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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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  • 0
    mapj1:

    Ah - a picture speaks 1k words . Not my mental picture at all.

    Leave the outer enclosure street box as it is - the DNO kit is in effect double insulated or reinforced insulation, and no further action needed on that score.

    I'm with Andy B above. Ignore the company earth terminal altogether but get an RCD in there - it can be a high current one assuming the TT earth is good enough.

    A 300mA ordinary one will detect cable faults between it and the destination, and will not be pushed off balance by anything much less than 100mA of DC - this sadly is not a parameter written  in the specs, but given the way magnetic cores operate, is undoubtedly true. Closer protection can be afforded at the load end. Something like the second label might be a good remineder (these are Irish but a UK variant must exist.)



    Was thinking a type S (time delay) 100mA type A RCD with 30mA RCBO's on all final circuits to prevent multiple RCD's tripping on a fault, are you suggesting 300mA (non time delayed?) up front and 30mA RCBO's on the final circuits may be more appropriate ? 

    noted the TT earth label


Reply
  • 0
    mapj1:

    Ah - a picture speaks 1k words . Not my mental picture at all.

    Leave the outer enclosure street box as it is - the DNO kit is in effect double insulated or reinforced insulation, and no further action needed on that score.

    I'm with Andy B above. Ignore the company earth terminal altogether but get an RCD in there - it can be a high current one assuming the TT earth is good enough.

    A 300mA ordinary one will detect cable faults between it and the destination, and will not be pushed off balance by anything much less than 100mA of DC - this sadly is not a parameter written  in the specs, but given the way magnetic cores operate, is undoubtedly true. Closer protection can be afforded at the load end. Something like the second label might be a good remineder (these are Irish but a UK variant must exist.)



    Was thinking a type S (time delay) 100mA type A RCD with 30mA RCBO's on all final circuits to prevent multiple RCD's tripping on a fault, are you suggesting 300mA (non time delayed?) up front and 30mA RCBO's on the final circuits may be more appropriate ? 

    noted the TT earth label


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