An arrangement to detect a ground fault in an AC electric circuit including an electric machine having a neutral point connecting to a neutral point and a terminal side, and a unit transformer connected to the terminal side of the machine, wherein the arrangement includes a signal injection unit, a voltage transformer having a primary winding connected to the terminal side of the electric machine and a secondary winding that is open delta-connected, an instrument transformer having a primary winding connected to the neutral point of the electric machine and a secondary winding, and a ground fault detecting unit. The signal injection unit is configured to inject a signal to the secondary winding of the voltage transformer. The instrument transformer is configured to measure the resultant injected signal at its secondary winding The ground fault detecting unit is configured to detect a ground fault based on the measured signal.