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 4847 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
    Would you bond a steel framed steel clad building? There is no practical difference between that cabinet and a steel framed building clad with steel, apart from the scale of them. You can walk into a building, whilst you can only get your head and shoulders into the cabinet, but the risks are the same, if not higher with the cabinet.

    Scale can be important though. The advantage of an equipoential zone only exists within that zone - there're always problems on the boundaries. With a decent sized building you're probably far more likely to be completely within it (or outside) it at any point in time rather than straddling the boundary. With cupboard that's so small you can't actually get inside, you seem to loose almost of the advantages of bonding while maintaining all the disadvantages (it's all boundary, not useful interior). With bonding the cabinet we're solving the problem of someone touching the switchfuse and cabinet at the same time, but making the problem of someone touching the cabinet while stood on the ground (or touching something else metallic in contact with the ground) much worse.


    Some might consider this situation as similar to extraneous metalwork (e.g. steel railings) within reach of  the likes a (PME connected) steel street lighting column - and the regs specifically say we don't bond them,


    AMD1 has an interesting change - in that it's specifically only calling for bonding of extraneous-conductive-parts within buildings - which might also be a consideration if it goes though.


       - Andy.
Reply
  • 0
    Would you bond a steel framed steel clad building? There is no practical difference between that cabinet and a steel framed building clad with steel, apart from the scale of them. You can walk into a building, whilst you can only get your head and shoulders into the cabinet, but the risks are the same, if not higher with the cabinet.

    Scale can be important though. The advantage of an equipoential zone only exists within that zone - there're always problems on the boundaries. With a decent sized building you're probably far more likely to be completely within it (or outside) it at any point in time rather than straddling the boundary. With cupboard that's so small you can't actually get inside, you seem to loose almost of the advantages of bonding while maintaining all the disadvantages (it's all boundary, not useful interior). With bonding the cabinet we're solving the problem of someone touching the switchfuse and cabinet at the same time, but making the problem of someone touching the cabinet while stood on the ground (or touching something else metallic in contact with the ground) much worse.


    Some might consider this situation as similar to extraneous metalwork (e.g. steel railings) within reach of  the likes a (PME connected) steel street lighting column - and the regs specifically say we don't bond them,


    AMD1 has an interesting change - in that it's specifically only calling for bonding of extraneous-conductive-parts within buildings - which might also be a consideration if it goes though.


       - Andy.
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