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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
    I'm puzzled over a few points here.


    Normally, if the DNO allow their fuses to protect consumer's wiring at all, they stipulate a limit - usually around 3m. So I'm surprised there's no consumer's fuse before the 100+m submain.


    Another option would seem to be to continue with the DNO's earth until DB1 and make the TN/TT split there. That should remove the need for the 100mA RCD (but you'd still need some overcurrent protection) . But there might be other considerations against that.


    Then there's the cabinet itself - you seem to be implying that it's an exposed-conductive-part - i.e. it can be made live by a single failure of basic insulation. I would have thought that unlikely as cables associated with the cut-out and meter tend to be sheathed and other parts within enclosures. It may still be an extraneous-conductive-part - if for instance it is in contact with the soil - so bonding rather than earthing might be a consideration - but if everything inside is within insulating enclosures and cables are insulated sheathed - i.e. there are no exposed-conductive-parts - and no other (bonded) extraneous-conductive-parts in the vicinity then it doesn't sound like bonding will provide any benefits either.


       - Andy.
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  • 0
    I'm puzzled over a few points here.


    Normally, if the DNO allow their fuses to protect consumer's wiring at all, they stipulate a limit - usually around 3m. So I'm surprised there's no consumer's fuse before the 100+m submain.


    Another option would seem to be to continue with the DNO's earth until DB1 and make the TN/TT split there. That should remove the need for the 100mA RCD (but you'd still need some overcurrent protection) . But there might be other considerations against that.


    Then there's the cabinet itself - you seem to be implying that it's an exposed-conductive-part - i.e. it can be made live by a single failure of basic insulation. I would have thought that unlikely as cables associated with the cut-out and meter tend to be sheathed and other parts within enclosures. It may still be an extraneous-conductive-part - if for instance it is in contact with the soil - so bonding rather than earthing might be a consideration - but if everything inside is within insulating enclosures and cables are insulated sheathed - i.e. there are no exposed-conductive-parts - and no other (bonded) extraneous-conductive-parts in the vicinity then it doesn't sound like bonding will provide any benefits either.


       - Andy.
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