Which is live and which is not ?

[alexbilly2012]

Hello , Please can I have some help .
I took down the ceiling rose with the intension of changing it to a new light. like a fool I forgot to write down which wires come out of which holes in the rose . So now I have coming out of the ceiling , 3 x RED , 3 X BLACK and 3 X GREEN/YELLOW . I know by reading various posts that the 3 x REDS can go together in a block and the 3. x GREEN/YELLOW will go in the light fitting ( Earth ).
The 3 x BLACK are all black and not one of them has a clue to tell me which is live as none are sleeved with a bit of red or taped red . I have cut of power and done a test with a Screwdriver that lights up and the result was - 1 x Black did not make the S/driver light but both the other 2 did , I know that two of these blacks need to go together ( N )and one will be " live " but how can I tell ...... Is it safe to experiment with the random blacks and hope I pick correctly or will I do some damage , My fuses in the leccy cupboard are trip ones if that helps , sorry this is long but needed to explain all , and yes , I know I am a fool !!!! but would appreciate some advise , many thanks

 

[jj4091]

The switch wire is the one that is only live when the light is switched on. It's not 100% guaranteed with a neon screwdriver, but if you connect the 3 reds & disconnect the 3 blacks then switch the light on & off while testing each black you may find the one you are looking for. The best way to carry out testing properly is with a multimeter.

 

[ban-all-sheds]

Buy a multimeter.

 

[plugwash]

Firstly what exactly is this "screwdriver that lights up"? My first thought would be a neon screwdriver but that wouldn't light up if the power was off so that makes me think the screwdriver in question was probablly some sort of continuity tester.

It is possible to figure this out without any test gear. Connect the cables to the live and neutral of the light one at a time (with the other cables safely insulated). The one that makes the light stay on permanently is the incoming supply cable.

Once you have identified the incoming supply cable move it's live to the loop terminal and connect one of the other wires up as if it was the switch cable. Leave the final wire disconnected. If the light turns on and off with the switch you have the switch cable. If it doesn't then you have the cable to the next light. Either way you have now identified all teh cables.

 

[alexbilly2012]

Ok , I now have a multimeter and was told to turn off electric at the main box and then test a red and black toegther one at a time over the 3 sets , I have ........ with light switch on , - this I did but it made the multimeter buzz everytime ..... that is what I didnt want because I am still no wiser as to what the live wire is out of the blacks .......Am I doing it right ?

 

[EFLImpudence]

But only one pair will stop buzzing when you turn the switch off.

This will, obviously, be the switch cable.
The black of which should be sleeved red and connected to the Live of the lamp.

The other blacks should all be connected to the Neutral of the lamp.

ALL the reds should be connected together BUT to nothing else.

 

[alexbilly2012]

Thanks for all your imput but just to recap .... Do I need to use the multimeter on red/black over the three sets with the mains electric off but light switch on , when multimeter buzzes ( which it did on all of them in a previous try ) do I then turn the light switch off , and this will stop the buzzing ..yes ? , and then the set that does stop buzzing is the black one in this my live wire ?

All three reds then can go in a block together and all yellow/greens in the " E " together with the two black ones together in " N "

 

[jj4091]

Correct