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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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    14th Edition. Test V.O.E.L.C.B.s. Appendix 6. Fig.3.


    The test apparatus produces the 45 Volts a.c. test Voltage. At an installation socket the 45 Volts supply is connected to N and E. It is assumed that the supply N is earthed at source and is I imagine a reference "Earth." The 45 Volts appears across N and E and tries to raise the Voltage on protected installation metalwork (exposed conductive parts). If successful, the V.O.E.L.C.B. automatically opens its contacts and trips OFF. I have just bought an old Clare wooden boxed Multiple Earth Loop Impedance Tester V144. It can test both current operated and Voltage operated earth leakage circuit breakers it says on its instruction sheet by the Voltage injection method.  I need to research this more.


    Z.
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  • 0
    14th Edition. Test V.O.E.L.C.B.s. Appendix 6. Fig.3.


    The test apparatus produces the 45 Volts a.c. test Voltage. At an installation socket the 45 Volts supply is connected to N and E. It is assumed that the supply N is earthed at source and is I imagine a reference "Earth." The 45 Volts appears across N and E and tries to raise the Voltage on protected installation metalwork (exposed conductive parts). If successful, the V.O.E.L.C.B. automatically opens its contacts and trips OFF. I have just bought an old Clare wooden boxed Multiple Earth Loop Impedance Tester V144. It can test both current operated and Voltage operated earth leakage circuit breakers it says on its instruction sheet by the Voltage injection method.  I need to research this more.


    Z.
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