Which of the following describes the relationship between transformer impedance and available fault current?

• A. Lower impedance reduces available fault current
• B. Higher impedance increases available fault current
• C. Higher impedance reduces available fault current
• D. Impedance has no effect on fault current

Answer: (C)

Available fault current is set by the impedance in the path. Ohm’s law shows I_fault ≈ V_source / Z_total, so as the total impedance increases, the fault current decreases. A transformer’s impedance, usually given as a percent, represents how much the transformer itself adds to the circuit during a fault. If that impedance is higher, the total impedance the fault current must push through is larger, which limits the current more. If the impedance were lower, the fault current would be higher.

For example, a 10 MVA, 13.8 kV transformer with 5% impedance has a short-circuit current roughly equal to the transformer's rated current divided by 0.05 (I_sc ≈ I_rated / Z%), yielding about 8.4 kA. If the impedance were 2%, the fault current would be about 21 kA. This illustrates why higher impedance reduces available fault current.