Bill Hawkins wrote:
My first job was in a blasting cap plant in 1960. There were
military devices so sensitive they could be set off by turning
on a nearby fluorescent desk lamp.
I learned that the human body has a capacitance of 400 pico F.
Getting up from a chair could raise a couple of kilovolts. We
walked on conductive rubber floors wearing conductive rubber
shoes. Bench tops were conductive rubber. Nobody had thought of
the wrist strap yet.
In those days, rubber was made conductive with carbon black. It
was almost as effective as a pencil at marking things. If the
anti-static material is not black, maybe it won't be a marking
hazard.
A megohm and 400 pF has a time constant of 400 microseconds, but
you do get the kilovolt spike. The wrist strap looks really good
as long as your motion is the only source of static electricity.
It keeps your body from ever reaching kilovolt potentials.
Your finger and hand makes a 700ps to 1 ns risetime device. Slew-rate
wise you can be up in several milions of V/us. The arm has sufficient
induction that it takes a considerable time before the body discharges,
but the hand creats the leader and then the big blow comes from the body
discharge. Just as a cloud and a ligthning bolt, but in man-size.
However, being ESD aware does not mean going maniac about it. It's more
like don't finger on things which is very hot. You need to build good
habits to avoid doing something bad.
Cheers,
Magnus
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