Nigel Goodwin writes:
>Never touch any metal work without first testing it, just rub it gently
>with the back of your finger, it will feel 'rough' if it's live.

        I have noticed this effect on equipment that was
transformer-isolated, also.  I think it may be either due to capacitive
coupling between the primary of the transformer and the chassis.  It is a
valid warning sign and lets you know that it could bite.

        When I was a technician with the Audio Visual Center, here, I once
got hold of a film strip projector that had been equipped with an after
market motor drive which advanced the film at an adjustable rate for
paced reading exercises.  The projector had a grounded plug but the motor
drive did not and the connection between it and the metal body of the
projector was a plastic coupling.  For some reason, the mounting that held
the motor assembly to the projector was also non conductive.  The motor
developed a short between its field and the metal case around it.  The short
was somewhere between one end of the field coil and the other so that the
mains, the short, and neutral were like an autotransformer.  The body of the
rest of the projector was Earthed and the little pigtail that plugged in to
the rest of the projector from the motor was not polarized.  This meant that
one got a nasty shock between the film advance motor and the rest of the
projector no matter which orientation of the motor plug was used.  The
operator was 100% assured of a potentially fatal shock just by laying a hand
on the housing of the motor and touching the body of the projector at the
same time.  I have never since seen anything in public use as dangerously
designed as that projector.  We basically condemned it and I cut the pigtail
off flush with the case before returning it to the school system which
had sent it in.  The motor assembly had been professionally installed, but
somebody never thought of the fact that its metal case was electrically
separate from everything else.

Martin McCormick WB5AGZ  Stillwater, OK 36.7N97.4W
OSU Center for Computing and Information Services Data Communications Group