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 4759 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
  • 0
    I'm puzzled over a few points here.


    Normally, if the DNO allow their fuses to protect consumer's wiring at all, they stipulate a limit - usually around 3m. So I'm surprised there's no consumer's fuse before the 100+m submain.


    Another option would seem to be to continue with the DNO's earth until DB1 and make the TN/TT split there. That should remove the need for the 100mA RCD (but you'd still need some overcurrent protection) . But there might be other considerations against that.


    Then there's the cabinet itself - you seem to be implying that it's an exposed-conductive-part - i.e. it can be made live by a single failure of basic insulation. I would have thought that unlikely as cables associated with the cut-out and meter tend to be sheathed and other parts within enclosures. It may still be an extraneous-conductive-part - if for instance it is in contact with the soil - so bonding rather than earthing might be a consideration - but if everything inside is within insulating enclosures and cables are insulated sheathed - i.e. there are no exposed-conductive-parts - and no other (bonded) extraneous-conductive-parts in the vicinity then it doesn't sound like bonding will provide any benefits either.


       - Andy.
  • 0
    Where are the customers installation fuses protecting the feeder cable?


    Andy Betteridge
  • 0
    could the box be fibreglass ?
  • 0
    Cheers for the info/feedback guys, Im new to the discussion board so still trying to find my way on here and how the threads work.


    The isolator shown is in fact an enclosed fused isolator and is not the DNO's/supplier equipment my mistake, drawing does not yet show this detail yet, the enclosed fused isolator is well within the 3m requirement.


    I had thought about doing the TN/TT split in a insulated enclosure as you suggested just before DB1, but thought the RCD in the DNO cabinet would be the better solution since the clearance time for a earth fault may be above 5s with the current installation and fuses in place (still awaiting details of isolator fuses).


    Im still awaiting for some other information, but will come back to you soon.






  • 0
    So can the RCD go upfront of the fused isolator?
  • 0
    The RCD could go upfront of the fused isolator as it is not part of the DNO/supplier equipment
  • 0
    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.
  • 0
    This sounds a bit like an exam question. Surely the distribution circuit should be earthed at the TN-C-S/PME end and isolated at the other one.
  • 0
    The DNO do not want anything in their cabinets except theirs and suppliers equipment. Life has moved on , AMR equipment is the way forward and the cabinets need the space to accommodate that. 

    This situation requires an additional box, RCD because its a farm. and 3m to the protective device. 

    Easy enough, plastic M/SW/F. stake in ground, SWA out to plastic dis board.

    Its always difficult for the "regulation" preachers but on no account do you bond a steel DNO box out side the equipotential zone. Neither do you bond it for "wiring regulation" rules. 


    Of course, the DNO/supplier have no control over jobbing electricians and this is an ongoing problem for the companies when they come to upgrade their equipment in a box.


    Regards, UKPN
  • 0
    Finding a three phase distribution board with a plastic enclosure might be a bit of a challenge, but it would be acceptable.


    Edit- Because it is not a domestic installation.

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