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Help identifying switched live cable

Postby random_dave_1 » Mon Jun 16, 2008 2:21 pm

Hi, I've got a bit of a problem with my upstairs lights. Me and my housemate changed 3 light fittings from ones with fans to standard lights. Unfortunately, whilst the 2 I did are connected up ok (kept the wiring as before and the 3 red ones separate from the connections to the new light), my housemate decided that there must be a red cable going to the light so took it upon himself to create something a bit different!

Thus far it seems things can be rectified, however, the switched live is not marked so I do not know which it is. I am left with 3 triple core cables coming in, obviously the earth and permanent lives are fine but how would I be able to work out which is the switched live out of the 3 black wires? I have connected it up having guessed which might be the switched live but whilst that light works fine the others are now very dim and do not work at all when the offending light is on. Any help would be greatly appreciated. Thanks.
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Simply Build It

Postby ericmark » Mon Jun 16, 2008 5:30 pm

Regular question:- How does one wire a ceiling rose? The switch wires should be both red or at least have a red sleeve but in practice this is rarely the case. This is how to work out which black/Blue wire is which when all three cables are the same colours. These are:- The feed, The Switch, The next Lamp. Easy way is meter or buzzer across the cables with power all off and flick light switch until switch wire is found but often the DIY man has no meter or buzzer so next method.
Do not use energy saving bulbs as we need to be able to see bulbs working on half power use normal bulbs only. Do not use this method if you have dimmers or discharge lamps in any of the house lights.
1. With power off connect all cables except for neutrals and switch cable i.e. the black or blue are left disconnected. Put one into spare block connecter and other two in the two outer connectors of the ceiling rose i.e. one to each wire feeding lamp. Now turn on power and try if it works jump to 4. If not turn off power.
2. Swap either of two blacks (or blues) connected with one in spare block connecter and then turn on power again. If it works then jump to 4. If not turn off power.
3. Swap the wire in spare block connector with other black (or blue) in ceiling rose. It should now work one switched on again. If not there is another fault not at rose and it is beyond this simple check.
4. Turn off power again and mark cable in spare block connector with black or blue type or some other way to find it again. Then put it in spare hole with other Black (or blue) then turn on power.
5. Test all lights if all work OK then all is done on the other hand you may find odd things happening like with light you have been working on being switched on other lights don’t work and with it off the other lights work as glow only and light you were working on will also start to glow. If this is the case switch off power and swap the two unmarked black (or blue) wires. Now all should work.
So what are we doing? Well we can’t connect any of the three blacks (or blue) together as one is live when switched on so to start with we find out which wire connects to the next rose in circuit by time 1, 2, and 3 have been completed we know this is wire is spare block connector. At this point, we don’t know which of the other two blacks (or blue) is neutral and which is the switch wire. But if we connect the next lamp in series to switch instead of neutral it will do no harm but the next lamp can only get a return through the lamp in the light we have been working on so either the one we were working on will come on dim or the next in line will come on dim which is all to do with what size bulbs are fitted. But we now know exactly which two wires are the wrong way around. So finally we have in the eight holes in rose (earths normally under clamp screw) Feed to lamp, Feed from switch, break, Three live wires at all times all red or brown (one feed, one out to switch and one to next lamp), break, Neutral (in or out), Neutral (out or in), Neutral to lamp.

Postby stevenc1603 » Tue Jun 17, 2008 2:12 pm

You can either guess again twice more or use a tester to find the wire. Either a voltmeter to locate the live/neutral pair, or a continuity tester to locate the switch cable.

Also, if its an upstairs light, if you go into the loft you might be able to follow the cables back to see which one goes to the switch.
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