VOLTAGE BETWEEN THE GENERAL MASS OF EARTH AND A PME NEUTRAL/EARTH

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that the relaxation for domestic premises of connecting a vehicle charging socket to a PME earthing system has been withdrawn from the 18th edition.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!

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!