Short circuit ratings, labels and fault withstandability of molded-case and insulated-case circuit breakers and combination motor starters

In many cases, the interrupting rating of a device is very different from its interrupting capacity. The lack of understanding of the test methods used to determine the rating can lead to applications of interrupters that are outside the limits of their interrupting capacity. The author discusses application problems and the situations in which they may occur, and he proposes solutions to allow for the proper application of equipment. He stresses that it is the application engineer´s responsibility to recognize equipment listing and labeling test methods and to apply electrical equipment within these test guidelines. He points out that short-circuit withstandabilities can be easily calculated from their test method ratings. These values can then be compared to the peak and RMS (root mean square) symmetrical available short-circuit levels at the line lugs of the equipment. This becomes increasingly important on modern large distribution systems where high X/R ratios and high available short-circuit levels are common.<>