Comparison of ANSI and IEC 909 short-circuit current calculation procedures

A generalization of the methods used by both ANSI and IEC 909 is presented. The philosophical differences in calculation procedures are discussed. Although ANSI fault calculation methods are designed to determine a value to compare with the equipment ratings, the IEC 909 Standard presents a method that inherently more accurately models the fault currents that flow. The IEC method requires significantly more complex modeling of the power system fault contributions than the ANSI method requires. By understanding the calculation differences, the difference in equipment ratings becomes more meaningful