Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, about the use of metal conduit as the circuit protective conductor (CPC), is answered by SELECT:
The distribution board is some distance away, and the cables are run in near capacity conduit and trunking through several ceiling spaces. The existing ring has, as its CPC, the metal conduit. The earth for the extra sockets is taken from the socket outlet and extended as a ring. The maximum permited earth loop impedence is not exceeded and a 'safety electrical earth do not remove' label is adhered.
Best practice would, no doubt, be to run two earth cables back to the board. But cost is a factor and the boards contain no earth bar as they are the old 'Bill' type boards so.......is this OK or not? If not, where is the breach in the regulations?
Answer: From your explanation, it appears that everything is OK. It is still OK to use steel conduit as a CPC and, of course, in that instance there is no actual ring for the CPC. To extend the ring in dado rail will necessitate extending the CPCs and, in this section, the CPCs should be in the form of the ring connecting both ends of the CPC's to the conduit earth terminal.
I trust you know that when conduit is being used as the CPC, it is a requirement to ensure that any metal back box of accessories etc., will need to be bonded to the earth terminal of the socket or switch faceplate. There is probably no need to fit a safety electrical earth label, but it won't do any harm.
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