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VOLTAGE BETWEEN THE GENERAL MASS OF EARTH AND A PME NEUTRAL/EARTH

John Peckham
Other than under fault conditions or a small difference due to volt drop on a heavily loaded CNE cable can anyone explain why you may get a large potential difference (say 70V) between the general mass of earth and the MET on a an installation with a PME earthing system?


I have not seen this myself. If this does occur how rare or frequent might this circumstance occur?


If this potential difference does occur what sort of duration might this persist for?


Although a DNO may switch occasionally the HV ring for fault or maintenance works transformer neutrals remain bolted to earth and if the HV/LV earths are combined then an earth resistance of sub 1 ohm (in UKPN land that is what they want) so how can the neutral voltage float up more than a couple of volts above the general mass of earth?
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    It is assumed that the general mass of earth is always 0V, but is this always true?



    The general mass of the earth (i.e. the planet in general) is indeed 0V, but local patches of soil can easily be very much different - even under ideal soil conditions the rise of earth potential around an electrode can be into the hundreds sometimes even thousands of volts during a fault (take a look at BS 7671 table 44.2 for some "interesting" numbers as a starting point).

       - Andy.
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  • 0

    It is assumed that the general mass of earth is always 0V, but is this always true?



    The general mass of the earth (i.e. the planet in general) is indeed 0V, but local patches of soil can easily be very much different - even under ideal soil conditions the rise of earth potential around an electrode can be into the hundreds sometimes even thousands of volts during a fault (take a look at BS 7671 table 44.2 for some "interesting" numbers as a starting point).

       - Andy.
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