Short circuit calculations are done to ensure that fault current in short circuits does not exceed breaker’s and other equipment’s maximum ratings. Under short circuit, the mechanical stresses on bus-bars and cabling becomes much higher than under normal operation, and class rules and standards defines certain limits that the equipment shall be designed for.
A typical short circuit current waveform for a generator is shown in As seen, the initial current has a significant DC component, which together with the sub-transient short circuit current may give a high peak value
typically in the order of 10 times nominal current for the generators.
The short circuit currents are being reduced, as the DC component decays rapidly, typically with a time constant of 20-100 ms. After 300-500 ms, the transient short circuit current is typically reduced to 3-5 times nominal generator current, and dependent on the system design, this is normally the breaking current for a circuit breaker for a branch.
A sustained short circuit current will after a second or more, reach a stationary value, which according to rules and regulations shall be at least three times nominal current for a generator in order to detect faults reliably, for clearing faulty branches.
The short circuit current is found by numerical simulation or by analytical methods. IEC 61363 is normally applied for short circuit calculations in ships.