The detection of ground faults in the rotor of brushless synchronous machines is usually a difficult task due to the absence of available measurements. This article presents a new on-line rotor ground fault location method for brushless synchronous machines. The proposed method is able to detect and locate a ground fault in the excitation system without removing the rotor for testing. The method is based on the measurement of the exciter field current and on the analysis of a voltage. This voltage is measured at a high ohmic grounding resistor installed between the neutral of the exciter armature and ground through a slip ring. The method has been validated by computer simulations and by experimental tests using a special laboratory setup, obtaining excellent results.