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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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    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! 


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  • 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! 


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