Re: Variac Safety

[Leigh Palmer <palmer@SFU.CA>]

At 9:25 PM -0500 11/30/00, Chuck Britton wrote:
> NO ONE should use a variac that doesn't have a three wire (grounded)
> cord AND outlets that are also 'three prong.
>
> Old, two wire corded variacs are (CAN) be killers.
>
> Having properly installed grounded AND polarized input/output cords
> will insure that the Neutral AC wire is common between the input and
> output wiring.
>
> I've rescued a few discarded two-wire units and 'upgraded' them to
> three-wire cords/outlets.

Chuck's fix will work if and only if one uses the Variac to power
appliances with three-wire cords and plugs. I can't think of any
time I've used a Variac in that manner. I always want to do some
fiddling with the output. In one mildly dangerous demonstration I
hook the output to the ends of a three meter long piece of piano
wire suspended horizontally two meters above the floor. (I do this
before turning anything on!) I then vary the temperature of the
suspended wire by varying the voltage output of the Variac. Of
course this demonstration is potentially dangerous, but I don't
have my students do it; I do it. In this case and most others it
doesn't matter that the output plug is two wired and unpolarized.

A Variac can be made safer to use by connecting it to an isolation
transformer (1:1) which, in turn, is connected to the mains.

Leigh