Human Electrocution: How is the resistance not ridiculously high?
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Posted by =?ISO-8859-1?Q?Tom=E1s_=D3_h=C on April 3, 2008, 12:05 pm
Please log in for more thread options I've been doing electronics for three years now but I don't understand how a person can be electrocuted by touching one part of the circuit in a mains supply. If I hold one lead of an ohmmeter in my left hand, and the other in my right hand, it registers the resistance to be approximately 2 megaohms, which is ridiculously high. Now if I hold one lead in my hand, and dig the other into the grass, it doesn't even register -- I may as well be holding the leads apart in thin air. Current = Voltage divided by Resistance Current = 230 volts divided by 2 megaohms = 115 microamperes 115 microamperes is nowhere near enough to electrocute someone. So lets say I stick a metal rod into the socket on the wall. The current has to flow thru my hand, down to my foot, thru my cotton sock, thru my shoe, thru the wooden floorboards, thru the concrete, thru the clay down to the metal rod we call ground. Now excuse me, but is that not a RIDICULOUS amount of resistance, up in the gigohms somewhere? It may sound like I'm denying that people get electrocuted -- I'm not, I realise that people do get electrocuted. But I can't for the life of me understand how enough current can flow, given the massive resistances that are involved. Can anyone enlighten me? | |||||||||||||||||||||||||||||||
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Posted by Stephen J. Rush on April 3, 2008, 12:54 pm
Please log in for more thread options The resistance of the human body varies widely, depending on the nature of the contact. Dry skin does measure around some megohmes, but sweaty skin conducts much better, and if the contacts abrade or penetrate the skin, the internal resistance can be as low as 300 ohms, hand to foot. The effect of the current also varies with the path of the current through the body, with paths that include the heart being the most dangerous. For a brief shock, timing has some effect. A short zap at just the wrong point in the heart cycle can cause the heart to "stumble" into a fatal arhytmia. | |||||||||||||||||||||||||||||||
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Posted by =?ISO-8859-1?Q?Tom=E1s_=D3_h=C on April 3, 2008, 1:59 pm
Please log in for more thread options Stephen J. Rush:
> The resistance of the human body varies widely, depending on the nature
> of the contact. Dry skin does measure around some megohmes, but sweaty > skin conducts much better, and if the contacts abrade or penetrate the > skin, the internal resistance can be as low as 300 ohms, hand to foot. A lecturer in my college posed this argument to me before. He said that if you stabbed the ohm meter into your foot and also into your hand, then you'd measure a hell of a lot lower than 2 megohms. People are often very nonchalant about this idea of "getting past the skin", but it's not meager feat at all. I'm taking about simply touching the positive terminal of an electrical socket, not about slicing your hand open, then slicing your foot open, then touching the socket. And, even if the resistance from hand to foot was extremely low, even in the region of something like 4 ohms, then that still doesn't explain how the current flows from my foot, thru my sock, thru my shoe. I bet if I put one terminal of an ohmmeter inside my shoe, and the other on the sole of my shoe, that the resistance will be too high to measure. (If I had an ohmmeter I'd do it right now). I'd love to do an experiment with one of those corpses that gets donated to scientific research. I had considered using a dead mouse, like the ones you get from the petshop for feeding snakes, but they're far too small. A dead pig might do the trick. | |||||||||||||||||||||||||||||||
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Posted by John O'Flaherty on April 3, 2008, 3:49 pm
Please log in for more thread options On Thu, 3 Apr 2008 10:59:48 -0700 (PDT), Tomás Ó hÉilidhe
>Stephen J. Rush:
> >> The resistance of the human body varies widely, depending on the nature
>> of the contact. Dry skin does measure around some megohmes, but sweaty >> skin conducts much better, and if the contacts abrade or penetrate the >> skin, the internal resistance can be as low as 300 ohms, hand to foot. >
>A lecturer in my college posed this argument to me before. He said >that if you stabbed the ohm meter into your foot and also into your >hand, then you'd measure a hell of a lot lower than 2 megohms. > >People are often very nonchalant about this idea of "getting past the >skin", but it's not meager feat at all. > >I'm taking about simply touching the positive terminal of an >electrical socket, not about slicing your hand open, then slicing your >foot open, then touching the socket. > >And, even if the resistance from hand to foot was extremely low, even >in the region of something like 4 ohms, then that still doesn't >explain how the current flows from my foot, thru my sock, thru my >shoe. I bet if I put one terminal of an ohmmeter inside my shoe, and >the other on the sole of my shoe, that the resistance will be too high >to measure. (If I had an ohmmeter I'd do it right now). > >I'd love to do an experiment with one of those corpses that gets >donated to scientific research. I had considered using a dead mouse, >like the ones you get from the petshop for feeding snakes, but they're >far too small. A dead pig might do the trick. If you touch a live power wire, not touching anything else, standing in dry shoes, you won't get shocked. If your shoes and socks are wet, or even damp, and you are on a concrete floor, you could carry enough current to make your heart fibrillate. Consider the great surface area of feet compared to the contact area of an ohmmeter probe. If the feet are 12 inches by 2.5 inches, for 50 square inches total, that is 50/(39.37^2) = .032 m^2, and if the ohmmeter probe is 1.5 cm by 2 mm, or 3e-5 m^2, the feet will have 1000 times less resistance. To experiment, you don't even need a dead pig - use the ohmmeter while standing in damp socks on a metal plate, with the meter connected to another metal plate for the hand contact. -- John | |||||||||||||||||||||||||||||||
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Posted by John Popelish on April 3, 2008, 1:25 pm
Please log in for more thread options Tom=E1s =D3 h=C9ilidhe wrote:
> I've been doing electronics for three years now but I don't understand
> how a person can be electrocuted by touching one part of the circuit > in a mains supply. >=20 > If I hold one lead of an ohmmeter in my left hand, and the other in my > right hand, it registers the resistance to be approximately 2 > megaohms, which is ridiculously high. > Now if I hold one lead in my hand, and dig the other into the grass, > it doesn't even register -- I may as well be holding the leads apart > in thin air. >=20 > Current =3D Voltage divided by Resistance >=20 > Current =3D 230 volts divided by 2 megaohms =3D 115 microamperes >=20 > 115 microamperes is nowhere near enough to electrocute someone. >=20 > So lets say I stick a metal rod into the socket on the wall. The > current has to flow thru my hand, down to my foot, thru my cotton > sock, thru my shoe, thru the wooden floorboards, thru the concrete, > thru the clay down to the metal rod we call ground. Now excuse me, but > is that not a RIDICULOUS amount of resistance, up in the gigohms > somewhere? >=20 > It may sound like I'm denying that people get electrocuted -- I'm not, > I realise that people do get electrocuted. But I can't for the life of > me understand how enough current can flow, given the massive > resistances that are involved. >=20 > Can anyone enlighten me? Do the calculation again but with the other hand on the=20 water tap or the grounded case of an appliance. --=20 Regards, John Popelish | |||||||||||||||||||||||||||||||
> how a person can be electrocuted by touching one part of the circuit in
> a mains supply.
>
> If I hold one lead of an ohmmeter in my left hand, and the other in my
> right hand, it registers the resistance to be approximately 2 megaohms,
> which is ridiculously high. Now if I hold one lead in my hand, and dig
> the other into the grass, it doesn't even register -- I may as well be
> holding the leads apart in thin air.