst remains flat) with increasing fault currents (unlike circuit breakers). For some reason BS 7671 chopped off the data below 0.1s a few editions ago (possibly a hint not to use the adiabatic formula directly but compare manufacturer's energy let-though data - as per the 2nd paragraph of 434.5.2).


       - Andy.
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  • 0

    As a matter of interest and looking BS7671 Characteristics for the fuse, that for a 0.1 disconnection time that the fault clears at 1450 A which I take it is true up to 33kA.



    The fuse will certainly be much, much, faster than 0.1s at higher fault currents - if anything the energy let-though (I²t) of fuses declines (or at worst remains flat) with increasing fault currents (unlike circuit breakers). For some reason BS 7671 chopped off the data below 0.1s a few editions ago (possibly a hint not to use the adiabatic formula directly but compare manufacturer's energy let-though data - as per the 2nd paragraph of 434.5.2).


       - Andy.
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