Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, concerning consumer units and the possibility of using 30mA time delay RCDs, is answered by SELECT:
Question: I recently replaced a number of consumer units with one split RCD consumer unit fed via (existing) 6m long tails in the ceiling void. The tails were further than 150mm away from the ceiling and were fed via an 800kMF switched fuse.
If the tails had been located less than 50mm from the ceiling (BS 7671:2008 - Regulation 522.6.7), they would need a 30mA RCD installing before they entered the ceiling void. This would not be practical as one of the two RCDs in the consumer unit would trip before the upstream RCD feeding the tails.
The only practical solution I can think of that will satisfy BS 7671:2008 (apart from wrecking the property decoration and re-routing the tails) is to use a 30mA time delayed RCD adjacent to the electricity company cut-out to protect the tails. The installation would then conform to Regulation 522.6.7.
The problem I have is that I cannot find a manufacturer of such a device - there are lots of 100mA time delayed RCDs to choose from, but no 30mA devices. I would appreciate you pointing me in the direction of any 30mA time delay RCD suppliers. Alternatively, please advise what I should do if - in the future - I have an installation where the tails have been concealed in a wall to a depth of less than 50mm and should be protected with a 30mA time delay RCD.
Answer: You should note that meter tails generally should not be longer than 3m in length. However, I don't understand from your description whether the cables are concealed in a wall at a depth of less than 50mm from the surface, in which case Regulation 522.6.7 would be applicable in domestic type premises (you refer to 50mm from the ceiling).
If, however, you mean that the cables are installed above the ceiling, Regulation 5226.5 applies and requires the cables to be installed in such a position that they are not liable to be damaged by contact with the ceiling or its fixings, and there is no specific requirement for 30mA RCD protection in this situation.
The reason why you cannot obtain a 30mA time delayed RCD is that such a device - if one exists - would not provide a person with the neccessary protection against electric shock.
To see many more Q & A in Voltimum UK's Experts Area, please click on the link:
www.voltimum.co.uk/consult.php?universe=consult.index.questions