Human Body Capacitance and Resistance

There are standard human body models for various electrical purposes such as ESD (electrostatic discharge) testing.
I'm sure some carefull wording typed ingo Google will lead you to an answer.
Del

The human body static discharge "model" used to be 150 pF, 150 ohms, up to 30kV. I believe there was a change in the model over the last decade, but I cannot find newer numbers.

High moisture and humidity provide additional leakage paths which limit the accumulation of charge and reduce the peak voltage. Low humidity provides a better environment to accumulate and retain more charge which increases the voltage. Specific values are difficult to define since charge generation and leakage rates vary considerably.

For other body resistance measurements, I have personally measured skin contact as low as 2000 ohms and over 1000000 ohms. Typical published values can be around 50000 ohms (with conditional disclaimers) for dry skin contact resistance. As you can see, there is no simple single answer.

Hope that helps a little.

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 you 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 Buddy

I have also personaly measured the capacitance of the human Body and it measures to 150pF, this cpacitance measured by placing two electrodes of the smart tweezer on distanced to 1mm apart and in between that skin.

As regards capacitance is concerned, this has been replied by other member, I would just say that effect of moisture you can see the same by holding the probes of hand held multimeter (3v dc) in your moistened hand. See the value of resistance. Let the contact points dry (may be standing under sun or fan). Continue to monitor the value of resistance. You will notice that as skin dries, resistance will increase.

By definition a capacitor has 2 plates. Can someone help define the "plates" of the human body? It seems a bit nebulous to me.

That definition is oversimplified. There is capacitance from any conducting object to any other conducting object and/or the rest of the universe. Reference a text on Electromagnetic Theory or begin with a quick web review here:

http://en.wikipedia.org/wiki/Capacitance#Self-capacitance

So the human body itself (by the "simpler" definition) is one plate, and the object that can be discharged to or discharges to the body, is the other plate. Right?

Yes, that is basically correct.