Scott asks a question concerning earthing a generator supply connected to a house supply that is TN-C-S. His question has been answered by SELECT (Theme – Power):
Question: I have been asked to wire an emergency generator supply to an existing mains circuit which supplies two freezers and a fridge freezer. The house supply is TN-C-S.
I intend installing a two-pole two-way switch with a mid position 'off' to switch phase and neutral. The generator will be earthed with an earthing rod and the neutral & earth will be linked within the 16A plug.
In the event that a mains failure occurs and the grid earth is broken, would you agree that this circuitry provides a suitable earth?
I have obtained several opinions on the earthing of generators, but there doesn't seem to be a consistent response!
Answer: With the standby emergency system you describe, the generator does require a separate earth electrode, so that each source operates independently of the other. The earth electrode resistance should not exceed 20 ohms.
There is no requirement to link the neutral and earth at the plug, as operation in parallel with the incoming supply should not be possible. Therefore, the generator neutral should be connected to the generator framework, which will be earthed via the earth electrode and the main earthing terminal.
The generator supply would in effect be a TN-S supply. However, where the supply is PME, there may be no requirement to isolate both poles.
Further information could be obtained from BS 7671:2008 Chapter 55 and Regulation 551.6. See also BS 7430 'Code of Practice for Earthing'.
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