What is the difference between Utilization categories A and B in circuit breaker classification.

According to IEC 60947-2, Utilization category of a circuit breaker shall be stated with reference to whether or not it is specifically intended for selectivity by means of an intentional time delay with respect to other circuit breakers in series on the load side under short-circuits conditions.

Without an intentional short-time delay for selectivity under short-circuit condition and therefore without a short-time withstand current rating.

With an intentional short-time delay (which may be adjustable) provided for selectivity under short-circuit conditions. Such circuit-breakers have a short-time withstand current rating.

Refer the extract IEC document about the utilization categories.