Let me declare a blatant self-interest up-front as a PD test equipment vendor before adding another test. On-line partial discharge testing of MV & higher motors (> 3 kV) is a good 'condition-based' maintenance test. 'Condition-based' is more appropriate than 'predictive' maintenance as one cannot predict from PD tests when the motor insulation will fail. It does enable you, however, to base your maintenance & off-line testing decisions on the insulation condition rather than on time, operating hrs, # of starts, etc.
Vince Green, p.Eng.