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Single phase sub-main

GA

Evening, I'm seeking some advise pls. I have a 4core 25mm2 SWA and was going to use this for two single phase submains running from and to a common point, where the circuits will then go into separate consumer units. limited loads a garage and stable. The cable will have DP isolation and RCD protection on each circuit (2cores L+N per sub main) at the supply point. I'm also looking to run a separate earth cable of 25mm2 which would be common to both circuits and armouring of the SWA cable is also earthed and common. My question is I believe this to be complaint with 17th Ed; As I can't see anything saying its not. Second question, could I common the neutral for both submains and therefore use the fourth conductor in the SWA as an earth which would save installing the separate earth cable? Again, I can't see anything incorrect as limited loads to a couple of sockets and a few lights.  thanks for assistance. GA

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    In this context surely an earth fault will clear very quickly if an R.C.D. is installed at the origin of the S.W.A. cable, and disconnect the fault before serious damage due to cable  heating can occur.

    Fuses and MCBs can be significantly faster than RCDs at high fault currents - an S type won't open in less than 40ms - at say 6kA that's 1,440,000 A2s so by the adiabatic would need over a 10mm2 copper conductor (if k=115).

    By comparison a B-type MCB at 6kA should have an energy let-though of less than 54,000 A2s - so should be adequate with just a 2.5mm2 conductor from a fault protection perspective.

       - Andy.

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    In this context surely an earth fault will clear very quickly if an R.C.D. is installed at the origin of the S.W.A. cable, and disconnect the fault before serious damage due to cable  heating can occur.

    Fuses and MCBs can be significantly faster than RCDs at high fault currents - an S type won't open in less than 40ms - at say 6kA that's 1,440,000 A2s so by the adiabatic would need over a 10mm2 copper conductor (if k=115).

    By comparison a B-type MCB at 6kA should have an energy let-though of less than 54,000 A2s - so should be adequate with just a 2.5mm2 conductor from a fault protection perspective.

       - Andy.

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