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Single phase sub-main

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: 
     

    Thanks. How will my suggested upfront S type 300mA R.C.D. affect the overall situation?

     

    Z.

    RCDs do not provide protection against overcurrent … In a TN system it's a little easier to address protection against overcurrent based on adiabatic, or manufacturer's let-through energy. In a TT system, to fully answer the question, you would need to know the maximum prospective earth fault current (that occurs when all extraneous-conductive-parts are connected in TT, as opposed to max Ze which is the fault through the earth electrode system alone in a TT system) … 

Reply
  • Zoomup: 
     

    Thanks. How will my suggested upfront S type 300mA R.C.D. affect the overall situation?

     

    Z.

    RCDs do not provide protection against overcurrent … In a TN system it's a little easier to address protection against overcurrent based on adiabatic, or manufacturer's let-through energy. In a TT system, to fully answer the question, you would need to know the maximum prospective earth fault current (that occurs when all extraneous-conductive-parts are connected in TT, as opposed to max Ze which is the fault through the earth electrode system alone in a TT system) … 

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