1. Overview With the development of circuit breaker technology, vacuum circuit breakers have gained widespread application in the medium-voltage field due to their superior characteristics. Especially in recent years, with the implementation of "oil-free" circuit breaker retrofit projects, the market share of vacuum circuit breakers in the medium-voltage field of power systems has been increasing. The core component of a vacuum circuit breaker is the vacuum interrupter (vacuum bulb). During operation, the vacuum interrupter may leak to varying degrees, sometimes even to the point of malfunction within its expected service life. Operating under such conditions can lead to serious consequences, and most vacuum circuit breaker accidents are caused by this. Therefore, regularly inspecting the vacuum interrupter is a very important task. 2. Methods for On-Site Testing of Vacuum Interrupters The commonly used method for on-site testing of vacuum interrupters is the power frequency withstand voltage method. With the development of testing technology, the vacuum degree testing method is also gradually being widely used in the field. 2.1 Power Frequency Withstand Voltage Method The power frequency withstand voltage method involves placing the vacuum circuit breaker in the open state and applying a certain power frequency voltage (voltage value according to the manufacturer's technical standards, generally 42kV for 10kV level) between the moving and stationary contacts. The leakage current is then measured, and the discharge phenomenon inside the vacuum interrupter is observed. There should be no continuous discharge phenomenon inside the vacuum interrupter; otherwise, the vacuum interrupter should be replaced. The power frequency withstand voltage method is essentially an indirect method to determine whether the vacuum degree of the vacuum interrupter meets the requirements. If there is a leak in the vacuum interrupter, the gas pressure inside will rise to a certain level, and the relatively short gap between the contacts will not be able to withstand the test voltage, and even discharge breakdown may occur during the voltage increase process. The advantages of the power frequency withstand voltage method are its simple principle, convenient operation, and ability to qualitatively detect the quality of the vacuum interrupter. During acceptance testing, it is important to note that the mechanical stroke of the vacuum circuit breaker contacts should be adjusted to the required value before applying the power frequency withstand voltage test. 2.2 Vacuum Degree Testing Method (1) Vacuum Degree Testing Principle The vacuum degree testing method directly tests the vacuum degree of the vacuum interrupter (in this article, vacuum degree actually refers to the absolute pressure of the gas inside the interrupter, in Pa, which is different from the general meaning of vacuum degree) to judge the quality of the vacuum interrupter. Most of the vacuum degree testers used on site are non-disassembly type products, and the instruments adopt advanced synchronous pulse magnetic discharge and single-chip microcomputer technology. The brief mechanism is as follows: rarefied gas molecules are ionized by collisions with electrons excited by a strong electric field, thereby forming a large measurable ion current. Since the ion current is basically linearly related to the logarithm of the vacuum degree, the vacuum degree value can be obtained by measuring the ion current. The purpose of applying a magnetic field is to lengthen the electron's path, increase the probability of collision with gas molecules, increase the ion current, and thus reduce the strength requirement of the applied electric field. The measurement range and measured value of the ion current are related to the strength of the applied electric field and magnetic field, as well as the contact opening distance, but mainly depend on the strength and direction of the applied electric field and magnetic field. It should be noted that vacuum circuit breakers exhibit gas absorption and release effects when electric and magnetic fields are applied. To accurately measure the gas flow, a method of synchronously applying pulsed magnetic and electric fields is often used. This is the basic principle of the pulsed magnetic discharge method. (2) Test method: Separate the moving and stationary contacts of the vacuum interrupter, apply a pulsed high voltage, wind the electromagnetic coil outside the interrupter, and pass a large current through the coil to generate a pulsed magnetic field synchronous with the high voltage electric field in the interrupter. Under the action of the pulsed magnetic field, the electrons in the interrupter move in a spiral motion and collide with the residual gas molecules to