Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 70 replies
  • Subscribers 32 subscribers
  • Views 4778 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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
Parents
  • 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?


Reply
  • 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?


Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines