Rogue 12volts

Unfortunately I decided yesterday to add a simple switch on the attic lights which are fed from the Roof Terrace lights. A simple 1 hour task fitting the light, junction box and running a switching circuit between the light and switch. However! murphy’s Law kicked in.

Rather than go all the way downstairs to the fuse panel I ensured the terrace lights were off and cut in. With the 3 wires run for the attic light I put my digital multimeter over the terminals and to my horror found 12volts lingering in the Live circuit. Nothing else shows power.

So I took the switch for the terrace lights out, checked it, aired on the side of caution and fitted a new one. With the switch out I swung the meter over everything. No power anywhere.

New switch in, terrace lights on, Attic lights work beautifully. Terrace lights off. Swing the meter over the terminals. 12 volts on the positive.

Having searched the net most say it can be created by tube lights or low energy lights or because the switching circuit is long and close to the power line!

Any thoughts?

Basic points~

3 leads from consumer unit.

Switching circuit fitted to terrace lights

Cut into one terrace light conduit

Ran power and earth to attack light

Ran switching circuit to switch and light

I agree with Peter, also part of the problem is digital meters present no load unlike the old moving coil type so can pick up spurious voltage induced from elswhere but with no current flow. Putting a resistor in the circuit and then re testing or a cheap moving coil meter from a Brico as backup to your digital meter.

Martin -The simple answer sounds to me like the length of the run from the consumer unit to the terrace light switch, as you have mentioned. The DVM doesn't tell you how much current might flow - only that there is a 12 volt AC voltage present. Over a decent distance, that could simply be induced from adjacent cabling. If you have circuit breakers in your consumer unit, you could just try touching the live wire that shows 12v to the earth wire to see if enough current would flow to trip a circuit breaker or one of the safety RCDs. (RCD = Residual Current Device, formerly RCCB, Residual Current Circuit Breakers.) It wouldn't surprise me if nothing happened except that measuring across the live and neutral wires no longer showed 12v whilst the live was in contact with the earth wire. In this case the tiny induced current just runs off to earth.

If you were that interested in tracing the circuit causing the induced current, you could turn off other consumer unit circuits in turn to see which circuit might be causing it, but I'm not sure that I'd bother.

A more complex situation could exist if you (a) don't have a genuine earth; (b) have a three-phase supply to the house; or (c) your supply isn't well balanced across the phases, as happens from time to time in rural areas. Unfortunately you don't mention whether the 12V is present between live and earth and/or live and neutral. If it were anything to do with 3-phase, it should be possible to see a voltage between neutral and earth back at the consumer unit. (If you're not used to opening up consumer units, I would recommend leaving them well alone, for your own safety.)

Hopefully, this is just a case of induced current and you can happily ignore it.