How does circuit complete when using live wire tester?

[ki_2004]

TL;DR Summary

How does circuit complete when using live wire tester even if I am wearing insulation shoes or worst case not at all connected to the ground.

I raise this question because at a fundamental level I still do not understand how a Capacitor works and how a circuit completes thru capacitor. The live electric tester screwdriver uses stray body capacitance to lit a neon when one end comes in contact with live wire and another with a human body (generally the fingertip)
However what is hard to understand is how circuit can complete from ground to the tester when you are wearing rubber shoes or even not connected to the ground. The tester consists of a resistor and a neon or some such LED bulb in series.

I actually jumped in air while my finger was still touching the tester and it still glowed.

Secondly, if so then why doesn’t this stray body capacitance come into effect if I would be hanging on to a power line? The circuit should still complete thru stray capacitance of human body and ground which is say 50 feet below. Everywhere it’s said that if I am hanging on to only a live power line in air, I shall be safe, because there's no path to ground and that's how the birds sitting on a single power line remain safe.
Thanks.

 

[AZFIREBALL]

Maybe this will help. (Maybe not.)

A capacitor is nothing more than two conductive plates separated by a dielectric (just like its graphic symbol.) When not connected to a power source the two plates have no charge; they are neutral, not negative and not positive. That is to say they have an equal number of positive and negative charges in each plate (zero voltage). When a power source ( DC battery) is connected with the positive lead attached to one plate and the negative lead connected to the other plate. A current will flow through the circuit until the capacitor becomes fully charged. Being ‘charged’ means one plate has been charged with positive charge while the other plate is charged with negative charge. When fully charged, to the potential (voltage) level of the power source, the current stops flowing (zero amps, not considering leakage.) This is called ‘charging the capacitor’.

If the plates of a charged capacitor are connected together, since they are opposite in charge, current will flow between them until the plates reach a neutral, zero voltage, state. This is called ‘discharging the capacitor’. If there is little resistance in the circuit, discharging will occur very rapidly with a large current flow between the plates as they become neutralized.

If the plates of a capacitor are connected to an AC power source, the capacitor will be charged and discharged at a rate equal to the cycle rate of the AC source. The charge (voltage) on the plates will swop from positive to negative, following the voltage changes in the AC source.

Now to the tester.
The neon bulb is lit by the very small current (milliamps) that flows between the test lead and the human body, to ground, when the test lead is held on a live wire. The current that flows is below the body’s detection level. The reason the light stays lit while you jump up in the air is the human body acts as a capacitor and allows the small current, needed to illuminate the test light, to continue flowing for some time, while the body is being discharged, even though it is not grounded. A bird can land on the hot wire, without harm, because there is no ground connection to complete the current path. You can do the same, if you jump onto the wire without making ground contact while on the wire. That is why you must jump free and clear from your car to the ground, if your car is in contact with a live wire after an accident.

 

[tech99]

The neon bulb glows with a very small current. When you hang from a live wire, for instance, the current flowing into your body by virtue of your self capacitance is very small and you will not feel it, although it would light the lamp. Do not do this, by the way. The bulb has an inbuilt safety resistor and is rated only for domestic supplies.

 

[Guineafowl]

Have a look at this thread: https://www.physicsforums.com/threads/trying-to-understand-electric-shocks.883550/

Especially the video posted by @jim hardy

You’ll see how the sensation from hanging from a live wire really depends on the wire...

 

[jim hardy]

I raise this question because at a fundamental level I still do not understand how a Capacitor works and how a circuit completes thru capacitor.

I think you have to first come to believe in the electric field.
We psychologically resist concepts that go against our intuition or upset our idea of how the universe works, i think it's a self defense mechanism for our ego ... It's frequently held me back in my science.

So to make it intuitive - start with the familiar -
Surely you've felt the tug of static electricity when pulling a styrofoam cup from a stack of them or a sweater out of a dry cleaning bag. The latter will pull at the hair on your arm.
Here's a snip from a video showing the electric field between charged human hairs pushing them away from one other.

youtube number is jZEFuCxD7BE

Electric charges of like polarity repel one another just as like magnetic poles repel.
There's potential energy in the field between them.

When charging a capacitor, you create an electric field between its plates that contains whatever energy you gave the capacitor by charging it.
The plates of that charged capacitor attract one another because you added charge to one plate and removed it from the other, leaving them oppositely charged, and opposite charges attract.
If you pull the plates of a charged capacitor apart you do work against that attractive force,
W = F X D,
and that work shows up as increased voltage between the plates.

That's the electric field. Believe in it.

Now, as explained in that link @Guineafowl pointed to,
when you connect yourself to a voltage source through that neon bulb,
a teeny current flows to charge whatever picofarads of capacitance exist between you and local Earth .
Because the current is so feeble you don't feel it, but it is sufficient to make the gas in the tube emit a few photons.

That guy on the high line helicopter was connecting to a far higher voltage so the charging current would likely be enough for him to feel. That's why they wrap him in a conductive suit and gloves, so the charging current flows through the suit not through him. He's safely snuggled inside it.
The capacitance of the helicopter itself also has to be charged and that's why the arcs in that video are so large .

Any help ?

Believe in the electric field. Observe it on the next dry winter day when you encounter static electricity.

old jim

 

[sophiecentaur]

That Mains Tester only works for AC. The constant flow of a tiny current in and out will keep the neon bulb alight. Dabbing onto a 200V DC source will only give a single flash which you could easily miss. Reliable testing for dangerous DC Voltage levels is best carried out using a gullible and trusting friend. (Not really - that is a joke!)

 

[Guineafowl]

Those mains testers are not that great. They give false positives with ghost voltages, and can give false negatives as well.

If I posted a thread on PF suggesting I poke a resistor and neon in series into a live socket what would the mods say?

Their best use is to find a switch is live, turn the breaker off, and then confirm the previously live contact is now dead.

 

[jim hardy]

Yes, indeed.
DC "tingles your fingertips" and feels not much different at all from AC.
It takes discerning digits to tell the difference. I advise against such tomfoolery.

Those neon testers do give a clue to the attentive eye, though, if they're resistive coupled and can find a DC path through your shoes to 'ground'..
DC makes only one electrode of the neon lamp glow.
AC makes both glow, but alternately, and that looks continuous to the eye

Image is from an interesting article here
http://www.giangrandi.ch/electronics/neon/neon.shtml
old jim