Variac turn-on surge solution

Dear List:

I use a 15 Amp variac on my system, and after my house was upgraded from a
fuse box to circuit breakers, I had a problem.  About half of the time when
I turn on the variac, even though the knob was at 0%, the breaker would
trip.  The reason for this is that when the variac was last turned off, the
core may be left in either of two magnetic orientations.  If that
orientation is counter to the phase of the AC voltage the next time it's
turned on, the turn-on surge current can be huge, large enough to trip a 20
Amp breaker.

To solve this problem, I had been considering having a small series
resistance that is switched out by a time delay relay, but I recently came
across a much simpler solution.

Some of the power supplies that we use at work have surge-limiting
thermistors on them.  These devices look like black ceramic disc capacitors,
0.9" diameter.  At room temperature or below, they have a resistance of
about 1.0 Ohm, enough to limit turn-on current to a reasonable value.  When
they heat up when 20 Amps (max rating) is flowing through it, the resistance
drops to 0.015 Ohms.  Inserting one of these devices in series with the
input to the variac completely solved the breaker tripping problem.

I pulled my part off of a scrap board and can't comment on where others may
get some, but the part is made by RTI Electronics, part number SG301.  It is
a negative temp coefficient thermistor specifically made for limiting surge
currents.  See www.thomasregister-dot-com/olc/rtielectronics/surggard.htm
<http://www.thomasregister-dot-com/olc/rtielectronics/surggard.htm>  for specs.

Hope this helps,
Gary Lau
Waltham, MA USA