Replacing an upstairs ceiling light

[StuartB]

There are only 2 wires coming through the ceiling from the loft, one red and one black.
A mains tester screwdriver touching the red wire lit up when the light switch was on, as you would expect.
However when the switch was off, the tester light still glowed but not as bright.
Does this mean that the red wire is live all the time? Why would it be?
For now I have abandoned the job.
Any advice?

 

[breezer]

becsue tester screwdrivers are rubbish thats why

 

[davy_owen_88]

A mains tester screwdriver touching the red wire lit up when the light switch was on, as you would expect.
However when the switch was off, the tester light still glowed but not as bright.
Does this mean that the red wire is live all the time?

No, it just means that neon screwdrivers are crap!

Any advice?

Throw the neon screwdriver in the bin and buy a multimeter.

Anyway, I don't know why you abandoned the job. Just isolate the supply at the consumer unit and change the fitting.

EDIT: Bah! Too slow.

 

[Steve]

Stuart, the red wire that made the neon go dimmer when the switch was turn off - this is (probably) your switch return, it is (probably) a switched live.

By the way, if you're connecting a compact fluorescent light to this wire, expect it to flicker every now and then when switched off. Due to the neon glowing, there is a charge building up on the wire and this will disipate itself across the lamp. Nothing harmful! And its only a very faint flicker.

 

[Space cat]

Your neon driver lit up with the switch off because of something in the circuit that's not on your mental diagram, namely capacitive coupling between the cores of the cable going to the switch. A ball park figure for this capacitance is around 100pF per meter so a 10m length is 1nF. That will pass enough current to make a neon glow dimly. If it's big enough it can even make low energy lamps flicker.

But this raises another question; is that light on two way switches? The reason I ask is because a single switch would typically be wired with twin and earth. In this case the earth wire acts as a screen which intercepts the current - unless it's not connected to earth! If you do have two way switches you can get capacitive coupling even with all earths intact.

 

[StuartB]

Space cat

Thanks for the technical info. - I think I understand the gist.

I'm doing this job for my brother; his house is probably late 60s - the lighting circuit is not earthed! Is this VERY bad?? Should I advise him to have it rewired?

I think the light is part of a 2-way setup - there is a redundant pull-cord
lazy switch. Unfortunately access to the wiring in the loft is very difficult.

Incidentally the reason for the new light fitting is that the previous one failed (it was a low-voltage one but only about one year old); would the constant low current have damaged the transformer?

StuartB

 

[Steve]

No, the transformer would have dissipated the charge across its windings. Transformers can fail as often as a lightbulb.

Not having an earth is not immediately dangerous, however you must NEVER fit metal switches or light fittings to the circuit.

 

[StuartB]

Thanks to everyone who replied.
I now know what to do and have also acquired a multimeter.