Recurrent Surge Oscillography (RSO) is a diagnostic tool widely used to detect short interturn faults in the rotor winding of turbogenerators. The indications for short interturn faults in the RSO measurement appear in the peak-to-peak voltage values of the resultant RSO waveform. The greater the peak-to-peak voltage, the worse the condition of the rotor winding insulation is. Nevertheless, there is still no quantitative severity level for short interturn faults in RSO measurements. In this study, a QFSN-220-2 generator rotor that experienced ground faults is examined to measure the severity of short interturn faults. The peak-to-peak voltage value of the resultant waveform RSO measured in the failed generator rotor is proposed as the severe short interturn value. Deep experiments are performed to validate it by operating the spare generator rotor with a severe short interturn fault and conducting fault simulation tests to reinforce the diagnostic results regarding the acceptance criteria of the RSO. The acceptance criteria for the RSO measurement are also examined by performing a Weibull analysis on several turbogenerator rotors with specifications similar to those of the QFSN-220-2 generator rotor. Based on the empirical approach and Weibull analysis, the peak-to-peak voltage value of the resultant RSO waveform can serve as a reference to assess the condition of the rotor winding and can be used to predict the useful life of the generator rotor. © 1994-2012 IEEE.