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Light Switch Wiring Confusion One Comes on With Switch the Other Does not

Postby supersh » Sun Dec 20, 2015 12:30 pm

OK, so I have two light fittings. Currently one of them comes on when the switch is flipped, the other does not.

I opened up the light switch and what I found is very confusing:

There are 3 cables coming in to the back of the switch box: all two core + earth, The earths are all wired together in a block and have yellow/green covering.

One of the cables has its wires connected to the one way switch: red to the common, black to L1. This doesn't seem right to start with, but it does actually work the switch. But if I test between this red/black pair with a live tester, it lights up.

The other four cores are taped off. There is one red/black pair and one red/yellow pair.

So, how should this be wired up correctly? Seems that black and red should NOT be made to connect to each other, for a start, but I don't know what the yellow should connect to and if I need to change the switch to a different type or what!!
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Postby thedoctor » Tue Dec 22, 2015 9:24 am

Here is a link to several pages we have on wiring lights and switches ... ghting.htm

Please do not get confused with the black wire going to a switch, this is live and should be taped with either a red sleeve, brown sleeve or red or brown tape.

All is explained in our lights and switches section from where you shoule be able to diagnose your own problem. It's not safe for us, or anyone else, to advise you based on what you have written because it very much depends on how the cables are connected in teh first place and how well this has been done.
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Postby ericmark » Wed Dec 23, 2015 3:32 am

Back in the 1970's all houses were wired the same in the main with power going to the ceiling rose in a radial circuit then each ceiling rose had a cable going to the switch.

However there are a few problems and to be fair advantages with the old system.
Advantage less volt drop, (loop impedance) easy to fit emergency lights or ceiling fan.
Problem was the switch has no neutral, and when using extra low voltage transformers you did not have anywhere to terminate the wires so it needed a junction box to replace the ceiling rose.

As a result some went to the other system and used the switch back box instead of the ceiling rose as a junction box. Before this happened we knew the black at the switch was line even if colours showed it to be neutral so there was a tendency not to mark cables as they should have done. Today although still use twin and earth to the switch the electrician is more inclined to over sleeve as he should have always done.

The black (or today blue) has always been a problem where some times they are line and some times neutral, but today with the mix and match it is really important to mark and not remove the markings and to take photos before you start to give one some chance of working it out if you make an error.

The two way switching adds even more problems. School boy two way works great with a bread board and single cables, but it does not really work that well in the real world. So assuming marked Com, L1, L2, the two switches are wired with the switch pair going to L1 and L2 on first switch plus a three core to second switch wired like for like. So what ever colour goes to com on one switch also goes to com on second switch. The same is normally true for L1 and L2. However some times a cheat method is used, and the permanent line is not taken to second switch but borrowed from another switch. This can cause mains hum but also is only permitted when all the lamps come from the same fuse/MCB/RCD/RCBO. Otherwise you get what is called a borrowed neutral which can be very dangerous for anyone working on the system.

I was banned from this site for trying to help some one work out what wire was what without using a meter or other belling method. As a result all I now do is explain how it works. Sorry you have to work out how to test to find which wire is which.

Good luck Eric
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