Human Body Capacitance and Resistance

Winfield Hill · 2010-05-30T11:49:35+00:00

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

May 30, 2010 63 Replies

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Dirk Bruere at NeoPax 16 years ago

I always do that as well. Have to explain it to non-engineer types or they think I'm gay :-)

Dirk http://www.transcendence.me.uk/ - Transcendence UK http://www.blogtalkradio.com/onetribe - Occult Talk Show

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Archimedes' Lever 16 years ago

The flow in bulk mediums differs from that of solid, metallic conductors. The eddy current propagation and the flow itself differs. The 'skin effect' differs.

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Don Klipstein 16 years ago

I would apply to the above a combination of Fourier transform for a frequency spectrum analysis, combined with the skin effect formula.

Please keep in mind the DC component of a quick DC pulse or a quick asymmetric AC pulse! A short unipolar pulse is roughly as bad as 50-60 Hz AC! In addition to AC component at frequencies in spectral analysis at frequencies near/below 1 KHz or 400 Hz or so, and frequencies from around

1 to 20 KHZ get deweighted rather than discounted!

- Don Klipstein (don@misty.com)

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JosephKK 16 years ago

the

finally

model

capacitance

won't

spike

levels,

"load."

(a 1

=20

Oh yeah. Back in the early 1970s i kept sensitive MOS the cards in the factory black plastic bags and equalized (touching he other hands) before handing it over in the bag. So on and so forth. =20

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