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TT/PME Bonding of metal cabinet that houses the DNO Cutout (PME) and Private RCD (TT)

Steve UKBC
Initial Post Edited for clarity/updated info


If anyone could suggest the correct way forward on bonding the cabinet to either PME/TT or neither.


The HV supply comes in to a pole mounted TX, A TNCS/PME supply is provided in a adjacent metal cabinet (Cutout, meter, isolator & fused isolator), proposal is to replace the fused isolator with a MCB & type S 100mA RCD within a plastic enclosure to supply a agriculture/horticulture/residential/glamping site some +100m away.


The feeder cable to DB1 some +100m away has not got a low enough impedance to clear a earth fault with the 100A DNO fuses/fused isolator within 5s required by BS7671, hence the RCD protecting the cable with the cable CPC/swa, connected only at DB1 where the main earth rod is.


The feeder cable to DB1 cannot be replaced/paralleled up.


So we are left with a metal cabinet where the PME supply switches over to a TT.


The question is, do we bond the cabinet to the PME and protect the cabinet from becoming live if the tails where to make contact (blowing the DNO fuses) but in doing so a broken neutral pre cutout would make the cabinet live, or bond the cabinet to the TT earth via the feeder cable SWA to ensure that if a broken neutral occurs that the cabinet does not become live but if the tails where to make contact to the cabinet then cabinet would be live.


I suspect the most likely fault between a broken neutral and tails touching the cabinet would be a broken neutral due to the exposed cables from the pole etc? hence suspect we should connect the cabinet to the TT earth ensuring the tails within the cabinet are well secured?
circuit.pdf
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  • 0
    Hi,


    Attached photo of the cabinet as it currently stands, (proposed MCB/RCD not installed), proposal was to replace the fused isolator with a MCB and RCD in a plastic consumer unit and have the cable to the barn (SWA/CPC) terminated only at the barn /DB1 end.  The thought to this was that the barn/DB1 earth rod would not have a low enough impedance to earth to disconnect the fuses in the enclosed fused isolator in the cabinet (on a earth fault) and so potentially allowing the SWA of the cable, DB1(barn distribution board) and metal enclosed isolator to remain live on a earth fault.


    Also looking at the cabinet you can clearly see steel threaded bar emanating from the concrete plinth and I was not sure if to connect the cabinet to the TT or PME earth or not at all.  Thinking a broken neutral pre cutout would make the cabinet live if its connected to the PME earth, hence better not to connect or connect to the TT earth?


    Any comments on the current or proposed setup would be grateful.


    I have blanked out any DNO/meter details to protect the end user.


    Cheers


    Steve

    17ec3cd1c1a10bd36833fe5857a0c505-original-cabinet.jpg

     



Reply
  • 0
    Hi,


    Attached photo of the cabinet as it currently stands, (proposed MCB/RCD not installed), proposal was to replace the fused isolator with a MCB and RCD in a plastic consumer unit and have the cable to the barn (SWA/CPC) terminated only at the barn /DB1 end.  The thought to this was that the barn/DB1 earth rod would not have a low enough impedance to earth to disconnect the fuses in the enclosed fused isolator in the cabinet (on a earth fault) and so potentially allowing the SWA of the cable, DB1(barn distribution board) and metal enclosed isolator to remain live on a earth fault.


    Also looking at the cabinet you can clearly see steel threaded bar emanating from the concrete plinth and I was not sure if to connect the cabinet to the TT or PME earth or not at all.  Thinking a broken neutral pre cutout would make the cabinet live if its connected to the PME earth, hence better not to connect or connect to the TT earth?


    Any comments on the current or proposed setup would be grateful.


    I have blanked out any DNO/meter details to protect the end user.


    Cheers


    Steve

    17ec3cd1c1a10bd36833fe5857a0c505-original-cabinet.jpg

     



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