Terminating supply to outbuilding; is it acceptable to

Jan 24, 2024

Question:

I’ve been asked to terminate the already installed 16mm 3 core buried SWA supply only to a new external building which has been already wired and second fixed by others, and to test and inspect it for signing it off.

It contains a water supply for a cooler and has metallic conduit for a couple of switch drops. Source supply is TN-CS so it requires it to be on a TT system so as not to export the arrangement?

The consumer unit installed is a metallic split load 63A 30mA RCD arrrangement with a 100A mains switch.

My question is: is it compliant to terminate it into the mains 100A switch of the CU, or is it required to go directly through the RCD incomer with it being on a TT system?

Answer:

So, firstly, you may not need to TT the outbuilding. It is worth looking at the IET Guidance Note 8 on this as it explains it better than I can here, and with some nice clear pictures too. Essentially, you have to consider the size of the bonding in the outbuilding and ensure you meet that requirement. If there is no bonding required, then there is no issue there. But check the GN 8 as it is useful.

Regarding the RCD, that will depend on what you have to do for the earthing, as detailed above. If you are going to supply the outbuilding as a TT, then you must check the manufacturer's instructions as some manufacturers will say you can use a traditional main switch, others will require an RCD.

Hope this helps.

- Gary Parker, ECA