Abstract: The "General Technical Conditions for 35kV Indoor High-Voltage Vacuum Circuit Breakers" (ZBK97004-89) defines closing bounce as the time from the moment the contacts of the circuit breaker first make contact during closing until the moment the contacts stabilize. All direct-reading data switching characteristic testers are designed and manufactured according to this definition. Keywords: Vacuum circuit breaker, closing, closing bounce . The "General Technical Conditions for 35kV Indoor High-Voltage Vacuum Circuit Breakers" (ZBK97004-89) defines closing bounce as the time from the moment the contacts of the circuit breaker first make contact during closing until the moment the contacts stabilize. All direct-reading data switching characteristic testers are designed and manufactured according to this definition. The arc affects the electrical life of the arc-extinguishing chamber, and the arc only occurs when the moving and stationary contacts are not in contact; it does not occur when the moving and stationary contacts are in contact. Extensive practical and theoretical analysis shows that the factor that truly affects the electrical life of the vacuum is the contact disconnection time during the closing process, from the moment the contacts first make contact until the moment the contacts stabilize. During the closing process of a vacuum circuit breaker, the contact bounce time should not exceed 2ms, which differs from actual operation. The opening and closing of the vacuum circuit breaker is accomplished by the vacuum interrupter. The switch parameters must meet the performance requirements of the interrupter. For example, if the interrupter requires a closing speed of 0.4–1.0 m/s, and the switch speed can be between 0.3–0.7 m/s, then the closing speed of the vacuum circuit breaker equipped with that type of interrupter must be adjusted to between 0.4–0.7 m/s. Similarly, if the vacuum interrupter's closing bounce requirement is less than or equal to 5ms, then the allowable range for the closing bounce of the circuit breaker equipped with that type of interrupter is also less than or equal to 5ms. Whether the closing bounce should be uniformly set to no more than 2ms is worth discussing. Currently, many vacuum interrupters specify a closing bounce time greater than 2ms, requiring only less than 3ms, or even 5ms. 2. Hazards of Closing Bounce Closing bounce is an important parameter of the mechanical characteristics of vacuum circuit breakers. During closing bounce, the contact disconnection distance is small, the arc does not extinguish, leading to increased contact wear and affecting the electrical life of the arc-extinguishing chamber. However, because its duration is short, much shorter than the arc burning time during closing, the main hazard of bounce within a certain range is accelerated wear of the arc-extinguishing chamber contacts, thus shortening the electrical life of the arc-extinguishing chamber. 3. Countermeasures to Solve Closing Bounce How harmful is bounce to the electrical life of the vacuum arc-extinguishing chamber? During closing, bounce is caused by the inelastic collision of the moving and stationary contacts. The magnitude of the bounce is related to many factors, such as the spring force of the contact spring, closing speed, opening distance, and the contact material of the vacuum circuit breaker. Installation and commissioning quality, as well as the machining accuracy of components such as aluminum supports, arc-extinguishing chambers, shaft pins, and commutators, all affect the duration of closing bounce in vacuum circuit breakers. To reduce closing bounce to within the specified range, the following measures are usually taken: (1) Improve the machining accuracy of accessories to ensure tight fit between aluminum supports and shafts, commutators and steel pins, shafts, etc., and reduce clearance. (2) Strengthen the quality control of assembly process and improve the quality of assembly process. During the assembly of vacuum circuit breakers, pay attention to reasonable installation to prevent the vacuum interrupter from being subjected to additional stress. Adjust the position of the guide tube so that the moving contact of the interrupter moves freely on the axis of the interrupter without any jamming. (3) Appropriately increase the preload of the contact overtravel spring. By taking the above measures, the bounce time can be effectively controlled. The ZN23-35 vacuum circuit breaker, due to its large contact area, long stroke, fast closing speed, and large impulse, especially when equipped with a CT10 type spring operating mechanism, exhibits significant bounce and is difficult to stabilize. To reduce the bounce, measures such as adding a butterfly spring to the stationary end of the arc-extinguishing chamber are used to create a closing buffer. While this reduces the closing bounce, it leads to unreliable circuit breaker operation. In the Yancheng power supply department of Jiangsu Province, commissioning personnel reported that a ZN23-35 vacuum circuit breaker manufactured by a large switchgear factory had a very small bounce value before installation, only 1ms for all three phases. However, once connected to the busbar, the bounce became very large because of the use of a stationary support closing buffer. Before installation, the buffer effect was significant, resulting in a small bounce. However, after connecting to the busbar, the stationary end was fixed, losing the buffer effect and exacerbating the bounce. Generally, the busbar is removed for mechanical characteristic tests, making this situation relatively concealed and difficult to detect. After adding a closing buffer device, the stationary contact is no longer rigid during the closing process. The stationary contact moves a greater distance with the moving contact, increasing the amplitude of the arc-extinguishing chamber's vibration. This greatly increases the possibility of the arc-extinguishing chamber's outer shell cracking. Moreover, when interrupting fault current, the enormous electrodynamic force generated by the fault current can cause the arc-extinguishing chamber to swing laterally, leading to damage and potentially causing a circuit breaker explosion. Relying on reducing the circuit breaker's reliability to ensure closing bounce parameters and extend the already sufficiently long electrical life of the vacuum circuit breaker is counterproductive. 4. Conclusion China has over 20 years of experience operating vacuum circuit breakers. From the operational experience of domestic vacuum circuit breakers, failures due to the end of their lifespan are rare. The biggest problem with domestically produced vacuum circuit breakers is their poor reliability, especially the high rate of mechanical failure. Extensive data shows that various parameters of domestically produced vacuum circuit breakers, such as electrical life, have reached or even exceeded those of similar foreign products. Only reliability and appearance still lag significantly behind foreign products. High reliability of circuit breakers is the primary goal that domestic circuit breakers must strive to achieve. Before the reliability problem of circuit breakers is solved, excessively long electrical life is an unnecessary waste. Therefore, the bounce problem must be solved, but first, we must find ways to improve the reliability of domestic vacuum circuit breakers.