Abstract— Partial discharge (PD) testing has been used for over 60 years primarily as a method to detect aging of the stator winding insulation in motors and generators rated 6 kV and above. More recently it has also been used by some machine manufacturers as a means of assuring the quality of the insulation. The PD measuring systems in use work either in the low frequency regime (less than about 1 MHz) or in the very high frequency (30-300 MHz) range. By reference to several international standards, published work as well as some experiments described here, the advantages and disadvantages of the two approaches are reviewed. In most circumstances it is now clear that off-line PD tests should be done in the low frequency range. For on-line tests, either frequency range can be used, but with different advantages.