David Hearn wrote:
Can anyone advise what depth backbox I should use? I'm adding a 2G MK
socket, and was planning on using 25mm - however after reading the
instructions it said that for = 9mm face plates, use 25mm, and 9mm use
35mm. I think the 2G MK socket is about 9mm - but I just wondered what the
benefit was with going with 35mm, and whether 25mm is very cramped (2x 2.5mm
T&E). Its an internal block + plaster wall (1930's block), so I don't want
to go too deep (not to mention that the deeper I go, the more work it is!).

Is 25mm a bit shallow?

One final thing - when I fitted some before, I set them too deep as I
thought they should be plastered into place. I think they should be screwed
into place and made good with plaster (rather than 1/2 - 1" of plaster being
the thing to hold it in place) - am I correct?

25mm is typically OK; it depends most on how many cables are being
connected. Assuming 2.5T&E, a single one (end of a spur) is trivial, two
(typical case on a ring, and your specific case) not much of a problem,
three (2 x ring + 1 to a spur) starts to be a bit tight, and sinking a
35mmsq makes the job easier. One-off on plaster+brick/plaster+block,
I've never found the need for anything more than a masonry drill round
the edges and a cold chisel+hammer to knock out the Inner Bit; obviously
if you were a jobbing electrician, it'd be worth splashing out on an
SDS-fitting box sinker.

As for securing them - again there's a difference between 'pro' practice
and sensible one-off d-i-y. Many pros prefer to gash out a gert
rough-n-ready lump, and squash a backbox onto a well-judged amount of
stiff plaster mix, letting the plaster that squeezes through the
mounting holes spread out and hold the box, along with the
tightly-packed plaster around the sides. If you're rewiring a house, the
time saved doing it this way adds up (and you can production-line the
fixing - chop holes, pull cable leaving over-long lengths sticking out,
mix up one batch of plaster and fix 10-15 backboxes at once). But for
those of us whose domestic authorities require Making Good to a high
standard, a neat and well-sized recess (little bits of Polyfilla/repair
plaster round the edges if your bolster gets a bit too energetic), with
the backbox held in place with a coupla screws-into-wallplugs, is the
way to go.

HTH - Stefek