Circuit breaker interrupting capacity and short-time current ratings

Low-voltage circuit breakers have interrupting capacity ratings and short-time current ratings that an engineer uses for their application. Interrupting capacity and short-time current ratings define different circuit breaker performance characteristics. The short-time rating is used by the engineer to determine the ability of the circuit breaker to protect itself and other devices and to coordinate with other circuit breakers so the system will trip selectively. While interrupting capacity rating levels of circuit breakers are somewhat consistent throughout the electrical industry, short-time current rating levels are often inconsistent. It is important to understand the performance characteristics of the specific device in order to apply it properly. The present emphasis on higher interrupting ratings, current limiting, and series ratings has impacted the short-time current ratings of circuit breakers. This paper examines the interrupting capacity and short-time current ratings of molded-case circuit breakers, insulated-case circuit breakers, and low-voltage power circuit breakers and their effect on time-current coordination. It reviews resistive and reactive X/R ratios and explains the short-time current and instantaneous trip characteristics of microprocessor-based trip units. A good understanding of interrupting capacity and short-time current ratings allows the electrical engineer to make a proper comparison of various circuit breaker designs.