chapter  8
40 Pages

Short-Circuit Calculations according to IEC Standards

WithJ.C. Das

This chapter analyzes the calculation procedures in IEC and American National Standards Institute (ANSI) standards. Short-circuit currents and impedances can be determined by system tests, by measurement on a network analyzer or with a digital computer. Calculations of short-circuit currents in installations on board ships and airplanes are excluded. The short-circuit calculations in IEC and ANSI standards are conceptually and analytically different and the rating structure of the circuit breakers is not identical. There are major differences in the duty cycles, testing, temperature rises, and recovery voltages, though there have been attempts to harmonize ANSI/IEEE standards with IEC. In IEC calculations, the asymmetry at the contact parting time must be calculated to ascertain the asymmetrical rating of the breaker. The calculations of steady-state fault currents from generators and synchronous motors according to IEC take into consideration the generator excitation, the type of synchronous machine, salient or cylindrical generators, and the excitation settings.