I have attempted to change a light fitting in a bedroom. There are two sets of each colour of wires, but three black covered wires. The label stating which is the wire to control the light switch has fallen off. Now, although the light will illuminate I cannot turn it off at the switch. Help, what should I do?

Perplexed., September 2008

Get yourself a mains-tester screwdriver with a neon lamp in the handle (about £1). Remove the fuse or throw the circuit breaker for the light circuit. TAKE CARE.

A ceiling fitting will have Mains Live (RED) coming in and usually Mains Live (RED) going out to the next fitting unless it's the last fitting in the circuit and another RED wire going off to the switch. The Red wires all connect together and are permanently live and dangerous.

There will be the same number of Black wires as there are Red wires. Leave them all hanging in the air and not connected to anything and not touching one another or anything else. One of these Black wires is not what it appears to be!

Restore the power and carefully check with the mains-tester screwdriver that the red wires are live. Touch the blade tip against the Red terminal and observe the neon glow as you physically touch the OTHER END of the screwdriver (the top - it's quite safe).

Now very carefully touch the screwdriver tip against the copper of each of the black covered wires. One of them is the return from the switch and should ideally have a red sleeve around it to indicate that it's Switched Live (Switched Line). If no black wire makes the neon glow then operate the switch to the other position and check again.

When you have identified the black wire that's coming from the switch - remove the power from the circuit and connect that wire to the terminal strip together with the Brown wire that goes off to the lamp or lampholder.

The blue wire coming back from the lampholder connects to any remaining Black wires you may have - either one or more usually two. Restore everything and switch on.

In summary - the Red wires are all Live, Dangerous and connected together. One of the Black wires is sometimes Live and Dangerous but only when the switch says so and connects to flex Brown. Flex Blue connects to any other Black wires.

Septimus suggests a more elegant but more expensive solution - a meter and a long length of wire will cost more than a mains tester screwdriver - his way is safer though as you can stay well away from any live terminals.

Richard Buxton, September 2008

you must identify the switch line . a way of doing this is to isolate the circuit.One of the blacks is one end and one of the reds the other end, Using any continuity tester and a length of single core wire . (any wire will do as long as it reaches from the light to the switch ) connect 1 end of the wire to any black wire at the light . the other end to a lead of the tester . put the other lead to the black in the switch .
depending on the meter it will buzz beep or give a very low ohms reading when u are on the the right wire . move the lead in the light to each black until u get a continuity reading or indication on the meter . do the same for the red wire. when u know which are the switch wires at the light . then put the other red wires, to the live terminals, and the black, to the neutral terminals, in the rose .
put one end of the switch wire to the live terminals and the other to the terminal where the lamp brown wire terminates . should work from there

septimus, September 2008