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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
    Sparkingchip:
    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.  




    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.  >>>>>>   would this be by providing a barrier or asking the DNO to come out and remove it?


  • 0
    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.


    Agree the RCD would be the better solution.

    Re volt drop the initial calc was based on the DNO fuses only (as i didn't know what was installed in the fused isolator) and based on this (although wrongly) the disconnection time was over 5s.   The volt drop for the whole installation is a concern particularly for the first 100m due to the new expected loads etc. but the cables further down the distribution are being selected to keep the volt drop within the regulations of 3-3.5% lighting and 5-5.5% power (length depending)


  • 0
    Just because there is an earth connection supplied by the DNO you don't have to use it and if you do it your decision to make.


    It is the only TNCS earthed item in the whole TT installation, if you are kneeling on the wet ground and lean against the metal cabinet you could feel a tingle if you touch the DNO earth terminal or possibly a bit more. It's not lethal as in if you touch you will die, but it serves no purpose and could be a risk.


    Enclose it in a plastic box or something if you think someone may decide it could be required in the future, it doesn't need to be complicated.

    The RCD is not the better solution, it is the only solution. Unless you are going to take the TNCS-PME earth connection to where it does not need to be.
  • 0
    Sparkingchip:

    Just because there is an earth connection supplied by the DNO you don't have to use it and if you do it your decision to make.


    It is the only TNCS earthed item in the whole TT installation, if you are kneeling on the wet ground and lean against the metal cabinet you could feel a tingle if you touch the DNO earth terminal or possibly a bit more. It's not lethal as in if you touch you will die, but it serves no purpose and could be a risk.


    Enclose it in a plastic box or something if you think someone may decide it could be required in the future, it doesn't need to be complicated.

    The RCD is not the better solution, it is the only solution. Unless you are going to take the TNCS-PME earth connection to where it does not need to be.


    Agreed the RCD is the best option but would the alternative solution not be to take the PME earth to an insulated enclosure just before DB1 (change over to TT) as long as the disconnection times are within 5s with the protection within the fused isolator?


  • 0
    The RCD is not the better solution, it is the only solution

    Why? If the downstream installation happened to be TN we wouldn't think twice about using the DNO's earth for the switchfuse & SWA.


    We're trading CNE reliability with RCD reliability - it doesn't look clear-cut to me. (Especially for an RCD that's likely to be subject to condensation, frost, etc).


       - Andy.
  • 0
    I see no reason to earth the cabinet, but that switch fuse definitely needs to be earthed. Achieving that via 100 m of SWA and an earth rod seems very odd. In the absence of an RCD it is definitely no better than C2.


    What size fuses are in the switch fuse and what is the EFLI at the DB?
  • 0
    A few plastic  P clips makes the idea of a live outer box from a meter tail an utterly incredible fault, rather than just not very likely,  and still no need to earth it at all.


    Three phase RCBOs exist, and could go in a plastic box in place of the metal switch fuse. Or in a plastic DIN rail box, and separate RCD and MCB

    This is not indoors under the stairs so all the metal CU for domestic fires nonsense does not apply, and a plastic enclosure can neither come live nor rust
  • 0
    Take a step back, what if anything is wrong with the Crabtree Fusestar fused switch?


    It’s a switch with fuses in it, so let’s rule out the fuses as being faulty as they are a replaceable item and really either they are okay or they aren’t, there is a middle ground.


    So if the switch operates and there’s no sign of thermal damage to it it or the fuse holders it doesn’t need replacing, so as the feeder cable is not going to be replaced the Crabtree Fusestar and feeder cable can all be left as it is saving time and money.
  • 0
    Given the multiple uses of these premises,  let's consider it as a farm.
    86423ab7373af1f536151322da119a27-original-20201026_160632.jpg
  • 0
    Now explain why it makes sense to do anything other than install a 300 mA RCD in the tails between the DNO isolator and the Crabtree Fusestar fused switch and make the entire installation TT earthed?


    Andy Betteridge.

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