Abstract: This paper mainly discusses the selection of vacuum and SF6 circuit breakers in medium-voltage switchgear and lists typical application examples for product development and design reference. Keywords: circuit breaker, selection, application 1 Introduction SF6 and vacuum switchgear are favored worldwide. In Europe and most Middle Eastern countries, SF6 circuit breakers are preferred; in China, Japan, and the United States, vacuum circuit breakers are clearly preferred; while in other regions, both technologies are almost equally popular. Oil-filled and oil-reduced technologies are still used to a small extent in China, Eastern Europe, Indian regions, and Latin America, but the trend is very clear, and they will soon be replaced by SF6 and vacuum technologies. In recent years, the latest and most important development of the two technologies at home and abroad is the adoption of permanent magnet operating mechanisms and the integration of relevant sensing elements into the switchgear. 2 Arc interruption characteristics 2.1 SF6 circuit breaker SF6 medium-voltage circuit breakers currently adopt a self-blowing system. Automatic air-blowing circuit breakers have relatively low operating energy but excellent performance, such as low arc energy when interrupting short-circuit currents and a long electrical life; when interrupting small inductive currents, their overvoltage multiple is less than 2.5 times. 2.2 Vacuum Circuit Breakers Vacuum circuit breaker contacts must have high resistance to arc corrosion during opening and closing operations; when interrupting small currents, they should have low current-cutting capacity. Materials research shows that copper-chromium composite materials are the best and most suitable contact materials, and copper-chromium composite materials with a chromium content of 20%–60% are now used as the standard contact material. Initially, specially shaped contacts were used to prevent radial magnetic fields on the arc contact surface, forcing the arc root to rotate continuously along the contact surface, thereby preventing localized overheating and uneven wear. Further improvements to the contact shape include the development of axial magnetic fields, which can evenly distribute the arc root across the entire contact surface. 3 Development Trends of SF6 and Vacuum Circuit Breakers 3.1 Permanent Magnet Operating Mechanism On the surface, the operating mechanism of a circuit breaker is only to realize the simple "open-close" or "close-open" function. When the required position is reached, it is necessary to ensure that the contacts remain in that position until the command to change the position is received. Therefore, the operating mechanism is a typical bistable actuator, and its performance must be highly reliable. In the past, mechanical springs and cam mechanisms were used for a long time. The permanent magnet operating mechanism includes a fixed laminated iron core, permanent magnets, closing and opening coils and special electronic devices, which can provide all electronic auxiliary functions, such as interlocking, opening, closing, etc., and also includes self-diagnostic facilities. Electrolytic capacitors provide the impulse power required for opening and closing coils. The mechanical system of the permanent magnet operating mechanism is greatly simplified, and the reduction of its components reduces the probability of failure, thereby reducing the maintenance level of the operating mechanism [2]. The new vacuum and SF6 magnetic operating mechanism circuit breakers are completely interchangeable with each other, just like the previous designs. This interchangeability allows existing switchgear to be re-equipped with minimal investment. 3.2 Integration of Sensing Elements and Electronic Equipment: By using appropriate software and necessary current and voltage sensing elements, the protection function is integrated into the circuit breaker's control system, making the circuit breaker a complete intelligent automated device with protection and switching functions, achieving the highest reliability target and the fewest component interfaces for convenient application in medium-voltage secondary power distribution systems. This is also the current development status of low-voltage power distribution systems. 4 Technical Performance 4.1 Electrical and Mechanical Lifespan: Experience shows that both vacuum and SF6 circuit breakers can be considered maintenance-free. High-quality SF6 and vacuum circuit breakers fully meet the Class B circuit breaker standard requirements in IEC 60056. This means that Class B circuit breakers do not require maintenance on the breaking components during their expected lifespan, only on other components. IEC specifies the number of operations a circuit breaker should be able to complete based on the overhead line network to which it is connected and whether reclosing is used. 4.2 Operating Overvoltage SF6 circuit breakers have low operating overvoltage levels without the need for any external devices. Vacuum circuit breakers using modern contact materials also exhibit low breaking current levels. However, in special applications, detailed system parameter studies may be required to determine whether specialized overvoltage limiting devices are needed, depending primarily on the characteristics of the industrial installation equipment. 5 Application Examples (1) Overhead Lines and Cables: Both vacuum and SF6 circuit breakers meet the requirements when used for disconnecting and protecting overhead distribution networks. (2) Transformers: Modern vacuum and SF6 circuit breakers are suitable for disconnecting the excitation current of unloaded transformers with an overvoltage multiple of less than 3.0. In certain special cases, such as when using vacuum circuit breakers to disconnect dry-type transformers in industrial equipment, surge arresters are recommended. (3) Motors: When circuit breakers are used to disconnect motors, sufficient attention must be paid to the issue of operating overvoltage. The target overvoltage limit is less than 2.5 times, which can be achieved by both types of circuit breakers. When a vacuum circuit breaker is used to cut off a small motor (starting current less than 600A), overvoltage limiting measures may be necessary due to repeated arc reignition, although the probability of this phenomenon is very low. (4) Capacitor bank: Both technologies are suitable for capacitor bank switching without reignition. When switching in groups, reactors may be required to limit the inrush current. Synchronous control technology of circuit breakers is an effective solution to this problem. SF6 circuit breakers are particularly recommended for applications with rated voltages higher than 27kV. (5) Electric arc furnace: Switching electric arc furnaces is characterized by frequent operation, high current, and short intervals. Vacuum circuit breakers are particularly suitable for this application. (6) Parallel reactor: SF6 circuit breakers are suitable for switching parallel reactors, usually with an overvoltage multiple of less than 2.5. When using a vacuum circuit breaker, overvoltage limiting measures may be required in some cases. (7) Electrified railway traction: In principle, both types of circuit breakers can be used, but in some low-frequency power applications (such as 16.67Hz), vacuum circuit breakers are recommended. 6. Conclusion From a user's perspective, SF6 and vacuum technologies are complementary. Economic factors, user preferences, national customs, manufacturing capabilities, and special switching requirements are the main factors for decision-makers in determining the type of circuit breaker. Special requirements such as the matching of the circuit breaker to operating conditions and frequent switching may be additional factors influencing the selection of the circuit breaker type. In such cases, a comprehensive study may be necessary to make the best choice.