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In fault current analysis, transformers are modeled using leakage reactances, while transmission lines are represented by equivalent series reactances.

Synchronous machines are modeled as constant voltage sources behind subtransient reactances.

This analysis excludes winding and series resistances, shunt admittances, delta-Y phase shifts, armature resistance, saturation, saliency, non-rotating impedance loads, and small motors under 50 horsepower.

Consider a single-line diagram of a synchronous generator powering a motor via two transformers and a transmission line. A three-phase short circuit is analyzed using the positive-sequence equivalent circuit.

The pre-fault internal voltages of the synchronous machines are represented behind their subtransient reactances.

The fault occurs when the switch closes, causing a voltage drop at the fault point.

Superposition is used to calculate the resulting fault current. If the phasor matches the pre-fault voltage, the second circuit represents the system before the fault.

This method predicts system behavior during faults, ensuring stability and reducing damage.