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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
    So the options for the original posted question 


    1. Assuming the PME MET is removed from the cabinet and i bond the cabinet to the TT then i suspect i will in effect be connecting the PME earth to the TT earth due to the proximity/direct connection or the local earth electrode of the TNCS/PME supply to the cabinet via the steel&conc cabinet foundation, and i don't want to export the PME earth to the farm.


    2. If I bond the cabinet to the PME earth and insulate any TT earth in the cabinet (i.e. the insulate SWA cable gland/use a plastic enclosure) then all is good apart form if there is a broken neutral pre cutout, making the cabinet live and the RCD in the cabinet not able to detect any leakage due to the fault current existing before the cabinet RCD. 


    3. Don't bond the cabinet and in effect the cabinet floats at or close to the PME earth potential and so potentially have the same problem as point 2 above. 


    4. Is the obvious option here to replace the metal cabinet with a fiberglass cabinet, although the DNO may have something to say? 


    5. Report the issue to the DNO (if its there cabinet) and pass the issue to them, will they do anything?


    Who's responsible if the cabinet causes injury or death through it becoming live?


    I know the chances of injury or death are probably very small but i want to get this right.


    Is there a standard ideal solution to this?  Am I over thinking the whole issue? 


Reply
  • 0
    So the options for the original posted question 


    1. Assuming the PME MET is removed from the cabinet and i bond the cabinet to the TT then i suspect i will in effect be connecting the PME earth to the TT earth due to the proximity/direct connection or the local earth electrode of the TNCS/PME supply to the cabinet via the steel&conc cabinet foundation, and i don't want to export the PME earth to the farm.


    2. If I bond the cabinet to the PME earth and insulate any TT earth in the cabinet (i.e. the insulate SWA cable gland/use a plastic enclosure) then all is good apart form if there is a broken neutral pre cutout, making the cabinet live and the RCD in the cabinet not able to detect any leakage due to the fault current existing before the cabinet RCD. 


    3. Don't bond the cabinet and in effect the cabinet floats at or close to the PME earth potential and so potentially have the same problem as point 2 above. 


    4. Is the obvious option here to replace the metal cabinet with a fiberglass cabinet, although the DNO may have something to say? 


    5. Report the issue to the DNO (if its there cabinet) and pass the issue to them, will they do anything?


    Who's responsible if the cabinet causes injury or death through it becoming live?


    I know the chances of injury or death are probably very small but i want to get this right.


    Is there a standard ideal solution to this?  Am I over thinking the whole issue? 


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