In inverter drive systems, surge voltage between cable shield and ground (generally 0V in AC drive systems) caused from inverter switching is induced if cable length is longer and dv/dt of the inverter output voltage is much higher. This surge voltage isn't observed under AC drives. This unexpected surge voltage may cause partial discharge at cable sheaths and finally breakdown if the surge voltage exceeds partial discharge inception voltage (PDIV) of the sheaths. Therefore insulating characteristics of cable sheaths under inverter surge should be understood for optimal design of inverter-motor systems. This report states about insulating characteristics of the cable sheath, especially PDIV and V-t characteristics under AC and high dv/dt pulse voltage.