Application of primary distribution fuses

THE FUNCTION of the primary fuse is to clear short circuits, and to minimize disturbances to the system at times of fault. Continuity of service requires that short-time overloads should not cause primary fuses to blow, whereas fire hazard requires that fuses for secondary windings operate on an overload basis. The proper application of primary fuses is dependent upon the determination of the available short-circuit current as well as the value of load current. Another factor which must be given consideration is coordination of fuse and relay time-current characteristics to insure isolation of the fault at the fuse point rather than by the main feeder switch. Minimum time of operation should be sought in the selection of fuses, both to provide maximum flexibility in relay settings and to give minimum system disturbance. The time required for a fuse to blow in the short-circuit current range is more important than at the one or five minute points.