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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:

    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? 




    I have been pondering this question whilst awaiting an erudite response from the experts, but as Mike has posted above, I don't think that the voltage can rise by much under fault-free conditions. It takes only a few volts to shove the electrons back to the transformer. ?


    If the N potential did rise, would the L rise with it, in which case appliances would still work; or if not, would we have dim lights and kettles which takes ages to boil?


    Now then John, you must have had a reason for raising the question - what was it please?

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    Chris Pearson:




    John Peckham:

    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? 




    I have been pondering this question whilst awaiting an erudite response from the experts, but as Mike has posted above, I don't think that the voltage can rise by much under fault-free conditions. It takes only a few volts to shove the electrons back to the transformer. ?


    If the N potential did rise, would the L rise with it, in which case appliances would still work; or if not, would we have dim lights and kettles which takes ages to boil?


    Now then John, you must have had a reason for raising the question - what was it please?


     




    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).


    Z.


     

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    If the idea is for a trip for car charging, not withstanding the unicorn like nature of the device currently described in the regs, I'd be much happier  if it tripped well before 70V RMS , and well before 50V RMS as well, personally I think it really ought to be set to trip at more like the 25V we used to insist on as the maximum exposed voltage for conductive locations before it all got harmonised to 50V everywhere. Cars have large metallic surfaces, charger points may well be in bare earth car parks, or next to grass verges, and folk will at some point touch the car either in bare feet, or while standing in a puddle in shoes that have a leak, one can imagine a situation in a seaside car park  that is almost as good as an outside bathroom.?

    As in posts above, I don't think a healthy network should be anything like 70V off earth for more than a few tens of milliseconds at a time before some ADS sorts it out, and a charge point that puts itself out of order as a precaution during an abnormal event of more than 24V for perhaps half a second, is preferable tp an exposed lethal voltage,  (so long as there is a reset method  at least ).

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    There's also a time issue with the EVSE VOELCB solution. A PEN can 'validly' be pulled up to anything up to half supply voltage for up to 5s during faults (possibly longer if the installation reckons it's a L-N fault rather than a L-PE one or the fault's on the DNO's part of the system) - and such faults could be almost anywhere on the LV distribution network or any installation it supplies (with a shared PEN). To avoid nuisance tripping you'd want a delay of at least 5s - but for shock protection you'd want it to operate within 0.4s. Presumably faults anywhere on the mains or installations fed by a single substation aren't that uncommon - first fault is probably reasonably rare, but then it typically gets repeated several times (perhaps over several weeks) as the customer tries resetting the MCB or replacing a fuse a number of times before eventually giving up and calling in an electrician.


    In some ways it might make more sense for the EVSE's "VoELCB" not to "trip" as such, but just to disconnect for as long as the hazardous voltage persisted, and then automatically re-connect afterwards. That way the disconnection could be immediate, but the inconvenience limited. BS 7671 doesn't seem to take that approach though.


      - Andy.
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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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    We could design quite a good system given time.



    Indeed. There was a suggestion a while ago that smart meters could implement voltage range protection (both L-N and to an local earth electrode if wanted) - they already have enough processing power to easily implement any wait/retry/give-up logic you could think of, already have the communications to phone home to the DNO to let them know directly if the problem persists and even have an in-built contactor to automatically disconnect the installation (perhaps extra contacts needed for N/PE) if necessary - thus protecting not just EVSE but the entire installation. It also means that ensuring the safety of the supply (including the supplier's PE) remains the responsibility of the supply industry rather than the householder - which seems right somehow.


      - Andy.
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    Chris


    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.


    Then you can not use the PME earth and fit an isolating transformer. Big heavy expensive device with high inrush currents, although I did find a soft start one for marine use. Another problem is the charger is in the vehicle not in the socket enclosure and a lot of them want to "see" an earth before they will start charging. So having lashed out loads of cash for the Tx and overcome the inrush problem, some consumer unit manufacturers don't make Type D MCBs, all tests out fine but plug in Mr Smiths electric car and it won't start charging and he is not going to pay the sparks.


    That leaves  converting the socket to a TT island. as the best way forward. Problems with that with no accessible parts connected to the PME that can be simultaneously touched at the same time as the vehicle. Getting the rod in the ground and not hitting other services and drains, could use a mat but needs a bigger hole. Then some DNOs want 2.5 -3m distance from underground services connected to the PME earth so will need a bigger garden.


