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.
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.
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,
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
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.
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.
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.
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.
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.
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