Burn up trip coil, becoming mechinicaly stickey, broken parts, control cable failure, gas leak, DC source failure, excessive fault current, over worn contacts, bad arc snubber, etc.
Worse than failing to trip, is slow operating, where it opens after the fault was cleared by other breakers. The maintenance people will insist it operated correctly, that you have a relay problem.
One more thing that I saw once: New oil circuit breakers often come from the factory with the contacts closed and the trip mechanism blocked for transport.
I remember a case where a breaker was installed, the blocking pin removed for commissioning tests, the breaker was closed for service, and somebody (nobody knows who that might be) inserted the pin back into the mechanism of the closed breaker. happily, the issue was noted when a remote operation did not occur. I was dispatched with two relay technicians and two substation mechanics. The bright red pin in the mechanism was easy to spot.
Had the breaker been required to operate under a fault, it would've been much more interesting.
Bad experences are what make a good pessimistic engineer.
I had a former coworker who had a sign that read "learn from other peoples mistakes, because no one has time to make them all themselves".
And yes I have seen the 'Blame the relay engineer first' thing many times. And I would not have known the breaker was slow except for the micro-processor relays which showed the current remained until the other breakers had cleared. The maintenance people then tested the breaker and never found anything wrong. It did this three times I can think of, before I took another job (not because of this).
Lesson: Never trust first impressions, because they will always point to the relay as the problem.
