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.
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.
Mike,
If you were to view any of my posts on this topic going way back, I didn’t really think the additional measures required on TN systems were worth the effort and the likelihood of injury from banging in earth rods presented more of a risk than injury from loss of neutral. The integrity of the system neutral is essentially the responsibility of the DNO. In the rest of Ireland PME predominates, chargers are installed without any consideration to loss of neutral. Perhaps, the system there is more robust.
So my concern on these chargers, which fall under BS7671 rather than IS10101, was simple compliance, especially with the guidance given in the COP. How could I know for sure that the TT electrode was not connected well within the voltage gradient of some element that was effectively connected to system neutral.
For now I am more content that I have someone like you saying don’t worry brother. I very much thank you for your top class engineering input and for your apparent infinite patience.
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.
Protective equipotential bonding is only required to be provided to each building (Regulation 411.3.1.2) ... this [part of the] installation is outside buildings.
I think this is being confused with the paragraph in Regulation 411.3.1.1, which states:
Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups, or collectively.
But here still, the fence is NOT an exposed-conductive-part, so does not require connection to the TT system to comply with BS 7671.
If, however, the fence is connected to PME somewhere (perhaps for a good reason, perhaps by accident, such as contact with an exposed-conductive-part), then the "separate TT system" concept is perhaps defeated.
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.
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).
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