Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, about the earthing requirements for six joined together lock-up buildings of metal construction, is answered by the NICEIC:
The supply to the lock ups is from a separate main building containing the incoming protective multiple earth (PME) supply (Building A). The supply to the lock ups is from the PME supply (of Building A) via an underground SWA cable feeding a three-phase distribution board in lock-up 1 (of Building B) and its supply cable is then to be looped into the five other lock-ups (of Building B), so that each of the six lock ups has its own distribution board and final circuits.
This creates a TT system in each lock-up, so each lock-up's earthing system is separate to avoid disconnection down the earthing system - so protecting the integrity of each lock up individually.
Please could you clarify whether we need to separate the PME earth to the lock-ups?
Answer: Because you have metallic buildings, and the practical difficulties in bonding all accessible extraneous-conductive-parts etc, you cannot export the PME terminal to the outbuildings, the distribution circuit CPC (if any) must only be earthed in the main building and be insulated from the outbuildings TT earthing arrangement.
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