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RE: Variac turn-on surge solution

One thing I failed to mention - When I was using the fusebox, the route to
the fuse was long and through an extension cord as well.  Significant
resistance.  Now with the circuit breakers, I have an outlet in the garage
directly off of the breaker box, not more than 2 feet away from the breaker
itself.  Perhaps it was the length of the old wiring that was cushioning the
turn-on surge, rather than a difference between the fuse and circuit
breaker.  

I did go through the Home Depot circuit breaker isle looking for a slow-blow
version, but didn't see any different breaker options other than current
ratings. Perhaps a "real" electrical supply distributor might have had more
options.   I did try both a 15A and 20A breaker, same result.  I don't think
the 20A unit was defective.

Regards, Gary Lau
Waltham, MA USA
		
		Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>

		Hi Gary,

		Perhaps a slower acting circuit breaker would be the fast
and correct fix??
		The breaker may also be defective...  I have a 15 amp
variac too but the
		surge current has never been a problem but all my breakers
are thermal slow
		acting...

			Terry


		At 01:13 PM 10/27/1999 -0400, you wrote:
		>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
		>