Double Light to Double Dimmer Switch - Problem

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7 Sep 2017
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United Kingdom

I have messed up our living room double light switch a little bit whilst trying to fit a new double dimmer. Both lights are on the living room ceiling left and right from the door when coming in.

Made the beginner mistake of not remembering how the wires were connected when I took the switch off - never again, I know now!!

I tried to fit the dimmer in various combinations, but could not get any of the two living room lights to turn on. I then tried to reconnect the old double light switch and get it to work, so I would find out the old, original cable allocation.

Tried many combinations and I now have the situation that when both double light switches are in the 'on' position, no lights go on, but when I turn both in the 'off' position, the right light turns on. Pictures below - appreciate any help!

Current connection that is wrong -

If anyone might know how to either fit the old double switch correctly again or directly let me know how to fit the double dimmer shown below, that would be miraculous.

Dimmer I want to fit -

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The wiring is some way off and as it stands has no chance of working, the only way to correct it is to test out and find the function of each wire. With that number of wires there are many possible incorrect combinations. I have had a look and think I have the solution, but without testing it would be unwise to suggest it.
My first question is why are there three cables for two lights? Maybe wired like European lights with both line and neutral to the switch? Although all well and good using trial and error with British wired switches where all wires are line, but with neutrals mixed in you could get a direct short.

Second question has to be are they two way lamps? This could explain the three cables, however one would expect a three core and earth to the other switch.

So it will need the cables tracing, there may be some short cuts, but it would involve however you do it some sort of meter, even a standard lamp could be used to test with, or a battery and bell, but trail and error without something to limit the current is not the way forward.
You are correct, there will have been a link between the two COMs, but another wire (probably RED) will have been in there as well.

As above. Without test equipment, and some knowledge, you are stuffed.
Call an electrician.
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Disconnect the two reds, put the black into L1 on one switch, leave the loop wire. Connect one red to the looped terminals and the other red to L1 on the other switch, if that is wrong, swap the reds.
royale, you might like to consider anybody who suggests electrical-wiring-by-guesswork as a blithering idiot, best ignored.
Absolutely nothing wrong with my solution. The op has already proved there are no neutrals involved, because if there were he would have tripped the circuit. Therefore, two of the three wires are switchlines and the other is the live wire. It's a very simple problem.

And my solution is safer and simpler than using step ladders to disconnect light fittings to test for the switchlines.
Appreciate all of your replies - thank you very much! Against most advice I have tried mwatsonxx's set up and it works on my new dimmer switch! Cheers! Very pleased and now know to take pics before attempting anything.....

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