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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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    The transformer neutral is not anything like as well earthed as the metal framed barn by the sounds of it. That does not mean it is faulty - anything up to a 20 ohm resistance for the DNOs electrodes all in parallel is permitted. On an extensive installation you would expect it to be lower though, perhaps sub-ohm, the 20 ohm figure is only really true for isolated pole-pig transformers supplying a small hamlet via half a dozen wooden poles in regions where the ground is free draining and gravelly, in such a case only a couple of the poles will have a drop wire and electrode so the DNO earth connection is a bit thin on the ground.

    Actually you could have had the same fault between two TNC-s users on the same transformer, had it been the live core of a damaged extension lead snagged on that metal barn from  non RCD supply.

    As it happens a working RCD would have found the fault sooner, and the farm end would have been disconnected but I suspect there is nothing actually wrong on the DNO side, just an unfortunate combination of users installations and a line to terra-firma earth fault, rather than a line to CPC fault.
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    John Peckham:

    I am thinking that the ground is sitting at around 0V and the outside tap is at 187V with respect to the general mass of earth as I cannot see the line to earth fault lifting the voltage of the general mass of earth to 187V some distance away from the site of the fault. 

    I am glad that I was not the only one to think that.

    If the copper water pipe in the house is bonded it is directly connected to the transformer neutral so how can it sit at 187V to Earth?


    If the water pipe is not bonded and/or the transformer earth is disconnected then that is a different story.


    I would like to think that the DNO would have checked the transformer earthing.


    It was I who asked whether there was a common metal water supply and I don't think that Lyle has answered that one, but if that is the case, the absence of adequate main bonding in the house would explain the situation.


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    Lyle


    You say the house supply was TN-C-S? You say it was a copper water pipe connected to the incoming metal water supply pipe? Was the supply pipe bonded to the incoming supply neutral?


    I can see the line to earth fault on a TT system without a functional RCD not operating the 45A fuse. I am thinking that the ground is sitting at around 0V and the outside tap is at 187V with respect to the general mass of earth as I cannot see the line to earth fault lifting the voltage of the general mass of earth to 187V some distance away from the site of the fault. 


    If the copper water pipe in the house is bonded it is directly connected to the transformer neutral so how can it sit at 187V to Earth?


    If the water pipe is not bonded and/or the transformer earth is disconnected then that is a different story.
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    I am thinking that if all final circuits are 30mA RCD type then the requirement for others of higher allowed values and delays are met.

    If that is the case (I might be wong) then a lot of folk would rely on one.

    Around here it`s nearly all TN systems and on the odd occasion I do TT I always feel happier with two (I always think about the consequences of failure) because a total reliance on one RCD worries me


    In any event, an RCD or three on a circuit with an incomming c.p.c above earth reference would not mitigate that final circuit attached to it as OMS pointed out with the bathroom lighting portion  only fused down (RCD FCU)  from a lighting circuit with a possible 5s allowed disconnection time.


    Would similar considerations be in order with the OPs fault?
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    lyledunn:
     


    Indeed Ebee! Where enhanced resilience is required following the assessment of the risk, it might be difficult to contend that a single RCD is an adequate control measure. Just because 7671 doesn’t require it doesn’t mean it isn’t required!




    It is not clear whether the failed RCD was a 300mA, 100mA or 30mA. 705.411.1 mentions all three types.

    Assuming it was a 30mA, you would have thought, it being a barn, that 705.422.7 would come into play, and an additional 300mA RCD , possibly time-delayed, would need to have been fitted.


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    Back in March you asked (lyledunn) about the puzzling Note to 411.4.1 (in the Irish regulations), 
    https://communities.theiet.org/discussions/viewtopic/1037/25908 

    and I pointed out the original (correctly formatted) version in the respective IEC standard.


    This case is an example of why it's advisable to ensure the source has much better earthing than other potential electrodes.  It's presumably also why in Germany (I linked to a few examples) they want a LV network to be one type of earthing or the other (TT or TN*) but not mixed.  Less of a problem with a big network, with multiple electrodes, foundations, bonding, old cables etc. The rural transformer with a couple of customers is an extreme case.  Nasty, indeed.


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    ebee:

    Interesting Lyle,


    It does enhance a bit my perception of two cascading RCDs to a TT to add resiliance.

    In this instance even a Delayed 100mA RCD prior to a 30mA non delayed might have saved the day.

    Different makes in different locations might help a little too.


    OK a 100 mA and a delayed RCD does not count as personal protection but in instances such as this a bad fault and a non functioning RCD might have caused the TD 100 to trip seconds before touching occoured thereby giving a warning that something was wrong


    Indeed Ebee! Where enhanced resilience is required following the assessment of the risk, it might be difficult to contend that a single RCD is an adequate control measure. Just because 7671 doesn’t require it doesn’t mean it isn’t required!


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    presumably this is a case where the TT earth is a better connection to terra-firma than the one the DNO have for the TNC-S part of the network.

    For example if the DNO had the maximum permitted 20 ohms between their NE  star point and terra-firma proper then the  effective electrode resistance to earth to which the farm has faulted is significantly lower resistance, about 7 ohms,  so that the NE 240V is split such that 187 of the volts are dropped on the TNCS DNO electrodes, and the other 53V across the TT electrode.


    This is not a fault mechanism that is unique to TT or TNC-s systems - any system with a very low impedance fault from a live wire to true earth, such as steel piles or buried private pipelines can pull the whole mains off-balance. The problem is more likely to be serious with a rural installation with lower power pole pig transformers supplying a small number of subscribers per transformer, as the DNO side earthing is likely to be less extensive.

    There is an assumption in a most textbooks and regulations that the supply side is pretty much the most solidly earthed thing around - it is not always true, and especially a problem for temporary supplies from gensets and similar.


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    Defective water heater made outside tap live.

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