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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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    3. Don't bond the cabinet and in effect the cabinet floats at or close to the PME earth potential and so potentially have the same problem as point 2 above.

    If it's not directly connected to either earthing system then it'll tend to be at the same potential as the ground it is stood on - which is exactly what you want if you want to avoid shocking someone stood on the ground and touching the cabinet. If there is a PME influence on the ground around/under the cabinet then that (almost*) makes no difference as both the ground and the cabinet will still be at the same potential - so still no shock (like a bird perched on a bare overhead wire - it's not the absolute voltage that matters, just the difference in voltage the victim is exposed to).


    *I say almost as you can get some difference of potential between different points on the ground, so if the PME electrode is fairly near, there might be some potential difference between the ground to one side of the cabinet compared with the other - but still the potential of the cabinet is likely to be fair average of the ground around it, and you're not going to get much better than that.


    Remember that TT earthing systems aren't always safer than PME ones - there might not be a broken CNE risk - but all it takes is a simple L-PE fault and a sticky RCD to leave the entire TT earthing system at a hazardous voltage indefinitely. The ideal would be to avoid all exposed-conductive-parts (and bonding anything) out doors - and the mitigation if you can't is often a buried grid. We're not in an ideal world however.


       - Andy.
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  • 0
    3. Don't bond the cabinet and in effect the cabinet floats at or close to the PME earth potential and so potentially have the same problem as point 2 above.

    If it's not directly connected to either earthing system then it'll tend to be at the same potential as the ground it is stood on - which is exactly what you want if you want to avoid shocking someone stood on the ground and touching the cabinet. If there is a PME influence on the ground around/under the cabinet then that (almost*) makes no difference as both the ground and the cabinet will still be at the same potential - so still no shock (like a bird perched on a bare overhead wire - it's not the absolute voltage that matters, just the difference in voltage the victim is exposed to).


    *I say almost as you can get some difference of potential between different points on the ground, so if the PME electrode is fairly near, there might be some potential difference between the ground to one side of the cabinet compared with the other - but still the potential of the cabinet is likely to be fair average of the ground around it, and you're not going to get much better than that.


    Remember that TT earthing systems aren't always safer than PME ones - there might not be a broken CNE risk - but all it takes is a simple L-PE fault and a sticky RCD to leave the entire TT earthing system at a hazardous voltage indefinitely. The ideal would be to avoid all exposed-conductive-parts (and bonding anything) out doors - and the mitigation if you can't is often a buried grid. We're not in an ideal world however.


       - Andy.
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