IEC 60909 Short-Circuit

I"k is the initial symmetrical short-circuit current — the RMS value of the AC component immediately after fault inception, calculated using the equivalent voltage source method of IEC 60909 §4.2. It is the foundational quantity from which all other fault currents are derived.

κ is the peak factor relating the peak short-circuit current ip to the initial symmetrical value: ip = κ × √2 × I"k. It depends on the R/X ratio: κ = 1.02 + 0.98 × exp(−3R/X) per IEC 60909 §4.4. For a purely inductive circuit (R/X → 0), κ approaches 2.0; for high R/X (> 5), κ ≈ 1.02.

Use cmax = 1.10 for worst-case fault current assessment: switchgear breaking capacity, protection relay current settings, bus rating. Use cmin = 0.95 (LV) or 1.00 (HV) for minimum fault current calculations required to verify relay sensitivity and detect high-impedance faults. Both are per IEC 60909 Table 1.

Ith is the thermal equivalent short-time current: the RMS value that would produce the same Joule heating as the actual fault current over duration Tk. Ith = I"k × √(m + n), where m captures AC component decay and n captures DC component decay, per IEC 60909 §4.7 and Figs. 22a/22b. It is used to size conductor cross-sections for short-circuit withstand.

JIS C 4620 §3 (Japanese standard for metal-enclosed switchgear) references IEC 60909 as the required calculation method for rated short-circuit current in LV and MV systems deployed in Japan. The same voltage factor c, κ formula, and equivalent voltage source method apply directly.

GB/T 15544.1 is the Chinese national standard for three-phase short-circuit calculation, harmonised with IEC 60909. It uses the identical equivalent voltage source method, voltage factor table, and κ factor formula, making results directly comparable across Chinese and international projects.