"Grant Erwin" wrote in message
...
I have a 20 amp variac. Yesterday I tried plugging it into a 20A circuit
with its output voltage set to 30% of input voltage, and it (consistently)
popped
the breaker. When I set it to 100% of output voltage, however, it didn't
pop
the breaker. Obviously the magnetizing current is higher with the variac
turned
down. The question is why? What's going on here?

I believe the variac and my electrical circuits are both in good working
condition, in other words I don't believe this is faulty behavior.

Grant Erwin
Kirkland, Washington
We used a lot of 20A variacs in the calibration labs where I worked. They
will occasionally trip the breaker when energized and I think this depends
on the part of the AC cycle present at the instant the switch closes.

It is not possible that you have the variac wired backwards as it would
dramatically raise the output voltage and the input current when turned
down.

It is likely that you have the input leads connected backwards. In that
case, when turned down both output leads (the hot and the neutral) would be
120volts to ground and this might cause an input overcurrent depending on
what you have connected and its isolation from ground. I strongly suspect
this is the case if you can reliably reproduce the problem. If so, it
creates a serious unrecognized hazard and should be corrected immediately.
When the output is turned to zero, both output leads can kill you if you
contact one of them and are grounded.

Don Young