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we know the extension cord is plugged in (unsafe) or it is not plugged in
(safe).
But the point he is missing is .... With the old variacs we look at it
and see the dial is turned down and/or the switch is turned off and we
think we are safe, ...
Your variac-powered equipment has a 50-50 chance of being "live" with
respect to ground (full line voltage) even when the dial is turned to zero
if you have a non-polarized variac cord/plug.
Worse, this could still happen even if we turn the switch off instead of
just turning the dial down, because some old-fashioned variacs have
single-pole switches instead of double-pole switches.