Question 1

What is the formula for three-phase symmetrical short-circuit current?

Accepted Answer

From the Thevenin equivalent circuit: I_fault = V_prefault / Z_total (rms). Z_total is the series combination of source impedance (derived from system short-circuit MVA), transformer impedance (%Z × V²/MVA), and cable impedance. Per IEC 60909, a voltage factor c = 1.0 to 1.1 is applied.

Question 2

What is the peak factor κ?

Accepted Answer

The peak factor accounts for the DC offset component of the fault current: I_peak = κ × √2 × I_rms. κ is a function of the X/R ratio; the IEC 60909 approximation is κ = 1.02 + 0.98 × exp(-3/(X/R)). A higher X/R ratio gives a larger κ (approaching 2.0), which is critical for breaker interrupting capacity selection.

Question 3

Why does single-line-to-ground current differ from three-phase fault current?

Accepted Answer

Single-line-to-ground (SLG) faults are analyzed using the symmetrical component method: I_SLG = 3V / (Z1 + Z2 + Z0), involving positive, negative, and zero sequence impedances. In solidly grounded systems, Z0 is small and SLG current can exceed three-phase fault current. In ungrounded or resistance-grounded systems, Z0 is large and ground fault current is small.

Question 4

How do you select a circuit breaker interrupting capacity?

Accepted Answer

Circuit breaker (CB) rated interrupting current must be at or above the maximum short-circuit current at the installation point (typically three-phase symmetrical fault). IEC 62271 specifies rated short-circuit breaking current I_sc [kA rms]. The peak withstand current I_pk ≥ κ√2 × I_sc must also be satisfied. It is common practice to select a device rated at approximately 120–125% of the calculated value for safety margin.

Short-Circuit Current Calculator (Symmetrical & Asymmetrical Faults)

Short-Circuit Current Formulas