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I'm not able to go through the regs right now but I think it has something to do with properly earthing the sleeve of the SWA. Then you can lay it on beds in the garden so I can't see why it needs to be buried deep.
On the other hand the whole job needs sign off from building control so whoever is doing that should be able to advise you:)
Try searching the IET forum, I've found they are the best for electrical advice.
Airhead-
I tried the IET forum but it’s a pain to navigate.
Electricians forum said there was no need for the conduit and laying it in a groove was totally fine.
The supply SWA’s armour will be earthed as it comes out the supply DB (using a gland and a banjo, something along those lines).
I’m just finishing my C and G 2365 level 2 and 3, so I have sufficient understanding to do the job. The practical experience is what I need to build - fairly handy, but electrical installation experience is clearly lagging behind other aspects. A course mate is a registered competent person so he’s agreed to sign it off. I’ll obviously do a full test to make sure it’s alright. And I’ll do the cable calcs beforehand for practise anyway.
My main concern is that the Zs at the cabin DB will be too high and I’ll have to upgrade the SWA, or have to faff around with an earth rod. But otherwise should be straightforward.
Nef
I've had a look through the regs, and only found stuff about buried cables needing to be of sufficient depth and with mechanical protection - SWA or conduit to equivalent level of protection. Quite excited about getting it done tbh.
I reckon it'll be fine. Convincing the FIL is the main hurdle.