Circuit breakers covered by specification sheet 1 through sheet 12 and sheet 20 through sheet 22 are intended primarily for use in the protection of electronic circuits, and should be used where moderate but unprolonged starting overloads may be expected and where the optional speed of response provided can be matched to the characteristics of the equipment to be protected. These circuit breakers should not be used in circuits, such as primary power circuits of electric systems, whose rupture current potential is greater than the interrupting capacity of the breaker, unless the circuit is properly protected by other means. Circuit breakers covered by specification sheet 13 through sheet 19 are physically larger than the other breakers covered by this specification; however, they offer higher current ratings and interrupt capacities, and may be used in primary power circuits providing their interrupting capacity is not exceeded. Many of the specification sheets now include high-inrush features that provide tolerance for short duration of high inrush or transient currents without decreasing steady-state protection.

Circuit breakers should continuously carry normal load current; however, the value chosen should be the lowest rating that will not open the circuit breaker inadvertently. The following factors should be considered in selecting the proper current rating and time delay curve of a circuit breaker:

a. The average (steady state) conditions of the load.

b. The initial start-up current and duration of the current of the equipment to be protected.

c. Transient overloads and their duration.

d. Comparison of the time delay characteristics of the circuit breaker with the time current characteristics (including starting or overload surges) of the equipment, component, or wire.