Human Body Capacitance and Resistance
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From: Robert Baer on 30 May 2010 16:55 Paul Keinanen wrote: > On 30 May 2010 04:49:35 -0700, Winfield Hill > <Winfield_member(a)newsguy.com> wrote: > >> As for the effect of high moisture and humidity, as said, these can >> affect things, e.g., lowering resistances to the low levels we see >> in Fisher's Severe Model, but it also means the maximum electrostatic >> voltage developed on the 400pF capacitor is likely to be much lower. >> I suspect Fisher would prefer to take the dry-air high voltage with >> the moist-skin low resistances for his Severe case. > > Those resistances seem to be quite low for the resistance of the skin. > If the resistance would be that low, you would get severe burns each > time you touched the 230 V mains. > > Those resistances in the order of a few hundred ohms would make sense, > if we assume that the spark penetrates the skin and the current > propagates in fluids under the skin. > > RF burns from touching an antenna connected to a 100 W transmitter can > be quite painful, put it can be hard to detect, where the RF current > penetrated the skin. > > With the component values given, it would form a lossy resonant > circuit with a resonant frequency in the upper HF range, thus > expecting a few cycles at that frequency, until the oscillation dies > down. > Those resistances may seem to be low but i think are in the correct order of magnitude for a "worst case" based on "average". One person died of excess current thru the chest by "merely" (for him) getting in series with a six volt car battery.
From: Robert Baer on 30 May 2010 16:58 legg wrote: > On 30 May 2010 04:49:35 -0700, Winfield Hill > <Winfield_member(a)newsguy.com> wrote: > >> A repost of a comment I made elsewhere, for discussion here. >> >> The subject is ESD Human Body model values. I'm enamored by >> a 1989 symposium paper by Richard Fisher, of Sandia Nat'l Labs, >> where he created a "Severe Human ESD Body Model." His model >> had worst-case numbers meant for use in electrostatic-discharge >> circuit-protection analysis, etc. >> >> Fisher's Severe Body Model consists of two parts, the body and an >> arm with hand reaching out to zap something. The body part has >> 400pF of capacitance in series with 250 ohms and 0.5uH. Then the >> arm and hand part bridges the body terminals with 10pF, and finally >> we have another 110 ohms and 0.1uH in series to complete the model >> and connect it to the poor real-world victim. The body capacitance >> is higher than you may see elsewhere first because the body is >> sitting down, and second because it's a worst-case body. We won't >> go further into what that means. :-) >> >> You charge the 400pF capacitor to a voltage of your choosing. >> 20kV is a nice high number. During discharge we get a fast spike >> of current from the 10pF, with sub-ns risetime to dangerous levels, >> with up to 5A peak current, and lasting up to 5ns into the "load." >> This is followed by a slower discharge of the 400pF capacitance, >> lasting up to 200ns. >> >> This would be followed by, ahem, a postmortem. >> >> As for the effect of high moisture and humidity, as said, these can >> affect things, e.g., lowering resistances to the low levels we see >> in Fisher's Severe Model, but it also means the maximum electrostatic >> voltage developed on the 400pF capacitor is likely to be much lower. >> I suspect Fisher would prefer to take the dry-air high voltage with >> the moist-skin low resistances for his Severe case. > > So0me references, if you don't already have them: > > http://www.aecouncil.com/Papers/aec1.pdf > http://www.globalsmtindia.in/documents/ESD_DAMAGE_MODELS_AND_CHEMICAL_KINETICS-PART_I.pdf > http://www.barefoothealth.com/science/body_voltage_study.pdf > > Combining dry ait high voltage with moist skin low resistance sounds > like a typical solutiion, when a committee avoids the use of it's > individual brains. > > RL Do *NOT* bet on that combo as being unlikely..
