Protection of low-voltage circuits by air circuit breakers in cascade arrangement

This paper deals with and outlines a new solution to the problem of providing adequate short circuit protection for low-voltage distribution systems and circuits operating at 600 volts or less. Recent tests and field experience have dispelled the illusion that protective devices of very limited interrupting ability will satisfactorily protect any low-voltage circuit. A recently published calculating procedure makes possible the accurate determination of the short circuit currents obtainable in low voltage circuits or systems. Modern types of low-voltage air circuit breakers in a wide range of interrupting ratings are now available. While the best short circuit protection is obtained by selecting those of interrupting ratings at least equal to the calculated short circuit current, such a procedure is not always justifiable. In many instances a relatively large number of main feeder circuits, emanating from a bus fed through one or more main breakers or incoming line breakers, supply an even larger number of branch feeders. The short circuit currents obtainable at the branch-feeder breaker locations may not be materially less than those obtainable at the main bus. Yet the selection of branch feeder breakers, or possibly even main feeder breakers, with interrupting ratings high enough to match these obtainable currents, may prove completely impractical when viewed with any reasonable regard for space requirements or cost. Air circuit breakers of varying interrupting ratings, properly coordinated in design may be used in a “cascade” or series connection in such manner that in event of a severe fault, the main breakers “back-up” the smaller main feeder breakers, and these, in turn, “back-up” the branch feeder breakers. Only the main breakers need have interrupting ratings equal to the obtainable short-circuit current, yet the system is adequately protected, within the limits set forth in the paper. Cascading also makes pos- ible a simple means of selecting air circuit breakers to provide economical yet adequate protection for relatively small installations, where a complete short-circuit study might be difficult to justify and where a poor selection of protective devices would result in either an unnecessarily expensive installation or a serious hazard — generally the latter. An application table for such installations, based upon the principle of cascade operation is included.