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This paper discusses partial discharge (PD) propagation, degradation and lifetime characteristics of magnet wires for inverter-fed motor under surge voltage application. Experimental results showed the time variation of PD activity, i.e. intermittent PD, successive PD, critical PD and breakdown (BD), under repetitive surge voltage application with a fixed peak value. Critical PD was a drastic change of PD activity and was identified as the partial BD of magnet wires. Since the final BD was confirmed to be always induced at the critical PD location, the critical PD was regarded as an important indicator to determine the lifetime of magnet wires. The experimental results also clarified that the lifetime of magnet wires could be determined by the total number of PD generation from its inception to breakdown. PD activity was interpreted by the time transition of PD generation frequency, and its mechanisms were discussed.