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A 35-kWh superconductor flywheel energy storage (SFES) system using hybrid bearing sets, which is composed of a high-temperature superconductor bearing and an active magnet damper, has been developed in the Korea Electric Power Corporation Research Institute (KEPRI). To keep the flywheel system safe, backup bearings were adapted, and a ball bearing was selected through simulations. The 35-kWh SFES was tested up to 6500 r/min successfully. During rotation tests, there were few accidents. An accident occurred while decreasing speed, at around 4000 r/min. Although the backup bearing was damaged to keep the flywheel system safe, the shaft was also damaged. Contact due to the elongation of the flywheel shaft was considered as the reason for the accident. Some results of the accident are described and analyzed.