It is true that the protection scheme provided for the generator should not operate for through fault (faults beyond protective zone) conditions. But the generator is the source by which any fault on the system is going to feed by it only, protection against such abnormal conditions should be provided.
For generators above 1 MW where primary protection to stator winding is provided by differential protection and the overcurrent and earth fault protection give back up protection for external phase to phase faults and earth fault.
Since the faults in stator winding are fed by the stator winding itself, their influence on current in the outgoing terminals of the generator depends upon the fault level of the main bus.
Hence over current and earth fault relays do not provide satisfactory protection against internal faults.
However, the overcurrent and earth fault relays provide backup protection to generator against external faults (faults in a bus zone, transmission zone)
The setting of over current and earth fault protection is selected such that the relay does not operate for through faults.
However, if fault F continues for a long time due to failure of line protection (1) the fault will be fed by the generator. Hence the overcurrent and earth fault protection of generator (3) may be set to operate with due time lag for higher values of external fault currents.
Hence high set, definite minimum time, induction type inverse over current, earth fault relays are recommended for generator backup.