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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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    Some yers ago when inspecting & testing a show cave open to the public, I tried to explain this to the HSE Inspector for Mines & Quarries who had requested the I&T.

    He couldn't get his head around the fact that the earth return path to PME was via the SWA arnour, and that due to it's resistance, a potential difference could result between it and the 'earthiness' of the installation in the cave itself. At that time, the electrical installation at the destination-end was situated in all metalic enclosures. I suggested that a supplementary rod should be installed at the cave/mine-end but he couldn't grasp the potential difference between the two!
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    Some yers ago when inspecting & testing a show cave open to the public, I tried to explain this to the HSE Inspector for Mines & Quarries who had requested the I&T.

    He couldn't get his head around the fact that the earth return path to PME was via the SWA arnour, and that due to it's resistance, a potential difference could result between it and the 'earthiness' of the installation in the cave itself. At that time, the electrical installation at the destination-end was situated in all metalic enclosures. I suggested that a supplementary rod should be installed at the cave/mine-end but he couldn't grasp the potential difference between the two!
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