How do test lights (mains tester) work?

Question

I was wondering how one-contact test lights work.

Obviously there is a small current running from an energized pole through the human body somewhere that is still large enough to produce a visible light.

German wikipedia talks about the current being in the range of µA.

What I find puzzling is where the current actually goes. It seems to work on whatever material I stand on (eg. tables, chairs) and whatever thick rubber shoes I wear.

I can't find a source that explains that.

Would http://electronics.stackexchange.com/ be a better home for this question? — Qmechanic, Feb 06 '14 at 17:36
@Qmechanic I'm not sure - I asked a different mains related question there and they told me their site is only about electronics design. — John, Feb 06 '14 at 17:43
http://www.eevblog.com/forum/beginners/how-do-electrician's-mains-test-lights-work/ may be, ahem, enlightening. — Carl Witthoft, Feb 06 '14 at 18:02

score 1 · Answer 1 · edited Oct 16 '14 at 04:16

AFAIK these gas-discharge lamp things can work only with AC of a sufficiently high voltage. An industrial 50 Hz or 60 Hz frequency is enough to generate a noticeable current because a human body acts as a capacitor. This means that the AC circuit with the ground closes,even though no current goes through the body. The voltage does matter because a low voltage (such as 3 V) just can’t ignite a discharge even in a very small lamp.

score 0 · Answer 2 · answered Oct 16 '14 at 04:33

I have used a similar two-leads test light in a class-room demo/lab to show the electric current produced by waving plate sized pieces of statically charged styrofoam close to metal pie plates connected by wires. These lamps light up with very small currents.

For a single lead tester, the tester's body acts as a one "plate" of a capacitor on the end of the AC wire, with the other "plate" of the capacitor being effectively the rest of the universe. The small back and forth current allowed through by the high resistance of the test lamp is enough to light the bulb, but not enough to kill the tester (or even be noticable by the tester), unless there is a fault in the device as mentioned by the wikipedia link.

Please note that it is the voltage that is necessary to be high enough that the energy levels in the atoms in the media making up the test lamp can be excited and release photons . Static electricity can give high voltages but very little current. — anna v, Oct 16 '14 at 04:52

How do test lights (mains tester) work?

2 Answers2