Lighting Circuit and Dimmer Switching Confusion


Postby StephenDolphin » Mon Aug 29, 2016 6:40 pm

Hello all,

Firstly I've read quite a few of others' posts here about lighting, dimmer switches and the general chaos - most of them have been incredibly helpful and have certainly given me useful knowledge about what I'm faced with. I have one outstanding issue that I can't seem to get over.

I have a double-dimmer (although the fact it's a dimmer is only of limited relevance) which controls a ceiling light and two wall lights (togther). I have replaced the ceiling light with a standard rose and flex today but didn't get the previous wiring noted down before I went ahead and took the old one out. This has led me to a little trial and error.

Following the other posts on here I've taken the switch off the wall and inspected it. This shows me a red wire going into both Cs on the switch (so this is the mains in) and two different coloured switched lives coming out of the two L1s - which makes sense.

The colours coming out of L1s are blue and yellow, the C is red.

In the ceiling it's clear which cable comes from the switch, not least because it contains the blue and yellow cables as I've identified in the switch, but also because the sheath is a slightly darker grey.

Therefore it's quite easy to see the 'ring in', 'ring out' and 'switch' flexes and which is which. Good start!

I know therefore that all the 'lives' go together (including the 'live' that is the C cable in the switch), all the 'negatives' go together all the earths go together, and to the earth terminal. What's left then is what to do with the two switched lives: yellow and blue.

On the wall lights there is no indication as to which is which, because there's only a pair of cables.

And so I did a little trial and error...

[*] Yellow in rose switched live and blue disconnected makes centre rose off/on work (with LHS dimmer), and nothing on wall (RHS dimmer)
[*] Blue in switched live and yellow disconnected makes cenre rose do nothing, wall lights off/on
[*] Putting either blue or yellow into live, and the other into switched (so one of each) in the rose behaves the same way as if the 'live' cable was disconnected (above) and the other cable switches as before too.

And so, contrarty to what I'd expect looking at the switch, which shows the LHS dimmer having the blue L1 output and the RHS having the yellow L1 output, having the yellow connected to the switched live controlls the LHS / ceiling rose and having the blue connected to the switched live controls the RHS / wall.

And so I've wired BOTH yellow and blue into the switched live in the ceiling rose. Now, when I turn on the LHS the rose comes on (weirdly, and this is the only reason I mentioned dimmers before, it doesn't dim, just on/off); and when I turn the RHS on the wall works. Super. What's gone wrong is that if I turn the LHS on AND the RHS on the wall doesn't work, only when the LHS is off & RHS is on do I get wall lights.

I'm sure that I could have both on before I attempted this little job.

Gallery of some pictures here: http://imgur.com/a/u67gd
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Postby kbrownie » Tue Aug 30, 2016 10:58 am

You need to do some testing of the circuit to confirm what is doing what. You are assuming cables are directly attached to one another end to end because the colour cores are the same. There could quite easily be junction boxes in the ceiling void that change that.
And what is wrapped up in the duct tape?
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Postby StephenDolphin » Tue Aug 30, 2016 11:09 am

kbrownie wrote:And what is wrapped up in the duct tape?


An earth cable (unsheathed) - the wall fixings don't have an earth connector.
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Postby StephenDolphin » Tue Aug 30, 2016 11:13 am

kbrownie wrote:You need to do some testing of the circuit to confirm what is doing what. You are assuming cables are directly attached to one another end to end because the colour cores are the same. There could quite easily be junction boxes in the ceiling void that change that.


Yes, I suppose that's true... But the lights worked before so I have a limited number of places that cables can go - I think it's really safe to assume the 'last light' and 'next light' cables are correct, because I've checked another ceiling rose and they're identical.

So my only 'unsure' cable is the one that appears to go to/from the switch (granted possibly via a junction box) - but either way, there's only four cables, one of which is earth, and there's only three places that they can go: neutral, live, or switched live. So three cables to three places is all I can wire to return it to the previous set up - I was just wondering whether someone had a better idea than the trial and error I've already done. I did also connect yellow to the mains neutral supply and it flipped the RCD when I turned the switch, so I'm assuming that yellow is indeed switched live - this fits in with my previous thoughts too.
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Postby kbrownie » Tue Aug 30, 2016 5:43 pm

Without a means of testing, it can only ever be trial and error!
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Postby StephenDolphin » Tue Aug 30, 2016 9:08 pm

kbrownie wrote:Without a means of testing, it can only ever be trial and error!


Okay. perhaps a different tack then - can you think of a way that I can trial and error to more success than the way I've highlighted?

Do you know what would make a double switch power one light, and the other, but not both together?
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Postby kbrownie » Wed Aug 31, 2016 9:13 pm

The problem is, that you are unsure that they both worked correctly before your intervention.
It would be much easier testing out the switch lines, to find out what the core colours doing?
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Postby kbrownie » Wed Aug 31, 2016 9:36 pm

Looking at your opening post, your are saying if the blue core is terminated at the switch live of the ceiling rose, that the wall light will then switch on/off but the ceiling light will not! Are you sure? As that does not make any sense.
Am I reading that correctly, are there any hidden cables in the ceiling void?
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Postby Mcleodcow » Fri Sep 09, 2016 5:56 pm

Hi all,

Really need some help. I'm needing to fit a dimmer switch in the bedroom.

After taking the old socket off, there is a red wire and a black wire. I have tried fitting according to the instructions but the breaker keeps blowing. The light is on but won't dim and when I try to switch the light of, it blows.

What am I doing wrong? I'm really confused
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Postby kbrownie » Sun Sep 11, 2016 10:29 am

You should have really started a new topic of your own!
What type of dimmer is it? A link to instructions would be nice.
Do you take a note of how the original switch was wired?
Do you know if the lights you are trying to dim are capable of being dimmed? not all are!
If the circuit is tripping out, what device is tripping? Fuse, MCB, RCBO or RCD/RCCB?
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