Jonathan Baker-Bates wrote:
I feel sure this must be in the FAQ somewhere, or in the archives, but I
can't find the answer...

I have an Ikea light (a LEDING its called) that I need to fit to a cieling
rose. There are two sets of wires in the ceiling: one I assume is the
switch, the other the mains (each have L/N/E wires).

OK. Here's the ugly way to do it - but if you do get tired of the light
and want to switch back to an ordinary pendant later (or grow so
attached you want to take the new fitting to a new house) you'll thank
me for it:
- leave the ceiling rose in place
- mount the IKEA fitting to the ceiling close by
- run the existing pendant cable, or a short length of new cable
(heat-resistant if you can lay your hands on some) to the new fitting.

This way you haven't monkeyed with the fixed wiring at all.

The light fitting has three wires L/N/E, and that's it. No block like you
normally get with friendly labels on.

So, my question is: what wires do I fit where? I assume the earths all go
to the same place, but what about the rest?

OK. If you want to make the new connections "inside" the new fitting,
your job is basically to faithfully replicate the pattern of connections
inside the existing rose. So, label the wires of the existing cables
carefully. There are four, yes four, sets of cable cores which are being
connected together; so to replicate the existing interconnection you
need a 4-way section of "chocolate block" (terminal block) - if we're
talking doing the best job, try to find the sort which has little leaves
of spring metal clamping down the cable groups instead of just the bare
screw.

One way is for all the earths. One is for "all" the neutrals - if you've
just two cables as the moment, there's most likely just one black N to
which the existing pendant cable's blue core is connected, at one of the
outer of the three blocks in the ceiling rose. That's the one you
connect the N wire of the new fitting to, in the second way of the
choccie block. Next way is for all the permanent lives; again with just
two cables there should be just two of these, one in each cable,
currently connected to the middle one of the three long blocks in a line
across the middle of the rose, and each coloured red - though it's
possible that one's black instead, with or without a bit of red sleeving
or tape on it, if the switch wiring is done "the wrong way round". None
of the wires from your new fitting connects here. Final way is for the
"switched live" or "switch return" - should be just one core of one of
the cables, in the best case black with the bit of red tape/sleeving
mentioned above, if wired "the wrong way round" it'd be red instead, but
in either case currently connected to the other outer block of the rose
and where the brown core of the pendant flex is also connected; this way
is where you'll connect the L wire of the new fitting.

If this doesn't make sense, leave the job alone and get someone who does
have a grasp of what they're doing to wire it up!

Cheers, Stefek