I think I might be able to explain why some of us old timers have a fear of
variacs, and why things have generally gotten better in this regard.
As Leigh Palmer originally explained, the typical variac is not an isolation
transformer because one of the input wires and one of the output wires are
connected. This by itself would not be a problem if we could guarantee the
common connection is the neutral connection. But older variacs do not have
polarized plugs. This means when you plug in one of these you have a 50-50
chance of getting the common connection connected to the building's hot
wire.
If you end up with the common connected to hot, then your equipment is
"live" with respect to earth ground even when the variac dial is turned to
zero. (I tried to decide if I should "shout" that by putting it in all
caps. Instead let me say it again.) 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.
So some of us are aware that we have turned the dial down to zero, have
begun to manipulate our equipment, and have gotten a good shock, and have
scratched our heads and said, "What happened? The dial is zero. How did I
get shocked?"
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. With these
poorly-designed units you still have a 50-50 chance of having your equipment
"live" (with respect to earth ground) even when the variac switch is turned
off.
There were essentially only two safe ways to touch your equipment with this
old type of variac. (1) Use the one-hand rule and make sure you are not
grounded. (2) Unplug the equipment from the variac, or unplug the variac
from the wall, before touching the equipment. Because... repeat again...
the equipment could be "live" with respect to ground even with the dial set
to zero, and perhaps even with the switch turned off, if the variac and
equipment are plugged in such that the common-connection is connected to the
building's hot wire.
Modern variacs do not have this problem. They use three-wire power cords
and have three-wire outlets. This guarantees the building hot wire is the
wire that goes to the switch and through the slide-contact of the
autotransformer. Your equipment will not be energized if the dial is turned
down or if the switch is turned off (assuming the building is wired
correctly). To help with incorrectly-wired buildings the switch in modern
variacs is almost always a double-pole switch.
John Denker's comments about extension cords is valid. But the point he is
missing is we know the extension cord is plugged in (unsafe) or it is not
plugged in (safe). 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, but we
are not.
Of course labs today ought to have GFCI outlets. Our newer labs do, but our
older labs do not.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817