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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 am looking at this because I am getting a lot of stick when teaching the topic of vehicle charging points on 18th Edition courses. You will know having done one of these courses that the relaxation for domestic premises of connecting a vehicle charging socket to a PME earthing system has been withdrawn from the 18th edition. I have had it more than once, "you have told us all the problems now give us the solution".



    Any thoughts on letting the EVSE manufacturer shoulder the problem? It seems that some are claiming that their units are suitable for use on PME - presumably via in-built VoELCB type mechanism. (Although I am curious about some of the attempts at least which seem not to need a local electrode for a true earth reference.)


      - Andy.
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    I am looking at this because I am getting a lot of stick when teaching the topic of vehicle charging points on 18th Edition courses. You will know having done one of these courses that the relaxation for domestic premises of connecting a vehicle charging socket to a PME earthing system has been withdrawn from the 18th edition. I have had it more than once, "you have told us all the problems now give us the solution".



    Any thoughts on letting the EVSE manufacturer shoulder the problem? It seems that some are claiming that their units are suitable for use on PME - presumably via in-built VoELCB type mechanism. (Although I am curious about some of the attempts at least which seem not to need a local electrode for a true earth reference.)


      - Andy.
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