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RCD failure causes shock in neighbours house

lyledunn
Large rural home, family members getting shocks from water pipes when water was running. Owner got a severe shock from the outside tap. Their contractor did some investigation and ruled out a fault in the installation. As the system was TNCS it then looked like a classic case of DNO lost neutral so contractor called them in. Investigation revealed a fault in a nearby farm which was on the same single-phase transformer but was TT. They duly cut off the the offending circuit which they established was supplying the barn. They removed a 45A fuse from the distribution circuit and stuck warning tape over the fuse carrier but left the fuse. Problem solved. I was called in by the home owner when the shocks returned. Unfortunately it was late yesterday afternoon and I didn’t relish the prospect. I stuck a bit of reinforcing bar in the garden and measured 187v between that and the outside tap which was connected to a copper supply pipe. I went to the farm and the old farmer kindly gave me access. He had replaced the fuse that the DNO removed as he needed light to feed his animals but forgot to remove it again. Anyway, his own contractor had apparently dismissed the DNO diagnosis. I pulled the fuse and found that the fault voltage at the house disappeared. Further investigation revealed an almost dead short between phase and earth on a circuit in the barn. The RCD had failed. Given that it was a TT system the fault current was insufficient to blow the 45A fuse. The fault voltage in the house, I speculate, was the manifestation of the voltage drop across the DNO earth electrode. 

The situation does reflect an issue in TTing installations on a TNCS system.
  • 0
    Alan Capon:
    Chris Pearson:

    My point was that it may not be PME at all. Where are those multiple electrodes and what are they achieving?


    If you look at the ESQCR regulations, there only needs to be two earths to make a PME supply - one at the substation end, located at or before the connection to the first service connection, and at the far end of the PME distribution main, at or after the final service connection. 




    Well yes, 2 > 1 = multiple, but the reality is that we don't know where those electrodes (if any) are situated. What I don't quite understand is that if the water supply is through a metal pipe, why has it not pulled the surface potential towards the distributor's earth.


  • 0
    mapj1:

    I stared  hard at 'equip one tail' and then had an AHA moment and  realized that this was the spelling chequer at work.


    I hadn’t spotted that one! Corrected now thanks. 


    Regards,


    Alan. 


  • 0
    So (I think).

    If we have L that can be any voltage (including zero) above or below true earth and

    If we have N that can be any voltage (including zero) above or below true earth and

    we take our earth as local ground reference then an RCD might save us from Live potentials to local ground giving enough current to flow to trip it in the required time.

    If we take our earth as from something other than local ground then we might not have that
  • 0
    I stared  hard at 'equip one tail' and then had an AHA moment and  realized that this was the spelling chequer at work.


    and what it should have been was equipotential. zone

    The EEB in  EEBADS,  for those old enough to recall "Earthed Equipotential Bonding". An excellent idea indoors, but really hard to extend the E/P zone outside  for sockets supplying a lawn mower, so rather than run a neutral potential mesh under the front garden,  we normally make anything electrical that is for use outside double insulated. Except electric cars, caravans and boats where we try and pick up the local terra-firma potential for use as CPC, not network neutral.
  • 0
    You are correct Alan, the last house is next door at the end of line about 150m away from the house in question. On the line running left on my sketch the first section is overhead to a pole where it feeds another TT property. At the same pole the cable goes below ground and feeds 3 more domestic properties on a run of about 500m to the last.
  • 0
    lyledunn:
    f1f51356309a9b365513f2d33749dfcc-original-ac0d728d-ff66-47a2-bd20-a8eea3202b7c.jpg


    The only change I would make to Lyle’s drawing, is to extend his TNC-S supply past the “TNC-S” house and add one more earth electrode (obviously with a further resistance) to the Supplier’s neutral conductor. 


    Regards,


    Alan. 


  • 0
    mapj1:

    . . . We have a 187V potential difference between the supply neutral  and house CPC, relative to the general mass of earth 

    Agree and note addition in bold. CPC earth is not terra-firma earth however.

      in the house. If every thing Electrical in the house works that suggests the  general mass of earth is rising up to 187V.
    NOPE,  true earth is in the right place. Relative to terra-firma Live will be falling to about 60V and neutral and CPC are at -187 such that the LN difference and CPC difference both remain 240V or what ever.

