Short-Circuit current

This is the infinite-bus method with a series correction, not a full IEC 60909 study. It sums source, transformer, and cable impedances and reports c x Un / (sqrt(3) x Z) as a three-phase figure; there is no motor contribution, no arc resistance, and no DC time-constant decay. The voltage factor c (1.05 for 400 V supplies under IEC 60909) prices the highest-voltage condition. Cable resistance carries a linear temperature correction from 20 degrees C, and the asymmetric peak uses the IEC factor k = 1.02 + 0.98 x e^(-3R/X), which reaches 2 x sqrt(2) x I on a purely inductive path. A pass against Icu says the device can break the prospective current at that point; it is not a coordination study and not a design certificate. Small transformers also feed large faults through their terminals: check the device rating where it is actually installed.