EV commercial installation

If the the installation is earthed via an electrode and the appropriate RCD fault protection is in place I see no issue.

A separate issue is the bonding of simultaneous exposed conductive parts which O=PEN devices provide no protection for. 

Neil Bridgeman said:

If the the installation is earthed via an electrode and the appropriate RCD fault protection is in place I see no issue.

I think that's an over-simplified situation ... especially in cases where the fence is [perhaps fortuitously] connected to PME. I don't think it's always clear.

Neil Bridgeman said:

A separate issue is the bonding of simultaneous exposed conductive parts which O=PEN devices provide no protection for. 

Agreed, but that's covered in 722.411.4.1 and the IET CoP. The bonding downstream of the O-PEN only matters if you put it there (or there is an accidental connection).

Sorry will someone indulge me and explain what exactly is the problem here.

Mike.

mapj1 said:

Sorry will someone indulge me and explain what exactly is the problem here.

Mike,

• the TNCS neutral is connected to the TT rod via an apparent resistance of 28 ohms. That means the body of the cars are also so connected to neutral, loss of which would cause touch voltage which the separating TT system was supposed to prevent. I can’t quantify the risk and you clearly don’t have concern but I am mindful of the concerns raised in the COP regarding this. This is a grant application so the COP must be complied with, is it?

Surely this is correct behaviour ? - the TNC-s substation and various street main joints etc have electrodes, connecting that neutral wire to terra-firma, and the TT electrode is also picking up a connection into the same planet, just a bit further along.

Now from that test you do not know how much can be allocated to each electrode, but you would hope that all the DNO's electrodes in parallel would be quite a bit lower resistance than the TT one - if we could measure to a plate of infinite area at the far end of the planet we could get an accurate figure for that, but realistically we do not care that much.

(Note that as a point of visualisation you do not really need an infinite electrode that is infinitely far away - once you move electrodes more than a few physical extents apart the current path cross section rises faster than the separation and the point to point resistance levels off to a value more or less set by the material around each end, and practically  independent of the separation - so SWER transmission systems manage a few ohms to tens of ohms over links of tens of km distance with fairly normal sized substation electrode arrangements.)

Mike

Accepting of all that Mike. You will know by now that I have great respect for your opinion, but are you suggesting that the situation that I presented is not of concern?

If the fence is bonded to the PME earth then in an open PEN situation the fence would rise to 230v in theory yes but this is why supplementary bonding is needed so that there is no difference in potential and nobody should get an electric shock.

The earth electrode arrangement is not only to prevent vising touch voltages, it is to prevent different potentials between CPC and Earth also.  The fact that the PME is earthed to Earth which could be only a few meters away there is of course going to be a reading between the neutral connected to the PME and Earth as that is the very essence of a PME.

The earth electrode arrangement is not only to prevent vising touch voltages, it is to prevent different potentials between CPC and Earth also.  The fact that the PME is earthed to Earth which could be only a few meters away there is of course going to be a reading between the neutral connected to the PME and Earth as that is the very essence of a PME.