The relay measurement unit shall be blocked out of 10-40 HZ, because in this frequency range a zero voltage can also be generated by generators starting up or slowing down. The 100-% stator earth fault protection detects earth faults in the stator windings of generators which are connected with the network via a unit transformer. This protection function, which works with an injected 20 Hz voltage, is independent of the system-frequency displacement voltage appearing in earth faults, and detects earth faults in all windings including the machine star point. The measuring principle used is not influenced at all by the generator operating mode and allows to perform measurements even with the generator standing still. The two measuring principles used