Earthing for generators
The electrical networks installed in our facilities is of the francophone type. The neutral and earths are not connected anywhere. Most of our installations are supplied with generator power (3 phase 5Kva - 25Kva). There are earths installed (now) in all our facilities.
Should we connect the neutral and earths at the distribution board or at the generator supply?
Do we measure the earth resistance at all earths separately?
What would constitute a satisfactory reading?
Is bonding of pipework recommended?
Should we install separate lightning earths and if so what would constitute a satisfactory measurement of earth resistance for such an earth?
I have discussed this with a colleague much better versed in earthing matters. He suggests that you:
a) connect neutral and earth at the generator only
b) use an RCD (say 100mA) for the main isolator
c) use further RCD protection (30mA) on socket circuits
d) bond pipes to earth where they enter the building; supplementary bonding (metal sinks, etc) is not required
Images and details can be found in http://www.pat-services.co.uk/iee-guidance-note-7-bs7671-1697.htm
Earth loop impedance (Ze) at the load should be less than 0.35 ohms (this will be via the generator) Test the resistance of the earth electrode separately. With damp soil a 4 foot electrode should give less than 20 ohms. If not, bond it to a second one about a foot away.
Lightning earth system should be kept separate from distribution earth and resistance as low as possible, certainly no higher than distribution earth resistance.
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