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So my light switch is connected to my smoke alarm in such a way that the smoke alarm isn't on/charging unless the light in the room is also on.

I live in the United Kingdom. This is an upstairs bedroom light. Smoke alarm in adjacent hall.

Pretty sure that black is neutral, red is live and green/green+yellow is earth.

I've a feeling I should only need to move one wire to make it so the smoke alarm is always powered, and my light switched, but not 100% sure how to identify which wire?

Any help would be appreciated.

Here's the switch: Switch photo A wiring diagram: Wiring diagram for switch The alarm wiring: Alarm wiring photo The lamp wiring: Light wiring photo

Daniel Griscom
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brightemo
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1 Answers1

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My guess would be

enter image description here

I notice someone (you?) marked the switched-live wires with black dots (single and double). If you are especially lucky, whoever did this also similarly marked the wires in the light fitting and in or behind the smoke alarm.

So for example it might be something like

  • switched live 1 - light fitting
  • switched live 2 - smoke alarm
  • permanent live - unused?

The bottom red wire is most likely the live feed in from the fusebox/consumer-unit.

If there are two reds behind the smoke alarm, it may be that one is the upper permanent live and the other is one of the two switched lives. If so you can rearrange wiring at the alarm. Otherwise you need to move wires at the switch.

If all else fails I would turn off the breaker for that circuit, disconnect the three red wires that exit the top of the back box, open up the smoke alarm, remove the light bulb (in case the alarm piggy-backs from the light fitting) and then use a continuity tester (e.g. Cat II multimeter) to work out which wires go where.

I have a cheap DT830B digital multimeter that I've never used.

Not ideal, but here's what I would do. Take great care as 240VAC is potentially lethal.

  • disconnect all the red wires (R1-R4)
  • plug black lead into bottom socket marked "COM"
  • plug red lead into middle socket labelled VΩmA
  • Set dial to V~ 750 range
  • prop the meter up somewhere you can see it which is near to the switch.
  • hold the probes keeping your fingers at least 1" away from the metal tips.
  • press the black probe tip firmly into the top of the black connector where there is a brass screw clamping the wires together.
  • press the tip of the red probe against each of R1 to R4 in turn and make a note of which ones show 230-250 VAC.

With luck that will identify which of the red wires is the live feed in. You can then try connecting the live wire to each of the other reds one at a time to see what turns on. That will identify what the other wires go to.

Connecting R2+R3 puts the light on ...

Test results:

Connections            |switch |light  |alarm |other |conclusion
-----------------------|-------|-------|------|------|--------------------------
top=R1+R2+R4 bottom=R3 |off    |off    |ON    |-     |R3 is not live-in nor alarm
"                      |on     |ON     |ON    |-     |R3 to light
R1 + R2 + R3 + R4      |-      |ON     |ON    |-     |-
R2 + R3                |-      |ON     |-     |-     |R3 is light, R2 is live-in     
R2 + R4                |-      |-      |ON    |-     |R4 is alarm, R2 is live-in
R1 not connected to R2 |-      |-      |-     |OFF   |R1 is other rooms

So in summary:

  • R1 is to other rooms.
  • R2 is live from fusebox/consumer-unit
  • R3 is light
  • R4 is alarm

Your Crabtree switch has a top connector, often this will be labelled COM, in your case at appears to be marked COMMON. I would connect R1, R2 and R4 here

It also has two lower connectors - it is a two-way switch (US nomenclature: three-way) used where you have two switches contolling one light (e.g hallway). In this case it is being used as a normal one-way switch. The two lower connections are usually labelled L1 and L2†. I would connect R3 to L1 here.

Summary 

top (COMMON) = R1 + R2 + R4
bottom (L1) = R3

It doesn't matter if you swap the roles of the top COMMON and bottom L1 connectors if that makes it easier to fit the wiring in.

If the normal ON position of the switch has the lights off, just move all the wires that are in L1 over to L2.

†Other manufacturer's switches sometimes have the common position labelled as L1 and the other positions as L2 and L3.

RedGrittyBrick
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  • I've added more photos [link](http://imgur.com/a/Aa2Cv). If I connect R4 to R1+2, the alarm gets power and the light doesn't - does that help? Connecting R3 to the same place or turning on the switch then turns the light on also. Does that sound right? – brightemo Nov 13 '16 at 11:19
  • Also, the dots were me, so I could at least put it back together the same way if all else failed. – brightemo Nov 13 '16 at 11:27
  • From your experiment, it seems that R3 is to the light. That suggests R1 or R2 is the live feed in. When you say "same place" it isn't clear whether you mean "same place as before" or "same place as R1+2" or some other same place. Anyway, do you have a [voltage tester](http://diy.stackexchange.com/q/90038/2815) of any sort? – RedGrittyBrick Nov 13 '16 at 11:47
  • I mean with R1+2+4 at the top of the switch and with R3 at the bottom. (Only moving R4). The alarm gets power, and the light only comes on when the switch is on - which is what I want. I have a cheap [DT830B digital multimeter](http://www.hobby-hour.com/electronics/dt830b-digital-multimeter.php) that I've never used. – brightemo Nov 13 '16 at 11:54
  • Maybe you have solved the problem to your satisfaction then. If not ,see updated answer. Be careful. Hope that helps. Good luck. – RedGrittyBrick Nov 13 '16 at 12:09
  • Thanks for helping with this! I've done what you suggested with the multi-meter - I think I need a new one as it kept only showing 0 or 1. R2 is the only one showing anything > 0, and once it did flash ~230 for a second, so I think that is the only live one. – brightemo Nov 13 '16 at 12:39
  • Connecting R2+R3 puts the light on. So I guess R3 is the light. Connecting R2+R4 puts the alarm on. So I guess R4 is the alarm. I think then that R1 is the connection to the rest of the upstairs rooms? As without R2+R1 connected, the other rooms lights don't go on? Does that sound right? Given that... how should they be connected to the switch? R2+R4+R3 to the top and R3 to the bottom? – brightemo Nov 13 '16 at 12:45
  • Correction: Given that... how should they be connected to the switch? R1+R2+R4 to the top and R3 to the bottom? – brightemo Nov 13 '16 at 13:09
  • @brightemo Yes, see updated answer. – RedGrittyBrick Nov 13 '16 at 19:53