fixing a new ceiling rose.

[Jmck1947]

i am attempting to fit a new light fitting but can`t seem to get the circuit right.it either stays on all the time ie does not switch off or the other lights on the circuit work but this does not.the switch cable is not marked and it is just a case of trial and error.any way to test which is the mains cable and which is the switch cable

[Lurch]

Turn off the power, turn off the light switch, test between live and neutral for continuity at the cieling rose. Chances are 2 will test as short circuit and 1 will test as open circuit. Remember which is open circuit. Turn on the light switch and retest the cable that was open circuit, if it reads as short circuit this is your switch wire.

SJW A.C.S. Ltd.

[Gary]

Lurch scribbled :

If they test as short circuit then surely the fuse/mcb with blow/trip? I assume you meant to say you will see a resistance if any of the other switches on the same circuit are in the ON position?

[Lurch]

I was just over simplifying it a little. On a basic continuity tester you would see a short or open circuit. Obviously the OP has little electrical knowledge so I thought keeping it simple would be the best idea. If other switches on the other circuit are on there will be a very slight resistance but not really measurable on a cheapo meter. Are you sure you understand your question, I''m not too sure what you're on about.

[Gary]

Lurch scribbled :

I'm not too sure you do if you expect to see a short circuit on in incoming L+N and outgoing L+N in a ceiling rose. You will see a resistance from the other bulbs on the same circuit if their respective switch is switched on if you measure the resistance between L+N. Even a cheap meter will measure the resistance of an incandescent lamp.

[Lurch]

I can't be arsed.

SJW A.C.S. Ltd.

[Owain]

"Jmck1947" wrote | i am attempting to fit a new light fitting but can`t seem to get | the circuit right.it either stays on all the time ie does not | switch off or the other lights on the circuit work but this does | not.the switch cable is not marked and it is just a case of trial | and error any way to test which is the mains cable and which is | the switch cable

Assuming (which may be foolish) you have three twin-and-earths at the ceiling rose and that the wiring is conventional, do the following tests

*very carefully*. If you have a non-contact voltstick type tester so much the better.

The three reds (lives) should be commoned.

Separate the blacks.

Put the lightswitch on and test each black for voltage. One should be live - that will be the switched live and goes to one side of the lamp. Put a bit of red sleeving on it to indicate it's (switched) live.

The other two blacks should be neutrals and should be commoned to the other side of the lamp.

Owain

[Lurch]

Actually all the blacks could be live, the switch wire and the neutrals, if there are other lights switched on. Don't do this unless you really know what yoou're doing, obviously the OP doesn't so should ignore that idea. I suppose the moral of the story is mark the cabless *BEFORE* you dismantle thee old fitting.

[Ed Sirett]

To determine the black which is the switched live from the switch without the power on:

Seperate all the blacks. Use a simple continuity tester to find out which black wire becomes un/joined to the reds when the switch is operated (this works only if the switch is not a dimmer).

[Lurch]

I said that but some pillock started arguing with me over the correct amount of resistance that should be in the circuit.

[Gary]

You said there would be a short circuit between L+N, there will not be unless an incompetent fool has wired it up (you?) There in fact will be a low resistance, across the various lamps filaments. Tosspot.

-- Gary Mort [e-mail]

-- All e-mails scanned with Norton Antivirus 2004 Professional on sending using the latest virus definitions

[Lurch]

I know exactly how it works, If you read my reply I said I was simplifying it for the OP. If you use a continuity tester it will indicate a short circuit. If you use an accurate, reasonable quality meter on a low ohm setting it will probably measurre a few ohms. Not that it matters now, he's probably given up. Fuckwit I'm ignoring this thread now.

[Gary]

Lurch scribbled :

Go on, take your ball home softarse.

[Dave Plowman]

First, can the cables be seen, or just the wires? If the cables can, then it simplifies matters.

** Switch off at the mains.**

Connect your DVM set to ohms or continuity buzzer to one black and one red. Switch the light switch on and off. You should get a short with it on, no (or very high resistance) reading with it off. You're likely also to get a reading if you go across the feed, but this won't change with the switch, so make sure there is a definite change when you switch on and off.

Even with six wires as you might have in the middle of the lighting radial circuit, it only takes a few minutes to find the correct pair.

Then all the reds should be connected together, but not to the light fitting. The black return from the switch - now marked with some red tape or sleeving, please, goes to the line on the light fitting. All the other blacks should be connected together and to the neutral of the fitting.

If you don't have a DVM, get one from Maplin for well under a fiver.

[s]

if there is a light swithed on futher on in the circuit then 2 of the blacks will be live......s a qualified and experienced electrician i find it bset not to advise,especially to the techniclly incompetent..

[Lurch]

I know I said I was ignoring this thread once but... Would it not be more likely that a live to another light would be red and the neutral black? As a qualified and experienced electrician I find it best not to offer daft suggestions.

SJW A.C.S. Ltd.

[Gary]

Lurch scribbled :

So if thats the case why don't you know about split neutrals becoming live if there is a load on the circuit? Electrician my arse.