Small voltage at light fitting with switch disconnected and at least one bulb fitted

JohnB47

This is a very odd story about a mystery voltage, initially involving a dimmer switch but now involving no switch connected at all.

I have a central light fitting in the bedroom. It has sockets for 5 BC type bulbs, currently fitted with three working led filament bulbs (each around 5 watt I think) and two duff halogens.

The fitting has been there many years, with no problems apart from the halogens gradually burning out. The leds have been there for a couple of years.

The other night I saw that the three leds were glowing very dimly, so I checked that the dimmer switch had been turned off completely. Today I removed the dimmer switch completely and investigated using my digital meter.

There are just two mains wires in the switch socket, a red insulated one and a black one (well there's another set of wires in there that are just taped up - presumably that's an old bit of wiring).

The situation I have now is that with the dimmer switch completely disconnected and with all bulbs removed, I'm getting around zero volts AC across the pins of one of the sockets. If I fit one led bulb the voltage measured is 3.7 volts AC. If I add a second led bulb, it measures around 2.6 volts and if I add a third, it's 2.0 volts.

2.0 volts is the voltage I measured right at the start of testing, with the dimmer fitted, switched off and with all bulbs fitted except for one (where I was testing). At that point I was sure the dimmer was the problem.

I've tried fitting each one of the leds, on it's own, in each of the sockets but the reading is always the same - 3.7 volts.

Any idea what's going on here? What other tests can I do? Is this some sort of induction thing?

Perhaps I'll find a working halogen or old filament bulb and try measuring while I have one of those fitted.

I should add that I'm taking all necessary precautions (mains off except when measuring - rubber gloves, test screw driver and digital meter).

Edit: I've just been internet searching about this. It seems that it's a well known inductance phenomenon. Not sure where I go from here. Probably buy another two led bulbs and see what happens when I fit them. Perhaps the voltage will drop to the point where they aren't glowing - or maybe I imagined it!

grumpy_codger

I'm no expert, but I'd expect the opposite - zero voltage with a lamp fitted and some low induced voltage when all bulbs removed. The latter - because the disconnected switched wire runs somewhere along a live (line) wire. E.g. in the cable connecting the switch to the fitting one wire is (permanent) 'line' that remains such even when the switch is removed completely. The second wire is 'switched line' and has to be marked accordingly in the switch backbox (with red/brown sleeve).

JohnB47

Thanks. I decided that maybe the dimmer wasn't faulty, so I re installed it. Now it won't fully turn the lights off!

When I press the button that should do that, there's 140 volts AC at one socket. So I've just replaced it with a simple one gang on/off light switch for now and I'll monitor the lights for a few days.

Perhaps the phantom voltage has always been there and the dimmer was gradually going faulty?

I've also read now that my multimeter is probably not the most appropriate tool to measure AC voltage accurately. It's probably OK for comparison purposes though.

Ectophile

Unless they are specifically marketed as dimmable LEDs, they are unlikely to work properly with a dimmer.  The same goes for the dimmer and LED compatibility.

If you have several wires running in parallel for a long distance, then it's possible for electricity to "leak" from one wire to another by capacitance alone.  This is most common on stairwells with two or more switches. The leakage can give rise to phantom voltages if you use a multimeter to check.

An LED will light dimly on even a tiny current, so it's not unknown for LEDs to glow dimly or flicker on and off.

JohnB47

Ectophile said:

Unless they are specifically marketed as dimmable LEDs, they are unlikely to work properly with a dimmer.  The same goes for the dimmer and LED compatibility.

If you have several wires running in parallel for a long distance, then it's possible for electricity to "leak" from one wire to another by capacitance alone.  This is most common on stairwells with two or more switches. The leakage can give rise to phantom voltages if you use a multimeter to check.

An LED will light dimly on even a tiny current, so it's not unknown for LEDs to glow dimly or flicker on and off.

Thanks. The LED's weren't glowing last night, so it looks like the dimmer was just faulty and my testing, with an inappropriate meter, simply showed up something that was always there - referred to online as a phantom voltage.