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

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?

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.

well "independent" means a slighhtly different thing to 'isolated' - what we really need is that a fault to earth in one network does not endanger someone holding kit that is earthed in a different way, and that the slope in voltage gradient along the ground  is not dangerous to someone or any animals walking along it.

I'm not sure what Ra not overlapping means, unless you have separate planets - I'd agree that is impossible. The only other way to do it maybe the joke way with an  earth rod in the insulating flower pot. That is not wise as an ADS method either ;-)

Personally I'd be more worried about thinks like that fence which is clearly a much better ground connection tha a bare foot or hand on the tarmac,  and may well  run near but not bonded to things that are earthed to the PEN - but even then a broken PEN is only a problem if you end up straddling the break, either directly or indirectly with a foot or hand holding something connected to it on either side.

The driver for keeping zones of earthing apart is all about safe voltage gradients, limiting step voltages if you like

A few metres is all that is needed if the maximum voltage offset is 230v

I hope that your mind is somewhat set at ease. It is tricky but  not as bad as perhaps you first thought.

Mike.

All true yes.