Interruption of short-circuit currents in MV networks with extremely long time constants

Short-circuit testing of MV circuit breakers is usually performed according to IEC standards 56-4 with a circuit time constant of 45 ms. However, in modern MV utility and industrial networks, specific situations may be found with extremely large time constants of the DC component in the initial short-circuit current. Therefore, the actual DC component in a 3-phase short circuit-to-interrupt may be substantially larger than the breaker´s testing ratings. The authors examine the theoretical background, the influence of the DC component on interruption severity and the calculation of practical networks. A number of practical X/R and time constant values are given, and IEC and ANSI standards are compared in this respect