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. 

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.

Bang on.

Yes, exactly, the reading of 28 ohms as such is of no concern at all.

I'd be far more worried if it showed

either 

1) no connection -as that would mean either the TT electrode was defective, or all those of the DNO, or

2) if it showed a very good connection, say fractions of an ohm, as that would suggest the spike electrode had hit the feed cable or something connected to it.

So again, I seem to have missed something - what on earth  (TT or TNC-s)   is everyone worrying about?

independent earth electrodes  are always a few tens of ohms apart, even if separated by yards or continents.. It is just harder to do the wander lead test in the 2nd case.

Mike.

So is there any point in converting to TT in such situations other than box ticking?

I have re-read this before posting and it is late and I do not want it to seem confrontational, but it does feel a bit so please do not read it in that way.

OK what metal part or parts, that you can touch when the cabinet is shut, is/ are  connected to the neutral ?

And can you touch it/them  at the same time as having a bare foot on the ground, or hold of the fence ?

Now to me,  the only important  ideas here  are

1) The car is never at a dangerously different voltage to a person who may touch it, regardless of what else is happening on the network, including credible network fault conditions involving PEN failures..

2) this means that the car is not that TNC-s neutralled object, and yet for reasons of ADS

3) that the car must still be  earthed in a way that will fire an RCD if the car electrics develop a live  to chassis fault.

Connecting the metal of the car to something at the same voltage as the same region of ground that the person will be standing on, regardless of any  voltage offset that has relative to the network earth, pretty much guarantees that.

And that I think, is what we mean by TT for the chargers.

But you have a concern, and I cannot see it for the life of me, and I am not confident that I have not missed something.

M.

Mike.

I guess that is the answer, the electrodes have to be independent in so much that their Ra does not overlap. The error I seem to have been making was to assume that  a relatively low resistance value between TNCS neutral and independent TT electrode was an indication of overlap, where in fact, it is not. The transfer of potential to the TT rod in the event of a loss of TNCS neutral is a function of distance rather than being indicated by a resistance value.
Error or not, it is surely a difficult task to establish a TT system in such circumstances. 

The earth electrode is to get the vehicle chassis and Earth at the same potential in the event of a fault so when someone is standing on Earth and touching the car there is no electric shock risk. 

Don’t think so Neil. The purpose of the TT electrode is to operate automatic detection, which, in this case is 30mA RCDs in the charge points with 100mA S-type upfront. The Ra value is relatively small so Ud across Ra will be small but during an earth fault Ud can be full mains voltage. Trade off is disconnection time. The cars could be 25m away from the electrode with the car body connected to that electrode.

Don’t think so Neil. The purpose of the TT electrode is to operate automatic detection, which, in this case is 30mA RCDs in the charge points with 100mA S-type upfront. The Ra value is relatively small so Ud across Ra will be small but during an earth fault Ud can be full mains voltage. Trade off is disconnection time. The cars could be 25m away from the electrode with the car body connected to that electrode.

All true yes.