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!
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!
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