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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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    Sparkingchip:

    Just because there is an earth connection supplied by the DNO you don't have to use it and if you do it your decision to make.


    It is the only TNCS earthed item in the whole TT installation, if you are kneeling on the wet ground and lean against the metal cabinet you could feel a tingle if you touch the DNO earth terminal or possibly a bit more. It's not lethal as in if you touch you will die, but it serves no purpose and could be a risk.


    Enclose it in a plastic box or something if you think someone may decide it could be required in the future, it doesn't need to be complicated.

    The RCD is not the better solution, it is the only solution. Unless you are going to take the TNCS-PME earth connection to where it does not need to be.


    Agreed the RCD is the best option but would the alternative solution not be to take the PME earth to an insulated enclosure just before DB1 (change over to TT) as long as the disconnection times are within 5s with the protection within the fused isolator?


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  • 0
    Sparkingchip:

    Just because there is an earth connection supplied by the DNO you don't have to use it and if you do it your decision to make.


    It is the only TNCS earthed item in the whole TT installation, if you are kneeling on the wet ground and lean against the metal cabinet you could feel a tingle if you touch the DNO earth terminal or possibly a bit more. It's not lethal as in if you touch you will die, but it serves no purpose and could be a risk.


    Enclose it in a plastic box or something if you think someone may decide it could be required in the future, it doesn't need to be complicated.

    The RCD is not the better solution, it is the only solution. Unless you are going to take the TNCS-PME earth connection to where it does not need to be.


    Agreed the RCD is the best option but would the alternative solution not be to take the PME earth to an insulated enclosure just before DB1 (change over to TT) as long as the disconnection times are within 5s with the protection within the fused isolator?


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