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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
  • 0
    If it is a box installed by the DNO then ask them what, if anything to do about earthing it if it is not earthed, because if they installed it it should already be installed correctly.
  • 0
    Hi,


    Attached photo of the cabinet as it currently stands, (proposed MCB/RCD not installed), proposal was to replace the fused isolator with a MCB and RCD in a plastic consumer unit and have the cable to the barn (SWA/CPC) terminated only at the barn /DB1 end.  The thought to this was that the barn/DB1 earth rod would not have a low enough impedance to earth to disconnect the fuses in the enclosed fused isolator in the cabinet (on a earth fault) and so potentially allowing the SWA of the cable, DB1(barn distribution board) and metal enclosed isolator to remain live on a earth fault.


    Also looking at the cabinet you can clearly see steel threaded bar emanating from the concrete plinth and I was not sure if to connect the cabinet to the TT or PME earth or not at all.  Thinking a broken neutral pre cutout would make the cabinet live if its connected to the PME earth, hence better not to connect or connect to the TT earth?


    Any comments on the current or proposed setup would be grateful.


    I have blanked out any DNO/meter details to protect the end user.


    Cheers


    Steve

    17ec3cd1c1a10bd36833fe5857a0c505-original-cabinet.jpg

     



  • 0
    Urgent remedial work required Code 2 ?


    Insert a 300 mA RCD in the tails between the DNO isolator and the Crabtree Fusestar as soon as possible.


    As to the type of RCD it will probably have to be a Type A off the shelf, but presumably there is a potential issue with people using granny leads to charge electric vehicles on the camp site pitches.


    Andy Betteridge
  • 0
    And I would not rush to connect the cabinet to the DNO earth bar, but would connect it to the Crabtree Fusestar earth bar.


    I am not seeing any reason to replace the Crabtree Fusestar, insert the RCD in the tails and connect the cabinet to the fused switch earth bar seems to be all that is required. Removing the DNO main earth conductor from the intake seems like a good idea, it's of no use and is a hazard to be avoided by anyone working in the cabinet.
  • 0
    Ah - a picture speaks 1k words . Not my mental picture at all.

    Leave the outer enclosure street box as it is - the DNO kit is in effect double insulated or reinforced insulation, and no further action needed on that score.

    I'm with Andy B above. Ignore the company earth terminal altogether but get an RCD in there - it can be a high current one assuming the TT earth is good enough.

    A 300mA ordinary one will detect cable faults between it and the destination, and will not be pushed off balance by anything much less than 100mA of DC - this sadly is not a parameter written  in the specs, but given the way magnetic cores operate, is undoubtedly true. Closer protection can be afforded at the load end. Something like the second label might be a good remineder (these are Irish but a UK variant must exist.)

  • 0
    So if I'm interpreting that photo correctly, the DNO earthing facility is completely unused, and the amour of the 100m SWA cable is earthed at the load end to an earth rod?


    If it was me I would change it so that that the armour is connected to the DNO earth at the feed end and isolated at the load end.
  • 0
    wallywombat:

    So if I'm interpreting that photo correctly, the DNO earthing facility is completely unused, and the amour of the 100m SWA cable is earthed at the load end to an earth rod?


    If it was me I would change it so that that the armour is connected to the DNO earth at the feed end and isolated at the load end.




    That creates more issues than it resolves.


    Insert a 300 mA RCD in a plastic enclosure in the tails downstream of the DNO isolation switch and lose the DNO earth terminal, if you cannot easily access the conductor at the intake end take the bar off and put some heat shrink over the end of the conductor. 


    The earth terminal is the only part of the DNO installation that is exposed and likely to be a risk, it's better off out of there.


  • 0
    Sparkingchip:

    And I would not rush to connect the cabinet to the DNO earth bar, but would connect it to the Crabtree Fusestar earth bar.


    I am not seeing any reason to replace the Crabtree Fusestar, insert the RCD in the tails and connect the cabinet to the fused switch earth bar seems to be all that is required. Removing the DNO main earth conductor from the intake seems like a good idea, it's of no use and is a hazard to be avoided by anyone working in the cabinet.


    Just realised what the quote button is for on here doh.


     Its been suggested that the crabtree unit is faulty, but have not confirmed this myself yet. Also space may be a bit tight for a enclosed RCD in addition to the fusestar unit without jiggling things about.


    UKPN suggested the cabinet should not be bonded outside of the equipotential zone or for wiring regulations?


  • 0
    mapj1:

    Ah - a picture speaks 1k words . Not my mental picture at all.

    Leave the outer enclosure street box as it is - the DNO kit is in effect double insulated or reinforced insulation, and no further action needed on that score.

    I'm with Andy B above. Ignore the company earth terminal altogether but get an RCD in there - it can be a high current one assuming the TT earth is good enough.

    A 300mA ordinary one will detect cable faults between it and the destination, and will not be pushed off balance by anything much less than 100mA of DC - this sadly is not a parameter written  in the specs, but given the way magnetic cores operate, is undoubtedly true. Closer protection can be afforded at the load end. Something like the second label might be a good remineder (these are Irish but a UK variant must exist.)



    Was thinking a type S (time delay) 100mA type A RCD with 30mA RCBO's on all final circuits to prevent multiple RCD's tripping on a fault, are you suggesting 300mA (non time delayed?) up front and 30mA RCBO's on the final circuits may be more appropriate ? 

    noted the TT earth label


  • 0
    wallywombat:

    So if I'm interpreting that photo correctly, the DNO earthing facility is completely unused, and the amour of the 100m SWA cable is earthed at the load end to an earth rod?  


    If it was me I would change it so that that the armour is connected to the DNO earth at the feed end and isolated at the load end.


    DNO earthing is unused.

    Armour of SWA cable is currently connected at both ends i believe, but earthed via the earth rod at the load end.

     


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