It is uncommon to see significant partial discharge (PD) activity in machines rated below 6 kV. For that reason, PD monitoring is usually confined to machines (electric motors and generators) rated 7 kV or higher.

Partial discharge monitoring instrumentation takes several forms. Ozone detection has a drawback: it does not detect PD activity within sealed voids in the coils. Ozone levels can also be influenced by the machine enclosure. More airflow through the machine will reduce ozone levels as compared to a machine with the same PD activity but 'stagnant' airflow.

Direct PD monitoring can be done with instrumentation that detects the actual discharges. Companies who produce this equipment include Iris Power Engineering, Doble, and Adwel. PD activity can be monitored on-line ("live") at the terminal box or inside the machine enclosure. The instrumentation will report higher values (i.e., more incidents) of PD activity when measuring activity inside the machine enclosure, so comparing PD activity detected within the enclosure of one machine to that inside the terminal box of another machine, is misleading. Consistency in location of PD monitoring equipment is helpful for consistent results and realistic comparisons of different machines.

Winding condition can also be evaluated for PD activity, when the machine is out of service, using instrumentation also available from the above-mentioned companies.

One word of caution: The PD activity may vary considerably for machines of different ratings/manufacturer, so direct comparison to identical motors is most meaningful. Work with the vendor you select; they have extensive databases to help you interpret the PD results.