Abstract: Regularly monitor the status of the switchgear, including transient earth voltage (TEV), Ultra high frequency partial discharge detection, ultrasonic, infrared thermal imaging and other technologies. Sometimes, TEV, ultrasonic and infrared thermal image data are normal, and Ultra high frequency detection finds abnormalities, which may lead to insulation defects. Therefore, TEV and Ultra high frequency detection should be conducted at the same time to judge whether the equipment operates normally in an all-round way. When defects are found, a comprehensive analysis of the defect signal is carried out and an oscilloscope is used to accurately locate them. After opening the warehouse for maintenance, it is found that there is an insulation discharge signal in phase C of the TV component of the switchgear.