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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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    There's also a time issue with the EVSE VOELCB solution. A PEN can 'validly' be pulled up to anything up to half supply voltage for up to 5s during faults (possibly longer if the installation reckons it's a L-N fault rather than a L-PE one or the fault's on the DNO's part of the system) - and such faults could be almost anywhere on the LV distribution network or any installation it supplies (with a shared PEN). To avoid nuisance tripping you'd want a delay of at least 5s - but for shock protection you'd want it to operate within 0.4s. Presumably faults anywhere on the mains or installations fed by a single substation aren't that uncommon - first fault is probably reasonably rare, but then it typically gets repeated several times (perhaps over several weeks) as the customer tries resetting the MCB or replacing a fuse a number of times before eventually giving up and calling in an electrician.


    In some ways it might make more sense for the EVSE's "VoELCB" not to "trip" as such, but just to disconnect for as long as the hazardous voltage persisted, and then automatically re-connect afterwards. That way the disconnection could be immediate, but the inconvenience limited. BS 7671 doesn't seem to take that approach though.


      - Andy.
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    There's also a time issue with the EVSE VOELCB solution. A PEN can 'validly' be pulled up to anything up to half supply voltage for up to 5s during faults (possibly longer if the installation reckons it's a L-N fault rather than a L-PE one or the fault's on the DNO's part of the system) - and such faults could be almost anywhere on the LV distribution network or any installation it supplies (with a shared PEN). To avoid nuisance tripping you'd want a delay of at least 5s - but for shock protection you'd want it to operate within 0.4s. Presumably faults anywhere on the mains or installations fed by a single substation aren't that uncommon - first fault is probably reasonably rare, but then it typically gets repeated several times (perhaps over several weeks) as the customer tries resetting the MCB or replacing a fuse a number of times before eventually giving up and calling in an electrician.


    In some ways it might make more sense for the EVSE's "VoELCB" not to "trip" as such, but just to disconnect for as long as the hazardous voltage persisted, and then automatically re-connect afterwards. That way the disconnection could be immediate, but the inconvenience limited. BS 7671 doesn't seem to take that approach though.


      - Andy.
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