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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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    I would bond the cabinet to the TT installation, the cabinet manufacturers think it is important to bond the door of the cabinet to the actual cabinet enclosure having installed an earth pig tail between them, just in case there is high resistance on the door hinges, now you want to leave the entire cabinet floating in relation to the customers installation equipment within it.


    If you are really concerned that it is close to the DNO electrode do a high current loop test on the cabinet before you connect it to the earthing system, I cannot imagine that it will be a particularly low test result, that plinth and studs hardly count as foundation earthing.
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    I would bond the cabinet to the TT installation, the cabinet manufacturers think it is important to bond the door of the cabinet to the actual cabinet enclosure having installed an earth pig tail between them, just in case there is high resistance on the door hinges, now you want to leave the entire cabinet floating in relation to the customers installation equipment within it.


    If you are really concerned that it is close to the DNO electrode do a high current loop test on the cabinet before you connect it to the earthing system, I cannot imagine that it will be a particularly low test result, that plinth and studs hardly count as foundation earthing.
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