Abstract: Currently, one of the bases for replacing the arc-extinguishing chamber of a vacuum circuit breaker in the field is the number of rated short-circuit current interruptions in the type test. Considering that delayed replacement may lead to interruption failure and accidents, some users only replace the arc-extinguishing chamber when the calculated cumulative interruption current reaches 80% of the rated short-circuit interruption count, which actually results in a significant waste. By analyzing over 400 type test reports, original records, and test waveforms from 2002 to 2005, and tracking the interruption performance of the auxiliary circuit breaker used for composite testing over nearly a year, we boldly predict that, given the current manufacturing process and technology, the interruption potential of vacuum circuit breakers far exceeds the assessment values ​​in the type test. Keywords: Vacuum circuit breaker, vacuum arc-extinguishing chamber, electrical life, electrical wear, short-circuit interruption test 1 Introduction Vacuum circuit breakers are favored by users due to their strong arc-extinguishing capability, short arcing time, low contact wear, long mechanical life, low maintenance, and easy arc-extinguishing chamber replacement. They dominate the market for voltage levels of 405kV and below. Currently, the maintenance and arc-extinguishing chamber replacement principles for vacuum circuit breakers are based on the mechanical life test count and rated short-circuit current breaking count (e.g., 20, 30, 50, etc.) conducted at large-capacity test stations. However, how many short-circuit currents can a vacuum circuit breaker actually break? In the mid-1990s, some manufacturers' 12kV vacuum circuit breaker products passed the rated short-circuit breaking current test of 75 times. As for more breaking counts and higher voltage level products, the high testing costs prevented manufacturers from pursuing further in-depth research. This author analyzes approximately 400 type test reports, original records, and test waveforms from 2002 to 2005, and tracks the breaking performance of a single-pole auxiliary circuit breaker used in a composite test over nearly a year, and proposes some ideas. 2. Analysis Conclusions of Type Test Reports An analysis of approximately 400 type test reports for vacuum circuit breakers and switchgear from 2002 to 2005 revealed problems with 63 vacuum circuit breaker test samples during short-circuit breaking tests. Analysis of the number of breaking tests of the problematic samples revealed a pattern: the failure rate decreased with increasing number of breaking tests. Specifically, 67.4% of the problematic tests occurred in the range of 1-10 breaking tests, 17.4% in 11-20, 15.2% in 21-30, and 0% in more than 30. Details are shown in Table 1. The main reasons for breaking failures were: (1) defects in the arc-extinguishing chamber, such as substandard vacuum or insufficient aging; (2) poor switch assembly, such as an unsatisfactory stroke curve, misaligned arc-extinguishing chamber installation, or loose screws; and (3) poor mechanism reliability. As can be seen from Table 1, most problems in the breaking test occur at the beginning of the electrical life test. Once this process is passed, the probability of failure begins to decrease. Of course, the failure cannot rule out the possibility of problems with the arc-extinguishing chamber itself, but now the production of vacuum arc-extinguishing chambers basically adopts the vacuum furnace one-time sealing and exhaust technology, which is quite mature and has a very high pass rate. Moreover, it undergoes several high-voltage, low-current aging tests and vacuum degree tests before leaving the factory, so the probability of problems should be very small. In other words, if the breaking failure is indeed caused by defects in the arc-extinguishing chamber itself (such as weld bridging, failure to extinguish the arc), it should be that with the increase of the number of breaking tests, the large amount of gas generated by the contact material causes the vacuum degree to drop too quickly, or the metal particles splashed out by the melting of the contact material reduce the arc-extinguishing performance and increase the probability of failure. However, in fact, the opposite is true. All the test products that broke more than 30 times passed the electrical life test without exception. Some products even successfully completed two rounds of electrical life tests in a row, that is, simultaneously meeting the national standard requirement of 274 E2-level extended electrical life test and the electrical standard requirement of 30 full-capacity breaking tests. Therefore, the main problem causing short-circuit breaking failure lies not in the arc-extinguishing chamber itself, but in carelessness and human negligence during assembly and commissioning. It can be said that a properly assembled and well-commissioned switch, provided it uses a qualified vacuum arc-extinguishing chamber, should theoretically be able to break dozens or even hundreds of rated short-circuit currents. 