The short circuit ratio (SCR) of ship synchronous generator is defined as the ratio of the field current required to generate rated voltage at open circuit to the field current required to circulate rated armature current with short circuit. Thus, SCR is merely the reciprocal of the synchronous reactance Xs expressed in the per unit system:
Thus, the short circuit ratio is a measure of the steady-state short circuit current in case of a terminal fault; it is the ratio of the fault current to the rated current of the generator, that is,
Steady-state terminal short circuit current = SCR × Generator rated current
However, for the ship generator circuit breaker selection, the transient fault current using the ship generator’s transient reactance not the synchronous reactance must be used.
The SCR also indicates ship machine’s sensitivity to the change in rated load. A machine with higher SCR is larger in physical size and weight, and hence costs more.
However, it has smaller Xs resulting in smaller internal voltage drop and smaller voltage regulation. The machine design engineer balances the SCR value for an optimum design.