Lethality of Van De Graffs and HV low capacitance capacitors

Discussion in 'General Electronics Chat' started by Austin Clark, Aug 8, 2012.

  1. Austin Clark

    Thread Starter Member

    Dec 28, 2011
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    Basically, this is a "It's not the volts that kill you, it's the amps!" question.

    Why does a high voltage shock from a van de graff, or doorknob, or even a stun-gun etc; not kill you? Sure there aren't many charges to be moved, but the amount of current should still be dictated by ohms law. You'd get an extremely high current for an extremely short period of time. Why is such extreme current safe for short periods of time? What would happen if you tried to shock yourself like this many times a second? At what point would things start getting dangerous?
     
    Last edited: Aug 8, 2012
  2. Dyl71

    New Member

    Aug 5, 2012
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    When you post the video of you shocking yourself, we will let you know when it gets dangerous. :D

    No in all seriousness, volts and amps are not the same, it's the amps that kill you but the volts are needed to push the amps through your body.

    Think of a garden hose and a slow moving river, hose has high pressure and not much current (water) (amps). I can hit you with the water and you won't really budge, unless it's really cold.

    Take a river about 4 feet deep moving at 3 MPH and it will take you for a ride, half that depth will move a vehicle. That's the amps, and low pressure.

    Amount of amps to cause harm to the body depends on where it flows through you, it only takes one tenth of an amp to stop your heart, again it depends on placement.

    Stun guns are very high voltage but very low amps, they can cause burns to the skin but the flow is from one probe to another. Safe bet that a jolt to the side of the head could possibly cause more damage but not sure on that.

    Static shock is again very high volts but miniscule amp rating, if even measurable.
     
  3. Austin Clark

    Thread Starter Member

    Dec 28, 2011
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    I thought this in the past as well, but now I see it as an incorrect understanding of ohms law, really. You can't have 1M volts conducting 1uA through a resistive load less than 1 trillion ohms (If I did my math right).
    What would keep a 1MV 1nF capacitor from discharging everything it's got through my body of about 1M ohms ( 1 amp for a very short period of time, 1 billionth of a second I believe.)? Maybe it has to do with inductance, but then truly you wouldn't have time to get the full voltage across your body before the cap discharged completely.
     
  4. Dyl71

    New Member

    Aug 5, 2012
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    Im not the expert on this, but I believe it has to do with the time it discharges also. I super-quick "pop" of a capacitor really isn't enough to penetrate the skin and especially to pass through the heart.

    There are stories of large caps blowing the ends of your fingers off and the likes but again the current needs to flow through the heart. The current will take the path of least resistance, and boring a path to the center of your body is not likely.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    This reminds me of a very funny story about testing a Tazer someone emailed me some years ago. Don't know if it's true but, if you haven't read it, here it is for your amusement.
     
  6. Markd77

    Senior Member

    Sep 7, 2009
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    Maybe part of the story is the spark through the air. Perhaps this limits the current and by the time you touch metal the capacitance has been mostly discharged.
     
  7. Austin Clark

    Thread Starter Member

    Dec 28, 2011
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    So, does anyone know for sure or have any ideas or not? a fairly detailed explanation would be appreciated :)

    In the case of DC resistive loads, current is always proportional to voltage. Somewhere the model isn't perfect, and there's something we're not taking into account.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    I believe the unknown factor as regarding lethality is the duration of the current. I suspect that even if the instantaneous current from a stun gun is higher that what might be considered a lethal current value, its duration is too short to affect the heart muscle and nerves adversely.
     
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  9. #12

    Expert

    Nov 30, 2010
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    This is a very old conversation. Thousands of people have had this discussion. Anyway, here's an idea:

    A human is mostly water with conductive salts dissolved in it.
    A human is 3 dimensional.
    Try modeling the human body as layers of parallel, conductive fibers.
    The first layer has a certain resistance and the next layer is in parallel with it.
    How many layers will be involved in conducting the current?
    How many layers have to become involved to conduct current through the deepest layer, where the heart is?
    How much current will be conducted on that deepest level?

    Just a thought experiment.
     
  10. Austin Clark

    Thread Starter Member

    Dec 28, 2011
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    That's what I was thinking as well, but wasn't really sure if that made enough sense or not.

    Basically, the instantaneous current is extremely high, but the average current over even 1 second is extremely low. Now it begs the question, what's the AVERAGE current per second that's lethal? Is it more or less lethal if the current is pulsed or constant? All interesting questions... any guinea pigs?
     
  11. Dyl71

    New Member

    Aug 5, 2012
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    Really depends on the test mule....1% body fat or 20%?

    Current flowing from one hand to another? Current flowing from front of chest through center of back? Get up close and personal to the ticker and like I said before, one tenth of one amp will give you cold toes.

    There are too many variables but it seems you are looking for a somewhat attainable value of current that is lethal. I don't know.

    I know you can be hit by lightening and live and changing a light bulb carelessly can put your heart in defib. Noone there to assist and you're a goner.

    The current needs to get to the heart and disrupt the electrical impulses, I honestly don't think you can get an accurate low average value unless you know the placement of the probes and the consistency of the body in question, among other variables.

    No test mule here....;)
     
  12. strantor

    AAC Fanatic!

    Oct 3, 2010
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    My guess is that a "50KV tazer" is onl 50KV when shooting pretty blue arcs through the air. As soon as you give it a load (a warm body), per ohms law, the voltage must drop.

    I just pinched leads of my DMM between thumb and forefinger of both hands and read 270Kohms. Then I licked my fingers and tested again (to simulate sweat) and read 40K. so, If I consider myself a 40K resistor, and I put an infinite capacity 50KV supply between my fingers, my body would draw 1.25A. 1.25A * 50,000V = 62.5KW. Am I really to believe that a handheld device containing a 9V battery is capable of producing 84HP? No way! Not even for a mS! and these devices stay energized for a long time (relative to a mS) - I have seen in videos cops taze bad guys for 10 seconds or more. I don't care how many caps are in that thing, there's no way it can put out that much power. The only answer that pops out to me, is that voltage drops as soon as there's a load. I imagine that it's a constant current device, that is set to a nonlethal current value. So let's say (going with previous poster's 1/10th of an amp #) that it's set to a "safe" value of 50mA; it sits there throwing pretty blue arcs until it meets my 40Kohm body, at which point it instantaneously drops to 2,000V. So that's 2000V * 50mA = 100W. That still seems pretty high, so I imagine that the current limit is even lower. and BTW these probes, especially the type that shoot out from the gun-like device, penetrate the skin. The skin is the highest resistance part; the meat underneath is very low resistance, so the voltage probably drops to some very unimpressive number.
     
  13. strantor

    AAC Fanatic!

    Oct 3, 2010
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    This article probably has all the information that you want and more, but I chose to excerpt this part:
    so it's a square wave with .19% duty cycle. If I tazed you for 10 seconds, you would only receive a total of .019 seconds worth of shock, at 1200V.

    Also, all the information about why it doesn't kill you is further down in the link. basically, the pulse width of the wave is not long enough to cause fibrillation.
     
    Last edited: Aug 10, 2012
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