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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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    How about a "buried grid" (solidly bonded to the PME earth) below the parking area? We seem to permit that as one option in other special locations where PME is seen as dodgy.

    Presumably it needn't be a full grid as such - a series of electrodes or just a perimeter ditch tape should suffice. OK still a lot more digging but perhaps a viable option in some circumstances (especially if it becomes well known, so people start burying grids when paving a parking area).


    I'm not sure we can exclude fault conditions - while open CNEs might be reasonably rare, there are lot of other kinds of faults that can raise the voltage on a CNE and I suspect some of those will be much more common.


      - Andy.


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  • 0
    How about a "buried grid" (solidly bonded to the PME earth) below the parking area? We seem to permit that as one option in other special locations where PME is seen as dodgy.

    Presumably it needn't be a full grid as such - a series of electrodes or just a perimeter ditch tape should suffice. OK still a lot more digging but perhaps a viable option in some circumstances (especially if it becomes well known, so people start burying grids when paving a parking area).


    I'm not sure we can exclude fault conditions - while open CNEs might be reasonably rare, there are lot of other kinds of faults that can raise the voltage on a CNE and I suspect some of those will be much more common.


      - Andy.


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