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What earthing arrangement is this?

Former Community Member
Former Community Member
The supply is from a private transformer in a four core cable 3ph + n. The cable armour is earthed and connected to the MET. However there is also a green and yellow cable connected to the neutral terminal at the main isolator going back to a the transformer casing. The transformer is only 5 or 6 metres away. I think this must have been intended to make it a tncs supply but seems to me to just create parallel neutral conductors. Or is it tn-s-c-s?  I have only been able to go off visual inspection because I could not disconnect the supply..
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    Ah, but it isn't!


    A line to PE fault will travel back to the N-PE bond and then back along the N conductor to the star point.



    Which is why I said thinking about it in term of the complete earth fault loop isn't helpful. If you follow that logic you conclude that an entirely TT system is also TN-C-S because the connection to the star point carries both N and earth fault currents.

       - Andy.
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    Ah, but it isn't!


    A line to PE fault will travel back to the N-PE bond and then back along the N conductor to the star point.



    Which is why I said thinking about it in term of the complete earth fault loop isn't helpful. If you follow that logic you conclude that an entirely TT system is also TN-C-S because the connection to the star point carries both N and earth fault currents.

       - Andy.
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