How do electrician's mains test lights work?

[dog80]

I mean the type where the user touches with their thumb the end of the screwdriver
http://en.wikipedia.org/wiki/Test_light#One-contact_neon_test_lights

I do not understand the part about body capacitance. Does that mean that some tiny current passes through the user to the ground?

Do these test lights work on galvanically isolated mains power?

[Alana]

Those neon light probles do have metal contact to be touched while probing to close the circut. And yes - some current passes trough human body but its so small it is harmless.

Think of circut like this:

mains L --- [probe -- light -- 100kohm resistor -- metal contact] --- human --- earth

and replace "human" with captitor of very small value

Neon light probes do light up way too often for them to be reliable probes of "there is no voltage here" - basically they light up for any leakage current, for example between "live" and "unconnected" wire. So i think answer is yes, but this is not reliable.

[IanB]

A tiny current flows through the screwdriver to the user. The current doesn't necessarily have to continue through the user and out the other side to ground.

Consider, the hot or live wire on a 120 V mains supply is alternating between +170 V and -170 V, 60 times a second. If you connect a human to that wire through a neon tester then the human is trying to change in voltage between +170 V and -170 V in step with the wire. For this to happen the human must be charged up and discharged repeatedly, and this causes current to flow through the screwdriver back and forth, back and forth, lighting up the neon lamp.

This will work even if the human is wearing rubber shoes and standing on a rubber mat.

[Mechatrommer]

This will work even if the human is wearing rubber shoes and standing on a rubber mat.

right! i just made a quick dumb experiment. took my tester connect it to mains and i jump, and stand on one foot on the chair, the light almost out or barely visible. so the connection to ground (earth) plays more important role.

[Spawn]

Here in the Netherlands electricians are not allowed to use and trust this, most electricians from last 10 years don’t have this in their toolbox, guys like me from old days still have it but don’t use it anymore either, we mostly grab one of those “voltsticks” but even that is not allowed to make sure if there is electricity, according NEN3140 (Dutch norm) we need to use dual pole tester (Duspol), I know this is going to sound stupid but before we test we need to make sure dual pole tester is working on another source and then test the installation we need to work on and after that we need to test dual pole tester again on another source, and guess how many electricians do that.

[madires]

mains L --- [probe -- light -- 100kohm resistor -- metal contact] --- human --- earth

For 220/240V AC the resistor is about 1Mohm.

and replace "human" with captitor of very small value

Nope, the human is simply another resistor.

Take the lovely Ohms law: I = V / R = 220V / 1Mohms = 0.22mA

Actually the current is even less since the human adds some kOhms too. And 200µA don't hurt :-)

[IanB]

and guess how many electricians do that

They probably lick a finger and quickly touch the wire to see if it bites? 

[madires]

Here in the Netherlands electricians are not allowed to use and trust this, most electricians from last 10 years don’t have this in their toolbox, guys like me from old days still have it but don’t use it anymore either, we mostly grab one of those “voltsticks” but even that is not allowed to make sure if there is electricity, according NEN3140 (Dutch norm) we need to use dual pole tester (Duspol), I know this is going to sound stupid but before we test we need to make sure dual pole tester is working on another source and then test the installation we need to work on and after that we need to test dual pole tester again on another source, and guess how many electricians do that.

That's a really good rule! With a high resistance towards the earth the neon lamp won't light. Another problem is a coupled current from a parallel wire. That may be large enough to light the lamp. For that reason the professional Duspol has a switch to add a load. And the checks before and after make sure that the tester works. It might be damaged before or during testing.

[SeanB]

I knew an electrician with a voltage tester called Clifford. Calibrated yell when touching a live conductor with bare back of hand.....