The on-line monitoring method for mechanical characteristics of vacuum circuit breakers is researched in this paper. Angular displacement sensor is used to measure the travel curve of the movable contacts indirectly

which solves the problems of sensor assembling and high voltage isolation effectively. The closing and opening instants of the movable and the fixed contacts are successfully obtained under on-line conditions by monitoring the vibration signal during the operation. Holtz current sensor is used to monitor the current in tripping coils. Furthermore

combining with the displacement curve of the movable contact

many characteristic parameters of mechanical conditions in vacuum circuit breaker have been obtained.