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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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    AJJewsbury:
    AJJewsbury:
    RCD in the DNO cabinet would be the better solution since the clearance time for a earth fault may be above 5s

    Provided everything is OK for voltage drop and R2 isn't massively larger than R1, I would be surprised if you couldn't get 5s disconnection times from the usual overcurrent protective devices.

      - Andy.


    Come to that what is your voltage drop like? 100m is quite long for a LV submain and your diagram seems to suggest it's just 35mm² - on what's presumably something close to a 100A/phase system? While the tabulated v.d. of 1.1mV/A/m will be an over estimate as you'll be running the cable a bit cooler than 70 degrees there doesn't feel to be much scope for achieving the usual 3% overall for downstream lighting circuits - even an overall 5% to the end of final circuits looks rather tight (even if you claim the extra 0.005%/m beyond 100m).


       - Andy.




    Hi Andy,


    As previously stated, the feeder cable cannot change and the volt drop has always been a concern (especially lighting), But my calcs have provided confidence that the volt drop is within the limits, some oversized cables down the line have been used purely for the volt drop.

    A load assessment and final circuit voltage readings are to be recorded on completion for this very fact. 


Reply
  • 0
    AJJewsbury:
    AJJewsbury:
    RCD in the DNO cabinet would be the better solution since the clearance time for a earth fault may be above 5s

    Provided everything is OK for voltage drop and R2 isn't massively larger than R1, I would be surprised if you couldn't get 5s disconnection times from the usual overcurrent protective devices.

      - Andy.


    Come to that what is your voltage drop like? 100m is quite long for a LV submain and your diagram seems to suggest it's just 35mm² - on what's presumably something close to a 100A/phase system? While the tabulated v.d. of 1.1mV/A/m will be an over estimate as you'll be running the cable a bit cooler than 70 degrees there doesn't feel to be much scope for achieving the usual 3% overall for downstream lighting circuits - even an overall 5% to the end of final circuits looks rather tight (even if you claim the extra 0.005%/m beyond 100m).


       - Andy.




    Hi Andy,


    As previously stated, the feeder cable cannot change and the volt drop has always been a concern (especially lighting), But my calcs have provided confidence that the volt drop is within the limits, some oversized cables down the line have been used purely for the volt drop.

    A load assessment and final circuit voltage readings are to be recorded on completion for this very fact. 


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