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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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    But if the stable has no extraneous-conductive-parts and is of wooden construction, perhaps with a blue plastic water pipe only, that puts a different complexion on things.

    Just as an aside - a modern TT installation may have a CT2 wired SPD before the first RCD - so what happens if the N-PE SPD element fails to short? (as I gather is possible, which is why no element is connected L-PE in the CT2 scheme). The TT system then ‘degrades’ to TN, no protective device opens (as the N-PE voltage difference is low) and the “fault” remains until probably the next EICR (whenever that might be). What's the magnitude of L-PE fault currents now?

        - Andy. 

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    But if the stable has no extraneous-conductive-parts and is of wooden construction, perhaps with a blue plastic water pipe only, that puts a different complexion on things.

    Just as an aside - a modern TT installation may have a CT2 wired SPD before the first RCD - so what happens if the N-PE SPD element fails to short? (as I gather is possible, which is why no element is connected L-PE in the CT2 scheme). The TT system then ‘degrades’ to TN, no protective device opens (as the N-PE voltage difference is low) and the “fault” remains until probably the next EICR (whenever that might be). What's the magnitude of L-PE fault currents now?

        - Andy. 

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