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ONE of the aims of the Joint Sections Committee on Air Circuit Breakers of the National Electrical Manufacturers Association (NEMA) is to propose standards of satisfactory performance for low-voltage (lv) air circuit breakers under short-circuit conditions. To determine the requirements for satisfactory circuit-breaker short-circuit performance, it is necessary to obtain information about the various short-circuit conditions that exist in electric power systems where the circuit breakers may be installed. Sponsored by the NEMA Joint Sections Committee, studies have been made to permit the maximum probable short-circuit duties to be determined at nearly any point in a wide range of low-voltage electric power systems. This paper gives the basic assumptions and procedures used in the calculations and presents the results as a series of curves that can be used to determine short-circuit duties in lv systems.