Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, concerning a supply authority protective multiple earthing (PME) supply, is answered by SELECT:
|IEE Guidance Note 8 'Earthing and Bonding'.
A supply authority 3ph+N PME supply
feeds a main distribution board in a metering cubicle. This distribution board feeds a second subsidiary distribution board in a nearby building, which - in turn - feeds a third distribution board in a separate building.
Q1: Do the bonding requirements under PME conditions extend throughout the distribution sub-circuits?
Q2: Should the main bonding at each of the subsidiary distribution boards be made to a local MET, or should the bonding be back to the incoming supply MET?
Answer: The answer to Q1 is 'yes' - main protective bonding conductors should be applied in each building and the CPC of the sub main will be used as part of the main protective bonding system.
The answer to Q2 is that the main protective bonding (MPB) in each building would be made at the local MET. Note that the bond size should be in accordance with Table 54.8 of BS 7671: 2008, and be sized to suit the incoming supply, ie: not based on the size of the sub-main conductors.
Alternatively, a sub-main feeding a separate building could be altered at the incoming sub-main position to make the supply earthing system of a building a TT earthing system - a good source of information on this subject is IEE Guidance Note 8 'Earthing and Bonding'.
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