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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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    John Peckham:

    My interest is how can the neutral potential on a PME system rise more than a few volts under fault free conditions given the neutral is connected to a low resistance Earth electrode at the transformer and in addition there are electrodes connected to the neutral along the service cable, the M in PME, which are designed to keep the neutral at Earth potential? 




    I have been pondering this question whilst awaiting an erudite response from the experts, but as Mike has posted above, I don't think that the voltage can rise by much under fault-free conditions. It takes only a few volts to shove the electrons back to the transformer. ?


    If the N potential did rise, would the L rise with it, in which case appliances would still work; or if not, would we have dim lights and kettles which takes ages to boil?


    Now then John, you must have had a reason for raising the question - what was it please?

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    John Peckham:

    My interest is how can the neutral potential on a PME system rise more than a few volts under fault free conditions given the neutral is connected to a low resistance Earth electrode at the transformer and in addition there are electrodes connected to the neutral along the service cable, the M in PME, which are designed to keep the neutral at Earth potential? 




    I have been pondering this question whilst awaiting an erudite response from the experts, but as Mike has posted above, I don't think that the voltage can rise by much under fault-free conditions. It takes only a few volts to shove the electrons back to the transformer. ?


    If the N potential did rise, would the L rise with it, in which case appliances would still work; or if not, would we have dim lights and kettles which takes ages to boil?


    Now then John, you must have had a reason for raising the question - what was it please?

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