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PME vs TN-C-S

AncientMariner
Are PME and TN-C-S supplies exactly the same?  Or does PME also have requirements regarding the number and relative location of suppliers earthing rods?

I am thinking here where the DNO replaces a length of failed 4-core (3-phase + Neutral) PILC cable with Wavecon or similar cable which has a combined Neutral and Earth conductor.  The average consumer would not know that their electricity supply's method of earthing had been changed.

Clive.
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  • 0
    gkenyon:
    Chris Pearson:

    Can it ever be PME if there is only one earth rod?


    Yes - at the very least PNB. See G12/4, Clause 4.11. The reason PME conditions apply if there is only one electrode is that there can be fortuitous electrodes such as extraneous-conductive-parts. Since up to 4 customers can be supplied with the arrangement, diverted neutral currents can flow through these ...




    This seems to be the operative sentence:

    Earth terminals provided using PNB shall be treated in all respects as PME earth terminals.

     



    AJJewsbury:
    In my head, PNB clearly can never be PME - any system has a certain number of points where the N is deliberately earthed - if it's one it cannot be PME, if it's greater than one it cannot be PNB. The definitions do not overlap.

    Same in my head! The get-out is "treated as".


    My confusion stems in part from the fact that my step-daughter's house is PNB (only earth is close to the transformer) but well over 40 m from the transformer with presumably a TT installation at the end of the line some distance away. Officially the DNO says that it is PME.
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  • 0
    gkenyon:
    Chris Pearson:

    Can it ever be PME if there is only one earth rod?


    Yes - at the very least PNB. See G12/4, Clause 4.11. The reason PME conditions apply if there is only one electrode is that there can be fortuitous electrodes such as extraneous-conductive-parts. Since up to 4 customers can be supplied with the arrangement, diverted neutral currents can flow through these ...




    This seems to be the operative sentence:

    Earth terminals provided using PNB shall be treated in all respects as PME earth terminals.

     



    AJJewsbury:
    In my head, PNB clearly can never be PME - any system has a certain number of points where the N is deliberately earthed - if it's one it cannot be PME, if it's greater than one it cannot be PNB. The definitions do not overlap.

    Same in my head! The get-out is "treated as".


    My confusion stems in part from the fact that my step-daughter's house is PNB (only earth is close to the transformer) but well over 40 m from the transformer with presumably a TT installation at the end of the line some distance away. Officially the DNO says that it is PME.
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