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
On 28 Jan 2004 17:27:17 GMT, firstname.lastname@example.org (Jmck1947) wrote:
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.
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
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.
Please remove #NOSPAM# if replying via email
| 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 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.
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
I suppose the moral of the story is mark the cabless *BEFORE* you
dismantle thee old fitting.
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).
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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.
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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.
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..
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
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.
*He who laughs last, thinks slowest.
Dave Plowman email@example.com London SW 12
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