In many facilities, the breaker maintenance program consists of "replace when nuisance tripping becomes intolerable." Yet, nuisance tripping isn't the worst failure mode. It's far worse when the breaker just doesn't trip. This failure mode means you have no circuit protection.
Breakers fail in this "no trip" mode for a variety of reasons, all of which can be addressed in a way that nearly guarantees this mode will not occur. Among the causes:
Dust accumulation.
Grease absent, hardened, or contaminated.
Fatigue of metal parts.
Trip linkage is broken or out of adjustment.
Dashpot leaked.
If you look at that list, you can see these are essentially "time in service" issues. You can't prevent them, but you can correct them via "fix or replace." That is generally preferable to the spectacular fireworks method of determining a breaker went bad.
