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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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    Lots LV faults can raise the N voltage - not just broken CNEs, but L-N & L-PE faults, line conductors coming into contact with true earth (either damaged/downed cable or an uncleared earth fault in a TT consumer's installation) - some of which can persist indefinitely, but as we're excluding fault conditions I'll skip them (not to mention HV faults). My thoughts then drift towards conditions that might affect the voltage of the ground around the LV electrode(s) - transferred voltages from say electric trains, or probably more likely these days, tram systems (as they will often run parallel with LV mains) - traction currents returning along the rails will surely alter the local ground potential. Hopefully substation earthing design minimises such effects, but perhaps not always, especially when new tram systems are built along existing streets.


    I guess thunder storms aren't "faults" - so I could perhaps also offer the effects of lightning as another possibility - especially as SPDs make it more likely that CNE conductors will be pulled up to half the surge voltage. very short duration of course.


       - Andy.
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    Lots LV faults can raise the N voltage - not just broken CNEs, but L-N & L-PE faults, line conductors coming into contact with true earth (either damaged/downed cable or an uncleared earth fault in a TT consumer's installation) - some of which can persist indefinitely, but as we're excluding fault conditions I'll skip them (not to mention HV faults). My thoughts then drift towards conditions that might affect the voltage of the ground around the LV electrode(s) - transferred voltages from say electric trains, or probably more likely these days, tram systems (as they will often run parallel with LV mains) - traction currents returning along the rails will surely alter the local ground potential. Hopefully substation earthing design minimises such effects, but perhaps not always, especially when new tram systems are built along existing streets.


    I guess thunder storms aren't "faults" - so I could perhaps also offer the effects of lightning as another possibility - especially as SPDs make it more likely that CNE conductors will be pulled up to half the surge voltage. very short duration of course.


       - Andy.
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