Abstract: As a critical equipment in the substation, the reliability of the 10 kV vacuum circuit breaker is crucial to the safe and stable operation of the power grid. In order to ensure the safe and reliable operation of the power grid, on-site operation requires that the circuit breaker must trip dependably. The inability to trip reliably or incomplete tripping may lead to serious hazards. Focusing on a fault where oil leakage in the tripping damper led to incomplete tripping of a 10 kV vacuum circuit breaker, this paper conducts on-site inspections of the initial position of the circuit breaker's main shaft. Testing is performed on the opening and closing times and synchronization, closing bounce, opening rebound, and the minimum operating voltage of the opening and closing electromagnets. The root causes of the circuit breaker's incomplete tripping are analyzed, and a resolution plan involving the replacement of the tripping damper is proposed.