According to the standard IEC 947-2, there is actually no defined stipulation regarding the requirements of electrical life of a circuit breaker. The numbers of switching operations which a circuit breaker under no-load, normal load, overload and short-circuit conditions has to make and/or break are stipulated.
These numbers vary between two switchings (O-t-CO) for the rated ultimate short-circuit breaking capacity test and a few thousand purely mechanical operations under no-load condition.
The electrical life or durability (life of the contact tips) of a circuit breaker, similar to a contactor, depends mainly on the magnitude of the breaking current (neglecting the contact erosion due to bouncing during the making operation, which is kept at a minimum with appropriate constructive measures).
Lower currents in the range of normal service conditions or overloads in the range of the thermally delayed release mechanism has very little influence on the contact life in comparison with the effect of the short-circuit currents of the order of the breaking capacity. Due to a few high current short-circuits close to the bus bars, the electrical erosion of the contacts could be so high that the circuit breaker may have to be replaced.
In the case of a circuit breaker which is already installed, it is not easily visible from outside how many short-circuits it has already interrupted or what the actual amplitudes of the short-circuit currents were and whether the circuit breaker has already reached the end if its electrical life.
In spite of the above, the circuit breaker indicates indirectly when it has reached the end of its useful life. The thermal overload release tends to trip earlier, already at the rated value of the current setting on the scale. Thus the protective function of the device remains fully active right up to the end of its life.
The reason of the early tripping is because of the higher temperature-rise of the current path already at the rated current due to the increased contact resistance of the strongly eroded main contacts. In the long run, it affects the tripping characteristic of the thermal release. Early tripping is the result.
In other words, the circuit breaker remains reliable and safe up to the end. A normal operation indicates that the circuit breaker is functionally in order. If there is no early tripping after the breaker is switched on, it signifies that it can interrupt a short-circuit as well.
Miniature circuit breaker clearing downstream fault (VIDEO)
Reference: Basics of circuit breakers – Rockwell Automation (Download guide)