Variac SAFETY!

[Fred Olsen]

> Mark Hughes wrote:
> Variac's (AKA autotransformers DO NOT PROVIDE ELECTRICAL ISOLATION!!!

EXACTLY SO!  Since this is a critical safety issue, I feel obliged to
reinforce it.  A "Variac" (TM of the former General Radio) is a
"variable autotransformer".  Autotransformer implies ONE winding; which
implies NO isolation, AC or DC (except for the reactance of the coil).

The output is taken across the sliding brush and common.  Since the
brush can rotate to the 'high' end of the coil, obviously there IS NO
ISOLATION.

There are isolated variable autotransformers, but they have TWO
windings.  If your 'Variac' does not have a completely separate second
winding, and MOST don't, there IS NO ISOLATION.  The near-end taps on
the winding are NOT isolated, they are merely for 'boost' output
operation in either the CW or CCW direction of rotation.

Please forgive my heavy-handedness, but there was too much dangerous
nonsense flying around this safety issue.

As for component order, for whatever it's worth my preference is to have
the isolation transformer upstream of the 'Variac', assuming they have
reasonably well-matched VA capacity, so that I may draw a load from
either straight or variable line with appropriate switching.

The most important thing to do to prevent damage to your expensive
'Variac' is to fuse its output lead, the one from the brush, at the
manufacturer's recommendation.

Open case 'Variacs' are designed to be panel mounted without
insulation.  The knob shafts are insulated from the rotor in all that I
recall.  Indeed a 'Variac' SHOULD BE GROUNDED for safety purposes, to
its grounded panel and enclosure.  That is all which will protect you in
case of insulation fault in the 'Variac' itself.

Fred

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