[ I_f = \fracV_thZ_th ] where ( V_th ) is the prefault voltage at the fault point (usually 1.0 pu), and ( Z_th ) is the Thevenin equivalent impedance seen from the fault point (resistance + reactance). In high-voltage systems, resistance is often neglected, giving:
[ Z_util = \fracS_baseS_sc = \frac101000 = 0.01 , \textpu ] fault current calculation
[ V_pu = \fracV_actualV_base, \quad I_pu = \fracI_actualI_base, \quad Z_pu = \fracZ_actualZ_base ] where ( Z_base = \fracV_base^2S_base ), ( I_base = \fracS_base\sqrt3 V_base ) (three-phase system). For a three-phase fault at a bus, the fault current magnitude is: [ I_f = \fracV_thZ_th ] where ( V_th
