Human Body Capacitance and Resistance
in [Design]
|
Prev: New Search Engine for Electronic Components
Next: I can't figure out the proper name of something
From: Archimedes' Lever on 30 May 2010 12:37 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.
From: Archimedes' Lever on 30 May 2010 12:42 On Sun, 30 May 2010 06:59:37 -0700 (PDT), Didi <dp(a)tgi-sci.com> wrote: >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 I got 'charged up' by a 50kV X-ray supply once. It was a very good thing I had good shoes on and provided no path as an outlet for the charge feed. Otherwise, instead of charging me up, it would have charged me up to the point where the exit location sparked across the gap to the return point. I would likely not be around to reflect on the event. HV bench operations... one hand... the other behind you... remain separated from other bench elements. Ideally, energize the device inside an HV containment cage. We never did that for anything at 50kV or below, but we did do it for all of our stuff that went up to 400kV.
From: legg on 30 May 2010 13:51 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
From: BobW on 30 May 2010 16:33 "Paul Keinanen" <keinanen(a)sci.fi> wrote in message news:kmt406pmn7ercv8n5l0iautvdd672uka05(a)4ax.com... > On 30 May 2010 04:49:35 -0700, Winfield Hill > <Winfield_member(a)newsguy.com> wrote: > [snip] > > 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. > How many times are we able to touch 230V mains in order to find out? Bob -- == All google group posts are automatically deleted due to spam ==
From: Robert Baer on 30 May 2010 16:49
Winfield Hill 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. > > I have known of two extreme cases of skin resistance: one person could feel a slight "tingle" of he placed his hands between 120VAC (himself in middle), and the other got killed when he accidentally got himself in series with a SIX VOLT car battery ("good old days") - the current thru the chest was more than sufficient to do the job. |