John Peckham:
In your diagram the earth resistance at the farm needs to be very low and the earth resistance of the transformer electrode a lot higher in proportion to get a 187V rise and that assumes any PME electrodes are not there or also very high resistance.
Who said anything about PME? It could well be a TN-C-S version of PNB, or anything else which the DNO fancies. If there are multiple earths, why are they not pulling down the local ground potential?
the earth resistance at the farm needs to be very low and the earth resistance of the transformer electrode a lot higher in proportion to get a 187V rise
Sparkingchip:
So f we install foundation earthing for an installation with a lower Ra than the DNO network earthing what happens to the touch voltages in the installation, if anything?
In normal circumstances, not a lot. During conventional L-PE faults, not too much either (the effect is still small compared with the normal metallic return path). In a broken CNE event on a PME system you'd still have 230V nominal between two different points each connected to true earth (possibly more than 230V on a 3-phase system in some peculiar circumstances) - just how that 230V (or whatever) divides between the DNO's and customer's systems will differ - the lower Ra the less the customer sees and the more the DNO's system (including N/CNE) will be affected.
To be honest I can't see it being worse than the original PME system for a CNE break near the substation where the DNO's electrode was a few tens of metres of copper buried within the substation compound while the consumers side was connected to literally miles of metallic gas and water pipes.
- Andy.
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