Both methods basically provide the same results. There are no theoretical reasons to prefer one to the other, only practical reasons. The ANSI/IEEE approach is the evolution of a method conceived in the '70s in the United States, when no computer-assisted calculations were available. ANSI/IEEE C37.010-1999 can only be used at medium or high voltages and only at 60 Hz. Calculation programs have been developed to determine fault currents that apply the multiplier factors called for in this standard. In fact, some clients may ask for the application of this calculation methodology by contract. Manufacturers may also recall the ANSI/IEEE standard in their catalogues. The classical method is used mainly in low-voltage studies and can also be applied at 50 Hz. It's a well-known procedure because it's a common topic in every “power system” college course.

Mitolo is an associate electrical engineer at Chu & Gassman Consulting Engineers in Middlesex, N.J.