Dimmer Switch Wiring

[Kroma]

Hi all,

My parents managed to get their double dimmer switch damp (with wallpaper paste) and it seems to have broken so I have bought them a new one.

However, upon looking at the connections of the old one (which has worked fine for about 15+ years) I saw the following connections:

this be right? I was expecting reds to L1s and blacks to COMS. I wasn't expecting just one black, a short red hopping between 2 terminals (an L1 and a COM) plus a red going to the other L1.

There is only one switch in the room, one of the dimmer controls will control a ceiling light, the other the wall lights.

Your observations will be gratefully received.

Many thanks,

Kroma

[Colin Wilson]

The top two are the "outgoing" switch wires to the lights, the bottom incoming cable is the incoming live, and its looped to energise the second dimmer. Are the cables all single insulated cores, or is one a T&E (it might help us figure out how / why they did it like this - if its in conduit in the wall it might have been a quick and easy way to add the fan circuit)

Ideally the black core should have a red oversleeve to indicate its a switched live - not sure about the other switched core (i'm a bit rusty, I haven't done any house wiring for 16 years or so !)

[Dave Plowman (News)]

The switch is two way, but of course the dimmer isn't, and it will only dim one of the two 'outputs' from the comm terminal - in this case L1. And dimmers aren't sensitive to which way round they are connected, so the original wiring is fine, if not perhaps how most would do it for the sake of tidiness. However, I've come across a dimmer where the dimmed output is L2, so it might be worth checking the instructions for the new one.

As regards the colours, my guess is a TW&E pair for the switch returns, and the switch feed possibly taken from another switch close by?

[Woby Tide]

Bizarrely I've come across almost the identical problem recently but was replacing a horrendous light fitting with a normal ceiling pendant. Took apart the original fitting and the wiring was done via a block, I got distracted midjob and lost what was connected to which. I've 3 sets of wires up there 1 pair of black and red, then 2 sets again of black and red but the red between the two is not 'split' and just loops away from the fitting. I've now through a process of elimination (not the safest way I know) got the remaining 3 blacks and 1 red wire connected so that the 2nd fitting (i.e. wall lights which I haven't touched) now works correctly and dims. The light I changed however now only switches on and off and does not dim which is fine just confusing as to why it does not dim. No matter which way I alter the wiring I can't get past this now, if I reverse anything I either get neither light working or occasionally only one light works.

[Dave Plowman (News)]

Sorry to reply to my own post, but it is rubbish. I really shouldn't post late at night when tired and listening to something interesting on the World Service. ;-)

A dimmer with a two way push on/off switch normally has the dimmer part inserted between the 'comm' terminal and the contact of the switch. So the connections are comm and *either* L1 or L2, if the two way function isn't needed. Doesn't matter which is feed and which return.