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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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    Chris Pearson:

    I see no reason to earth the cabinet, but that switch fuse definitely needs to be earthed. Achieving that via 100 m of SWA and an earth rod seems very odd. In the absence of an RCD it is definitely no better than C2.


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


    Not sure what's in side the fused isolator yet but agree it needs earthed as it stands and with a upfront RCD, my own thought was to replace it with a plastic enclosure and a MCB & RCD (100mA Time delay) within, as the fuse isolator has been noted to be potentially faulty anyway.

    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.


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  • 0
    Chris Pearson:

    I see no reason to earth the cabinet, but that switch fuse definitely needs to be earthed. Achieving that via 100 m of SWA and an earth rod seems very odd. In the absence of an RCD it is definitely no better than C2.


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


    Not sure what's in side the fused isolator yet but agree it needs earthed as it stands and with a upfront RCD, my own thought was to replace it with a plastic enclosure and a MCB & RCD (100mA Time delay) within, as the fuse isolator has been noted to be potentially faulty anyway.

    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.


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