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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:
    Chris Pearson:

    What size fuses are in the switch fuse and what is the EFLI at the DB?


    The earth fault loop impedance is way too high for anything currently protecting the cable i think my MFT recorded around 10ohm / 25A, but it had been raining a while so the ground was soaked so this was probably best case, but generally speaking i thought it was a good low TT reading.




    Sorry, that's not really what I meant to ask. What is Ze, and what is R1+R2 of the distribution circuit.


    If you don't know what size fuses are in there, how do know "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 ..."


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  • 0
    Steve UKBC:
    Chris Pearson:

    What size fuses are in the switch fuse and what is the EFLI at the DB?


    The earth fault loop impedance is way too high for anything currently protecting the cable i think my MFT recorded around 10ohm / 25A, but it had been raining a while so the ground was soaked so this was probably best case, but generally speaking i thought it was a good low TT reading.




    Sorry, that's not really what I meant to ask. What is Ze, and what is R1+R2 of the distribution circuit.


    If you don't know what size fuses are in there, how do know "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 ..."


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