Human Body Capacitance and Resistance
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From: Don Klipstein on 31 May 2010 22:00 In article <86iudiFmtmU4(a)mid.individual.net>, Dirk Bruere at NeoPax wrote: >On 30/05/2010 12:49, 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. > >Well, since people regularly charge themselves up to at least 30kV (a 1 >cm spark) I would say not. 30 KV per centimeter is the breakdown gradient of air. But that number only correlates to spark gap length when the eectric field is even just before breakdown. In the usual case of people charging themselves up by shuffling their shoes on carpet, they make sparks in gaps with uneven electric field. So, less than 30 KV can make a 1 cm spark. If one end of the spark gap is a sharp point or the tip of a wire maybe AWG 22 (approx. .63 mm) or smaller, then a 1 cm spark can occur from about 11 KV, easily from 12KV. Also, these sparks often appear bigger than they are. I find 1 cm to be uncommon, but I find 8 mm fairly easy to achieve with favorable shoes and a fairly favorable carpet and favorable humidity. So, I think 10 KV is common but much more is not. However, I remember experiencing one apartment with one exceptionally favorable carpet and I somewhat remember making 15 mm, possibly 18 mm sparks (corresponding to probably about 17 to possibly 20 KV). -- - Don Klipstein (don(a)misty.com)
From: legg on 31 May 2010 23:26 On Sun, 30 May 2010 13:58:34 -0700, Robert Baer <robertbaer(a)localnet.com> wrote: >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.. I don't think that's the issue. It's a brainless attempt to cover all bases with the fewest tests. 'Betting' would make just as much sense. Rationally, you'd have to examine the effect of the range of conditions, to see what actually had the most serious impact on varying targets. There could, for example be a joule-second effect, rather than simple breakover 'pinholes', that high voltages, greased by local moisture, couldn't produce. Being able to anticipate committee-think and to suggest it right off the bat, however, is the sign of a real up and comer. RL
From: Shaun on 31 May 2010 22:54 "Archimedes' Lever" <OneBigLever(a)InfiniteSeries.Org> wrote in message news:l99606p33e73hgn5m69c8iajl38soges8n(a)4ax.com... > On Sun, 30 May 2010 19:57:54 -0500, "Shaun" <rowl(a)nomail.com> wrote: > >>Also it take the same amount of current through the body (dry skin, moist >>skin or open wounds) to cause their heart to fibrillate. > If you had read my hole post you would have seen that I mentioned that right after quote above. See below Quote: "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. end quote > > Wrong. The key is the pathway, and how much of the total current > actually flows through that part of the pathway that is also comprised of > the heart. The heart only needs a couple milliamps through it to > fibrillate or defibrillate. As one gets further and further from the > heart as far as the current source and exit is concerned, the amount of > current needed to get the heart at 2mA increases, because the pathway is > millions of parallel resistors of which only a few thousand relate to > current flowing through the heart. The current in is one value, and the > current out is that same one value, but the current in each of the > millions of parallel resistors differs for each and not all contribute to > any flow that would relate to fibrillation or pass through the heart. So > entry point and egress point are very important. There are pathways that > would not cross the heart at all. Mode of entry is also important. Dry > skin has a high resistance. Blood does not. If punctures by the voltage > or by other means is involved, the current to cause fibrillation lowers > because the skin is not longer in the model. > > You lose. Again. I never lost in the first place, but you always lose! You must have taken that paragraph right out of a book, bravo! I know you don't have the brains to come up with that your self or even put together a paragraph like that.
From: Shaun on 31 May 2010 22:58 "Archimedes' Lever" <OneBigLever(a)InfiniteSeries.Org> wrote in message news:0cb606hljsk5dgsdkjm2rvbk0699ehq7j0(a)4ax.com... > On Sun, 30 May 2010 19:57:54 -0500, "Shaun" <rowl(a)nomail.com> wrote: > >> because there is a direct connection to the internal fluids of the >>body, > > > That is what I said, dumbfuck. No it's not. You talked about all sorts of variations.
From: Archimedes' Lever on 31 May 2010 22:59
On Mon, 31 May 2010 21:54:05 -0500, "Shaun" <rowl(a)nomail.com> wrote: >If you had read my hole post you would have seen that I mentioned that right >after quote above. See below The problem with your hole post is that it was as wholly insignificant as this last post of your was. |