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I'm in the UK. This is 240V mains wiring.

I have two bulbs (D and E), one at the top and one at the bottom of some stairs. Controlling these bulbs are two sets of light switches (A, B, C).

The switch at the top of the stairs (A) is a 'single' switch, and the one at the bottom of the stairs is a double switch plate (B and C), with the right hand switch controlling a third bulb near the front door (not shown) (does not affect bulbs D or E)

The issue is that the switches A and B do not properly (as I would like) control the bulbs D and E. Bulbs D and E only come on when A is off (up) and B is on (down)

What I want is for either switch to make the lights toggle. That is to say, if the lights are off, then changing the position of either switch should make the lights come on, and if the lights are on, then changing the position of either switch should make the lights go off.

Stairwell light switch wiring

Can anyone suggest what might be happening, and how to correct it?

PS: I have currently no idea where these wires run. If someone can suggest how I can 'bell them out' safely then I can make the diagram better.

As requested here is a photo of the inside of the bottom switch (B and C)

bottom switch

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  • These should be (UK/Commonwealth) 2-way switches (US calls the same thing 3-way.) If they are, the wiring to terminals is probably incorrect if they are operating not as they should. I suspect (but don't know) that there may be the similar issue of using the (old-standard) neutral color as a traveler between switches. Can you provide pictures of the switches involved showing their wiring (power off for the circuit first, of course?)
    – Ecnerwal
    Apr 12 at 1:01
  • What other wires are present in the switch junction boxes? Apr 12 at 1:06
  • If, (and that's BIG if) the diagram is accurate as drawn regarding switch terminals, it appears that the bottm red into B is the live feeding C as well, and the black and red on the top of B are travelers to A, but the black needs to be switch to the top of A, and you need to figure out which red is a traveler to B (should be in the same cable) and which is the light (should have a neutral connection) and connect that red to A's bottom. But I repeat, that is based only on the drawing mapping accurately to real life. Which is why pictures showing wires, terminals, & cables would be helpful.
    – Ecnerwal
    Apr 12 at 1:17
  • I guess the behavior indicates that the right-side Red at A and the Black at A would be what gets swapped, if the up/down/terminals is all accurate and the same, as drawn.
    – Ecnerwal
    Apr 12 at 1:20
  • @Ecnerwal The diagrams are an accurate representation of what is inside the switch patresses. I just don't know where those wires go off to once they disappear into the walls.
    – Richard
    Apr 12 at 1:26

4 Answers 4

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This looks like the older UK style of 2-way wiring, where the switched terminals of each switch are connected together and then the common terminals are run separately to a junction box. The switched wires were usually in one twin-and-earth cable running only between the 2 switches, and the common wires as single core cables (you can see the single core red in your photo of the bottom switch) It looks like in the upstairs switch someone has swapped the common wire for one of the switched lives, probably when replacing the switch plate.

It is likely to fix the issue if you swap the black wire in switch A into one of the other terminals (most likely the one currently occupied by the wire not sharing a grey outer sheath with it)

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So, What I think you have going on from your symptoms, if we make the "reasonable but not guaranteed" assumption that this was once a correctly working circuit.

Diagram as it seems to be.

Live in at B common, if B is off Live goes to Red Traveler, but Red Traveler can't connect to Red Lights, only to Black Traveler back to B.

If B is on, Live goes to Black traveler, A can switch that to Red Traveler back to B, or to Red Lights. Lights have a Neutral connection somewhere, not shown here becasue not shown on your diagram.

If you swap A Black & Right Red:

Diagram as it should be to work

Live in at B common, if B is off Live goes to Red Traveler, A on can switch Live to Lights. If B is on, Live to Black Traveler, A off can switch live to Lights. Changing ether switch changes the state of the light.

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  • Common goes to common - that's why it's called common. L1 & L2 come initially from the ceiling, common doesn't. Common connects to nothing except 'each switch'. L1&L2 start in the ceiling, then also pass to the next switch, like for like.
    – Tetsujin
    Apr 12 at 8:36
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Someone previously exchanged one or both of those switches, e.g. for aesthetic reasons. They didn't know anything about 2-way circuits, did not note how the previous wiring had been connected, and just slapped 3 wires on 3 terminals.

To correct this, you need to learn a little more about 2-way circuits. Here's how they work.

enter image description here

(we call them 3-way in the States and use white for neutral and black for always-hot).

The important concept is the travelers. One is hot, and one is not. (sorry, that's 'live' to you). A 2-way has 3 terminals: one common and two travelers. The UK doesn't use special screw colors, so you'll need to look at the markings on the switch to see which is "common" and which are "travelers".

The traveler wires are easy to spot in your case: both of them are in the same cable. They are usually the same colors on both ends.

If you want to use tape, paint or shrink tube to identify wires by their function, feel free to do so (consistent with local laws). There is no need to distinguish travelers from each other.

While you're looking at the switches, make sure they have the BSI Kitemark or other reputable testing lab insignia. Many homeowners are installing cheap garbage they get off eBay, Banggood, Amazon Marketplace, or other dodgy mail-order sources. This stuff has no safety testing; the CE mark is faked without consequence (since the manufacturer is beyond reach of EU authorities).

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  • Anything old enough to be using red/black in the UK won't have the permanent hot to the switch. It will be in the ceiling.
    – Tetsujin
    Apr 12 at 8:17
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This is a partial answer until we can get more info…

Can we have photos? I need to see how these wires are bundled into cables to have a better guess at where each is from/to.

UK lighting does not run on a ring usually. It's a loose collection of spurs. The 'power' is always in the ceiling, switches are powered by drops from one of the lamps in each circuit*. D & E are connected in the ceiling & you shouldn't need to trace that to get this to work. A & B are then connected from this drop & to each other. Basically, between A & B we should have L1 -> L1, L2 -> L2 & Com->Com. Simple. These are all switched lives, there is no neutral in the circuit. Colours are differentiated differently compared to regular plug & socket wiring.

To visualise how each switch works, when you flip the switch one way, the mechanism connects Com to L1, then when you flip it the other way, it connects Com to L2. That's all it does, it's an 'either/or' switch.

So, one of the switches should have an extra cable [containing a set of appropriate wires] for each light circuit, coming from the ceiling. That really must be initially to B/C which implies you should have two sets coming from the ceiling to each of B & C, then a third [which ought to be a 4-wire cable] going off to A. Nothing more needs to leave C to your off-screen lamp, that's self-contained in its own drop from the ceiling.

Someone has jumpered the C's L1 to B's Common. That's not a good start. I'm not really sure what they were thinking.

If we look at B to start with, ignoring C, we should have 3 wires from the ceiling [hopefully black, red, green].
We should then have four new wires going off to switch A. A should not be connected to anything except B. B is the 'master', A the 'slave'.

enter image description here

This is your basic UK 2-way wiring [ignoring Earth for now]. The two lamps are connected to each other in the ceiling, and that's where you 'real' live is too. If we look at only A/B, then the black goes com->com, fortunately, the reds can go to either of L1/L2 so you don't need to trace those.

Your B/C switch, though, implies there is only one red coming from A. The 'far end' of a 2-way [A] must have three wires plus earth, unless someone has tried to rewire it to a permanent live for a Smart switch, in which case it will start to look more like the American system shown in the other two answers.

I'd be tempted to start doing some continuity testing, so you can actually find which of those is which. There are two ways of getting it right & many ways of getting it wrong. You got the lucky one, the one that doesn't go bang.

*This is changing in new builds. Smart switches won't work with the old UK structure, so the system is moving towards the American system with a true live to the switch. I thought we shouldn't confuse the issue on old wiring.

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