This standard applies to all low-voltage power-circuit breakers that are intended for use in field circuits of apparatus such as generators, motors, synchronous condensers, or exciters and embodying contacts for establishing field discharge circuits. In performing their function of interrupting field circuits of rotating electric machinery, field discharge circuit breakers are subject to the following conditions not met within the interruption of ordinary direct-current power circuits. In alternating-current machine field circuits, in addition to the problems usually encountered in switching highly inductive directcurrent circuits, there is superimposed upon the voltage generated by the exciter, a voltage induced in the field windings by the transient current in the machine armature under alternating-current short-circuit conditions. These result in large pulsating currents in the field circuit as illustrated in Fig .Due to the large amount of magnetic energy stored in the machine field circuit, the interruption of the circuit results in an unusually high transient voltage under alternating-current short-circuit conditions, unless special means are provided for dissipating the energy.A discharge resistor must be connected across the terminals of the machine field by the discharge contacts of the circuit breaker just prior to disconnecting the field from its excitation source in order to provide for dissipation of the stored energy of the machine field and to minimize the transient voltage. The voltage across the field discharge circuit breaker main contacts following interruption of the excitation source current is illustrated in Fig .In addition to the above conditions, there is the usual direct-current short-circuit condition wherein the field circuit may become short-circuited. To cope with these unique conditions, this standard for field discharge breakers has been developed to establish the rating structure and test requirements not encountered in other ANSI Standards which apply to other low-voltage circuit breakers. The dielectric test values given in Table 7A, ANSI C37.16-1979 , Preferred Ratings, Related Requirements, and Application Recommendations for Low-Voltage Power Circuit Breakers and AC Power Circuit Protectors, reflect the change in dielectric test requirements in ANSI C50.13-1977 Requirements for Cylindrical Rotor Synchronous Generators.