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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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    We could design quite a good system given time.



    Indeed. There was a suggestion a while ago that smart meters could implement voltage range protection (both L-N and to an local earth electrode if wanted) - they already have enough processing power to easily implement any wait/retry/give-up logic you could think of, already have the communications to phone home to the DNO to let them know directly if the problem persists and even have an in-built contactor to automatically disconnect the installation (perhaps extra contacts needed for N/PE) if necessary - thus protecting not just EVSE but the entire installation. It also means that ensuring the safety of the supply (including the supplier's PE) remains the responsibility of the supply industry rather than the householder - which seems right somehow.


      - Andy.
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  • 0

    We could design quite a good system given time.



    Indeed. There was a suggestion a while ago that smart meters could implement voltage range protection (both L-N and to an local earth electrode if wanted) - they already have enough processing power to easily implement any wait/retry/give-up logic you could think of, already have the communications to phone home to the DNO to let them know directly if the problem persists and even have an in-built contactor to automatically disconnect the installation (perhaps extra contacts needed for N/PE) if necessary - thus protecting not just EVSE but the entire installation. It also means that ensuring the safety of the supply (including the supplier's PE) remains the responsibility of the supply industry rather than the householder - which seems right somehow.


      - Andy.
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