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Capacitance between human body and ground

  1. May 4, 2014 #1
    Why does a voltage tester (that looks like a screwdriver) turn on even when you are "heavily" insulated from the ground e.g. sitting on a wooden chair.
    Is it the result of capacitance between the human body and the ground? (i.e. the body and the ground act as conductors, whereas the insulated gap between them behaves as a dielectric does in a capacitor.)
    If capacitance can cause the tester to glow, how then do insulated/EH (electrical hazard) safety shoes work? I don't have those shoes yet so didnt get the chance to use a tester while wearing them. But I was wondering why capacitance doesnt cause current to flow through the human body when wearing those shoes?
     
  2. jcsd
  3. May 4, 2014 #2

    mfb

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    The capacitance is "human body against everything else" - floor, walls, ceiling, whatever is in the room (and even in free space, a body has a capacitance).
    The capacitance is tiny, and therefore the currents are tiny as well. Safety shoes protect you from larger currents.
     
  4. May 4, 2014 #3
    But the tiny current was sufficient to make the tester glow quite brightly - about as bright as you see when you're standing on ground with no shoes!
     
  5. May 4, 2014 #4

    berkeman

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    It's a bit hard to help you answer this question without a lot more information about your test situation. Can you post what tester you were using, and what exactly were the test situations you tried (Where did it not light up, where did it light up, etc.)?
     
  6. May 4, 2014 #5

    jim hardy

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    Such testers that i've seen include a high ohm resistor that limits the current to less than you can feel. It also protects the neon lamp.
     
  7. May 4, 2014 #6

    Averagesupernova

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    I think the one the OP means does not even use metal to metal contact. I used to have one. Lost track of it for a while and now it does not work. Soooooo, it was rather pointless to start with. If I can't trust it to light up then why would I trust a new one?
     
  8. May 4, 2014 #7

    davenn

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    yup likewise Jim and sounds like what the OP is describing
    Haven't owned one for years but they are effective :)
    With the high resistance in the screwdriver tester body and the high resistance of the person to ground via shoes and sox there's still enough current to light the neon globe

    cheers
    Dave
     
  9. May 4, 2014 #8

    mfb

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    You can get some visible light with 1mW. At 100 V, that corresponds to 10µA. Below 1mA, it is hard to feel a current, so we have a safety factor of two orders of magnitude.

    To get 10µA over 10ms (50 Hz, similar value for 60 Hz) with 100V, you just need 100nC or a capacitance of ~1nC. Give or take a factor of two as I did not do this calculation properly, this is a small capacitance value.
    To get the same as DC current, you need a corresponding resistor of 10MOhm. Regular shoes, the floor, and various other objects have a higher resistance, so DC current is probably negligible.
     
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