Abstract: The correct operation of the relay protection device is the prerequisite to ensure the safe and stable operation of the power system, and the missetting of the protection setting value will lead to the protection rejection or misoperation. This paper analyzes a case of missetting leading to an out-of-area fault trip and a case of missetting leading to load fluctuation leading to false trip of the circuit breaker, and discusses the causes of false trip of the circuit breaker. Finally put forward the prevention measures to prevent the recurrence of such incidents.