Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day, concerning an outbuilding, a TNC-S supply and its earthing arrangements, is answered by the NICEIC:
1) The Circuit Protective Conductors (CPCs) of the outbuilding need to be connected to an MET as per Regulation 542.4.
The means of earthing is an electrode connected to the general mass of earth, so as per Regulation 542.1.4, we need to connect the MET via an earthing conductor.
If we look at the definition of 'Electrical Installation' and 'Origin of Installation', and if we are to assume that there is only one earthing conductor per electrical installation, would it be correct to then state that the TT outbuilding - by default - may become an electrical installation in its own right?
Answer: Generally speaking the situation you describe would be considered to be one installation having two earthing systems.
The origin of the installation would be the point where the electrical energy was delivered to the consumer by the supplier. The sub-main to the outbuilding would be part of the consumer's installation.
IEE Guidance Note 8 contains detailed information on various earthing arrangements.
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