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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
    So you are making the SWA feeder circuit TT after all.


    That's a set up that neither Andy J or myself had in mind ?
  • 0
    Steve UKBC:

     supply a agriculture/horticulture/residential/glamping site 


     




    Go back to first base, list the types of special location is it. 


    I make it two in total for the overall installation: 

    705- Agriculture and horticultural premises.

    708- Caravan and camping parks.

    But parts of the installation will be in more specific special locations such as the showers.


    It's 705 that requires the 300 mA RCD upfront of all the circuits, so is it required or not? What's on the farm?


    Andy Betteridge.


  • 0
    IET Wiring Matters: 708
  • 0
    Sparkingchip:
    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.




    Indeed - but each of them is backed up by another within the caravan or camper van or in the tent's mains kit.


      - Andy.


  • 0
    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

    I wouldn't be particularly worried about discrimination between the DNO's fuses and your up-front overcurrent protection - everything goes off either way - there's no additional loss of service as it were if the DNO's go first (or as well). There's just the inconvenience of having to get the DNO to replace the fuses rather than your local electrician (but the cable will probably need fixing first either way).


    I would be concerned about lack of discrimination between your up-front overcurrent protection and downstream MCBs (or RCBOs) - in practice there's simply no guaranteed discrimination between MCBs regardless of their ratings. Unlike fuses where having an upstream device rated 2x (or 1.6x) the rating of the downstream one will usually provide discrimination, there's no such relationship between MCBs. If the fault current is high enough both MCBs will see the fault, and both will de-latch before either have fully opened.


       - Andy.
  • 0
    I’d argue the toss about the tents having a RCD in addition to the pitch socket RCD, also there could be someone charging a car off a pitch socket using a granny lead.


    The TNCS-PME earth  can be utilised for a caravan and camping sites permanent buildings, but not the pitches and the installation doesn’t need an upfront RCD, so a hybrid earthing system is acceptable. 


    But is this a farm?


    Whilst everyone is discussing it a RCD could have been fitted into the tails and most of the issues would almost certainly disappear. 


    The installation should never have been constructed like this and the RCD should really be installed tomorrow morning or the earthing arrangement altered.


    I would fit the RCD in the tails.
  • 0
    I’d argue the toss about the tents having a RCD in addition to the pitch socket RCD, also there could be someone charging a car off a pitch socket using a granny lead.

    In my experience tents usually use a hookup unit which has an RCD in-built (often loaned by the site in exchange for an extra payment for use of electricity) - likewise a granny lead will have a 30mA RCD or equivalent within the control box.


       - Andy.
  • 0
    Sparkingchip:

    So you are making the SWA feeder circuit TT after all.


    That's a set up that neither Andy J or myself had in mind ?


    Id prefer not to take the PME earth to the farm buildings and keep the TT/PME switch away from the area and in the cabinet in the field adjacent to the TX pole, as you cannot know the future use or these farm buildings, there is currently no livestock but who's to say this does not change in the near future.

    Any outer damage of the feeder cable if the PME earth were to be brought to the farm building (i.e exposed SWA carrying the PME earth) would introduce the PME earth potentially closer to the farm building earth electrodes would it not?


  • 0
    Sparkingchip:
    Steve UKBC:

     supply a agriculture/horticulture/residential/glamping site 


     




    Go back to first base, list the types of special location is it. 


    I make it two in total for the overall installation: 

    705- Agriculture and horticultural premises.

    708- Caravan and camping parks.

    But parts of the installation will be in more specific special locations such as the showers.


    It's 705 that requires the 300 mA RCD upfront of all the circuits, so is it required or not? What's on the farm?


    Andy Betteridge.




    To clarify, there are no tent pitches or shower/toilet blocks, just glamping pods, hot tubs and a sewage treatment unit.

    Again the the farm is currently more horticulture, but that's not to say the use of the farm will not change in the near future.


  • 0
    AJJewsbury:
    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

    I wouldn't be particularly worried about discrimination between the DNO's fuses and your up-front overcurrent protection - everything goes off either way - there's no additional loss of service as it were if the DNO's go first (or as well). There's just the inconvenience of having to get the DNO to replace the fuses rather than your local electrician (but the cable will probably need fixing first either way).


    I would be concerned about lack of discrimination between your up-front overcurrent protection and downstream MCBs (or RCBOs) - in practice there's simply no guaranteed discrimination between MCBs regardless of their ratings. Unlike fuses where having an upstream device rated 2x (or 1.6x) the rating of the downstream one will usually provide discrimination, there's no such relationship between MCBs. If the fault current is high enough both MCBs will see the fault, and both will de-latch before either have fully opened.


       - Andy.


    Andy, there is only so much we can do to mitigate these issues, selectivity studies have been carried out to ensure best practice and to limit multiple devices de-latching/tripping on a fault.


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