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Partial discharge (PD) testing has been used for over 50 years to detect insulation problems in motor and generator stator windings. By periodically measuring the on-line PD activity, usually at least two years warning is given before the risk of stator winding failure is high - which is sufficient time to plan corrective maintenance or a rewind. Such testing is now routinely applied to more than 8000 machines around the world. To overcome the limitations of periodic PD testing, an inexpensive continuous PD monitoring system was developed 15 years ago that removed the need for visits to the plant, and which would automatically record the PD and collect operating data from the plant computer. The monitors use all the noise separation methods that are incorporated into the most common test instruments, to reduce the risk of false indications. The continuous PD monitors also produces PD quantities that are the same as recorded by portable instruments, so that the results can be interpreted in the same manner. In addition the monitors produce an alarm if the PD activity is very high or increasing rapidly. This paper describes how the system is implemented, as well as how the PD monitor communicates with head office or plant maintenance staff. There is now over 14 years of operational experience with the continuous PD monitor on over 1000 machines. Some case studies are presented.