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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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    For the self service charger in the layby or supermarket carpark,  if it is not to be out of order 364 days a year, a user operated  reset, or an auto reset, is essential. Street furniture goes for months between service visits - look how long it takes street lamps to be fixed, and you can see those !

    A system that relies on sending a body out from head office  in some sort of vehicle just to flick an RCD back on is a dead loss, it will be hard enough keeping track of plugs being run over and general wear and tear and keeping it going at reasonable expense.


    ( here is an example of a cautionary tale of a damaged public charger lead)


    I can imagine a problem with an auto-reset if the disconnection of the load is just enough to remove the fault voltage to below threshold, so it oscillates between tripping and resetting. Maybe a few  retries spaced at 30 seconds in any one hour and then give up and wait for user intervention.


    There is already  a need for telemetry home for billing purposes, so it may as well make a few more calls and report repeated the faults to the DNO and to the HSE while it is at it.

    We could design quite a good system given time.

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    For the self service charger in the layby or supermarket carpark,  if it is not to be out of order 364 days a year, a user operated  reset, or an auto reset, is essential. Street furniture goes for months between service visits - look how long it takes street lamps to be fixed, and you can see those !

    A system that relies on sending a body out from head office  in some sort of vehicle just to flick an RCD back on is a dead loss, it will be hard enough keeping track of plugs being run over and general wear and tear and keeping it going at reasonable expense.


    ( here is an example of a cautionary tale of a damaged public charger lead)


    I can imagine a problem with an auto-reset if the disconnection of the load is just enough to remove the fault voltage to below threshold, so it oscillates between tripping and resetting. Maybe a few  retries spaced at 30 seconds in any one hour and then give up and wait for user intervention.


    There is already  a need for telemetry home for billing purposes, so it may as well make a few more calls and report repeated the faults to the DNO and to the HSE while it is at it.

    We could design quite a good system given time.

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