    So I am exploring how big a problem the divergence of the PME earth potential is from the general mass of earth and how frequently this may occur on a PME system (Not TT) under faulty free conditions. I think the 4s is in the Regulations for this divergence issue which may be a rare event. If it is then my 3 pole VOELCB to BS 842,which is a current BS but needs a polish, would do the job. Just got to get the BEAMA boys to make them again.


    Spookily I looked at a design job today for a block of 6 new flats which will have a 3 phase PME supply. The flats will have 5 parking spaces side by side, someone is going to be unlucky in years to come,with VCPs. Supply is to be from each individual flat. So best thinking trousers on for that one! Any ideas?


    The Essex Unicorn was the last known to be alive and David Attenborough wanted to film it. I blew it up with my Variac set to the wrong end of the scale on switch on, so if David asks it died peacefully in it's sleep!


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    Actually the choice of not more than 5 seconds not less than 4 thing is as arbitrary (read silly) as the choice of a permitted touch voltage of 70V.  It has the feel of being a spec being devised by a committee that do not normally look at such matters, and in effect re-inventing the wheel, and in the process coming up with a rather unusual square shape. ? It is not a safety of life spec like the ADS times for fuses and circuit breakers.


    If the voltage is there for 4 seconds it might as well be there for 40, as far as electrocution is concerned, the question is how much current will flow in a victim for the time it takes to trigger  a heart fibrillation, which is a function of current flow and duration. That current has only a loose connexion to the applied voltage and a more lot to do with size of contact areas on the live metal and the ground, and the dampness of the exposed skin, or if there is clothing in the way, if it is wet, and if the wetness is fresh water or salty.

    (more info  on shock times http://www.industrial-electronics.com/elec-safety_6.html)

    In terms of building one, the electronics to detect any voltage you like and introduce any delay you like is a matter of pennies in bulk, and a few tens of pounds for a one off design. The more expensive part would be the contacts,  though a 3 pole contactor, or a 3 pole MCB or switch strapped to  a shunt trip coil to act as the actuator from the electronics would be standard parts.  The harder  step would be deciding what to test it and declare compliance  to, as the standard is not properly  fixed.
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    Does anyone have a photo of voltage operated device to replace the existing photo on Wikipedia?
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    John Peckham:

    Chris


    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.


    Then you can not use the PME earth and fit an isolating transformer. Big heavy expensive device with high inrush currents, although I did find a soft start one for marine use. Another problem is the charger is in the vehicle not in the socket enclosure and a lot of them want to "see" an earth before they will start charging. So having lashed out loads of cash for the Tx and overcome the inrush problem, some consumer unit manufacturers don't make Type D MCBs, all tests out fine but plug in Mr Smiths electric car and it won't start charging and he is not going to pay the sparks.


    That leaves  converting the socket to a TT island. as the best way forward. Problems with that with no accessible parts connected to the PME that can be simultaneously touched at the same time as the vehicle. Getting the rod in the ground and not hitting other services and drains, could use a mat but needs a bigger hole. Then some DNOs want 2.5 -3m distance from underground services connected to the PME earth so will need a bigger garden.


    So I am exploring how big a problem the divergence of the PME earth potential is from the general mass of earth and how frequently this may occur on a PME system (Not TT) under faulty free conditions. I think the 4s is in the Regulations for this divergence issue which may be a rare event. If it is then my 3 pole VOELCB to BS 842,which is a current BS but needs a polish, would do the job. Just got to get the BEAMA boys to make them again.


    Spookily I looked at a design job today for a block of 6 new flats which will have a 3 phase PME supply. The flats will have 5 parking spaces side by side, someone is going to be unlucky in years to come,with VCPs. Supply is to be from each individual flat. So best thinking trousers on for that one! Any ideas?


    The Essex Unicorn was the last known to be alive and David Attenborough wanted to film it. I blew it up with my Variac set to the wrong end of the scale on switch on, so if David asks it died peacefully in it's sleep!


     




    We must not forget that 722.411.4.1 (iii) refers to 543.3.3.101(ii) and requires the protective conductor to be disconnected last and connected first when the supply is respectively broken or made by the "device". A normal 3 pole V.O.E.L.C.B. of old probably can't do that.


    Z.

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