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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.
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    Indeed, if we measured the fault current as well as the voltage split we have enough info to infer both the barn and the DNO electrode resistances, in exactly that way.
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    As in the Sonel article.
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    With the farm as the auxiliary current probe.
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    Alan Capon:
    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. 






    With another electrode placed beyond the house the diagram is starting to look a lot like a diagram of three pole earth resistance testing.


    Andy B.


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    I am assuming these installations are north of the border and supposed to comply with BS7671.
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    Chris Pearson:
    lyledunn:

    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.


    How many of the three further properties are TN-C-S? Is it likely that after at least 650 m, Ze could be < 0.35Ω?




    Very good point Chris, but I cannot answer that. The lengths I gave were all estimates based on how far I can hit a golf ball!. I had to get into my van and drive to the farm from the house. The overhead from the transformer near the farm went as the crow flies to the first property then went underground and along the road before being teed off to the house in question. I know where the tee is on the side of the road because the first DNO crew dug it up a week earlier as they suspected a neutral failure at that point. The front lawn is at least a 60m pitch shot with a good whack with my hybrid and perhaps another pitch to get back to the transformer, so maybe 300m .  What I can tell you is that when the fault was fixed Ze at the house was  0.28 ohms with P to N displaying the same. Relatively small transformer so I guess the conductors must have been at least 50mm2. The last house looked further away than it actually is on Google maps so maybe they are just on the threshold.  By the way, the incoming water service is MDPE with copper thereafter. And dont go doing resistivity and impedance sums as my golf shots often fly well over the green!


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    From the OP - "family members getting shocks from water pipes when water was running. "

    Was it only when the water was running Lyle?


    Excellent sketch - a picture is worth a thousand words


    Regards
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    Chris Pearson:
    ......


    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.




    Well it will do, a bit, but presumably the ground there is not so conductive (or the DNO electrodes would also be better connected to terra-firma too. It may also be a plastic water main in the road, and only a few metres of copper underground. In this case bonding the pipes to the neutral at the MET makes plumbing indoors safer, but the outside tap more dangerous.

    Actually when PME first came in there was a recommendation for outside taps to have an insulating break in the pipes, but I think plenty never did and even plenty of new ones still do not. It was mentioned in "PME Regulations", which was a statutory document (a companion to the Electricity Supply Regulations in the UK).  The later Requirements for the Application of Protective Multiple Earthing to Low Voltage Networks  diluted that advice to "An insulating insert may be incorporated in the pipe to an outside water tap."

    Certainly in older versions of Guidance note 5, there is stronger advice,

    "It is recommended, however, that a plastic insert be provided in the pipe to the outside water tap." near the bit about PME and caravans.

    I presume they are reserving judgement that outside taps for anything other than water (oil, propane, milk, beer etc) may come under other rules.


    If each DNO electrode is 30-50 ohms and the water pipe is similar then the PEN to terra-firma resistance will still be some tens of ohms.

    (The Zs and the PEN, being all-copper will be fractions of an ohm, but that is not the loop we are looking at.)


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    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?




    Thinking about it the fault stays constant, adding more earthing on the other side of the divide has no effect on the actual cause of the problem.


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    lyledunn:

    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.


    How many of the three further properties are TN-C-S? Is it likely that after at least 650 m, Ze could be < 0.35Ω?


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