ionize. The resulting ion current is approximately proportional to the residual gas density, i.e., the vacuum degree. For vacuum bulbs of different diameters, the magnitude of the ion current is also different under the same vacuum degree conditions. Through experiments, the corresponding curves of vacuum degree and ion current for various tube types can be calibrated. After the ion current is measured, the vacuum degree of the tube type can be obtained by querying the ion current-vacuum degree relationship curve of that tube type. This process is automatically completed by the microcontroller. (3) Standard for vacuum degree of vacuum interrupter The performance of vacuum interrupter has an important impact on the safe operation of vacuum circuit breaker, and vacuum degree is a key parameter of vacuum interrupter. The relevant regulations and standards have made clear requirements for this. The industry standard "Technical Conditions for Ordering 10～35kV Indoor High Voltage Vacuum Circuit Breaker (DL403-91)" stipulates that the vacuum degree of vacuum interrupter shall not exceed 6.6×10-2Pa at the end of the validity period of 15～20 years.[l] The industry standard "3.6～405KV Indoor AC High Voltage Vacuum Circuit Breaker (JB3855-1996)" stipulates that the gas pressure in the vacuum interrupter used for assembling vacuum circuit breakers should be lower than 1.33×10-3Pa.[2] 3. Advantages and Disadvantages of Power Frequency Withstand Voltage Test and Vacuum Test Method 3.1 Vacuum Interruptor Leakage Faults Vacuum interruptor leakage faults can be divided into two types: one is a "hard fault," which is caused by an outer casing rupture or bellows damage leading to air ingress and loss of vacuum, allowing the interruptor to communicate with the surrounding atmosphere; the other is a "soft fault," where the interruptor is not actually connected to the atmosphere, but due to manufacturing processes, transportation, installation, and maintenance, the gas pressure inside the interruptor exceeds the allowable value, preventing the interruptor from meeting normal breaking capacity and indicating a latent fault. The traditional testing method—the power frequency withstand voltage test—is relatively effective for "hard faults" in vacuum interruptors, qualitatively distinguishing between good and bad vacuum interruptors; however, it is ineffective for "soft faults." Because when the vacuum level is between 1×10⁴ Pa and 1×10⁻³ Pa, the power frequency withstand voltage test can pass, making it impossible to distinguish the differences. A vacuum level tester can accurately measure the vacuum level within the range of 1×10⁻¹ Pa to 1×10⁻⁵ Pa, allowing for the monitoring of changes in the vacuum level of the interrupter chamber and understanding the development of leakage. This elevates the detection of vacuum interrupters from a qualitative to a quantitative stage. Furthermore, the service life of the vacuum interrupter can be calculated based on changes in vacuum level over a certain period, providing a technical means to ensure the reliability of vacuum circuit breaker operation. For example, in a substation, most of the 10kV vacuum circuit breakers in the same batch have vacuum pressures below 6.60×10⁻⁴ Pa in their vacuum interrupters. However, one has a vacuum pressure of 3.12×10⁻³ Pa. Although this is within the acceptable range and passed the power frequency withstand voltage test, its vacuum level is lower compared to similar interrupters. This type of vacuum interrupter should be closely monitored to understand its vacuum level trends. 3.2 Vacuum Degree Test The vacuum degree tester is limited by its testing range (1×10⁻¹ Pa to 1×10⁻⁵ Pa). Outside this range, the approximately proportional relationship between the ion current and residual gas density (i.e., the vacuum degree) upon which the vacuum degree tester relies changes, thus compromising the accuracy of the test results. This is especially true for "hard faults" such as total leaks (open to the atmosphere), where the test value is very close to the value when the vacuum degree is good, easily leading to incorrect judgments. The reason for this can be explained by the gas collision theory and the principle of vacuum testing instruments: When the gas pressure in a vacuum interrupter increases, it can still result in only a small ion current flowing between the interrupter's fracture surfaces, similar to when the gas pressure is very low. This is because when the gas pressure is high, the gas density increases, and the mean free path of electrons decreases. Although the number of collisions increases, the lower kinetic energy accumulated by electrons reduces the likelihood of gas molecule ionization. This leads to the vacuum tester incorrectly indicating a good vacuum (low pressure). Before