    The only place there is a step voltage will be evident is near the DNO LV electrodes and again at the farm, not at the point of shock. . . 


    Exactly. What most are forgetting, is that a “PME Earth” is not a real Earth. The Supplier has allowed their neutral conductor to be used instead of an earth, providing certain conditions are followed, such as conductor size, bonding requirements, use only within an equipotential zone (usually taken as the walls of the main building). Everyone always quotes the “broken neutral” as the greatest risk with PME. Lyle’s scenario adequately demonstrates a much more likely scenario, and there are more scenarios that are more likely than a broken neutral, where the Supplier’s neutral conductor is displaced from being close to earth potential. You also need to consider the other two thirds of customers (assuming a three-phase transformer) where their phase conductor is 300V or more above earth potential. 


    Regards,


    Alan. 


    Edited due to an over-zealous spelling checker! 


  • 0
    Chris Pearson:

    My point was that it may not be PME at all. Where are those multiple electrodes and what are they achieving?


    If you look at the ESQCR regulations, there only needs to be two earths to make a PME supply - one at the substation end, located at or before the connection to the first service connection, and at the far end of the PME distribution main, at or after the final service connection. 


    Regards,


    Alan. 


  • 0
    I'm with Lyle's sketch  on this one, and for some odd reason that pits my view of what is happening against that of JP - unusual but there we go. No offence meant, but I will argue this one, as I think it is important to consider this, especially in a world where we are considering adding additional earthing , possibly better than the DNOs earthing, at the load ends to the distribution network in the DPC.

    We have a 187V potential difference between the supply neutral and the general mass of earth in the house. If every thing Electrical in the house works that suggests the  general mass of earth is rising up to 187V. I know about Hot Sites in respect of HV but not an LV fault to Earth causing a voltage rise of earth some distance away.


    We have a 187V potential difference between the supply neutral  and house CPC, relative to the general mass of earth 

    Agree and note addition in bold. CPC earth is not terra-firma earth however.

      in the house. If every thing Electrical in the house works that suggests the  general mass of earth is rising up to 187V.
    NOPE,  true earth is in the right place. Relative to terra-firma Live will be falling to about 60V and neutral and CPC are at -187 such that the L to N difference and live to  CPC difference both remain 240V or whatever.

    The only place that a step voltage will be evident is very near to the DNOs LV electrodes and again at the farm, not at the point of shock.



    -As it happens I have previously been bitten by this mechanism exactly, and so have had some time to think about it. Not on a fixed installation though. A mobile genset, not that well earthed, and a cable to a remote load - flood lights I think, but not really relevant, and the cable went over and through a hedge, and un-observed caught on barbed wire, piercing the outer and snagging one of the lives. Further along the barbed wire drooped with the rotting remains of a fence post into a very wet ditch, making a much better earth than the  half-heartedly fitted rod at the genset.  The  effect was to ground the whole 3 phase triangle by one corner, making grabbing the door handle to climb into the genset vehicle a shocking experience, luckily one that passed enough extra current to fire the genset RCD.


    regards Mike

    Edited for grammar 5/10/20
  • 0
    Sparkingchip:

    So f we install foundation earthing for an installation with a lower Ra than the DNO network earthing what happens to the touch voltages in the installation, if anything?


    In normal circumstances, not a lot. During conventional L-PE faults, not too much either (the effect is still small compared with the normal metallic return path). In a broken CNE event on a PME system you'd still have 230V nominal between two different points each connected to true earth (possibly more than 230V on a 3-phase system in some peculiar circumstances) - just how that 230V (or whatever) divides between the DNO's and customer's systems will differ - the lower Ra the less the customer sees and the more the DNO's system (including N/CNE) will be affected.


    To be honest I can't see it being worse than the original PME system for a CNE break near the substation where the DNO's electrode was a few tens of metres of copper buried within the substation compound while the consumers side was connected to literally miles of metallic gas and water pipes.


       - Andy.


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