Re: Variac Safety

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

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.

Please correct me if I am wrong in identifying what can possibly
happen or not happen. Like Leigh I also use a variac which has
only a two-wire output. For example, to dim a light bulbs. The
~110 V input is a standard three-wire AC cord.

1) I suppose that the grounded input wire is connected to the
body of the variac only and that the remaining four wires (input
and output) are floating. In fact, one of the output wires is always
at the same low potential as one of the input wires. Another output
wire is at a higher potential, as chosen, for example, between 10
and 150 V.

2) Suppose the variac is used to control light from a metallic lamp.
Normally this is OK because the body of the lamp is not connected
to an output wire. But an abnormal connection can occur. That is
where the danger is. The lamp would continue functioning but a
person touching it will place himself in parallel with the filament,
perhaps at 150 V.

3) What else can be dangerous in using a variac with a lamp?
4) What else can be dangerous in using a variac with something else?

> 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