The Generator Neutral Breaker is used in systems, which are grounded through low resistances or solidly grounded (without a resistance). In such systems, the fault current in the line due to an earth fault will be high.
The current flowing through the equipment due to an earth fault can be limited if a breaker is connected in series with the neutral. This breaker is opened simultaneously with the armature and the field breaker. This will bring the fault current to zero quickly.