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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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    Zoomup: 
     

    QUOTE: “(b) the TT system is derived from a TN system, and unfortunately shares extraneous-conductive-parts with that system. Again, can't rely on that for protection against electric shock … but this time we definitely need to size our cpc's in a similar manner to TN systems for protection against overcurrent (earth fault current). Again, the RCD is not an overcurrent protective device according to BS 7671, it requires back-up of a suitable overcurrent protective device, and the cpc sized accordingly.”

    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.

    Z.
     

    It's very difficult … you know the site layout and the earthing arrangements, sadly we don't.

    Does the stable have its own earth electrode and effectively you're only exporting live conductors? Or are you exporting the protective conductor from another part of the installation that does have extraneous-conductive-parts?

    And regardless …  even if it has its own electrode and only imports live conductors, you know the earth electrode resistance (whether it's 1 ohm with Ipef = 230 A, or 100 ohms and Ipef = 23 A) and we don't.

    And of course, I don't know whether the stable has a concrete floor or wooden floor … if concrete, re-bar recommended to be main-bonded regardless of supply earthing arrangement (not just PME) see 705.415.2.1.

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  • 0

    Zoomup: 
     

    QUOTE: “(b) the TT system is derived from a TN system, and unfortunately shares extraneous-conductive-parts with that system. Again, can't rely on that for protection against electric shock … but this time we definitely need to size our cpc's in a similar manner to TN systems for protection against overcurrent (earth fault current). Again, the RCD is not an overcurrent protective device according to BS 7671, it requires back-up of a suitable overcurrent protective device, and the cpc sized accordingly.”

    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.

    Z.
     

    It's very difficult … you know the site layout and the earthing arrangements, sadly we don't.

    Does the stable have its own earth electrode and effectively you're only exporting live conductors? Or are you exporting the protective conductor from another part of the installation that does have extraneous-conductive-parts?

    And regardless …  even if it has its own electrode and only imports live conductors, you know the earth electrode resistance (whether it's 1 ohm with Ipef = 230 A, or 100 ohms and Ipef = 23 A) and we don't.

    And of course, I don't know whether the stable has a concrete floor or wooden floor … if concrete, re-bar recommended to be main-bonded regardless of supply earthing arrangement (not just PME) see 705.415.2.1.

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