I have had various shocks over a number of years from various items, but I
think the worst one was probably when I was doing a callout one night. The
company I worked for serviced microfiche printers and copiers. We took over
the servicing of a couple of machines that we did not normally handle, so we
had to get used to the idiosyncrasies of these.

this particular night I was called out to a problem with exposures on the
duplicate fiche. I ascertained that the exposure tube needed replacing, and
being on my own in the place (apart from an operator in later stages of
pregnancy), was being very careful.

The exposure lamp in this machine is a gas discharge lamp which has about
3kV DC across it, and when it comes time to turn on the light, an additional
AC voltage is turned on to get the lamp above its striking voltage. When
this AC voltage is turned off the lamp goes out as the 3kV is not sufficient
to sustain the arc. To further complicate matters the lamp is in an acrylic
tube which has water flow through it to keep the lamp temperature down to a
point where the film does not burn up.

I proceeded to turn off the power at the mains switch on the machine, at
which point all the pumps went dead, indicators went out etc, and I then
dismantled the lamp housing. Once the lamp assembly was accessible, and free
of the chassis, I manoeuvred the assembly so that the chassis touched each
end of the lamp in turn to discharge any stray volts left behind lurking in
capacitors. Proceed to unbolt things, and then while getting the lamp out
managed to touch the terminals with both hands. The resulting shock caused
me to sit on my backside between the machines. The operator was at this
point standing on an upturned rubbish bin, attending to some DLT tapes in a
tape drive which was an add-on sitting on top of the machine, and she was
quite short, and could not otherwise reach it. She turns around and asks if
I am all right, to which I said yes.

having done this less than elegant manouvour, I went investigating somewhat
more thoroughly, and found that although the main switch I had turned off
removed power from almost everything, it left this DC supply still on. My
attempts at trying to be safe by shorting the voltage to earth were thwarted
as the supply was fully floating from ground, with some 10's of Megohms
resistors each end to the chassis. Hence my attempts at shorting things to
ground still left several 10's megohms load on the supply, and did not
achieve anything. The one thing I did not do was short the two ends of the
lamp together, but this was hard to do as it was a straight lamp with
terminal on the ends, and I could not manoeuvre it to have both ends touch
chassis simultaneously.

Moral of the story is to make sure the machine is fully isolated from the
mains before doing anything remotely dangerous.

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