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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?
  • 0
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    • I refuse to join any club that would have me as a member.


    Z.

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    I blame the radio hams for upping the ground voltage.
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    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?




    The reasons I can think of include:


    • Fault on the HV network

    • Fault transferred from another HV network

    • Switching impulses

    • Lightning strike

    • Fault in another installation, fault in PEN conductor, or in the PEN conductor to another installation

    • PME earth rod deterioration



     

  • 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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    sorry to hear you have blown one - in normal use the resistance of the ground around the electrode (Ze) would limit the peak current, so a dead short L-E fault on a TT system (the common use case for these trips) would normally be current limited and the trip coil would have  survived. I suspect your Variac near the 100% setting is far lower impedance, approaching that of the mains L-N loop, and the fault current too high, and the opening time too long to save the coil. (As like an MCB the mechanism won't speed up with increasing current beyond a certain point.)


    It is an interesting failure mode to have, as it is very much fault to danger, as if this was in a real installation it would now have lost all earth reference to terra-firma, as well as all ADS.


    I think the great unclear quantity in normal use would have been the source impedences of the fault (mains wiring plus a rather nebulous weather dependent earth path)  and maybe the impedance of the  ELCB coil - together these are forming a voltage divider of the permissible touch voltage.  If for example the assumed electrode impedance was 100 ohms, and if we assume the trip coil reactance similar, then for a 50 V touch voltage the coil would need to go nearer to  25volts.


    Have you a feel for the operating current as well as the voltages of the ones you have ? I'd be assuming a significant impedance in the trip, to intercept enough V * I to operate it, or it would need a very good low resistance electrode  to operate properly,  but this  may be my intuition misguided , they have been  a thing of the past all my life pretty much, and therefore not given much time, other than perhaps the time to unscrew them.

    From discussion with aged relatives, tripping when there was a thunderstorm was not uncommon. Equally, given casual mention of shocks from sinks and taps when a neighbour first got an electric kettle, there may have been other issues at that time.

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    Hello Zoomup.


    Thank you for your notes about the Crabtree E60 VOELCB and the link to John Ward's video.  I have fitted scores of these in past years and never had any trouble with any of them.  Note how good the 60 amp twin-screw tunnel terminals are compared to the rather poor "100 amp" single-screw cage clamp efforts we encounter today!


    Regards,


               Colin Jenkins.
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    John Peckham:

    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? 


    There is an old publication produced by Crabtree called: "The Protection of Domestic and Industrial Installations" which covers the use of V.O.E.L.C.B.s in great detail. Publication No. B1181.




    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.




    R E P L Y>>>>>>>>>>>>>>


    Hello John,

                          I do not have a V.O.E.L.C.B. installed anywhere at home. I do remove them from older installations. I partially dismantle them and service them just for fun and add them to my own collection.


    Please note that a row of country terraced houses back in the 70s may have had a V.O.E.L.C.B. installed in each house. If the common metal gas pipe serving each house had a Voltage rise on it from house A, then house Bs V.O.E.L.C.B. could trip off, but the fault could still prevail on the gas pipe, so a minor shock risk could exist in house B even with its own electrical supply cut off, in certain situations. A person could feel tingles if they touched both the metal gas pipe and a better earthed metal water pipe.


    Nuisance tripping did occur years ago with these devices.


    The E.60 device had an E.L.  tripping coil of impedance 500 Ohms and  a minimum tripping current of 35mA.


    The V.O.E.L.C.B. was tested with a 45 Volt a.c.  supply connected between N and E. A short circuit rated 750VA double wound step down transformer was used to provide the test supply of 45 Volts..


    Z.



     




     

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    14th Edition. Test V.O.E.L.C.B.s. Appendix 6. Fig.3.


    The test apparatus produces the 45 Volts a.c. test Voltage. At an installation socket the 45 Volts supply is connected to N and E. It is assumed that the supply N is earthed at source and is I imagine a reference "Earth." The 45 Volts appears across N and E and tries to raise the Voltage on protected installation metalwork (exposed conductive parts). If successful, the V.O.E.L.C.B. automatically opens its contacts and trips OFF. I have just bought an old Clare wooden boxed Multiple Earth Loop Impedance Tester V144. It can test both current operated and Voltage operated earth leakage circuit breakers it says on its instruction sheet by the Voltage injection method.  I need to research this more.


    Z.
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    Zoom


    I can see that test working on a TT system with the neutral as being at Earth (capital E) and the test voltage applied between the installation earth ( lower case e). Effectively applying the test voltage to the operating coil with the earth resistance in series.


    If using a VOELCB on a PME system the neutral is effectively shorted to installation earth in the supply head and the M in PME is intended to keep the supply neutral at or near the same potential as the general mass of Earth so how would the VOELCB work using that test if bonding is in place and there are parallel paths.


    My interest is how can the neutral potential on a PME system rise more than a few volts under fault free conditions given the neutral is connected to a low resistance Earth electrode at the transformer and in addition there are electrodes connected to the neutral along the service cable, the M in PME, which are designed to keep the neutral at Earth potential?
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    I think, fault free, it cannot by more than a slack handful of volts. The problem is the level that  it can rise under certain fault conditions that may persist for some time as they operate no modern ADS.

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