3. Verification Tests and Results The main factor affecting electrical life is electrical wear, including the arc-extinguishing chamber, the arc-extinguishing medium, and the contacts. Contact wear is generally considered to play a decisive role, mainly manifested as net contact loss, metal transfer from the contact material, and chemical corrosion. Net loss is mostly caused by the fluid medium washing away or splashing away the contact surface that has melted or vaporized under the high temperature of the arc. Contact electrical wear depends on the arc energy, i.e., the breaking current and the arcing time. Numerous experimental results show that, considering the cumulative electrical wear of circuit breakers, although the arcing time is random for a single interruption, the average arcing time is relatively consistent (averaging 6–10 ms). Therefore, the influence of the first and subsequent phases can be ignored, and the interrupting current can be used as the sole reference. According to vacuum arc theory, the vacuum arc voltage is an approximate value, unaffected by the magnitude of the applied voltage. Only a certain applied voltage is needed to maintain the vacuum arc voltage. Therefore, as long as the short-circuit current meets the requirements, the voltage can be reduced for electrical life interruption tests, and the contact wear should be equivalent to that under full voltage conditions. Based on this principle, the auxiliary circuit breaker used in the synthetic test participates in interrupting the short-circuit current each time and also withstands a high recovery voltage, thus still meeting the requirements for contact wear. Therefore, from October 2005 to July 2006, the author meticulously recorded the interruption performance of the auxiliary circuit breaker used in the in-station synthetic test to verify the electrical life limit interruption capability of the vacuum circuit breaker. The test principle is shown in Figure 1. The arc-extinguishing chamber of FD in the diagram is model TD-40.5/1600-31.5 (serial number 0402578). Nearly a year of follow-up testing was conducted on the newly replaced vacuum arc-extinguishing chamber, recording each interruption. To ensure test equivalence, the arcing time for each interruption was specifically set to 8-10 ms. Unlike the currently popular equivalent cumulative method, which calculates all interruption currents equivalently to full capacity for life testing, this record completely disregards cases with short-circuit interruption currents below the rated value. It only records the number of interruptions of the rated short-circuit current, i.e., 315kA. In other words, the actual operating conditions are even more demanding than the recorded conditions. As of the end of July 2006, approximately 20 units of various products at 405kV and 315kA levels were tested, with a total of 211 interruptions of 315kA current, a cumulative interruption current of 6600kA, and a cumulative arcing time of approximately 1900 ms. On July 27th, this arc-extinguishing chamber was used as a test sample for a 40.5kV, 315kA combined breaking test, with a total of three successful breaking operations. These included one symmetrical current breaking test with an arcing time of 85ms; one large half-wave arc breaking test with an arcing time of 10ms; and one large half-wavelength arc breaking test with an arcing time of 12ms (the T100A is arguably one of the most demanding short-circuit breaking tests). All three breaking operations were successful. Figures 2 and 3 show the waveforms of the breaking tests. Subsequently, an insulation test was conducted on the arc-extinguishing chamber. The power frequency withstand voltage reached 100kV, while the impulse withstand voltage level was slightly lower than the standard requirement (185kV), reaching 182kV. This fully demonstrates that this arc-extinguishing chamber still possesses strong arc-extinguishing capabilities and can still meet the conditions for continued use. Dissection revealed that the arc coverage of the contacts was relatively uniform, most of the grooves on the contact surface had melted and adhered, and the contact surface was burned by approximately 1.4 mm, with localized pitting. The contact burn condition is shown in Figure 4. Furthermore, the splashing of metal vapor onto the shielding cylinder was very slight, indicating that the longitudinal magnetic field structure electrode has a strong arc control capability. 4. Conclusion The above verification tests demonstrate that, given current manufacturing processes and technologies, a full-capacity electrical life breaking count of 20, 30, or even 50 cycles does not accurately reflect the actual electrical life breaking capacity of a vacuum circuit breaker. For the E2 class with an extended electrical life breaking count of 274 cycles, dissection results after conducting breaking tests on the same product according to two different standards show that the contact burn degree is only equivalent to the effect after 20-30 full-capacity current breaking cycles. If all aspects are properly coordinated, the potential for the maximum electrical life breaking count of a vacuum circuit breaker is very large. Therefore, replacing the vacuum interrupter based solely on the electrical life breaking count verified at the test station would undoubtedly result in considerable waste.