Vacuum circuit breakers are among the most promising electrical devices in medium-voltage electrical equipment. Due to their small size, light weight, high reliability, and simple maintenance, they are widely welcomed by the power sector and are currently among the most modern power circuit breakers. All 10kV circuit breakers in our city's power system have been replaced with vacuum circuit breakers. The overvoltage problem of vacuum circuit breakers has, to some extent, affected their development speed. Therefore, it is essential to study and discuss the causes of overvoltage in conjunction with actual production, and to take certain protective measures. 1. Types of Overvoltage 1.1 Current-Cutting Overvoltage When a vacuum circuit breaker interrupts a small AC current, due to the nature of the arc-extinguishing chamber itself, when the current drops from its peak value before reaching its natural zero point, the arc extinguishes, and the current is suddenly interrupted. The remaining electromagnetic energy on the inductive load will generate an overvoltage, which we call current-cutting overvoltage. Current-cutting overvoltage is not unique to vacuum circuit breakers; it occurs in circuit breakers of other media as well. However, it is more likely to occur in vacuum circuit breakers, especially when interrupting small inductive currents, where the current-cutting value and its overcurrent multiple will be higher, potentially causing harm to the power system, especially high-voltage electrical equipment. 1.2 Repeated Re-ignition Overvoltage: When interrupting large inductive currents (such as motor starting current), vacuum circuit breakers often experience overvoltage hazards, even if current-limiting overvoltage is not a problem, leading to breakdown of the motor's inter-turn insulation. This is mainly caused by overvoltage generated by repeated re-ignition of the vacuum circuit breaker, and is called repeated re-ignition overvoltage. Many conditions must be met for repeated re-ignition overvoltage to occur, so the probability is very small. However, once it occurs, its damage should not be underestimated, and necessary preventative measures should be taken. 1.3 Capacitive Load Overvoltage: Vacuum circuit breakers have better performance than other types of circuit breakers in interrupting capacitive loads. However, when switching power capacitor banks, due to the insufficient stability of the dielectric recovery strength after the arc of the vacuum circuit breaker gap and the reduction in DC withstand voltage, breakdown may occur, resulting in overvoltage. 2. Preventive Measures Overvoltages occurring during the use of vacuum circuit breakers can damage the insulation of electrical equipment. Therefore, appropriate measures should be taken according to the type of overvoltage to reduce its occurrence and magnitude. Besides issues related to the manufacturing process of the vacuum circuit breaker, protective devices can be installed to modify load parameters, thereby achieving the desired effect. 2.1 Capacitor Protection: Connecting a capacitor in parallel with the inductive load effectively reduces load impedance, thereby reducing the amplitude of the overvoltage surge and slowing down the rise of the overvoltage. This not only protects inductive loads from damage caused by overvoltage surges but also mitigates the damage to motor insulation caused by repeated overvoltage surges. Connecting the vacuum circuit breaker to the transformer or motor using a cable, due to the large distributed capacitance of the cable, has the same effect as a parallel capacitor, with excellent results. 2.2 RC Protection: Connecting a resistor R and a capacitor C in series as protective elements in parallel at the load input terminal forms an RC overvoltage suppressor. The capacitor can slow down the rise of the overvoltage and reduce the load impedance, thus reducing the overvoltage surge. The function of the resistor is: when current is cut off, its presence increases the attenuation coefficient of the high-frequency discharge circuit, which can reduce the number of reignitions and reduce the overvoltage of multiple reignitions, and can even effectively prevent them from happening. Using RC suppressors to protect loads such as motors is the most effective. 2.3 Nonlinear resistor protection (1) Using ordinary surge arresters in parallel with capacitors, ordinary surge arresters can limit the overvoltage amplitude, and capacitors can slow down the steepness of the overvoltage rise. (2) Using metal oxide surge arresters, which use ZnO varistors, are surge arresters without arc-extinguishing gaps and have the stable characteristics of semiconductor transistors. Under normal operating voltage, the resistance is very large and the current is very small. When the voltage increases to a certain value, the resistance decreases and exhibits stable characteristics. It should be noted that when using metal oxide surge arresters for overvoltage protection, their model should match the system voltage and the capacity of the inductive load or capacitor bank should be properly matched. 2.4 Inductive protection uses an LR overvoltage suppressor, consisting of a series reactor coil (or saturated reactor) and a parallel resistor, between the vacuum circuit breaker and the motor power supply cable, to suppress the steepness and peak value of the overvoltage rise.