EV commercial installation

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.

Bang on.

I have just had a look at the fence on Google Earth, I assume it runs around the perimeter from the front gate right around, but stopping short of running across the back of the building with the PV on it, which measures as 154 metres using the measuring tool on Google Earth.

I cannot see any electrical equipment mounted on the fence that could be hardwired to the TN-C-S earth terminal, but assume there is. That already probably makes the fence one of the biggest earth electrodes in the local distribution network, bearing in mind it’s in an industrial estate with lots of steel fences and steel buildings.

Picking up on what Graham said last night, I guess there’s enough of it that it could carry a significant diverted neutral current without an excessive touch voltage being present, for all we know it might already be doing so!

If the steel fence carries on across the back of the building and is fixed to the steel cladding by the air conditioning units it will be effectively within the earthing system of that building, has someone actually bonded the fence to the sir conditioning units thinking someone could touch both at the same time?

I cannot see that installing Open-PEN protection to TT earthed EVSE will achieve anything, but the fence warrants further investigation and consideration.

If the fence really is that good an earth electrode would it actually be better if either everything was TT or TN-C-S, both building and EVSE, other than it is not a dedicated earth electrode?

Edit- it’s probably unlikely but there could be a new gate inserted into the fence or other alterations which would drastically reduce its effectiveness as as earth electrode.

I'm confused. If it's TT, how the separation between the TT and TN system achieved? Or is it just an additional electrode? Is that a corner of a car I see touching the feeder pillar door in the second picture?

   - Andy.