testing the vacuum level, we cannot accurately determine whether the vacuum level of the interrupter is within the test range, thus compromising the accuracy of the vacuum level measured by the vacuum tester. Therefore, the power frequency withstand voltage test cannot be omitted in on-site vacuum interrupter testing. If the vacuum interrupter passes the power frequency withstand voltage test, ensuring that the vacuum level is within the test range of the vacuum tester, then the accuracy of the vacuum level test can be guaranteed. In conclusion, vacuum level testing and power frequency withstand voltage testing should complement each other to make an accurate diagnosis of the vacuum interrupter. In our field work, we encountered three examples of vacuum interrupters with defects but good vacuum performance: #1 The glass cover of the interrupter was broken, but the vacuum performance test value was normal (local altitude 1700m, atmospheric pressure 8.4×10⁴ Pa); #3 The interrupter broke down when the voltage between the breaks was increased to 10KV, but the vacuum performance test value was normal; #4 The insulation between the breaks of the interrupter was only 2Mo, and the current surged immediately after the voltage was increased, but the vacuum performance test value was normal. This shows that judging the quality of a vacuum interrupter solely based on vacuum performance testing is unreliable. 3.3 Difficulties in conducting power frequency withstand voltage tests on vacuum interrupters in the field: If the busbar is not energized, it is difficult to implement withstand voltage tests on the breaks of the vacuum interrupter. Taking a 10kV vacuum circuit breaker as an example, the manufacturer requires a test voltage of 42kV on both sides of the vacuum interrupter break. Since the 10kV busbar is energized, voltage cannot be applied from the upper terminal block of the circuit breaker (the lower terminal block is grounded). The distance between the upper disconnecting switch break and the upper terminal block of the circuit breaker does not meet the test voltage requirements. Although adding an insulating partition at the disconnecting switch break can solve this problem, the risk is too high and it is not easily adopted by field test personnel. Applying voltage from the lower terminal block (upper terminal block grounded) is also problematic because the lower terminal block connects to dry-type CTs and circuit breaker insulating rods, whose withstand voltage standard is 27kV. Therefore, the vacuum interrupter break can only be tested at 27kV. This low test voltage reduces the sensitivity of the test for detecting defects in the interrupter. 3.4 Difficulties in On-Site Vacuum Interrupter Testing: Vacuum testing is relatively convenient for handcart-type vacuum circuit breakers; however, on-site vacuum testing for fixed cabinet-type vacuum circuit breakers is troublesome and time-consuming due to the complex wiring and disconnection process. The cable room of the high-voltage switchgear is small and has high-voltage cables connected to it. Some circuit breakers also have arc-blocking plates installed between phases. The space for the test personnel to move around in the cabinet is small, and the wiring is inconvenient. During the test, the high-voltage outgoing cables must be disconnected. If there are surge arresters or RC absorbers installed at the outgoing terminals of the circuit breakers, they must also be disconnected, which is time-consuming and laborious. 4. Conclusion (1) The development of vacuum degree testing has enabled us to move from qualitative to quantitative testing of vacuum interrupters. We can grasp the development and changes of leakage in vacuum interrupters and estimate their service life. Testing is more convenient for handcart-type vacuum circuit breakers, but not so convenient for fixed cabinet-type vacuum circuit breakers. (2) Vacuum degree testing cannot replace the traditional power frequency withstand voltage test. Due to the limitation of the test range (1×10-1Pa×104Pa), vacuum degree testing must be combined with power frequency withstand voltage testing to make an accurate diagnosis of vacuum interrupters. (3) Before testing, the appearance of the vacuum interrupter should be carefully checked. If there is any damage, no other test is needed and it should be replaced immediately. The interrupter that has a good vacuum degree test but fails the power frequency withstand voltage test must be replaced. The interrupter that has passed the power frequency withstand voltage test but fails the vacuum degree test must also be replaced. The interrupter that has passed the withstand voltage test and has a qualified vacuum degree but has a rapidly decreasing vacuum degree test should be strengthened.