Any power system network (Figure 1) can be represented by a simplistic Thevenin's equivalent circuit (Figure 2). Using the Thevenin's equivalent circuit of the network, it will be very easy to calculate the fault current at any point of the the power system.

Figure 1 Power System Network
Figure 1 Power System Network
Figure 2 Thevenins Equivalent Circuit
Figure 2 Thevenins Equivalent Circuit

where:

V_s=Thevenin's~Eqvuivalent~Voltage~Source
Z_i=Thevenin's~Equivalent~Impedance
I_f=Fault~Current
Z_f=Fault~Impedance
From Figure 2, the fault current will be

I_f={V_s}/{(Z_i + Z_f)}

Most faults are considered to be bolted faults thus Z_f=0, which will simplify the equation to

I_f={V_s}/{Z_i}

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