1 Introduction A relay is an electrical device that connects or disconnects a control circuit based on changes in external input signals, such as electrical quantities (voltage, current) or non-electrical quantities (heat, time, speed, etc.), to complete a control or protection task. It has three basic parts: sensing mechanism, intermediate mechanism, and actuator mechanism. This article describes the troubleshooting methods for their faults. 1 Troubleshooting of the sensing mechanism For electromagnetic (voltage, current, intermediate) relays, the sensing mechanism is the electromagnetic system. Faults in the electromagnetic system are mainly concentrated in the coil and the moving and stationary iron cores. (1) Troubleshooting of coil faults Coil faults usually include coil insulation damage; mechanical damage causing inter-turn short circuits or grounding; and low power supply voltage causing poor contact between the moving and stationary iron cores, resulting in excessive current through the coil, causing the coil to overheat and burn out. When repairing, the coil should be rewound. If the armature does not engage after the coil is energized, it may be due to the coil lead wire connection coming loose, causing the coil to break. After checking the loose part, it can be soldered back on. (2) Repair of Iron Core Faults The main iron core faults include the armature failing to attract after power is applied. This may be due to a broken coil, foreign objects between the moving and stationary iron cores, or low power supply voltage. Repair should be carried out according to the specific situation. The armature makes a lot of noise after power is applied. This may be due to uneven contact surfaces between the moving and stationary iron cores, or oil contamination. During repair, the coil should be removed and its contact surfaces filed or ground smooth; if there is oil contamination, it should be cleaned. Loud noise may be caused by a short circuit or a broken ring; repair or replace the short circuit ring. The armature cannot release immediately after power is cut off. This may be due to the moving iron core being stuck, the iron core air gap being too small, spring wear, or oil contamination on the iron core contact surface. During repair, the cause of the fault should be addressed differently, such as adjusting the air gap to maintain a protection level of 0.02–0.05 mm, replacing the spring, or cleaning the oil contamination with gasoline. For thermal relays, the sensing mechanism is a thermal element. Common faults include burnt-out thermal elements, malfunctions, or failure to operate. (1) Thermal element burnout. This may be due to a short circuit on the load side or the thermal element operating too frequently. During maintenance, the thermal element should be replaced and the setting value readjusted. (2) Thermal element malfunction. This may be due to the setting value being too small, operating before overload, or strong impact and vibration in the application environment causing the operating mechanism to loosen and trip, resulting in malfunction. (3) Thermal element not operating. This may be due to the setting value being too small, causing the thermal element to lose its overload protection function. During maintenance, the setting current should be adjusted according to the load operating current. 2 Maintenance of the actuator Most relay actuators are contact systems. They perform certain control functions through their "on" and "off" states. Common faults in contact systems include contact overheating, wear, and welding. The main causes of contact overheating are insufficient capacity, insufficient contact pressure, surface oxidation or uncleanliness, etc.; the main causes of accelerated wear are too small contact capacity, excessively high arc temperature causing contact metal oxidation, etc.; the main causes of contact welding are excessively high arc temperature, or severe contact jumping, etc. The contact inspection sequence is as follows: (1) Open the outer cover and check the contact surface condition. (2) If the contact surface is oxidized, silver contacts do not need to be repaired, while copper contacts can be smoothed with an oil file or the oxide layer on the surface can be gently scraped off with a knife. (3) If the contact surface is not clean, it can be cleaned with gasoline or carbon tetrachloride. (4) If there are burn marks on the contact surface, silver contacts do not need to be repaired, while copper contacts can be repaired with an oil file or a knife. It is not allowed to use sandpaper or abrasive cloth for repair, so as to avoid residual sand particles and poor contact. (5) If the contacts are welded, the contacts should be replaced. If it is caused by too small contact capacity, a relay with a larger capacity should be replaced. (6) If the contact pressure is insufficient, the spring should be adjusted or replaced to increase the pressure. If the pressure is still insufficient, the contacts should be replaced. 3. Inspection of intermediate mechanisms (1) For air-type time relays, the intermediate mechanism is mainly the air bladder. Its common fault is inaccurate delay. This may be due to the air bladder not being sealed properly or leaking air, which shortens the action delay or even causes no delay; it may also be due to the air bladder air passage being blocked, which lengthens the action delay. During repair, for the former, the air bladder should be reassembled or replaced with a new one, and for the latter, the air chamber should be disassembled and the blockage removed. (2) For speed relays, the bakelite lever is an intermediate mechanism. If the motor cannot stop when the reverse braking is applied, it may be because the bakelite lever is broken. It should be replaced during inspection. 4. Conclusion Summarizing the common fault inspection methods of relays is beneficial to the good operation of power equipment and systems, and can also provide reference experience for peers.