Specifying current limiting fuses to protect molded case circuit breakers

In many industrial and commercial installations, electrical distribution systems can deliver short-circuit currents approaching 200000 A, RMS symmetrical. The overcurrent protective device must safely interrupt any overload or short circuit and protect the electrical components from damage and/or destruction. The factors involved in protecting molded-case circuit breakers applied beyond their withstand ratings are presented, and engineering solutions to the problem are provided. The focus is on modern current-limiting fuses. It is shown that actual molded-case circuit breaker withstand ratings can be significantly less than the marked interrupting ratings. Proper, safe applications would require the designer to be aware of these reduced ratings and design systems based on these values. When current-limiting fuses are used to protect circuit breakers, the I/sub p/ let-through of the fuse should be less than I/sub p/ withstand ratings of the circuit breaker. When molded-case circuit breakers are used, safe design practice dictates that the short circuit and power factor not exceed the tested values as determined by the UL test circuit.>