ABSTRACT: This paper describes the capability of the PDA test [1.2,9] to distinguish between the various operative mechanisms of Stator winding insulation deterioration (if any). The ability to distinguish between the various mechanisms yields signiﬁcant beneﬁts in scheduling priorities for inspection, repairs, and possible locations of deterioration. The application of this partial discharge test to Ontario Hydro Generator Maintenance has been discussed brieﬂy in Reference . Two Normalized Quantity Number (NQN) [4,5] charts are presented; one for polyester and epoxy Stator insulations systems, and one for asphalt-mica and mica foliwn Stator insulation systems. Their stated insulation damage relationships are based on Ontario Hydro’s operating experience over the past 15 years.