Abstract: The relay protection device is a critical component of the secondary equipment in substations. It performs functions such as monitoring, measurement, and control of the primary equipment, and is capable of promptly isolating faults to minimize the scope of accidents. This paper presents two cases: one involving external fault tripping caused by missetting, and another involving tripping caused by missetting during initial power supply to the line. Through two cases, the problems in the process of secondary technical reform are reflected, and relevant measures to prevent missetting by relay protection are proposed.