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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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    How about a "buried grid" (solidly bonded to the PME earth) below the parking area? We seem to permit that as one option in other special locations where PME is seen as dodgy.

    Presumably it needn't be a full grid as such - a series of electrodes or just a perimeter ditch tape should suffice. OK still a lot more digging but perhaps a viable option in some circumstances (especially if it becomes well known, so people start burying grids when paving a parking area).


    I'm not sure we can exclude fault conditions - while open CNEs might be reasonably rare, there are lot of other kinds of faults that can raise the voltage on a CNE and I suspect some of those will be much more common.


      - Andy.


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    John Peckham:

    I am looking at this because I am getting a lot of stick when teaching the topic of vehicle charging points on 18th Edition courses. You will know having done one of these courses that the relaxation for domestic premises of connecting a vehicle charging socket to a PME earthing system has been withdrawn from the 18th edition. I have had it more than once, "you have told us all the problems now give us the solution".


    The problems are.

    1.  Connecting a VCP to a balanced 3 phase system is difficult to comply with and most VCPs are going on single phase domestic supplies. So not much of an option.

    2. The second indent says you can fit an additional electrode to hold down the touch voltage in the event of a PEN loss to 70V. Depending on the load getting an earth resistance in single figures is going to need a lot of metal in the ground so difficult to achieve. So again not much of an real   option.

    3. The special device that disconnect in 5s if in the event of a PEN loss and disconnects live conductors and earth but not less than 4s. This is my Unicorn device, not currently made. I have a 3 pole VOELCB that will do this but no time delay for the 4s. So not an option.





    Zoomup:

    John mentioned 70 Volts earlier. Is he investigating converting a V.O.E.L.C.B. for E.V. charging I wonder? Will we get a cut when he patents it? 722.411.4.1 (iii).



    Spot on Zoomy!


    At first glance, 3-phase looks good and during charging, the loads will probably be adequately balanced, but one would do well to get a domestic installation sufficiently balanced otherwise.


    Had my crystal ball been functioning six or seven years ago, I would have had a foundation electrode installed when my garage was extended. I advised my step-daughter and her husband to have one installed in their new garage, which is nearing completion, but they chose not to do so. 


    As far as I can see, we cannot get away from the fact that some sort of independent earth electrode is required. So I think that the answer to the class must be that you have to make a TT island.
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    At first glance, 3-phase looks good and during charging, the loads will probably be adequately balanced



    But I gather that some cars only use a single phase even when connected to a 3-phase supply. So not only does installing a 3-phase EVSE not solve the problem you might be well advised to rotate the phases if you have several of them.


      - Andy.
  • 0

    I am looking at this because I am getting a lot of stick when teaching the topic of vehicle charging points on 18th Edition courses. You will know having done one of these courses that the relaxation for domestic premises of connecting a vehicle charging socket to a PME earthing system has been withdrawn from the 18th edition. I have had it more than once, "you have told us all the problems now give us the solution".



    Any thoughts on letting the EVSE manufacturer shoulder the problem? It seems that some are claiming that their units are suitable for use on PME - presumably via in-built VoELCB type mechanism. (Although I am curious about some of the attempts at least which seem not to need a local electrode for a true earth reference.)


      - Andy.

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