A 20Hz voltage generator, injects 20Hz voltage into the neutral point of the generator via the neutral ground transformer (NGT), and this 20Hz signal is continuously available in the stator, under both idle and running condition of the generator. In case of any earth fault due to insulation breakdown or any other cause, the 20Hz current from the generator increases, thus triggering the protection systems. It is also observed that the 20Hz fault current is proportional to the extent of the fault/ damage. It has been observed that most of the earth fault occurs during startup or energizing the generator after major overhauling. The 20Hz injection method is well suited for such conditions tool