2 gang 2 way switch with only one common terminal

[Itisonlyme]

Hi all, new here and after some advice on a twin dimmer switch purchased from a large warehouse type diy store.

It is marked as a two gang two way dimmer, so I was expecting to find six terminals; one common and two switched live on each of the two sides.

Yet there is only one common.

Now, I am using it for what I think is a pretty standard use of replacing the two gang two way switch that controls; gang 1- one way control of lounge lights gang 2- two way control of landing light upstairs. I have wired it as prescribed by the instructions and it works fine. I have also added earths to the metal faceplate which were not present on the original metal switches!

So here is the question.....live is running down a red core of three core cable from upstairs and connected to the single common terminal, also connected to this terminal is the live side of the lounge lights downstairs. So surely if I pull the fuse to isolate upstairs (though I always switch off all power) lights the switch will still be live upstairs courtesy of the live being supplied from the downstairs supply to the lounge?

I did originally connect all the cables as they had been connected in the original switch which had two commons but this resulted in the light not being switchable from the other location/staying on when the should have been off e.t.c.


Is this arrangement allowed?

Grateful for any advice.

 

[telectrix]

what i'd call a shared L. this arrangement used to be popular some years ago as it negated the need for 3 core strappers. nowadays it is not done because 1. you turn off the MCB for the upstairs lights, but still have a live feed. 2. you can't split the 2 circuits to separate RCDs.

 

[Itisonlyme]

what i'd call a shared L. this arrangement used to be popular some years ago as it negated the need for 3 core strappers. nowadays it is not done because 1. you turn off the MCB for the upstairs lights, but still have a live feed. 2. you can't split the 2 circuits to separate RCDs.

Is there a way this can be wired to get round the two issues you mention?

Thinking that even if I had two commons some part of the circuit is still live if I pull just one fuse anyway unless up and down lighting circuits are on one fuse. E.g even with two commons there is still a live feed into the switch from the downstairs circuit, though at least this route would mean upstairs had no supply. Pulling the downstairs fuse would still leave the upstairs supply to the downstairs switch live.

 

[telectrix]

only way would be to replace the strappers with 3 core/E.

 

[Itisonlyme]

There is a three core and earth at the downstairs switch, with a twin and earth and 3 core in the upstairs switch.

 

[telectrix]

so reconfigure the wiring. so that the L for the landing light comes from the upstairs. edit: unless the 3 core in the downstairs switch feed L into the switch. without seeing it, it's a lot of guesswork.

 

[Lucien Nunes]

But surely, if the neutrals at the two lights are from two different circuits, the only possibility that avoids a borrowed neutral would be to send the neutral from one of the two related lights all the way to the other light via the second core of the switch drop, and remove the loop through from its rose and put it in a separate JB.

Whichever direction you send it, one cable will need one more core than at the moment. Is it not easier to get a 2-gang dimmer with two separate modules?

 

[Itisonlyme]

But surely, if the neutrals at the two lights are from two different circuits, the only possibility that avoids a borrowed neutral would be to send the neutral from one of the two related lights all the way to the other light via the second core of the switch drop, and remove the loop through from its rose and put it in a separate JB.

Whichever direction you send it, one cable will need one more core than at the moment. Is it not easier to get a 2-gang dimmer with two separate modules?

lucien - there is only one light, with a straight two way switch upstairs and a two gang two way dimmer downstairs which is the problem.

Seems like the best option is to swap it for an alternative as I am not at all happy with it , as I've just replaced all of the switches with the same style I'll probably have to go with a twin switch instead of dimmers which is also not great.

Don't know why they are selling something that in a reasonable use of the product is not safe when wired as per the supplied wiring diagram.

P.s. If I replace with a two gang switch with separated common terminal is this situation avoided where one terminal is live if either the upstairs or downstairs circuit fuses are remove? I think it isn't but at least if I pull the upstairs fuse the supply to the upstairs portion is isolated.

 

[Itisonlyme]

so reconfigure the wiring. so that the L for the landing light comes from the upstairs. edit: unless the 3 core in the downstairs switch feed L into the switch. without seeing it, it's a lot of guesswork.

Live for the upstairs light is coming to the downstairs switch from upstairs.

Will take photos of both switches tomorrow.

thanks for your help.

 

[Lucien Nunes]

there is only one light

But you said:

gang 1- one way control of lounge lights gang 2- two way control of landing light

By two lights I meant lounge and landing. My point was that the dimmer with a shared common requires that both lights are on the same circuit, as they would be with two lounge lights individually dimmable etc. Yours appear not to be due to their locations, so you would have to remove one light from its existing circuit and connect it to the other.

That's not as hard as it sounds, as the line and CPC can already be extended all the way from one to the other via the switch drops. E.g. for the landing light to be run from the lounge circuit, one core in the 3+E to the upstairs switch and the 2+E from there to the light can be used as the neutral instead of the PL. But as Tel points out, in the lounge, there probably isn't a core available to get the N from the light to the dimmer, so it comes unstuck.

If it were me I'd open the dimmer and see if I could separate the two, or fit alternative dimmer modules to the matching face plate. But I probably shouldn't recommend modifying products on account of voiding safety approvals. YMMV.

 

[Itisonlyme]

Thanks for the advice, I am thinking this is way past my level of competence and needs a professional. As I also want the CU changed and juice run out to the summerhouse there seems little point me mucking about with this as it is clearly not a straight swap out of the original fitting I had hoped for.

 

[Itisonlyme]

Update on this is that the dimmer has gone faulty anyway and I replaced with what would have been a like for like replacement two gang two way switch and rewired back to the original circuit. Simples!

Spec on the dimmers said a minimum load of 60 watt which may be the cause of the fault as I was loading below that.

Many thanks to all who offered advice.