Edwin asks this question, which is about stand-by genset earthing arrangements when an automatic changeover switch is used. It has been answered by SELECT (Theme – Regulations and Legislation):
Question: We have a small processing plant running on a three-phase supply (PME).
We are installing a small 12kVA single-phase generator set to act as a stand by power supply for the IT equipment on site. It will supply and single-phase consumer unit via an automatic change over switch. The generator is auto start on loss of the main grid supply.
What earthing arrangements do we need for the generator, and also at the consumer unit it will be supplying?
My understanding of the regulations (which may be wrong!!) is that when the system changes over to the generator, the connect to the generator has to become a TT system. Therefore, consumer unit would require its own earthing rod and the change over switch would need to change over the earth to this rod upon taking it's power from the generator set. Is this so?
Answer: Where a generator is used as a standby supply as a switched alternative to a TN system, a suitable means of earthing of the standby generator is required in accordance with BS 7671:2008(2011) Regulation 551.4.3.2.1 - e.g. via an earth electrode.
The changeover switch should not switch over the earthing arrangements.
Reference should also be made to the Note to Regulation 551.6.2 and Clauses 7.2.1 and 7.2.5 of BS 7430:2011 'Code of practice for protective earthing of electrical installations.'
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