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Being very honest, that needs to be deeper. I agree with @JonoMarshall that 5mm is fine, a bit more would be even better.
I don't know how much of the theory you know but cables in walls run in permitted zones, and the protection should come from mechanical protection (conduit or capping) plus a 30 mA RCD on your ring main. I believe there is no minimum depth under the regs, but there is a maximum (50mm) - this makes sense because you could just run conduit over the wall. I mean, it would look shit, but you could.
I'm presuming you already know about extending ring mains, not to run a spur off a spur etc.
I think you're using oval conduit there(not sure?), if so it's OK but trunking with a clip on cover is much easier as you can run the cables then pop the cover on afterwards. I'm deeply unsure about the screws (how do you avoid the cables?) through the conduit but if you're thinking about capping anyway that style of trunking could be an alternative.
I am not an electrician (so usual pinch of salt on my advice) and you should get this worked checked by one and a minor works cert issued, but I feel your pain as I'm currently in the middle of moving five 2 gang sockets and installing two new ones because architects...
Fox-
Cheers. I've gone back over it today to make it deeper.
It's extending the end of the ring in that room which is on a RCD.
My understanding was that as it's in line with a socket at the same level on the adjacent wall a mechanical protection wasnt needed. However, from reading forums it sounded like it was advisable.
I went for oval as whenever I bought everything that seemed like the most commonly recommend option.
I've added clips that I forgot I had. But the screwing is very secure. I screwed through with a step drill bit so the top hole is bigger. To allow the screw head to fit through the top. I was going to put blanking plugs in the holes to cover them.
TBH I wish I have just used metal capping now I've seen how deep the plaster is. It might have been a bit wider but I wouldn't have had to chase into the brick.
Actually I wish I hadn't bothered at all.
hugo7
I'm adding two sockets in a room. The whole room will be reskimmed.
Right now I'm chasing in a channel to run the cables in conduit.
How deep does the conduit need to be?
Currently it is maybe 1mm below the existing surface. No idea how to photograph this but here is a pic.
Any quick answers appreciated as I only have a small window of time left for the messy work before everyone gets home.
(the reason for running them through conduit was general Internet research showed this as the best belt and braces approach)