Series ratings of protective devices

The objective of this paper is to clarify "series-ratings" for fuse/breaker and breaker/breaker combinations. The paper compares "series-rated" systems to "selectively coordinated" and "fully-rated" systems, and references the appropriate Underwriters Laboratory standards and National Electrical Code clauses. Appropriate reference is also made to Canadian Standards Association standards and the Canadian Electrical Code. The UL and NEC requirements are extensive and relate to the performance of the individual protective devices, to the combination performance, and to the marking/labeling of the final enclosures. The inadequacies of the up-over-and-down method for assessing the series-rating capability of fuse/breaker combinations are discussed, and combination testing is emphasized. Years of experience have demonstrated that series-ratings provide an economic method of gaining the application advantages of molded case circuit breakers. The combinations are strictly evaluated and controlled with the consequence that series-ratings provide an excellent system-application.