EV commercial installation

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.

AJJewsbury said:

What sort of distance might we be looking at to get a dangerous p.d. (70V say)?

As little distance as a couple of metres - the electrode being on one side of the car, vs someone standing on the other side of the car.

(this is Figure H.4 from the IET CoP for EV charging equipment installation, 4th Edition, but link to image in E&T article here: https://electrical.theiet.org/wiring-matters/years/2020/80-may-2020/the-iet-code-of-practice-for-electric-vehicle-charging-equipment-installation-4th-edition/)

Whilst this is a great line of thinking, sadly, it lets us down basically on what BS 7430 tells us about what is, in effect, the inverse-square law degradation of potential (shown in Figure H.2 of the IET CoP for EV Charging Equipment Installation, 4th Ed)

In many ways the fact it is a  car on that drawing is not important. A metal swing gate with a planted metal post instead of the TT electrode would give a similar shock. The thing that makes it so is the low impedance and more or less un-divided connection between the feet and the buried live conductor of the defective PEN.   The advice has to be to pave over such things to increase the series resistance. Luckily  once you get a few burial depths to either side of it the voltage has fallen substantially.

M.

mapj1 said:

The advice has to be to pave over such things to increase the series resistance. Luckily  once you get a few burial depths to either side of it the voltage has fallen substantially.

Agreed ... the surface materials on which someone is standing definitely has a bearing. This is documented in BS EN 50522, but that's an HV standard, so obviously can't be considered for LV risks ?

There are further discrepancies between DNO standards and BS 7671 - i.e. 2.0 m for arms' reach (compare with 2.5 m in BS 7671), and 100 V AC touch potential (compare with BS 7671, 70 V AC in Section 722 and 50 V AC in general areas other than medical locations where it's 25 V AC ... also mirrored in the "Blue Book" for filling stations).

The drawing represents what I was on about although the charge point and earth rod were on the same side as the buried pipe with the person on the far side of the car where the charge point is shown. How can I confirm that such a situation does not exist or is unlikely to exist when using TT “island” on a TNCS site? 

How can I confirm that such a situation does not exist or is unlikely to exist when using TT “island” on a TNCS site? 

you don't unfortunately

But if you were to decide to earth the car to the same earth as the cable beneath, then while it reduces the risk to the person standing over the  cable during a PEN fault and opening the car door, sadly anyone approaching the same car from any other direction is more likely to get a shock from the car  instead... Of course a faulty PEN on a sub-main that is not the one feeding the car is not mitigated either...

As I noted in the post above the best is if the defective cable is as far away from the feet as possible, and given we cannot really move it, adding a bit of a meander to the current path with some paving slabs or a layer of tarmac is quite worthwhile.

(even if it is not in the standards ;-~)

Mike.

How can I confirm that such a situation does not exist or is unlikely to exist when using TT “island” on a TNCS site? 

you don't unfortunately

But if you were to decide to earth the car to the same earth as the cable beneath, then while it reduces the risk to the person standing over the  cable during a PEN fault and opening the car door, sadly anyone approaching the same car from any other direction is more likely to get a shock from the car  instead... Of course a faulty PEN on a sub-main that is not the one feeding the car is not mitigated either...

As I noted in the post above the best is if the defective cable is as far away from the feet as possible, and given we cannot really move it, adding a bit of a meander to the current path with some paving slabs or a layer of tarmac is quite worthwhile.

(even if it is not in the standards ;-~)

Mike.