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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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    Please not an MCB (or RCBO) upstream of other MCBs (or RCBOs) - you'll get little to no discrimination on L-N or L-L faults and you don't want the entire site blacked out for a simple fault on one final circuit.


    Likewise if you have more than one tier of RCDs you'll need all the downstream RCDs to be N switching if you want discrimination (many RCBOs aren't).


    The OP says the submain supplies a mixed agriculture/horticulture/residential/glamping site -  thus this submain doesn't feel to be entirely horticultural (after all the residential part and permanent building on the camping part are permitted to be PME) - it's merely a common supply to a number of (sub) installations, only one of which is agricultural. So I see no requirement for 705's 300mA RCD requirement to be applied to the submain.


    I still think that in a metal box outdoors is far from an ideal location for a delicate device like an RCD - especially one you're going to be totally reliant on for ADS, not just additional/fire protection. HBC fuses, with no moving parts and the fuse wire itself hermetically sealed in the cartridge feels like it's going to be a far more reliable solution (provided you get Zs low enough  of course - but that should be only a matter of using the DNO's earth for that bit).


      - Andy.

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  • 0
    Please not an MCB (or RCBO) upstream of other MCBs (or RCBOs) - you'll get little to no discrimination on L-N or L-L faults and you don't want the entire site blacked out for a simple fault on one final circuit.


    Likewise if you have more than one tier of RCDs you'll need all the downstream RCDs to be N switching if you want discrimination (many RCBOs aren't).


    The OP says the submain supplies a mixed agriculture/horticulture/residential/glamping site -  thus this submain doesn't feel to be entirely horticultural (after all the residential part and permanent building on the camping part are permitted to be PME) - it's merely a common supply to a number of (sub) installations, only one of which is agricultural. So I see no requirement for 705's 300mA RCD requirement to be applied to the submain.


    I still think that in a metal box outdoors is far from an ideal location for a delicate device like an RCD - especially one you're going to be totally reliant on for ADS, not just additional/fire protection. HBC fuses, with no moving parts and the fuse wire itself hermetically sealed in the cartridge feels like it's going to be a far more reliable solution (provided you get Zs low enough  of course - but that should be only a matter of using the DNO's earth for that bit).


      - Andy.

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