Strange problem

[KTECH]

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

[KTECH]

[Greg O]

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

[Pop]

Apparently you found the source of the problem if I read it right?

It's possible the reason the 30A didn't open but the

100A did was, because the 30A is a time-delay breaker? The 100A, not being time delay, could open before the 30A in that case. In which case, it did its job. OR Is it possible the 30A breaker was welded closed by the short? Very, very unusual, but possible. If so, you wouldn't be able to turn the breaker off and then back on or if you were able to, it may not turn back on.

So if the 30A isn't a time-delayed breaker, I think I'd check to be sure it's operational. It may not be able to open.

Probably worth looking around where the short occurred and at the panel to be sure no insulation melted, etc.. Any odors around that are new?

Just a couple thoughts; lots of possibilities, but those come to mind first.

HTH,

Pop

[Marty]

You should have an electrician come over and check the integrity of the breaker connections to the 'buss' If the 30 amp is loose it could cause heat to travel up the buss bar and trip the 100 amp.

Been there

Marty

[Art Todesco]

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.