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Re: Wimshurst generator question



On 04/01/2003 09:40 PM, cliff parker wrote:
Everyone in the chain receives a simultaneous shock.
.... wondering if anyone in the chain may be
placing themselves in danger.

Some comments (not a complete analysis):

1) Obviously it depends on details. If you wanted
to modify the apparatus to make it dangerous, you
certainly could. Some guy named Franklin set up
an electrostatic demonstration to kill turkeys and
inadvertently nearly killed himself.

The obvious independent variables are capacitance,
max voltage, and contact geometry. Safety depends
on establishing limits on these quantities. Any
particular generator will have some built-in capacitance
(probably not very large) and some built-in max voltage.

As for contact geometry, a localized spark hitting
bare flesh will cause pain and damage over a tiny
area. In contrast, the same spark hitting a hand-
held key or wand will distribute the current over
a larger area, decreasing the peak current density
by orders of magnitude.

The peak voltage is high enough to do unlimited harm.
The peak current is high enough to do unlimited harm.
Safety depends on limiting the capacitance so that the
energy and peak current are limited. The peak voltage
and peak current do not last very long.

If you add lots of capacitance (Leyden jars), things
could get seriously dangerous.

2) Obviously sparks should be kept away from flammable
vapors and dusts.

3) I looked without success for OSHA standards for
exposure to static electricity. That's a bit odd;
they seem to have standards for everything else.

But there are ANSI and Underwriters Laboratory standards
for electric fence chargers. Apparently you have to pay
to get access to these, alas.

http://www.google.com/search?q=electric-fence+peak-current+duration
http://www.egr.msu.edu/age/ET/ASAE983007.htm

4) I'm not sure what are the relevant properties that
directly govern perceptibility and/or safety. I suspect
pulse energy is not the right thing. Clearly current and
duration are both relevant. My first guess is that the
product of duration and peak current is the relevant
quantity over a moderately wide range, at least when the
peak current is large and the duration is small.

Note that having more people in the chain increases the
duration by increasing the RC time constant.

The voltage is readily adjustable according to how much
you crank the generator.

5) The obvious procedure is to start with a small voltage
and work your way up until it becomes perceptible.

Common experience suggests that there is a wide margin
between what is perceptible and what is dangerous, for
short pulses. This is true even for people with pacemakers
or other infirmities; otherwise they would never survive
the winter. You can pick up a goodly charge just by
shuffling across a carpet.