2 way wiring problem

27 Oct 2013
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United Kingdom
Sorry yet another 2 way lighting problem. This seems really strange to me.

Downstairs 3 gang switch controlling hall (1 way), landing (2 way) and outside light (1 way). Upstairs 2 gang 2 way switch controlling landing (2 way) and second floor landing (2 way).

Everything works perfectly except the 2 way control of the landing light.

This worked perfectly until today when I replaced the landing 2 gang 2 way switch. So before today I could control the landing lights from the hall and the landing as expected.

Having read around the subject and consulted the wiki, the wiring was as I expected.

I will only refer to the landing light connections. Downstairs in the hall is a 3 core and earth with yellow to com, red to L1 and blue to L2.

Upstairs on landing switch was yellow to com, red to L1 and blue to L2. Additionally a twin red and earth with one red to L1 and one red to L2.

I emphasise that all was working fine at this point. I then replaced the plastic switch with a plastic MK one, and since the cables had give I transferred the connections one by one.

I now have in the new landing switch yellow to com, red to L1 and blue to L2 from the "connecting cable" and one red each from the lamp cable into L1 and L2. I kept the pairs of wires going into L1 and L2 together and transferred each separately.

So in theory I have exactly the same wiring as before I replaced the switch. However I have clearly done something wrong because now although the landing switch controls the landing light, it only does so when the hall switch is in a certain position. If I flick the hall switch then the landing light goes out and the landing switch has no effect.

I have checked the connections at least 10 times. I have also not altered the downstairs hall switch at all, and I have rechecked the connections - definitely yellow com, red L1, blue L2.

Since it was working normally before I changed the switch, and now it works as I describe obviously I have changed something without realising. But I am at a complete loss as to what.
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I think you may have got the 'COMs' the wrong way round.

Is the switch exactly the same as the old one.
That is, two triangles - the same way up or - one up, one down.

The com relates to the opposite L1 & L2.
Thanks for your help.

No the switches are not exactly the same. The older switch is an older Crabtree. On the old switch COM A is at the top with L1A and L2 A at the bottom, and COM B is at the bottom with L1B and L2B at the top. In the new switch each gang is completely separate from the other, and each COM is on the top and L1 and L2 are on the bottom.

I was suspicious of getting the COM wrong as well. However 2 points : first I havent changed the hall switch at all, each connection is clearly labelled and yellow is definitely in com. If it is yellow in the lower switch surely it must be the same on the upper switch? Secondly L1 and L2 both had two wires in them. I wouldnt expect a com to have two connections?

Thanks again
Check that you have your three core cables in the correct sets.
You should have a two sets of three core cables entering the switch backbox, they should if done correctly enter the box in a grey or white PVC sheath, check that the three cores from each sheath is going to the same gang.
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Yes, your original description IS correct.

Therefore, the only way to get a cross connection between the landing and hall switches would be to get the coms the wrong way round or two of the reds the wrong way round.

If you say you have not done that, I cannot dispute it.
Thanks both.

Into the switch (landing) backbox enter 2 three core and 1 two core (two reds). I have checked the connections to the switch multiple times. Each cable enters the box as you say in a grey sheath. I have not checked the origins from the sheath as closely and as many times as the connections to the switch. I will recheck this tomorrow (when light!).

EFL - when you say two of the reds the wrong way around which reds are you referring to? This might be something else I should check.
Post clear photographs, when you can, of the switch wiring showing the conductors and their terminal positions.
EFL - when you say two of the reds the wrong way around which reds are you referring to? This might be something else I should check.
I think I may have been confused by your description.
When you said 'hall switch' I thought you meant the switch that operated the hall light.
I now realise you meant the switch downstairs which operates the landing light.

I can think of nothing else to cause the situation except a faulty connection or a faulty switch.

Does the second floor light operate correctly with no effect on the landing light?
Thanks I'll try and get some photos up, but will need to isolate the supply so will need to wait for daylight. And they say there's a big storm coming tonight so hopefully that wont complicate matters!

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