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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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    John Peckham:

    Andy


    Thanks. I am thinking that this is a very rare circumstance under fault free conditions. Maybe possible but not probable.


    I may put my PQA on my overhead PME supply in Norfolk where I have an Earth electrode to see what voltage I can record over a few days.


    For the old school forum members can you recall any nuisance tripping problems with VOELCBs? If you still have a VOELCB installed any nuisance tripping problems?




    Hello John,

                            the old reliable solidly made V.O.E.L.C.B.s were designed to monitor the Voltage on earthed metalwork within an installation compared to Earth. All earthed items within an installation were connected to the frame terminal of the V.O.E.L.C.B. This terminal was marked "F". The main earth terminal was connected to the "F" terminal of the device.


    The other terminal of the device was the Earth terminal which was connected to a remote independent earth electrode in the garden or by the front door. It was marked "E". The earth electrode was to be situated away from buried metal pipes like water pipes or gas pipes, out of the electrical influence of them.


    Nuisance tripping could occur if the device detected in practice a Voltage of between 20 to 50 Volts on earthed metalwork in a house. This Voltage could be imported from a house nearby along any communal metal pipework such as a water pipe or gas pipe if an electrically  leaky appliance such as an immersion heater or cooker element cause a Voltage to appear on the common metallic pipe. So an earth fault next door could trip off your V.O.E.L.C.B.


    Z.

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  • 0

    John Peckham:

    Andy


    Thanks. I am thinking that this is a very rare circumstance under fault free conditions. Maybe possible but not probable.


    I may put my PQA on my overhead PME supply in Norfolk where I have an Earth electrode to see what voltage I can record over a few days.


    For the old school forum members can you recall any nuisance tripping problems with VOELCBs? If you still have a VOELCB installed any nuisance tripping problems?




    Hello John,

                            the old reliable solidly made V.O.E.L.C.B.s were designed to monitor the Voltage on earthed metalwork within an installation compared to Earth. All earthed items within an installation were connected to the frame terminal of the V.O.E.L.C.B. This terminal was marked "F". The main earth terminal was connected to the "F" terminal of the device.


    The other terminal of the device was the Earth terminal which was connected to a remote independent earth electrode in the garden or by the front door. It was marked "E". The earth electrode was to be situated away from buried metal pipes like water pipes or gas pipes, out of the electrical influence of them.


    Nuisance tripping could occur if the device detected in practice a Voltage of between 20 to 50 Volts on earthed metalwork in a house. This Voltage could be imported from a house nearby along any communal metal pipework such as a water pipe or gas pipe if an electrically  leaky appliance such as an immersion heater or cooker element cause a Voltage to appear on the common metallic pipe. So an earth fault next door could trip off your V.O.E.L.C.B.


    Z.

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