I would guess that your setback
thermostat somehow turned off the running
AC compressor and immediately turned it
back on at the time you indicated.
Depending on the AC condensing unit,
there may not be electronics
to prevent this. I have seen a small
short on a 15 amp branch circuit cause
the running AC compressor to stall and
take out the 100 amp main without
tripping the 15 amp circuit with the
short or the 40 amp AC breaker.
Apparently, the current from the short
circuit and the overcurrent from the
stalling compressor added up and took
out the main. I know on my setback
thermostats, there is supposed to be
built in time delays that prevent doing
unfriendly things to the compressor.


Thought some of you might add to this/
Came home from work 5:30 PM Monday,no power.
Found 100 amp breaker tripped re-set all OK
Same thing Tues.
Same thing Wed. after checking lines etc and again taking whole house
amps (Max38w/ air on and fridge on.
On way back up I was resetting clocks when I noticed they all stopped at
the same time.After checking my setback thermostat sure as hell main
tripped at program change.Went to held temp. problem solved. Still a
little miffed that 30 amp air breaker held and 100 amp opened.
Frank


It will happen. I assume the 30 amp was for your AC condensing unit?

At a office building we had a 1000 amp, (yes one thousand!), main breaker
trip instead of a 40 amp for a AC unit. The compressor was shorted to
ground. The 40 was a time delay breaker of some sort, and the main was
not. Both breakers saw the short to ground, but the main was designed to
trip faster, so it did.
Greg