3 core lighting puzzle

[devans]

Hi guys

Just joined today - great forum! I've always had an interest in electrics and am considering becoming an electrician one day. Anyway... I was wondering if anyone could shed some light some wiring.

When the lighting switches were recently slackened off to allow for decorating, I was confused by the wiring. There is a 3 core and earth terminating in a 1 gang 2 way switch, but this switch only controls one light fitting. Why are there 3 (presumably live) cables entering a switch that only controls one light? (NB there are no other switches that control the light).

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• devans
• 6 May 2008
• 1

The 3 core goes up into the attic where it terminates in a junction box. From here is a feed for the ceiling rose which is simply one 2 core and earth cable which terminates in the ceiling rose. It appears that this junction box serves other lights on the circuit (and there are more of these junction boxes around the house serving the other lights) and I believe that the other switches in the house are wired similarly.

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• devans
• 6 May 2008
• 1

I am definitely not going to change anything, but was just wondering if anyone could explain how this set up works, its been puzzling me for ages!

Thanks

 

[THRIPSTER]

May have been part of an old two way switching system - how long have you lived in this house - are you aware of any alterations that made the need for a possible, alternative switch redundant? Or, perhaps there is another swicth that you have not yet discovered - what about in the attic? Or maybe one of the cores was just terminated at the switch but doesn't go anywhere else? Or maybe there are only two wires really but your camera is introducing a chromatic aberration that makes it look as though there are three wires? Or, perhaps you have imagined the whole thing?

 

[devans]

Thanks for the reply.

Definately 3 wires in there and all connected (NB lack off earth sleeving on earth to back box )! My parents have lived in the house for 20 years, and there have been no alterations to the wiring (apart from a couple of changes to new switch/sockets etc when redecorating). Also no switches in the attic or no other 2 way circuits apart from on the landing and stairs.

This switch is in a bedroom and was originally a dimmer which was changed a few years back. However no changes were made apart from a simple changeover of the switch. Other switches are wired this way in the house too so I think the dimmer was no different. I guess one wire could terminate amongst the others in that JB but all seems a bit over-complicated and pointless to me

According to my elderly neighbour, a sparky used to live in the house and he rewired it in the 80s when he lived here. maybe only he knows!

 

[devans]

actually was one other modification - the lighter grey cable at the front of the JB is 1mm2 two core and earth which goes to a wall light (with built in pull switch) on the landing!

 

[Spark123]

Maybe the previous owner planned to make it 2 way one day so put the wire in for it. You will probably find one of the cores is not used and terminated in the JB. Nothing wrong using 3 core and earth by the way, however it is a little bit naughty not using earth sleeving!!

 

[holmslaw]

..

 

[Steve]

You will find the same thing in my house. The reason I did it is that when the burglar alarm goes off a relay operates and livens up the "other" wire and all the lights in the house come and cannot be switched off.

Excellent!!!! Why aren't alarms in commercial premises wired this way? That way they'll catch crims on CCTV and get decent images! It'd be so simple too. Most larger stores have contactors controlling lighting anyway. But i suppose its getting co-ordination between sparks and alarm engineers.

 

[breezer]

some big well known companies do.

But it doesnt work too well on discharge lighting

but for the average joe its a waste of time

 

[ban-all-sheds]

There is a 3 core and earth terminating in a 1 gang 2 way switch, but this switch only controls one light fitting. Why are there 3 (presumably live) cables entering a switch that only controls one light?

Lots of 2-way switching circuits only control one light.

(NB there are no other switches that control the light).

But it's that that makes it odd. It's not a bedroom light, by any chance? Maybe there used to be a 2-way pull switch over the bed - have a rummage in the loft.

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• devans
• 6 May 2008
• 1

Where, talking of which, you need a lot more insulation laying...

actually was one other modification - the lighter grey cable at the front of the JB is 1mm2 two core and earth which goes to a wall light (with built in pull switch) on the landing!

Are the other cables black?

 

[dingodeano]

You will find the same thing in my house. The reason I did it is that when the burglar alarm goes off a relay operates and livens up the "other" wire and all the lights in the house come and cannot be switched off.

Circuit diagram please?

 

[Space cat]

Check the junction box. Is either the yellow or the blue wire sitting in a terminal all by itself? If it is then, as others have suggested, it's been put in with a view to adding two-way switching - or a double switch - later or it's been part of a circuit long since removed. Either way you can safely leave it alone.

On the other hand, if both of these wires go off somewhere else you should follow them and find out what they do.

 

[devans]

After the above posts, my guess is that the 3 core is for extending if required. This switch is in a bedroom but there has never been any pull cord switches in as long as my parents have lived in the house (20 years) and the house was only wired a few years before this. Btw the cables are all grey (no remenants left of the original 30s wiring). The house is a 30s semi and while the roof is still in good repair, it doesn't have an underlay under the slates and the attic is a bit dusty (and the cables a bit mucky). The white stuff on the loft insulation is the lime torching from the underside of the slates which has started to come away over the last year or so. And yes, it does need a bit more insulation up there!

Come to think of it, i believe there was a similar wiring set up in the lounge downstairs - when we moved in there was a main pendant and cabinet lights in the room which came off a double gang switch. The cabinet lights were removed shortly after and the switch replaced with a 1 gang switch (which now has the yellow disconnected in a choc block in the back box). When doing some unrelated work I found a junction box under the upstairs landing which had a 1mm2 twin and earth disconnected so I assume this is the old feed to the cabinet lights which were disconnected!

Thanks for your help, I am beginning to understand it at last!

PS clever idea with the burglar alarm system!