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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
    Mike


    Assuming the water pipe is bonded to the incoming neutral in the house which means that it is connected to one pole of the transformer. This is a single phase pole transformer so the HV side will not have an earth screen if an overhead HV supply. One pole is connected to an earth electrode to create a neutral which could have an earth resistance of up to 20 ohms. The house is TN- C - S so as the supply has more than one consumer will be PME but could be PNB. So probably a few electrodes along the cable run. The farm is TT so no connection to the supply neutral but could be a connection via a metal water pipe between the farm electrode and the house neutral.


    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.


    If the house water pipe is not bonded and that pipe is connected to the farm I can see the voltage rise.


    So how else can you get a voltage rise of the general mass of earth some distance away without an earthing or bonding issue?
  • 0
    lyledunn:

    Above is my interpretation. There are no common services.


    That's how I see it too. The point to note is that it's the supply CNE that's 187V adrift from normal, not the general mass of the earth.


       - Andy.


  • 0
    Lyle


    In your diagram the earth resistance at the farm needs to be very low and the earth resistance of the transformer electrode a lot higher in proportion to get a 187V rise and that assumes any PME electrodes are not there or also very high resistance.
  • 0
    John Peckham:

    In your diagram the earth resistance at the farm needs to be very low and the earth resistance of the transformer electrode a lot higher in proportion to get a 187V rise and that assumes any PME electrodes are not there or also very high resistance.


    Who said anything about PME? It could well be a TN-C-S version of PNB, or anything else which the DNO fancies. If there are multiple earths, why are they not pulling down the local ground potential?


  • 0
    Chris


    I did mention PNB a couple of posts back.
  • 0
    John Peckham:

    Chris


    I did mention PNB a couple of posts back.


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


  • 0
    the earth resistance at the farm needs to be very low and the earth resistance of the transformer electrode a lot higher in proportion to get a 187V rise

    But probably not implausibly so - say the DNO's electrode just met the 20Ω requirement - the farm need only be about 4.6Ω - if the barn was steel framed into decent ground (or the farm installation was bonded to some buried metallic water pipes) that's probably not implausible. The farm would only be seeing a voltage rise of about 43V on their protective system during the fault - so perhaps understandable it went unnoticed.


       - Andy.
  • 0
    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?
  • 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.


  • 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

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