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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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    AJJewsbury:

    Thus the neutral (star point) of the source could be distinct from the earthed point of the source.




    So in my diagram:
    cb4f0ba7a2819e2221fe9e9b04dd0042-huge-tn-s-paths_zpsyci5uu0c.png


    it doesn't matter how long the conductor between the star point ("0") and the N-PE link is - it's still just a neutral conductor.


    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. If it headed for the general mass of the Earth, there would have to be another connexion between the star point and the Earth otherwise there is no circuit. Even if it did have a pathway through the Earth, most of the current would be carried by the N conductor because of its lower impedance.


    In a TN-S system, the PE must be connected to the star point itself. If there is any connexion between N and PE anywhere else, it becomes TN-C-S because there are then two conductors wired in parallel.

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    AJJewsbury:

    Thus the neutral (star point) of the source could be distinct from the earthed point of the source.




    So in my diagram:
    cb4f0ba7a2819e2221fe9e9b04dd0042-huge-tn-s-paths_zpsyci5uu0c.png


    it doesn't matter how long the conductor between the star point ("0") and the N-PE link is - it's still just a neutral conductor.


    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. If it headed for the general mass of the Earth, there would have to be another connexion between the star point and the Earth otherwise there is no circuit. Even if it did have a pathway through the Earth, most of the current would be carried by the N conductor because of its lower impedance.


    In a TN-S system, the PE must be connected to the star point itself. If there is any connexion between N and PE anywhere else, it becomes TN-C-S because there are then two conductors wired in parallel.

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