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Generator short circuit fault current calculator

Calculates the short circuit fault current of a 3-phase AC generator.

See Also


  • Rated voltage (V). The rated phase-to-phase voltage of the generator in volts.
  • Rating (S). The power rating of the generator in kVA.
  • Impedance (Zk). The short circuit impedance of the generator as a percentage. The short circuit impedance can be specified for the sub-transient, transient or steady state phase of the generator fault. For simplification the resistance can be ignored and only the reactance can be considered. Typical reactance values are shown below.

What is the impedance of a generator?

Typical impedance values for generator short circuits.

Phase Equation Typical reactance (%) Typical time
Sub-transient \(Z_{k}= X_{d}''\) 10 – 20% 10ms
Transient \(Z_{k}= X_{d}'\) 15 – 25% 250ms
Steady state \(Z_{k}= X_{d}\) 200 – 350% > 250ms

How to calculate the fault current for a generator?

The simplified generator short circuit current formula is:

\(I_{fault-actual}= \dfrac{S_{r} \times 100}{\sqrt{3} \times V_{r} \times Z_{\%}}\)


  • \(I_{fault-actual}\) is the fault current in kA.
  • \(S_{r}\) is the generator power rating in kVA.
  • \(V_{r}\) is the generator rated voltage in V.
  • \(Z_{\%}\) is the generator impedance in base percentage.

How to calculate the fault current for a generator with the per unit system?

Step 1: Calculate the per unit fault current:

\(I_{fault-pu}= \dfrac{1}{Z_{pu}}\)

Note that \(Z_{pu} = \dfrac {Z_{\%} }{100}\)

Step 2: Calculate the nominal base current:

\(I_{nominal-base}= \dfrac{S_r}{\sqrt{3} \times V_r}\)

Step 3: Calculate the actual fault current:

\(I_{fault-actual}= I_{fault-pu} \times I_{nominal-base}\)