RSO test is performed to detect faults in rotor windings. The electrical faults in generator rotors fall into two main categories; faults from the winding to the rotor body (earth faults) and faults between parts of the winding (inter-turn winding faults). The existence of the faults will frequently display excessive mechanical vibration and cause serious concern. An RSO test identifies the type of fault and pinpoints the position of the fault.