Chris Pearson:
My point was that it may not be PME at all. Where are those multiple electrodes and what are they achieving?
If you look at the ESQCR regulations, there only needs to be two earths to make a PME supply - one at the substation end, located at or before the connection to the first service connection, and at the far end of the PME distribution main, at or after the final service connection.
Regards,
Alan.
mapj1:
. . . We have a 187V potential difference between the supply neutral and house CPC, relative to the general mass of earth
Agree and note addition in bold. CPC earth is not terra-firma earth however.
in the house. If every thing Electrical in the house works that suggests the general mass of earth is rising up to 187V.
NOPE, true earth is in the right place. Relative to terra-firma Live will be falling to about 60V and neutral and CPC are at -187 such that the LN difference and CPC difference both remain 240V or what ever.
The only place there is a step voltage will be evident is near the DNO LV electrodes and again at the farm, not at the point of shock. . .
Exactly. What most are forgetting, is that a “PME Earth” is not a real Earth. The Supplier has allowed their neutral conductor to be used instead of an earth, providing certain conditions are followed, such as conductor size, bonding requirements, use only within an equipotential zone (usually taken as the walls of the main building). Everyone always quotes the “broken neutral” as the greatest risk with PME. Lyle’s scenario adequately demonstrates a much more likely scenario, and there are more scenarios that are more likely than a broken neutral, where the Supplier’s neutral conductor is displaced from being close to earth potential. You also need to consider the other two thirds of customers (assuming a three-phase transformer) where their phase conductor is 300V or more above earth potential.
Regards,
Alan.
Edited due to an over-zealous spelling checker!
Alan Capon:Chris Pearson:
My point was that it may not be PME at all. Where are those multiple electrodes and what are they achieving?If you look at the ESQCR regulations, there only needs to be two earths to make a PME supply - one at the substation end, located at or before the connection to the first service connection, and at the far end of the PME distribution main, at or after the final service connection.
Well yes, 2 > 1 = multiple, but the reality is that we don't know where those electrodes (if any) are situated. What I don't quite understand is that if the water supply is through a metal pipe, why has it not pulled the surface potential towards the distributor's earth.
lyledunn:
You are correct Alan, the last house is next door at the end of line about 150m away from the house in question. On the line running left on my sketch the first section is overhead to a pole where it feeds another TT property. At the same pole the cable goes below ground and feeds 3 more domestic properties on a run of about 500m to the last.
How many of the three further properties are TN-C-S? Is it likely that after at least 650 m, Ze could be < 0.35Ω?
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?
Thinking about it the fault stays constant, adding more earthing on the other side of the divide has no effect on the actual cause of the problem.
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