The IET is carrying out some important updates between 17-30 April and all of our websites will be view only. For more information, read this Announcement

Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

Options
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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.
Register to replyAlready registered? Log in and reply
  • 0
    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
  • 0
    The situation does reflect an issue in TTing installations on a TNCS system.




    Definitely - it can work both ways, ground or earth potential rise from either system can affect the other either through the ground itself, and/or through extraneous-conductive-parts.
  • 0
    The fault voltage in the house, I speculate, was the manifestation of the voltage drop across the DNO earth electrode.

    Indeed - what you effectively have is 230V across the TT customer's electrode, general mass of the earth and the DNO's electrode all in series. The ratio between the two electrodes dictates how the voltage divides. The "better" the TT consumer's electrode, the more the system N is raised above true Earth.


    It gets even more interesting on 3-phase distribution systems as the non-faulty lines then drift towards 400V above true earth rather than 230V and insulation and other things (SPDs) get a bit stressed....


       - Andy.
  • 0
    I think you can get the same effect from a L-true earth fault - e.g. a downed overhead line.

      - Andy.
  • 0
    Are the two premises connected by a metal water supply, perhaps fed from a shared borehole?


    Lyle seems to be saying that the ground potential was 187 V above the earthed tap, but could it not be the other way around? Circuit would be L, CPC to MET, bonding to water pipe, pipe between premises, bonding to MET, earthing conductor to service head, PEN back to transformer. The resistance of such an earth loop might have been insufficient to blow the fuse.


    Just a thought!


    It seems odd that the farmer didn't report any shocks.
  • 0
    Some years ago in Worcester there was a similar issue and the DNO guy knocked on the neighbours door and spoke to the lady who lived there, as gave them free run of the house to investigate.


    The DNO found that her husband had rigged the bath to electrocute her and the intermittent shocks the neighbours received occurred whilst he was testing his execution equipment. The husband was arrested and confessed saying he planned to kill her, then remove his equipment and throw a hairdryer into the bath connected to an extension lead before going to the pub and returning to find the “tragic accident” caused by his wife foolishly using her hairdryer in the bath, despite being advised by him not to on many occasions.


    I believe she divorced him whilst he was in prison .
  • 0
    I would have thought that the farmer might receive quite an interesting electricity bill if he had anything approaching 45A continuously going from Live to Earth.
  • 0
    Harry Macdonald:

    I would have thought that the farmer might receive quite an interesting electricity bill if he had anything approaching 45A continuously going from Live to Earth.


    Harry don’t forget, this is a TT installation. I measured Ra with a loop tester at 80ohms. That would have included Re but not any parallel contribution due to the metal stanchions of the old barn. The combined resistance was 26 ohms. As AJ indicated, the combined consumer resistance will have been lower than Re to see that sort of voltage division; 187v across Re and circa 230-187 across the consumer connection to earth. There appeared to be about 1ohm in the fault but all in all a little over 8A of fault current. 

    To be honest, I am sorry that I didn’t spend more time on it as it was a very interesting situation. I am almost tempted to return and re-impose the fault, if nothing else to get a photo of the MFT displaying 187v with one probe on the water tap and the other on the re-bar in the flower bed! Might even have stuck a lamp on for effect! 


  • 0
    Defective water heater made outside tap live.
  • 0
    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.


1 2 3 4 5

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines