Abstract: The invention is directed to an arrangement for detecting ground leaks and includes two identical synchronous machines coupled to the rotor of a brushlessly excited synchronous machine. One of the identical machines is built as an outer-pole machine and the other as an inner-pole machine. The stator winding of the outer-pole machine is supplied with a constant direct current and its rotor winding is connected through a rectifier bridge to be between ground and the neutral point of the rotor of the brushlessly excited synchronous machine. The rectifier feeds the rotor winding of the inner-pole machine, and the voltage measured at the stator winding of the latter is a measure of the insulation resistance in the rotor circuit of the brushless synchronous machine.