Replacing ceiling light and dimmer switch (novice!)

Twincam1986 · 19 Feb 2011

I've just had my living room totally replastered with a new ceiling and have now come to fit a new ceiling light and replace the old dimmer switch for this light and just wanted to check a couple of things out, as what was supposed to be totally straightforward seems not to be.

I've got the lighting cable hanging through the ceiling (usual grey cable containing live neutral and earth), but on the wiring diagram for the ceiling light, it says as well as connecting these I should also be connecting a switch cable - I don't have this cable. The light fitted before only had 3 connectors, but this one has 4 - do I just leave the switch cable connector empty? (the ceiling light only works off one light switch if this makes a difference).

Also, I'm changing the old plastic dimmer switch for a chrome one. I thought this would be dead straightforward - just look what went where in the old one and replace like for like (which I can see how to do), but the new switch has an earth connector on the back. On the diagram supplied with it, it shows an earth cable connected to this as well as the one from the supply attached to the backplate. Am I supposed to fit an earth wire from the switch to the backplate, as one wasn't supplied, so do I need to supply it?

ELZ4742 · 19 Feb 2011

sounds like you have a switch wire - neutral & earth at the fitting... just wire accordingly.

the earth needs to be connected to the face plate, theres no reg. to say you need to flylead to the backbox (unless it was a surface one of course)

MrTinker · 19 Feb 2011

Twincam1986 said:
do I just leave the switch cable connector empty?

If there is only one cable yes.

Twincam1986 said:
Am I supposed to fit an earth wire from the switch to the backplate, as one wasn't supplied, so do I need to supply it?

Yes!

Twincam1986 · 20 Feb 2011

So, just so I'm clear - leave switch wire connector on the light fitting empty, and connect live neutral earth as normal (this is all that was connected to the old light fitting)

and detatch the earth from the backplate to the faceplate on the dimmer, or leave it connected to the backplate and fit an additional earth from the faceplate to the backplate?

(sorry if this is very basic, but I only attempted this as I thought I would be replacing exactly like for like and they seem slightly different)

MrTinker · 20 Feb 2011

The face plate of the switch must be earthed, if the back box is sunk into the wall, the earth wire from from face plate to back box, can be omitted.
So yes you've got it right, moving the earth from the backplate to the faceplate.

Twincam1986 · 26 Feb 2011

I've come to do this today and it isn't working.

At the dimmer switch, there are 3 wires - 2 red and an earth. I looked where the reds were connected on the old dimmer (one to L1 and the other to a wavey symbol) and connected them to the same points on the new switch and the earth to the earth connection on the faceplate.

The ceiling light itself has connections for live, neutral, earth and switch (labelled L, N, symbol for earth and S). I've left the S terminal blank as I don't have this switch wire at the ceiling and wired the rest accordingly. However upon trying the dimmer switch, nothing happens. I've re-checked all connections and they are secure.

Any idea where I'm going wrong? - I presume it's something to do with this switch terminal?

sherlockholmes · 26 Feb 2011

Twincam1986 said:
I've come to do this today and it isn't working.

At the dimmer switch, there are 3 wires - 2 red and an earth. I looked where the reds were connected on the old dimmer (one to L1 and the other to a wavey symbol) and connected them to the same points on the new switch and the earth to the earth connection on the faceplate.

The ceiling light itself has connections for live, neutral, earth and switch (labelled L, N, symbol for earth and S). I've left the S terminal blank as I don't have this switch wire at the ceiling and wired the rest accordingly. However upon trying the dimmer switch, nothing happens. I've re-checked all connections and they are secure.

Any idea where I'm going wrong? - I presume it's something to do with this switch terminal?

I have deduced the one in L should be in S. There must be a junction box somewhere doing the job of the L terminal, the wire coming out of the ceiling is a switched live and a neutral so the red goes in the switched live terminal.

The trick my dear boy, is that when you have eliminated the impossible, whatever remains however improbable will either operate the light or burn down your house.

Twincam1986 · 26 Feb 2011

Ok, I've swapped L to S and still nothing.

I've just tested the red wires at the dimmer switch to double check them and live is connected to L1 (presume this is correct)?

I tried to test the cables at the ceiling by turning the power back on and clicking the dimmer switch on and turning it on full - no cable was showing as live. Does this mean the new dimmer switch could be faulty?

sherlockholmes · 26 Feb 2011

Twincam1986 said:
Ok, I've swapped L to S and still nothing.

I've just tested the red wires at the dimmer switch to double check them and live is connected to L1 (presume this is correct)?

I tried to test the cables at the ceiling by turning the power back on and clicking the dimmer switch on and turning it on full - no cable was showing as live. Does this mean the new dimmer switch could be faulty?

I feel I should point out at this stage i'm not an electrician and probably know about as much as you. Just stopped by for advice myself.

So with the proviso that you shouldn't follow my advice, the switch should have L1, L2 and C. C is where the live goes, it connects to L1 or L2 depending on the position of the switch. If you're connected to L1 and L2, never the twain shall meet and the positions may well be different from the last switch you had. Did you check the labels rather than positions?

Twincam1986 · 26 Feb 2011

Yes, the terminals are in different positions on new and old, so I connected them from the symbols. In the old switch live went to L1 and the other wire to C, which is what I've repeated, but now nothing works

sherlockholmes · 26 Feb 2011

If you have the live in C and no power reaching L1 or L2 on the far side of the switch, you have a broken switch. Confirm by connecting the two switch wires together and seeing if the light comes on. Don't do it live and don't follow any of my advice in any case, for the aforementioned reasons.

Twincam1986 · 26 Feb 2011

So live should be in C? Because it's in L1 at the minute because this is how the old one was.

sherlockholmes · 26 Feb 2011

Twincam1986 said:
So live should be in C? Because it's in L1 at the minute because this is how the old one was.

The switch connects one t'other and doesn't care which way round they are. Stick them both in the same terminal and go see if the red wire on the fitting goes live.

And don't follow any of my advice.

Twincam1986 · 26 Feb 2011

Bridging the 2 wires at the switch doesn't make the ceiling live. This is doing my head in now. Looks like I'm spending another £50 for an electrician to sort it in 2 mins.

sherlockholmes · 26 Feb 2011

Yep, you need an electrician. I'm guessing there is an errant wire that has been misplaced above the ceiling and not pulled through again by the plasterer. That would make what you're seeing two ends of a switch line, without the feed I was previously guessing was away in a Junction box somewhere. Elementary my dear Twincam. If you're going to test my new theory by poking around for it in the ceiling, bear in mind it must be the live one since you have no power on any visible wires.

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Replacing ceiling light and dimmer switch (novice!)

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