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Electrocution threshold

  1. Apr 22, 2008 #1
    Understanding the amount of 'power' in Watts to kill a human. It is said that around 20ma is enough to kill. So I would have thought it does not matter about the voltage as long as 20ma is passing through the heart. obviously it would require different voltages depending of the resistance between the 2 contacts through the body. I've heard that 12 can be enough to cause a metallic taste in the mouth and dizziness. This would perhaps require the body to be very wet and a large area of it to be touching the contacts-perhaps leaning over a car bonnet without a shirt (Earth) and tightly gripping a positive connection to the battery. Basing my question on the 240v ac shock and that no more than 20 ma does pass through the heart do we just use Ohms law to work out the power used? W=VI This would give 240 x 0.02= about 5 watts. If this is near the truth what really puzzles me is if a circuit was set up so that a poor human volunteer was to encounter a huge voltage with a current limiter of 10ma max. Sounds like he would be spared his life due to the low current. What about 10ma at 1,000,000 volts! this sounds like 1000 watts to me. This is what I don't get. Any ideas! Thanks, Al
     
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  3. Apr 22, 2008 #2

    stewartcs

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    If the current is being limited by some other device and the fault current experienced by the person is less than the lethal amount (which is typically 0.1 amps for the average human), then the person is not "feeling" the full voltage.

    Current is the primary variable that will kill you in a circuit so don't think of the power being output by the source since you have a current limiting device in the circuit, but rather the current flowing through the person (specifically across the heart).

    The power calculation for AC is slightly different due to the time varying nature of it. Mathematically it would be [tex] P = VI \cdot \cos \theta [/tex], for average power that is.

    Hope that helps.

    CS
     
  4. Apr 22, 2008 #3
    stewartcs,
    Yes I should have realised about the full voltage not being felt just as across a light bulb or heater due to different amounts of resistance. Short of connecting a volt meter in between would the resistance of the circuit(through the person being shocked) need to be known? Also I realise that AC is an RMS value due to frequency. If just for arguments sake it would be possible to set up some sort of circuit where the current is limited to way below the fatal level and an enormous voltage was applied surely a huge amount of power in Watts could be achieved. Have you any figures for the wattage through a human at say 50ma 240v ac and to see the difference dc?
     
  5. Apr 22, 2008 #4

    mgb_phys

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    The mechanism for a fatal shock with AC and DC is different.
    With AC you are trying to force the heart to beat at the AC frequency - the hearts own muscles provide the power to damage it not the electricity. If you put electrodes directly on the heart you need only a tiny current.

    With DC you are doing damage only by the heating power - so with a high enough voltage you would be able to ionise the body, even with a low current.
     
  6. Apr 22, 2008 #5
    Ok yes that is also true so not getting to my point. a relatively small amount of current is needed from an ac shock due to sending the heart out of sync so to speak. I think it is a large charge of dc they use to take it out of fibrillation.
    If we stick to dc then and consider death by heating the internal organs. Again a current limiter device fixed at 10ma and million volts output. Surely at this voltage the resistance of the human body is irrelevant and please lets just assume he is feeling a full 1000,000. If 1000 watts is going through the poor soul would it cause the same damage as 1000 v with a current limiter at 1amp. again not including any resistance perhaps he is soaking wet for example! Would it be the same kind of heating and perhaps pain? A bit morbid but just want to understand how it works.
     
  7. Apr 22, 2008 #6

    mgb_phys

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    Sounds correct ( leaving aside second order effects like fluids in the body and changing resistance) it's just the total energy input that matters.
    A core temperature increase of 5degc will kill you. Assuming you are water and weigh 80kg it would take 30mins to absorb enough heat from a 1000W source to kill you (assuming perfectly insulated - no sweating etc)

    I don't know if the nervous system is sufficently conducting to transfer most of the enrgy into more delicate areas such as the brain - which would have a quicker effect.
     
  8. Apr 22, 2008 #7

    LURCH

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    I don't think we can ignore resistance. Isn't it the resistance of the bvody that is causing the heating? But I'm not talking about external resistance (if the guy is soaking wet), I'm talking about the internal resistance the human body creates by being a nig bag of salt-water.
     
