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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
    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. 


  • 0
    Sparkingchip:

    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.


     


    So are you saying bond the cabinet to the PME, TT or leave the cabinet as is  ? noting that the steel running from the cabinet into the conc plinth is probably in close contact if not connected to the TX/cutout earth electrode?


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    I would bond the cabinet to the TT installation, the cabinet manufacturers think it is important to bond the door of the cabinet to the actual cabinet enclosure having installed an earth pig tail between them, just in case there is high resistance on the door hinges, now you want to leave the entire cabinet floating in relation to the customers installation equipment within it.


    If you are really concerned that it is close to the DNO electrode do a high current loop test on the cabinet before you connect it to the earthing system, I cannot imagine that it will be a particularly low test result, that plinth and studs hardly count as foundation earthing.
  • 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.
  • 0
    So the options for the original posted question 


    1. Assuming the PME MET is removed from the cabinet and i bond the cabinet to the TT then i suspect i will in effect be connecting the PME earth to the TT earth due to the proximity/direct connection or the local earth electrode of the TNCS/PME supply to the cabinet via the steel&conc cabinet foundation, and i don't want to export the PME earth to the farm.


    2. If I bond the cabinet to the PME earth and insulate any TT earth in the cabinet (i.e. the insulate SWA cable gland/use a plastic enclosure) then all is good apart form if there is a broken neutral pre cutout, making the cabinet live and the RCD in the cabinet not able to detect any leakage due to the fault current existing before the cabinet RCD. 


    3. Don't bond the cabinet and in effect the cabinet floats at or close to the PME earth potential and so potentially have the same problem as point 2 above. 


    4. Is the obvious option here to replace the metal cabinet with a fiberglass cabinet, although the DNO may have something to say? 


    5. Report the issue to the DNO (if its there cabinet) and pass the issue to them, will they do anything?


    Who's responsible if the cabinet causes injury or death through it becoming live?


    I know the chances of injury or death are probably very small but i want to get this right.


    Is there a standard ideal solution to this?  Am I over thinking the whole issue? 


  • 0
    Steve UKBC:

    ... and i don't want to export the PME earth to the farm.


    That is fair enough, but on the basis of the data which you have given, I see no reason not to use it to protect the distribution circuit.


  • 0
    3. Don't bond the cabinet and in effect the cabinet floats at or close to the PME earth potential and so potentially have the same problem as point 2 above.

    If it's not directly connected to either earthing system then it'll tend to be at the same potential as the ground it is stood on - which is exactly what you want if you want to avoid shocking someone stood on the ground and touching the cabinet. If there is a PME influence on the ground around/under the cabinet then that (almost*) makes no difference as both the ground and the cabinet will still be at the same potential - so still no shock (like a bird perched on a bare overhead wire - it's not the absolute voltage that matters, just the difference in voltage the victim is exposed to).


    *I say almost as you can get some difference of potential between different points on the ground, so if the PME electrode is fairly near, there might be some potential difference between the ground to one side of the cabinet compared with the other - but still the potential of the cabinet is likely to be fair average of the ground around it, and you're not going to get much better than that.


    Remember that TT earthing systems aren't always safer than PME ones - there might not be a broken CNE risk - but all it takes is a simple L-PE fault and a sticky RCD to leave the entire TT earthing system at a hazardous voltage indefinitely. The ideal would be to avoid all exposed-conductive-parts (and bonding anything) out doors - and the mitigation if you can't is often a buried grid. We're not in an ideal world however.


       - Andy.
  • 0
    Steve, have you considered purchasing a copy of the "Wiring Regulations" This is a publication that would suit the project perfectly. The book is "the industry standard" (although they say it themselves) and are world leaders in advice on electrical installations. Their motto is "Achieve great things" Easy to understand, they have a special section on steel fuseboxes without an earth. This is a piece similar close to your current query. No ambiguity but they do emphasize at the end of the day you must rely on your own "skill and judgement" I know that answers your question "Whos responsible if someone dies"? but its a tough business, Steve. 


    Call the DNO, Steve, we have suffered enough.


    Regards, UKPN


  • 0
    The DNO will say it’s up to you to determine if a suppliers earth terminal is suitable for the purpose you intend to use it for.
  • 0
    When the original poster raised this in the "ask the community" forum, I suggested to him it would get better viewing and so more comments if re-asked here.

    I never expected it to be over the half century.

    I'm less sure if a clear consensus has emerged.? Still, at least we have made it clear there are a couple of right answers.

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