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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:

    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.




    Revise that, the RCD is not optional, regardless of the earthing arrangement.


    Andy Betteridge.


  • 0
    Chris Pearson:

    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?


    Not sure what's in side the fused isolator yet but agree it needs earthed as it stands and with a upfront RCD, my own thought was to replace it with a plastic enclosure and a MCB & RCD (100mA Time delay) within, as the fuse isolator has been noted to be potentially faulty anyway.

    The earth fault loop impedance is way too high for anything currently protecting the cable i think my MFT recorded around 10ohm / 25A, but it had been raining a while so the ground was soaked so this was probably best case, but generally speaking i thought it was a good low TT reading.


  • 0
    Sparkingchip:

    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.


    we have not been able to look inside the switch yet or operate it to see for ourselves and only had word from the end user that its lever/mech is faulty (repairable/faulty we cannot say yet)


  • 0
    A 100 mA RCD is three more times likely to trip than a 300 mA RCD with all the various combined and completely unknown loads.


    Better the 300 mA upfront, then more closer protection downstream of it.
  • 0
    Please not an MCB (or RCBO) upstream of other MCBs (or RCBOs) - you'll get little to no discrimination on L-N or L-L faults and you don't want the entire site blacked out for a simple fault on one final circuit.


    Likewise if you have more than one tier of RCDs you'll need all the downstream RCDs to be N switching if you want discrimination (many RCBOs aren't).


    The OP says the submain supplies a mixed agriculture/horticulture/residential/glamping site -  thus this submain doesn't feel to be entirely horticultural (after all the residential part and permanent building on the camping part are permitted to be PME) - it's merely a common supply to a number of (sub) installations, only one of which is agricultural. So I see no requirement for 705's 300mA RCD requirement to be applied to the submain.


    I still think that in a metal box outdoors is far from an ideal location for a delicate device like an RCD - especially one you're going to be totally reliant on for ADS, not just additional/fire protection. HBC fuses, with no moving parts and the fuse wire itself hermetically sealed in the cartridge feels like it's going to be a far more reliable solution (provided you get Zs low enough  of course - but that should be only a matter of using the DNO's earth for that bit).


      - Andy.

  • 0
    Steve UKBC:
    Chris Pearson:

    What size fuses are in the switch fuse and what is the EFLI at the DB?


    The earth fault loop impedance is way too high for anything currently protecting the cable i think my MFT recorded around 10ohm / 25A, but it had been raining a while so the ground was soaked so this was probably best case, but generally speaking i thought it was a good low TT reading.




    Sorry, that's not really what I meant to ask. What is Ze, and what is R1+R2 of the distribution circuit.


    If you don't know what size fuses are in there, how do know "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 ..."


  • 0
    AJJewsbury:


    I still think that in a metal box outdoors is far from an ideal location for a delicate device like an RCD - especially one you're going to be totally reliant on for ADS, not just additional/fire protection.


      - Andy.

     




    At the other end of the installation there may presumably be RCDs in boxes on top of poles at each camping pitch.


  • 0
    Steve UKBC:
    Sparkingchip:

    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.


    we have not been able to look inside the switch yet or operate it to see for ourselves and only had word from the end user that its lever/mech is faulty (repairable/faulty we cannot say yet)






    Possibly unfamiliarity with interlocking switches?


    Best go and have a look in the morning and put the RCD in the tails whilst you are there ?


  • 0
    Go back to the original drawing and the distribution board being supplied is in a barn.


    Okay, a barn could describe many types of building, indeed it could have been repurposed however in general terms it will describe a farm building. So it presumably needs a upfront RCD rated at no more than 300 mA regardless of the earthing arrangement.


    So why create a hybrid earthing system, with SWA bolted into plastic enclosures and other "safety measures" that are not actually required, that may well be creating unnecessary work and expense for the client?


    If adding an additional earth electrode at the meter cabinet it would be advisable to determine if it is close to the DNO earth electrode, There is not a green/yellow conductor going back into the ground from the DNO intake, but there could still be a DNO earth electrode within striking distance.
  • 0
    Thank you all for the suggestions, comments and debate and feel free to continue it if anyone has further use or need.


    So taking some of your comments/suggestions into consideration I have revised the proposed changes as per following attached drawing snippet, and again if anyone feels they need to comment please do. Consideration will be given to a cabinet heater to prevent/limit condensation as I think Ajjewsbury suggested.


    I know I'm probably going to get comments on the 100A MCB following the 100A fuses and I'm open to these comments as I know the selectivity lines cross between these two devices, but if I reduce the the MCB to 80A, then I have selectivity issues further down the line,  but I believe in the end that the MCB is more likely to trip before the fuses blow anyhow, and either of these devices tripping/blowing would be a major fault and rare occurrence hence I'm not to bothered about the selectivity here unless someone can say its against regs etc? 


    I was hoping to stick with the 100mA RCD (time delay) and 30mA RCBO's on final circuits to give that little bit more protection on the distribution circuits, but can see the possible need for the 300mA RCD (time delay and 30mA RCBO's on the final circuits instead, again open for comments here.


    de140706eb7c5b1430c0b7091fd8de67-original-updated-proposed-circuit.jpg

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