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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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    wallywombat:

    So if I'm interpreting that photo correctly, the DNO earthing facility is completely unused, and the amour of the 100m SWA cable is earthed at the load end to an earth rod?


    If it was me I would change it so that that the armour is connected to the DNO earth at the feed end and isolated at the load end.




    That creates more issues than it resolves.


    Insert a 300 mA RCD in a plastic enclosure in the tails downstream of the DNO isolation switch and lose the DNO earth terminal, if you cannot easily access the conductor at the intake end take the bar off and put some heat shrink over the end of the conductor. 


    The earth terminal is the only part of the DNO installation that is exposed and likely to be a risk, it's better off out of there.


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  • 0
    wallywombat:

    So if I'm interpreting that photo correctly, the DNO earthing facility is completely unused, and the amour of the 100m SWA cable is earthed at the load end to an earth rod?


    If it was me I would change it so that that the armour is connected to the DNO earth at the feed end and isolated at the load end.




    That creates more issues than it resolves.


    Insert a 300 mA RCD in a plastic enclosure in the tails downstream of the DNO isolation switch and lose the DNO earth terminal, if you cannot easily access the conductor at the intake end take the bar off and put some heat shrink over the end of the conductor. 


    The earth terminal is the only part of the DNO installation that is exposed and likely to be a risk, it's better off out of there.


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