Installing a new light fitting and can't identify the cables

[cluelesslass]

I'm trying to fix a new light fitting. I thought it would be straight forward (don't you always) as it is a central rose with one simple on off switch. I was surprised to find 3 cables all with red black and earth wires. From reading some of the other postings I've gleaned (I think) that two of these cables are coming/going to lighting in other rooms and one is going to the switch but unlike the other postings I have read the switch does not have two red wires. I can't work out how to identify the switch cable. Have I got the right idea or not? Can anyone help?
The setup looks like this diagram, which is from another posting but I'm not able to identify the cables.

 

[breezer]

but the diagram does not ave 2 reds for the switch either

get a multimeter, read the instructions then use it, its the best way

 

[cluelesslass]

Thanks for the prompt reply. I'm aware the diagram does not have two reds as the diagram is identical to the situation in my room but without the helpful annotation. The two reds comment came from reading lots of other postings to see if I could work it out for myself before bothering the experts -but failed!

 

[breezer]

as i said get a multi meter, for what you want you do not need anythng expensive, maplin sell them

 

[cluelesslass]

Unearthed one that I bought (from Maplins) a few months ago. Just reading upon how to use it. Thanks again.

+++++++++++++++++

I think I've got it! - but can I just check before I blow all the downstairs lights. I'm getting a fluctuating resistance reading on one of the cables when the light switch is in the "on" position and when I test the other two there is a constant of 1 which is also the reading when I am not testing anything (it is an ultra cheap multimeter, the instructions with it are minimal and designed for someone that knows what there doing). So I'm presuming this is the cable coming from the switch and should then wire the fitting up in a similar way to these instructions?

To find the switch return cable you will need to use a multimeter & test for continuity between cable ends at light fitting & switch, I would imagine it will be a black (should have red sleeving on it).

Join all reds (phase/live) together in a chockblock. the switch return when you find it goes to the 4 brown wires of light fitting. The other two black (neutral)wires go to the 4 blue wires of light fitting. All earth wires joined together (all should have green/yellow sleeving).

I suspect what you have done is to put the reds to the light fitting browns so making the light permanently on, the two neutrals have been joined to the switch live, now you've pushed the switch & put switch live to neutral tripping mcb or blowing fuse.

Dont forget to isolate the circuit before doing any of the above !

_______________________
moderator

please note 10a

 

[breezer]

1 what? it is important.

also set your meter to the lowest range it has, its probably metering you

 

[andy]

also set your meter to the lowest range it has, its probably metering you

done that the other day... with a megger...

 

[cluelesslass]

Since I was running out of daylight and being pestered for food by two small children, I risked it and wired it as above It all seems to be working O.K. but I think thats more by luck than judgement.
Many thanks for all your help!

 

[jtaunton]

Just for next time.....

1 is what your multimeter will say when the reading is above the range it is set for (or of course, if it is genuinely reading 1 of whatever you're measuring) - so if you are measuring resistance then a 1 indicates a very high resistance, or open circuit. If this then changes to something more like zero when you flick the switch, then that's the switch cable.

Before you do this though, you should identify any permanent live cables using the voltage range (separate out the three cables first) - Do not attempt any resistance measurments on these ones.