From: Mike on 30 May 2010 19:15 Winfield Hill <Winfield_member(a)newsguy.com> wrote: [...] > I'm sorry for not being clear. The 10pF is already charged up by > the 400pF cap's voltage via 250 ohms. Here's an ASCII drawing of > Fisher's Severe Human Body Model. > > 250 0.5uH 110 0.1uH > ,---/\/\---UUUU---+---/\/\---UUUU----> > > > > _|_ _|_ > ___ 400pF ___ 10pF > | | > '-----------------+-------------- GROUND > > View with a fixed font, notepad, etc. Oddly enough, I could not find any existing LTspice simulation, so I posted the above in abse under "LTspice Human Body Model". Interesting results. Where does the torso get 500nH? Here's a couple more ESD references for those who lack an extensive library: http://www.esda.org/esd_fundamentals.html http://www.st.com/stonline/books/pdf/docs/6970.pdf Mike
From: Shaun on 30 May 2010 20:57 "Archimedes' Lever" <OneBigLever(a)InfiniteSeries.Org> wrote in message news:sn4506pr9gp4c4u38orsth7ee9hu5addo4(a)4ax.com... > On 30 May 2010 04:49:35 -0700, Winfield Hill > <Winfield_member(a)newsguy.com> wrote: > >> A repost of a comment I made elsewhere, for discussion here. >> >> The subject is ESD Human Body model values. I'm enamored by >> a 1989 symposium paper by Richard Fisher, of Sandia Nat'l Labs, >> where he created a "Severe Human ESD Body Model." His model >> had worst-case numbers meant for use in electrostatic-discharge >> circuit-protection analysis, etc. >> >> Fisher's Severe Body Model consists of two parts, the body and an >> arm with hand reaching out to zap something. The body part has >> 400pF of capacitance in series with 250 ohms and 0.5uH. Then the >> arm and hand part bridges the body terminals with 10pF, and finally >> we have another 110 ohms and 0.1uH in series to complete the model >> and connect it to the poor real-world victim. The body capacitance >> is higher than you may see elsewhere first because the body is >> sitting down, and second because it's a worst-case body. We won't >> go further into what that means. :-) >> >> You charge the 400pF capacitor to a voltage of your choosing. >> 20kV is a nice high number. During discharge we get a fast spike >> of current from the 10pF, with sub-ns risetime to dangerous levels, >> with up to 5A peak current, and lasting up to 5ns into the "load." >> This is followed by a slower discharge of the 400pF capacitance, >> lasting up to 200ns. >> >> This would be followed by, ahem, a postmortem. >> >> As for the effect of high moisture and humidity, as said, these can >> affect things, e.g., lowering resistances to the low levels we see >> in Fisher's Severe Model, but it also means the maximum electrostatic >> voltage developed on the 400pF capacitor is likely to be much lower. >> I suspect Fisher would prefer to take the dry-air high voltage with >> the moist-skin low resistances for his Severe case. > > The skin is the big unknown. > > Once opened by a wound, etc., the resistance is VERY low. > > Open heart defib is only 2mA on the paddles. > > Arm-to-arm with open wounds is only 10mA as opposed to 40mA on dry skin > to cause a fibrillation. > > Area of contact is also a factor in lowering skin resistance (lowering > of the interface resistance into the salty blood conductor of our inner > body). > > As for an electrostatic event causing a fibrillation, it is not very > likely until one gets up into the lightning bolt voltages. The time of > stroke becomes a factor. > Once again Dimbulb is wrong, "always wrong". Don't you understand Ohms Law, if resistance goes up (dry skin), current goes down, YOU IDIOT!. Also it take the same amount of current through the body (dry skin, moist skin or open wounds) to cause their heart to fibrillate. The closer you are to the heart the less current it takes because more of that current will flow directly though the heart. Resistance goes down for open wounds(assuming the current goes into the open wound) because there is a direct connection to the internal fluids of the body, hence less voltage is needed to cause fibrillation. With dry skin (high resistance) it takes more voltage to cause dangerous amounts of current to flow. The current levels that you specified are also wrong! 20 uA of current is considered hazardous to the Heart if it passes directly through the heart. 500 uA of current is the maximum amount of leakage current allowed in medical equipment through ground and if the ground connection was broken, you touch the case of the device and ground, that current will flow though you. These specs are for patients at high risk, poor health condition.
From: Paul Hovnanian P.E. on 30 May 2010 21:42
Archimedes' Lever wrote: > [snip] > > As for an electrostatic event causing a fibrillation, it is not very > likely until one gets up into the lightning bolt voltages. The time of > stroke becomes a factor. Correct. 400pF doesn't store much energy at a few kV. At 3kV its 1.8 mJ. Getting stuck to a 3kV DC source is a different matter. According to http://en.wikipedia.org/wiki/Defibrillation, it takes hundreds of Joules to defibrillate a heart. -- Paul Hovnanian mailto:Paul(a)Hovnanian.com ------------------------------------------------------------------ What color is a chameleon looking in a mirror? |