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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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    gkenyon: 
     

    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) … 

    I am considering the armour as being suitable as a C.P.C. In this context surely an earth fault will clear very quickly if an R.C.D. is installed at the origin of the S.W.A. cable, and disconnect the fault before serious damage due to cable  heating can occur.

    If the cable is supplying a stable then the fault current may only be a few hundred Amps as buried metal will offer a poor earth return.

     

    Z.

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  • 0

    gkenyon: 
     

    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) … 

    I am considering the armour as being suitable as a C.P.C. In this context surely an earth fault will clear very quickly if an R.C.D. is installed at the origin of the S.W.A. cable, and disconnect the fault before serious damage due to cable  heating can occur.

    If the cable is supplying a stable then the fault current may only be a few hundred Amps as buried metal will offer a poor earth return.

     

    Z.

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