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Trying to replace broken light fitting in under stairs cupboard - 3 cables lead to it(one from light switch, two from consumer unit and third goes up through wall to the kitchen ceiling i think) - all with 1 black and 1 red wire inside. all three reds are twisted together, two of the blacks are twisted together (from consumer unit and cable from kitchen) leaving the black from the light switch as a single wire. The new batten lamp holder has 3 ports for wiring - which goes where please ? All help gratefully received - thanks!

Two of the three wiring ports in your new fitting should be connected to the two pins of the actual lamp holder that the bulb contact. The third one shouldn't be connected to anything. You need to establish which two ports go to the lamp holder and MAKE CERTAIN that the other one isn't connected to the metal sleeve of the lamp holder (if it has one) or anything else in the fitting for that matter. If you can't do that I'd get an electrician to connect it up for you. If you ARE able to establish all that, then connect the black from the switch to one side of the bulb holder and the two blacks twisted together to the other side. Fix the three reds into the remaining port that isn't connected to anything.

As I stated above, if you're in any doubt whatsoever about which two ports connect to the bulb holder or, perhaps more importantly, that the other port isn't connected anywhere else in the fitting DO NOT tackle this yourself. The three reds are all live, as is the single black wire from the switch when it's turned on. The other two black wires are the neutrals coming from the consumer unit and onto the next light in the circuit. Both the live and neutral wires are known as "line" wires as they form part of the circuit during normal operation of electrical equipment whereas earth wires, where there are any, do not.

I've had further thoughts about this. It's likely that the wiring port on your batten lampholder that isn't connected to one of the lampholder pins is meant to be for an earth connection. If so then the 3 red wires MUST NOT be connected to it. A better way to connect the fitting would be to get a junction box then connect the two lampholder pins to the single black and twin black wires via the junction box. The three red wires can then be clamped into a spare terminal on the junction box.

the three reds twisted together( should be in a cable connector) are the lives, main feed from cu, feed to the kitchin light and feed to the switch under the stairs.
the two blacks twisted together are the neutrals, again feed from the cu and feed to the kitchin plus neutral for the light under stairs.
the black on its own is the switch wire from the switch under the stairs, youve proberly got a swtich somewhere nere the cu.