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When calculated fault levels lie very close to circuit breaker interrupting ratings, a thorough evaluation of the asymmetrical currents involved may become the deciding factor between breakers of varied capabilities and costs. The increasing emphasis on efficiency tends to exacerbate concerns in this area. An attempt is made to help the reader visualize the events of the first few cycles after fault inception as well as providing a rigorous quantitative analysis of the magnitude of currents involved. Equations for sizing both molded-case and power circuit breakers are developed.