Human Body Capacitance and Resistance
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From: Winfield Hill on 30 May 2010 07:49 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. -- Thanks, - Win
From: Didi on 30 May 2010 09:08 On May 30, 2:49 pm, Winfield Hill <Winfield_mem...(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. > > -- > Thanks, > - Win With all disclaimers on - not having been in depth into that ever, just having designed numerous HV sources - I think I have a question. If the 10 pF is modeling the initial, pre-spark/ionization path (i.e the capacitance hand/victim), I can't see how it will be charged in sub-nS time, the hand has to approach the victim in that same amount of time (a really fast hand :-) ). So the 10 pF must stand for something else I do not understand. Now how much current limiting resistance is there in the ionized path, or how does it get ionized (perhaps the 10 pF are some part of that process that I do not understand, of course) I have no idea. Dimiter ------------------------------------------------------ Dimiter Popoff Transgalactic Instruments http://www.tgi-sci.com ------------------------------------------------------ http://www.flickr.com/photos/didi_tgi/sets/72157600228621276/
From: Winfield Hill on 30 May 2010 09:35 Dimiter Popoff asked... > > Winfield Hill wrote: >> >> 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. > > If the 10 pF is modeling the initial, pre-spark/ionization > path (i.e the capacitance hand/victim), I can't see how it will > be charged in sub-nS time, the hand has to approach the victim > in that same amount of time (a really fast hand :-) ). 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. -- Thanks, - Win
From: Didi on 30 May 2010 09:59 On May 30, 4:35 pm, Winfield Hill <Winfield_mem...(a)newsguy.com> wrote: > Dimiter Popoff asked... > > > > > > > Winfield Hill wrote: > > >> 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. > > > If the 10 pF is modeling the initial, pre-spark/ionization > > path (i.e the capacitance hand/victim), I can't see how it will > > be charged in sub-nS time, the hand has to approach the victim > > in that same amount of time (a really fast hand :-) ). > > 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. > > -- > Thanks, > - Win I get it now - thanks. Not having much to add I guess I can only mention *my* body (not a model of it :-) got somewhat charged by 3 kV once or twice last time I did such a design (older designs have also utilized it but this has been long enough ago to be forgotten :-). BTW, the last time it was a 60W thing, I must have had some luck staying undamaged as a model :-). ( http://tgi-sci.com/tgi/hv60w.htm ) Dimiter
From: Paul Keinanen on 30 May 2010 10:46
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. |