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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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    I can see that test working on a TT system with the neutral as being at Earth (capital E) and the test voltage applied between the installation earth ( lower case e). Effectively applying the test voltage to the operating coil with the earth resistance in series.


    If using a VOELCB on a PME system the neutral is effectively shorted to installation earth in the supply head and the M in PME is intended to keep the supply neutral at or near the same potential as the general mass of Earth so how would the VOELCB work using that test if bonding is in place and there are parallel paths.


    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?
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  • 0
    Zoom


    I can see that test working on a TT system with the neutral as being at Earth (capital E) and the test voltage applied between the installation earth ( lower case e). Effectively applying the test voltage to the operating coil with the earth resistance in series.


    If using a VOELCB on a PME system the neutral is effectively shorted to installation earth in the supply head and the M in PME is intended to keep the supply neutral at or near the same potential as the general mass of Earth so how would the VOELCB work using that test if bonding is in place and there are parallel paths.


    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?
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