Start by checking the ground first. A breaker is cheap. A burned-down house isn't.

 

right on jb - I'll do that first - thanks.

 

Are you sure they are tripping INSTANTLY and not 5-10 seconds after you flip the thermostat back on? You could leave the breaker off, turn the therno on, and then go flip the breaker on and watch how long it takes to trip again, if you havent done that already.

Also verify that if the thermostat is off and the breaker is on that it does not trip. If this is true, then when the thermostat calls for power and engages the contactor switch out on the compressor that is when it is tripping.

A direct short circuit will cause a breaker to trip instantly. A motor that wont start, or some other general overcurrent situation will usually take several seconds to get the breaker to trip. That is why I asked if you are sure it is instant, or just really fast. If it is not instant and takes several seconds, then I would start looking at why the compressor motor is not starting. Could be a motor, capacitor, contactor, or something lodges in the fan blades preventing them from spinning.

If it does in fact trip instantly, you have a short and need to find it. If it only trips after the thermostat calls for power, I would suspect the short to be outside in the compressor unit itself. That seems unlikely, but not impossible.

 

Yeah, it trips instantly. I turn the breaker off, set the thermo, and cycle the breaker back on and trips instantly. Luckily, its quite cool right now, so we're not desparate for cooling. I'm out of town tomorrow, so Friday I'll pull the panel off the breaker box and AC and see what I find. If all looks good, I'm gonna buy a new SquareD 40 amp breaker and see what happens.....

Edit: Sounds like I'm gonna need a new breaker anyway after the sound I heard last time I tried to reset it.