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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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    AJJewsbury: 
     

    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. 

    What is 3a on page 509 then?

    Also fault protection as defined in Chap 41 shall remain effective in the protected installation even in the event of SPD failure. 536.4.6.

    Also table 534.5 applies. SPD only downstream of R.C.D.

    Edit. Add. Having just spoken to a very nice young lady at Surge Protection Devices Ltd. she assured me that their S.P.D.s can not short circuit permanently. They have a ten year guarantee and can work for up to 100,000 hours.

    Z.

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  • 0

    AJJewsbury: 
     

    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. 

    What is 3a on page 509 then?

    Also fault protection as defined in Chap 41 shall remain effective in the protected installation even in the event of SPD failure. 536.4.6.

    Also table 534.5 applies. SPD only downstream of R.C.D.

    Edit. Add. Having just spoken to a very nice young lady at Surge Protection Devices Ltd. she assured me that their S.P.D.s can not short circuit permanently. They have a ten year guarantee and can work for up to 100,000 hours.

    Z.

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