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).
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.
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.
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