False positive with volt stick; about 3 V AC on a switched-off cable

[Adam Funk]

Almost 20 years ago I installed an electric shower in our house, with a continuous run of 10 mm^2 T&E from the CU to the big switch in the airing cupboard, then the same cable to the shower. A couple of years ago when we got a combination boiler & a new mixer shower, I switched off the breaker, disconnected the cable from the "load" end of the switch, & removed the electric shower. The breaker has been off since then.

Last weekend I was tiling part of the inside of the airing cupboard & needed to move the switch out of the way. My volt stick said the cable was live, but the neon on the switch didn't light up when I switched it on. I went downstairs & checked that the breaker labelled "SHOWER" was still off; the volt stick didn't beep at that end of the cable. (There's no other 10 mm^2 T&E in the house, so it has to be the right one.) I switched the breaker on & the cable beeped; switched it off & it stopped.

Back at the top end, the cable still beeped but the neon wouldn't come on. I very carefully opened the box & used a voltmeter on the switch "supply" terminals: between 3 & 4 volts AC! Anyway, I carefully took the box off the wall, put it back together, then tucked it & the cable out of the way.

Where did the mysterious voltage come from, & why did the volt stick only beep at the downstream end of the same cable? (I think the cable does run physically parallel to a few other live ones in the floor of the landing, but it's not coiled anywhere.)

[Tim Watts]

Capacitive or inductive coupling to another cable it's running next to - completely normal :)

[newshound]

Agreed, but unfortunate that the volt stick was so sensitive!

[Tim Watts]

Well - it's basically doing what a Neon did, but with less direct contact.

There is a magnetic detector I have that relies on being able to load the circuit with an amp or two - not so handy in this case, but extremely good at pinpointing cables under plasterboard - normally PB lights up detectors for many inches either side of a live cable.

[Harry Bloomfield]

newshound a écrit :

Better a false positive than a false negative (safely off) indication!

[tabbypurr]

It's capacitive pickup. The voltmeter loaded the 120v down to 3-4v.

NT

[Brian Gaff]

You still get induction even if the runs are straight, and indeed you still also get electrostatic coupling as well. If you had shorted it all to earth it would probably not have caused any issues and not been apparently live either. Have you ever run your finger over the metal case of a double insulated device with no earth? You can feel a buzz and a small tickle. this is just electrostatic coupling to it from the transformer or psu inside.

I can get a similar feeling with my exposed metal bits of my blind white cane when I'm were walking under electricity wires on a pylon. Indeed, in some places you can strike a fluorescent tube under the wires, but you need to be quite close..:-)

Brian

[ARW]

Hold one of those 2D work lights (they are class II) in one hand and your volt sick in the other hand.

[Adam Funk]

Yes, of course! But it's a bit of a delay in getting the job done.