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martysparky
Reg:411.3.2.4 p47
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Discuss Ze in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net
I have to answer this question with reference to 7671 and, well, can't find it.
Ze is covered by the ESQCR as it is DNO`s responsibility
The 21 ohms is the DNO earth at their supply transformer. No matter what system of earthing you have in your house the supply transformer will have an earth electrode and that is the DNO responsibilty
If you have TNS or a TNC_S then the DNO have the responsibility to maintain that earth.
If you have a TT then it's the consumers responsibilty to maintain that not the DNO so they don't have the responsibilty of your Ze at a TT. All they have to ensure is that the Electrode at the transformer is there at that 21 ohms. and it is your responsibilty to get the earth path back to it.
The Ze in a TT changes with the weather and is unreliable so we don't really know it; isn't that why its RCD'd
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