With the successful implementation of oil-free transformation of my country's power distribution system in recent years, the application of vacuum circuit breakers has become increasingly widespread. The core component of a vacuum circuit breaker is the arc-extinguishing chamber, which operates under vacuum conditions. Therefore, its vacuum level directly affects the operational safety of the power system. Compared to oil circuit breakers, vacuum circuit breakers have advantages such as larger breaking capacity, better arc-extinguishing performance, longer mechanical life, less operation and maintenance, less repair work, and longer maintenance cycles. Although vacuum circuit breakers have lower defect and failure rates, a prominent issue is the detection of the vacuum level of the vacuum chamber. Unlike oil circuit breakers and SF6 circuit breakers, it is not easy to check the dielectric level. Some vacuum circuit breakers exhibit varying degrees of leakage in their vacuum interrupting chambers during operation, and some may leak to the point of being unable to interrupt circuits properly within their lifespan. There have been reports of circuit breaker explosions, three-phase short circuits, and major accidents caused by insufficient vacuum levels in the vacuum chamber. Therefore, the detection of the vacuum level of the vacuum chamber must be given high priority. I. Basic Structure and Principle of Vacuum Interruptor Each interruptor in a vacuum circuit breaker is a non-removable unit, with the moving and stationary contacts welded to their respective conductive rods. The stationary conductive rod is welded to the upper flange, while a bellows-shaped tube is welded to the moving conductive rod and runs within a guide sleeve. The bellows and guide sleeve are welded to the lower flange. A metal cylindrical shield supported by porcelain pillars covers the moving and stationary contacts, forming a sealed cavity with the glass outer shell. This cavity is evacuated, typically to a vacuum level above 10⁻⁶ Pa. During closing and opening operations, the moving conductive rod fluctuates, compressing or stretching the bellows, thus maintaining the vacuum level within the interruptor. In a vacuum, due to the large mean free path of gas molecules, gas ionization is minimal, resulting in a much higher insulation strength than that of the atmosphere. When the switch opens, an electric arc is generated between the contacts. Metal vapor evaporates from the contact surface at high temperature. Due to the special shape of the contacts, a magnetic field is generated when current passes through. Under the influence of this magnetic field, the arc moves rapidly along the tangential direction of the contact surface, condensing some of the metal vapor on the metal cylinder (i.e., the shield). The arc extinguishes itself upon natural zero crossing, and the dielectric strength between the contacts quickly recovers. II. Basic Testing Methods 1. Traditional Method: Power Frequency Withstand Voltage Method. This method involves applying a certain pressure between the moving and stationary contacts while the vacuum switch is in the open state, detecting the leakage current or observing the discharge phenomenon in the arc-extinguishing chamber, thereby inferring the vacuum level. The advantage of this method is its simplicity. Its disadvantage is that it can only qualitatively detect the vacuum level. Furthermore, because the applied voltage is low, the vacuum level is between 10⁻⁵ and 3 Pa, making it impossible to distinguish between different levels. The test results of the withstand voltage method are essentially the same, so it cannot reasonably determine the extent of leakage (i.e., how much leakage has occurred compared to the previous test on the same vacuum switch). This method can only roughly determine the arc-extinguishing chamber with severely deteriorated vacuum levels; it is a qualitative test. 2. Recent Research Achievements: Excitation discharge, ionization charge sampling principle, belonging to qualitative and quantitative detection. III. Test Principle of Ionization Charge Sampling A pulsed high voltage is applied by opening the two contacts of the arc-extinguishing chamber at a certain distance. An excitation coil is wound around both the inner and outer sides of the arc-extinguishing chamber, and a large current is passed through the coil, thereby generating a pulsed magnetic field synchronized with the high voltage within the arc-extinguishing chamber. Under the action of this pulsed magnetic field, electrons in the arc-extinguishing chamber undergo helical motion and collide with residual gas molecules, ionizing them. The resulting ion current is approximately proportional to the residual gas density (i.e., vacuum level). For vacuum tubes of different diameters, the magnitude of the ion current varies under the same vacuum level. Experiments can calibrate the correspondence curve between vacuum level and ion current for various tube types. Once the ion current is measured, the vacuum level of that tube type can be obtained by consulting the ion current-vacuum level curve; this process is automatically completed by a computer. Furthermore, due to differences in the geometry and materials of vacuum arc-extinguishing chambers, the amount of discharge charge varies considerably when the internal vacuum level and external excitation power supply are constant. For accurate measurement, each type of vacuum interrupter must have a corresponding curve derived from ionization charge to vacuum level. Regarding these measurement curves, vacuum testing equipment from domestic manufacturers, when using the ionization charge method for sampling, varies significantly in the number of measurement curves they provide: one, three, five, and a maximum of 35. This leads to considerable controversy regarding the accuracy of the measured data. The main reason is the high cost and difficulty in collecting these curves. Generally, the vacuum level measurement range is 10⁻¹ to 10⁻⁴. According to national standards, a vacuum level below 6.6 × 10⁻² Pa is considered unacceptable and should be discarded. According to the "Technical Conditions for Ordering High-Voltage Circuit Breakers" (99), the vacuum level of the vacuum interrupter in operating vacuum circuit breakers must not exceed 6.6 × 10⁻² Pa, and the Ministry of Electric Power standard is 1.33 × 10⁻² Pa. When the vacuum level exceeds 10⁻² Pa, it is recommended that the vacuum tube be scrapped. The shelf life (including storage period and service life) of vacuum switch tubes is generally around 15 to 20 years. At the end of the permissible period, the vacuum level must not exceed 6.6 × 10⁻² Pa. Vacuum tubes in storage should also be inspected within the specified service life, and their vacuum level must not exceed 6.6 × 10⁻² Pa. The storage period can be calculated using the formula: T = 6.6 × 10⁻² Pa - Pn/Pn+1 - Pn × tn/365. Pn: Vacuum level measurement value; Pn+1: Vacuum level re-measurement value after settling; tn: Not less than 7 days. Based on the experience of vacuum switch users, it is recommended to conduct preventative tests at 3 months, 6 months, and 1 year after commissioning, followed by normal pre-testing cycles. This aims to strengthen operational monitoring during periods of vacuum switch instability. A comprehensive judgment of the vacuum bulb's operating status is made through measurement tests and statistical analysis. IV. Issues to Consider When Purchasing a Vacuum Tester: 1. Accuracy of measured values; avoid analog data. 2. On-site interference resistance; avoid system crashes. 3. Safety hazards; prevent electric shock injuries. 4. Choose a vacuum tester with a comprehensive measurement curve. V. Conclusion Vacuum switches, as increasingly common high-voltage electrical appliances in China's power industry, directly affect their breaking performance and the safety of power system operation due to their internal vacuum level. Therefore, vacuum testing of the arc-extinguishing chamber is essential and important. Choosing the right equipment from the numerous quality and price options on the market is also crucial.