A circuit protected by a 1,200A circuit breaker had reached the design maximum of 960A. With 20% headroom, no new loads were added. But with harmonics, power factor, and other contributing problems (e.g., poorly maintained mechanical gear boxes), the load crept up past 1,200A. The breaker didn't trip, but the power monitor triggered an alert. The alert allowed for time to fix several overload problems and schedule breaker maintenance. Wisely, you rented a breaker test kit and tested the breaker after maintenance. It performed flawlessly.

Due to a design oversight, the circuit recently acquired a total load of 1,250A, but the breaker didn't trip. What might be wrong?

Trying to save money via self-servicing this breaker yet again could prove costly to the facility and even lethal to personnel. Replace this breaker ASAP.

The history indicates that the most likely cause of this repeat failure is lubrication error. That's probably an issue with all the other breakers. So the question you should be asking is, "What about all the other breakers?" The only reason you knew this breaker failed to open under overload was because your power monitor alerted you to the overload that was within the ampacity of the circuit.

A fault, however, can impose current far exceeding the circuit ampacity. You need reliable breaker protection when the fault occurs. A nonfunctional breaker in that condition sets the stage for disaster. Test every breaker, and review your breaker maintenance training and procedures.