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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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    Steve UKBC:

    (originally)

    So we are left with a metal cabinet where the PME supply switches over to a TT.


    (later)

    Id prefer not to take the PME earth to the farm buildings and keep the TT/PME switch away from the area and in the cabinet in the field adjacent to the TX pole, as you cannot know the future use or these farm buildings, there is currently no livestock but who's to say this does not change in the near future.

    Any outer damage of the feeder cable if the PME earth were to be brought to the farm building (i.e exposed SWA carrying the PME earth) would introduce the PME earth potentially closer to the farm building earth electrodes would it not?




    I don't think it is at all correct to say that the PME supply switches over to TT. The distributor's earth is simply sitting there unused.


    If DB1 is in a farm building which has potential for housing livestock, I can understand the reluctance to take the PME supply to it, but as things stand you have exposed conducting parts of two earthing systems which are very obviously simultaneously accessible contrary to 411.3.1.1 On reflection, I may upgrade to C1!


    Get rid of that shoelace of an earthing conductor between the service head and "MET" and preferably, bung in another earth rod near the origin.


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  • 0
    Steve UKBC:

    (originally)

    So we are left with a metal cabinet where the PME supply switches over to a TT.


    (later)

    Id prefer not to take the PME earth to the farm buildings and keep the TT/PME switch away from the area and in the cabinet in the field adjacent to the TX pole, as you cannot know the future use or these farm buildings, there is currently no livestock but who's to say this does not change in the near future.

    Any outer damage of the feeder cable if the PME earth were to be brought to the farm building (i.e exposed SWA carrying the PME earth) would introduce the PME earth potentially closer to the farm building earth electrodes would it not?




    I don't think it is at all correct to say that the PME supply switches over to TT. The distributor's earth is simply sitting there unused.


    If DB1 is in a farm building which has potential for housing livestock, I can understand the reluctance to take the PME supply to it, but as things stand you have exposed conducting parts of two earthing systems which are very obviously simultaneously accessible contrary to 411.3.1.1 On reflection, I may upgrade to C1!


    Get rid of that shoelace of an earthing conductor between the service head and "MET" and preferably, bung in another earth rod near the origin.


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