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Partial discharges (PD) are small electrical sparks that can occur in liquid or solid insulation systems in high-voltage equipment, and can eventually cause failure of the equipment ?????????. Partial discharge testing has been used for more than 80 years as a factory quality control tool to find manufacturing defects that could eventually lead to equipment failure. We believe that Johnson was the first to measure PD on operating high-voltage equipment, in the 1940s . His aim was to find an online method to determine whether stator winding coils or bars were vibrating excessively in the stator magnetic core. These vibrating coils lead to abrasion of the high-voltage electrical insulation and to eventual failure. A symptom of the insulation abrasion process was that PD (or what he referred to as slot discharge) occurred between the surface of the coil and the stator core. By measuring the PD online, he could indirectly detect the movement of coils, which indicated that failure was likely. The measurement had to be made online because, if the generator were not operating, no magnetic forces would be acting on the coils; thus, the air gaps that are a necessary precursor of PD would not be as large. Johnson was successful in identifying those generators that were suffering the most from this problem, which was caused by the introduction of the first thermoset insulation systems and by workmanship variations that were magnified by an inadequate method of securing the coils in the stator slots for the novel insulation system. The success of the Johnson online PD measuring system inspired other machine manufacturers and even a few utilities to develop their own methods , . The main reason Johnson needed online PD measurement was that loose windings do not produce as much PD when the motor or generator is not operating. Thus one of the important reasons for performing online PD tests is to monitor the condition of the equipment under normal operating electri- al, thermal, and mechanical stresses. However, with the current emphasis on extending times between maintenance outages, and the push to reduce testing costs in general, the main reason now given for online PD measurement is to avoid shutdown of the equipment, which would be necessary for an off-line PD test or other diagnostic test. Although we believe online PD monitoring was first applied to rotating machines, the same reasons are valid for other electrical equipment, such as oil paper cable joints or terminations, distribution class switchgear, gas-insulated switchgear, and power transformers , .