EV commercial installation

That was the point of my post AJ. The separation between the two earthing systems is almost impossible in these types of situations. So why bother? Why dismiss a perfectly good TNCS earthing arrangement in favour of a dubious TT arrangement. 
I commissioned 4 similar systems this week, all displaying the same issues. Contractors are banging in electrodes and claiming that the requirements of a TN system as set out in 722.411.4.1 do not need to be met. That would seem to be an imprudent tack to take. So the thrust of my post was to suggest that in built-up areas TTing is not a feasible solution. Even if it is at the time of installation, things can change to thwart it. 
As Sparkingchip points out, there is no point in putting O-pen protection on the TT side. So if we are to comply with BS7671 and the COP, we are limited to those indents under the above mentioned regulation only one of which I would accept as a contender, that being the system devised by the MyEnergi crowd.

I am sorry I mentioned the fence, totally deflected from my point.

I didn’t realise you could pick up location from the photos I posted. I will have to ask the moderator for advice on how to avoid that. 

Something to be said for just putting Petrol, diesel and other hydrocarbons into vehicles via equipment and electrical installations developed overtime from 1928 and we dont have very many electric shocks, fire, and explosions on traditional filling stations. In 2022 the "Pressure" to shift to Electric vehicles and charging them up via EVSE suddenly becomes very complex, and with the "Competency" levels of contractors/tradesmen out there just now and clients just looking for the cheapest install, and people thinking installation of EVSE is easy then I can see unfortunately a few incidents are going to happen.......................... There is some real electrical engineering talent on this forum and reading the detail in replies below there are "Polite" statements over what is correct and safe in both theory and practice and application of the correct regulations and engineeringa and installation practices. So if the talent on this forum cant 100% agree there is no chance for the joe bloggs ordinary electrical contractor getting it right is there? E.G what did the survey for the installation of EVSE and location of the pillar and equipment highlight or say, or is the concern only coming up all too late at commissioning and certification?.

GTB

 lyledunn 

It doesn't take to much common sense to work out where they are, there's logos on the EVSE.

I am doing a job in Leominster, Herefordshire converting a shop into two shops and a flat.

Shops front and back, flat in the middle where the two round windows are,  parking around the back. 

A 100 amp three phase supply and a three phase E10 meter, I'm going down the logical route a phase for each shop and one for the flat, at the moment the customer wants two single phase landlords meters after suppliers meter for the shops and will split the bills. I suspect it won't be long before he applies for three suppliers single phase meters when he tried collecting the payments. 

Around the back there's plenty of room for several EVSE, but the back wall is clad with steel sheeting. 

The landlord does not want to install EVSE, but I have already made provision for the rear shop to have one as I am running new SWA distribution circuits, so it has a 25mm and 16mm SWA running to two consumer units allowing for all sorts of possibilities as the rear shop does not have a gas supply. 

I want to at least run a new SWA cable out of the flat whilst the building work is being done ready for EVSE even if it's not going to be installed. 

The best place for the EVSE is screwed to the steel cladding on the back wall.

The customer is not going to accept a bill for installing at long earth rod as foundation earthing for the TN-C-S supply at this stage and no decision is required now, but if and when EVSE gets installed someone,  which may not be me, is going to have to think long and hard about the earthing arrangements. 

I might retire before any EVSE is installed at this job or the very first tenants may ask for it as part of their rental agreement, so I need to give it a bit of thought now. 

They are not two earthing systems though. They are both PME but one circuit is using an earth electrode as the CPC. 

To my mind it's not so much a PME/TT issue, but getting as close as we can to equipotentiality - if the car, the ground it's on and anything within reach (including the charge point if it's metal cased) are all at the same potential, that it matters little if it 0V, or 230V or anything else. TT and TN-S systems aren't entirely immune to nasty voltages on the earthing system either - e.g. TT with an earth fault and a sticky RCD, or TN-S with a severed c.p.c. and some decent leakage current.

So I guess the 'ideal' is a buried grid under the car and everything bonded together. Next question is how far from that ideal is the TT island idea where the rod is in close proximity to the ground the car is on - even if that ground is under the influence of PME broken PEN voltages or whatever. In this case If the perimeter fencing is in effect bonded (deliberately or fortuitously) with steel posts in the ground, the PME influence might spread a fair distance. Obviously the surface voltage won't be constant over the compound, I'd guess more dish shaped - gradually falling from the fencing towards the centre, but does anyone have any feel for the numbers? What sort of distance might we be looking at to get a dangerous p.d. (70V say)?

Or we use the same tactic a old-school switchboards and just put some insulating matting down where the car will be (extending few metres each way).

   - Andy.

So this is the resistance between the neutral incomer to a EV feeder pillar from the TNCS intake and the disconnected end of the TT earth electrode also at the pillar. 28.5 ohms on the low resistance ohmmeter scale. 

Just a thought as I always avoid doing resistance/continuity on non-isolated conductors - might the voltage difference between the N/PEN conductor and the electrode be influencing the reading? (The N potentially being affected by v.d. on the N from any loads connected to the same transformer)

   - Andy.

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.

err I do not think so. There are two groups of objects fed by the substation.

Those using the dno electrodes and supply neutral for earthing, (those may be PME) and those that use a local electrode (TT). The latter group are far safer to touch if you are standing on the ground, as whatever happens the thing you can touch remains more or less at the same voltage as your feet (except for very short times limited by ADS). Ideally  you'd like not to be able to make good contact to one of each at the same time, but if you did, it would be no worse than bridging between say a PME lamp post and the bare earth or a non electrical  item such in the ground such as a metal gate post. 

During certain fault conditions there are voltage gradients along the earth between various items acting as electrodes, and the resistances you see allow you to estimate the currents that flow while that happens, but  it is not sinister, it is just a consequence of living on a moderately conducting planet

Mike