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Partial discharge (PD) activity has long been known to be an indicator of the presence of some deterioration mechanisms at work within, or on, the solid insulation of the stator windings of high voltage rotating machines. Specifically, the test can find loose, overheated, and contaminated windings, well before these problems lead to failure. As a result, on-line PD testing has become an important tool for planning motor maintenance. On-line monitoring of partial discharge results is preferred as it provides an indication of the activity while the machine is operating under normal thermal, electrical, ambient and mechanical stresses. However, when monitoring on-line, the influence of high frequency pulses from sources external to the stator winding must be identified and separated in order to prevent erroneous interpretations. In the past, this has required the use of a human expert. By using results obtained with sophisticated noise separation techniques, 272,000 partial discharge (PD) test results have been analyzed to establish the criteria for comparing results from different machines and the expected PD levels. This paper will highlight some of the benefits of on-line PD testing using these results with case studies and signal analysis from marine vessel applications.