Changing two single switches to a two-gang dimmer switch

[hawkeye]

As the heading suggests, this should be reasonably straightforward but I spent the best part of four hours last night going round in circles.

Situation is:- I have two wall light circuits in the lounge (5 lights on one and 2 on the other), both of which come of a fused spur from the downstairs ring main. I wanted to replace two adjacent single faceplates with a new two gang dimmer switch (I don't need it to be two way) so assumed I'd only have to wire red to common and black to L2 for each gang (there are two 2 core cables in the cavity). However, this only gives me two lights working. I believe that the former wiring had a further piece of cable that linked the two switches so does this mean one of the cables has a permanent live? I tried this out in just about every permutation and got an array of results (e.g. all lights working but two not responding to the switches, three lights working off one switch and 4 off the other but the 3 needed both switches on, the combinations were endless).

Please can anyone help? I probably sound pretty hopeless but I'm normally fairly competent!

Thanks

 

[Steve]

I have two wall light circuits in the lounge (5 lights on one and 2 on the other), both of which come of a fused spur from the downstairs ring main

so what cables do you have at the switches? by that scenario, i would expect 3x twin and earth cables (one supply, two to the lights) and if this were the case, you dont connect anything black to the switches. But from what you have said this is not the case.

And you probably do need the link wire, yes. This is most likely why it is not working. Link the 2 COM terminals together, but keep the "standard" wiring there too. Sounds like there should be 2 wires in each COM. Are you sure this is a spur from a ring main???

 

[hawkeye]

There are only 2x twin and earth cables and all four cores were attached to the original switches, in addition to the link wire (I just can't remember which terminals the link wire 'linked!).

As there are only 2 cables, is only one a permanent live? I am absolutely certain it is a spur so are you able to assess which cores need to go to which cables?

Thanks very much for your help so far.

 

[ban-all-sheds]

Use your multimeter to identify which cores are permanently live.

One thing I can 100% guarantee is that if there's a spur cable from a socket circuit coming to the switch, and you put red into COM and black into L2 you would have got a bl**dy great bang.

Were the old switches DP ones?

 

[securespark]

Strip the switch down so you have all your conductors ready for testing. Find the live (there must be one!).

Once you have found a live, assuming the switch is not two way at all, and that there are no neutrals present at the switch poistion, the other three must be switch wires. Connect each in turn to the live (safety first) and see what comes on.

 

[hawkeye]

I tried but failed to identify the live, probably because I'm not used to using a multimeter (any quick tips?).

If there is only one live do I use the link wire to take it to the COM of the second gang, and thereafter which cores go into the L2 terminals?

 

[hawkeye]

OK - some progress. I've identified the one permanent live and have tested the three remaining cores. These switch on three different sets of wall lights (i.e. 2+2+3).

Please can anyone advise how I now need to connect all four cores (+ the link wire?) so that each gang can work independently (say, in a 2+5 configuration).

Nearly there so any help would be appreciated.

 

[johnny_t]

Permanent Live goes to COM of one switch
Link wire goes between the two COMS

One switched live (2 lights) goes to L1 of one switch
Two switched lives (2+3 lights) goes to L1 of other switch

 

[hawkeye]

Excellent - all sorted now - thanks for all the helpful advice.