Electrical screwdrivers

[D.M. Procida]

I noticed that the wire into a ceiling lamp lit up an electrical screwdriver when I checked its terminal, and it did the same at the switch end - even when I completely disconnected it from both ends.

I'm pretty sure that this was a single uninterrupted wire from switch to lamp, with nothing else connected to it, but I can't be 100% certain.

I assume that there was a very low induced current in the wire from some other nearby wire; whatever it was, I couldn't actually feel anything myself when I touched it.

Is this normal behaviour for an electrical screwdriver, to be so sensitive?

Daniele

[Max Demian]

Yup. I've got one of those table lamps which turn on when you touch the metalwork, and that lights a neon screwdriver dimly, showing that they put mains (or near mains) voltage through you. Real electricians dislike them as they respond to leakage currents.

[tabbypurr]

Yes it is. It's voltage produced by stray capacitance, not inductance.

NT

[Brian Gaff]

Yes. They are and can only be a rough guide. My old Neon one when I could see could light up if you just laid a wire alongside where a mains cable went. I imagine the newer kind use some kind of fet and an led to do the same job, but either way, I'd not want to trust on myself. The only use I ever found for them was as a confidence check that an appliance was absolutely dead and disconnected. Anything involving runs of wire will be very ambiguous. Brian

[newshound]

Assuming you mean a *neon* screwdriver, yes it is not uncommon, although with experience you can usually differentiate between a "true" and a "false" live, the latter lights the neon more faintly and sometimes there is some flickering.

A useful tool in sufficiently skilled hands. The "volt stick" is also very useful in diagnosis since it will sense live through insulation, but some also seem to show false positives, and others can be a bit low on sensitivity.

[Brian Reay]

Depends on the 'electric screwdriver'- they are not the most 'reliable' or safe items. As you surmise, they can detect 'stray' fields / currents and give misleading results.