Jonathan Baker-Bates wrote:


That's right - none of the diagrams seem to apply. I have the following
coming out of my ceiling:

- L, N & E in one sheath from (I assume) the mains.
- L, N & E in another sheath.

Both the above are wired together - i.e. each L, N & E pair are together in
chock blocks (three of them).

- One black sheathed wire.

This is poking out between the above two bundles, not connected to
anything. The outside light and switch for it in the conservatory) is
working OK.

The original lighting fixture was removed by a plumber (fixing a pipe) and
I never thought to ask him how it was wired when he took it down.

Oh dear. Not looking simple, I'm afraid: I assumed you had an existing
fully-working all-wires-accounted-for ceiling rose, in one of the
sort-of standard configurations. Now it turns out you have a
partially-disconnected-by-some-bloke-or-other mess of wires :-(

When you say "one black sheathed wire", do you mean it's a single solid
copper wire with just one covering of black insulation; or that it's got
two layers of insulation - an inner black one, with a grey PVC outer
layer? If the former, ***with the circuit if not all the electricity in
the house isolated***, dig around more carefully, as it's quite likely
the black core of a third T&E you haven't found all of. If the latter,
look into that conservatory fitting you mention and its associated
switch - can you see a wire there which is "obviously" the other end of
the same wire?

The point is, you really *can't* wire in this new light to this existing
once-had-a-light position without working out what this black wire's
for. If you had a meter and know how to use it, you'd be unlikely to be
asking here. If you feel you're able to work safely, and bone up on the
principles of what to measure (or know them already), get a meter, and
work out (a) whether the L is a permanent live, or affected by some
switch somewhere, (b) if it is a permlive, which one is feed in and
which one is pass-it-on-downstream (this more for curiousity than
anything else ;-), and then (c) tackle the Mystery Black Wire. Is it
live when some switch (is there one which used to control the missing
fitting?) is switched on? Or is it a neutral to some other now-removed
fitting?

If all this is sounding too haed-hurting, time to swallow pride and line
a local sparky's pockets with a few notes...

Stefek