In Chapter 7, we discussed current interruption in AC circuits and the stresses that can be imposed on the circuit breakers, depending on the nature of fault or the switching operation. We observed that the system modulates the arc interruption process and the performance of a circuit breaker. In this chapter, we review the ratings of circuit breakers and fuses according to ANSI, mainly from the short-circuit and switching point of view, and examine their applications. While a general-purpose circuit breaker may be adequate in most applications, yet higher duties may be imposed in certain systems that should be carefully considered. This chapter also forms a background for the short-circuit calculation procedures in Chapter 10.