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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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    What is 3a on page 509 then?

    Yup - that's the SPD element connected N-PE - typically a spark gap/gas discharge tube type.

    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.

    534.4.6. - and the 2nd indent of that reg describes the permitted connection types for TT systems.  

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

    Which is why you'd usually use the right-hand column instead - CT2. S-type RCDs are relatively expensive and contain electronics, so it seems daft to omit them from SPD protection by putting the SPD afterwards and so drawing the full spike current (possibly many kA) through the delicate RCD.

       - Andy.

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  • 0

    What is 3a on page 509 then?

    Yup - that's the SPD element connected N-PE - typically a spark gap/gas discharge tube type.

    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.

    534.4.6. - and the 2nd indent of that reg describes the permitted connection types for TT systems.  

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

    Which is why you'd usually use the right-hand column instead - CT2. S-type RCDs are relatively expensive and contain electronics, so it seems daft to omit them from SPD protection by putting the SPD afterwards and so drawing the full spike current (possibly many kA) through the delicate RCD.

       - Andy.

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