Air Conditioning - A/C used to trip circuit breaker

Here's a strange problem, maybe you could help me with.

Some time ago I noticed that the second floor A/C (actually heat pump) unit stopped blowing cold air. The air was going, but not cold. I saw that the outside condenser unit fan was not turning. Digging further I noticed that the circuit breaker in the electrical box that controls this unit was tripped.

I tried to turn on the breaker, but it would shut off quickly after a second or so. During that second the condenser's fan would start turning though.

We decided to deal with this after vacation. WHen we came back I went to the basement and was able to turn on the circuit breaker without a problem. The A/C now works, but I feel uneasy. This problem could repeat.

Also, I noticed that the breaker says "40" on it, but the other breaker for the first floor unit says "50". I am not sure which rating is right for the heat pump and could this be the problem?

If this repeats who do I call - an HVAC professional or an electrician?
Does "40" rating seem right? What about "50"?

Thanks

You will need a HVAC professional on this, the problem could be a weak breaker but the unit should first be checked for compressor/fan amp draws and any other problems, the HVAC pro can change the breaker out if that turns out to be the problem. Be sure to ask for a copy of his data sheet that lists the unit parameters amp draws/superheat/subcooling/refrigerant pressures/airflow data etc. so you will have that info in case of any future problems. If your technician does not provide data sheets you did not get a pro probably just a jackleg, never call them again.