  9. Apr 22, 2008 #8
    Perhaps I am thinking about those taser guns and cattle prods. Trying to determine which hurts the most in a constant dc voltage. I do realise these devices use electronics to pulse or oscillate. Putting it a different way, assuming for some reason the shock is not fatal and went say just through one hand, if a 1000 watts went through your hand which would initially hurt more the higher voltage/lower current or higher current lower voltage. I am thinking it is the nerves that feel pain from higher voltages regardless of the current, though that is up to a point.
     
  10. Apr 22, 2008 #9
    after reading this i have a question... what are the limits to DC and AC to getting an electric shock... so when we touch the + and - of a battery we absorve current? since we have high impendance the current is approx 0. is this right?
     
  11. Apr 22, 2008 #10

    mgb_phys

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    Thats correct - the hand to hand resistance of your skin varies from a few*100K for dry skin to only a few K for sweaty skin. So a 12Volt car battery would only be capable of putting a milliAmp through you even under the worst conditions.

    See here for some more examples http://www.allaboutcircuits.com/vol_1/chpt_3/4.html
     
  12. Apr 22, 2008 #11
    'Possible heart fibrillation 500 MA Dc after 3 seconds' Also in this example in 'All about circuits' the writer says that with dry skin the resistance across his body is about 1 meg ohm. So lets say that to carry half an amp across this the equation would be V=IR. =0.5x1000,000. So that sounds like 500,000 V DC! I suppose if this would be correct in theory other factors would come into play such as the massive amount of voltage enabling it to penetrate the pours into the less resistive fluids of the body.
     
  13. Apr 22, 2008 #12

    mgb_phys

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    Yes the body isn't a simple resistor.
    Personally having a 1,000,000 source connected across me would lead to a pretty rapid increase in skin moisture!

    There a lot of other factors, AC frequency, muscles gripping onto a cable etc which can lead to huge variations in the danger of a shock. That's why industrial systems tend to be very cautious and treat anything over 30V as potentially dangerous.
     
  14. Apr 22, 2008 #13

    wolram

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    110v @ 5 amps can give a nasty burn and is quite painful.
     
  15. Apr 22, 2008 #14

    mgb_phys

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    The voice of experience?
     
  16. Apr 23, 2008 #15
    I have a faint memory (and could be wrong) that being wet on the skin increases the chances of surviving in lightning accidents. I guess that's due to the fact that most of the current will flow on the skin rather than through the vital organs.
     
  17. Apr 23, 2008 #16

    wolram

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    Yes, i have a (y) shaped scar on one hand as a reminder, and it is VERY difficult to
    do any thing (break free) even for 110v.
    Edit

    It is the circular scar not the (y) shaped one.
     
    Last edited: Apr 23, 2008
  18. Apr 23, 2008 #17

    Integral

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    Non lethal high voltage shocks are very common. Ever been Zapped by static after charging up on a carpet? Static voltages can easily exceed 10,000VDC. Since the current is very low, they are painful but non fatal. You could say that this is an example of a current limited high voltage shock.
     
  19. Apr 24, 2008 #18

    rbj

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    also, for people in the country with livestock, an electric fence has a high enough voltage to deal with variations in resistance of the connection to ground (and in the ground itself in wet or dry conditions), but if it was known to be lethal to people, they wouldn't let such be manufactured. there is a high resistance in the transformer secondary winding which, if you think about it as a Thevenin Equivalent, makes it into a (nearly) constant current source. and has been said before, it's the current that kills you (and "High Voltage" is often dangerous because it has all the current in the world backing it up).
     
  20. May 8, 2008 #19
    what i would like to know in understanding voltage and current is-if someone was subjected to 2 electric shocks say from hand to forearm one with a current limited to 10ma, the other unlimited. if the voltage was 1000 v ac and the same conditions, the resistance of the skin being so that 1000 watts can pass through the arm with the unlimited test, test to last 1 second and none fatal. would it be a similar pain as it has the same high voltage or does the pain solely come from the current passing through the body?
     
  21. May 8, 2008 #20

    mgb_phys

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    How much pain you feel is complicated. It depends not only on the voltage but on frequency and exact path the surface current takes.
     
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