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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 what Andy has drawn would be TN-C-S if the exposed parts in the installation are connected to a separate protective conductor. Any PNB system would also be described as TN-C-S in Ireland.



    I'm not sure that's right. TN-S vs TN-C-S and PNB vs PME are almost two different things. TN-S vs TN-C-S is about whether there's a shared neutral and earth conductor at some point in the system; PNB/PME is down to how many distinct connections to earth the N has (one vs two more).


    PNB could be either TN-S or TN-C-S depending on whether the N-PE link is upstream or downstream of the earthing of the N.

    b5d6b6dc8e4ab52c65f06dee2399d6f3-huge-tn-c-s-paths_zpshylchzz3.png


    The above would be the TN-C-S version of PNB, but add an extra electrode between A and B and it becomes PME. But the earlier diagram would be TN-S in Ireland as well I think - there being no CNE.


      - Andy.


    Yes, on reflection I think you are right Andy. So those diagrams in GN8 are wrong! I wonder if they do refunds!

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  • 0

    AJJewsbury:




    Thus what Andy has drawn would be TN-C-S if the exposed parts in the installation are connected to a separate protective conductor. Any PNB system would also be described as TN-C-S in Ireland.



    I'm not sure that's right. TN-S vs TN-C-S and PNB vs PME are almost two different things. TN-S vs TN-C-S is about whether there's a shared neutral and earth conductor at some point in the system; PNB/PME is down to how many distinct connections to earth the N has (one vs two more).


    PNB could be either TN-S or TN-C-S depending on whether the N-PE link is upstream or downstream of the earthing of the N.

    b5d6b6dc8e4ab52c65f06dee2399d6f3-huge-tn-c-s-paths_zpshylchzz3.png


    The above would be the TN-C-S version of PNB, but add an extra electrode between A and B and it becomes PME. But the earlier diagram would be TN-S in Ireland as well I think - there being no CNE.


      - Andy.


    Yes, on reflection I think you are right Andy. So those diagrams in GN8 are wrong! I wonder if they do refunds!

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