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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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  • 0
    Thank you for your input so far.


    Zoom


    You say you have a VOELCB, is that fitted or on the shelf? If it is fitted do you experience any nuisance tripping if so how frequent? 


    I have been carrying out some tests on my collection of VOELCBs from my black museum. I have also been scrutinizing the BS 842 which is still a current (excuse the pun) standard for VOELCBs.


    I am not getting this divergence of potential between Neutral and the true mass of earth under FAULT FREE CONDITIONS. I do get it under various fault conditions where it would be desirable for the VOELCB to trip.The required current to trip the device is dependent on the voltage between the mass of earth and the frame terminal of the device. How was this test carried it the good old days given the much quoted voltage of 50V is frequently refer ed to as the required test voltage? Or is an early example of making up test results for the test certificate? If 50V was applied directly between the F and E terminals it would trip in mighty quick time as my experiments get my collection of devices to trip at 12V when applied directly to the coil! I killed one of my collection when I accidentally applied the test voltage with my Variac at the wrong end of the scale, the coil is now open circuit.



Reply
  • 0
    Thank you for your input so far.


    Zoom


    You say you have a VOELCB, is that fitted or on the shelf? If it is fitted do you experience any nuisance tripping if so how frequent? 


    I have been carrying out some tests on my collection of VOELCBs from my black museum. I have also been scrutinizing the BS 842 which is still a current (excuse the pun) standard for VOELCBs.


    I am not getting this divergence of potential between Neutral and the true mass of earth under FAULT FREE CONDITIONS. I do get it under various fault conditions where it would be desirable for the VOELCB to trip.The required current to trip the device is dependent on the voltage between the mass of earth and the frame terminal of the device. How was this test carried it the good old days given the much quoted voltage of 50V is frequently refer ed to as the required test voltage? Or is an early example of making up test results for the test certificate? If 50V was applied directly between the F and E terminals it would trip in mighty quick time as my experiments get my collection of devices to trip at 12V when applied directly to the coil! I killed one of my collection when I accidentally applied the test voltage with my Variac at the wrong end of the scale, the coil is now open circuit.



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