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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
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  • 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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  • 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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