Abstract: When a vacuum circuit breaker is in the closed position, its insulation to ground is borne by the supporting insulator. If a permanent ground fault occurs in the line connected to the vacuum circuit breaker, and the ground fault point is not cleared after the circuit breaker trips, the insulation to ground of the live busbar must also be borne by the vacuum gap of the circuit breaker's break point. During various fault interruptions, the vacuum insulation gap between a pair of contacts at the break point must withstand various recovery voltages without breakdown. Therefore , the insulation characteristics of the vacuum gap have become a major research topic for improving the break voltage of the arc-extinguishing chamber and enabling single-break vacuum circuit breakers to develop towards higher voltage levels. Keywords: Vacuum circuit breaker, insulation characteristics, break voltage. Therefore, the insulation characteristics of vacuum gaps have become a major research topic for improving the breaking voltage of arc-extinguishing chambers and enabling single-break vacuum circuit breakers to develop towards higher voltage levels. Vacuum Degree Representation A space with a gas density below one atmosphere is called a vacuum space. The higher the vacuum degree, the lower the gas pressure within the space. Vacuum degree is expressed in three units: Torr (1 mm mercury column height), millibar (10³ bar), or Pascal (Pa). (1 Torr = 131.6 Pa, 1 millibar = 100 Pa). The vacuum degree of 10⁻⁴ Torr commonly referred to in vacuum arc-extinguishing chambers means that the gas pressure within the chamber is only "one ten-thousandth of a mm mercury column height," which is 1.31 x 10⁻² Pa. "Passen's Law" refers to the relationship between gap voltage withstand strength and gas pressure. Figure 1 shows that the relationship curve of Passen's Law is V-shaped, meaning that increasing or decreasing the filling pressure can improve the insulation strength of the inter-electrode gap. The breakdown mechanism remains unclear because the vacuum level inside a vacuum interrupter is higher than 10⁻⁴ Torr. In such a rarefied space, the free path of gas molecules is 10³ mm. Within the volume of a vacuum interrupter, the probability of collision is almost zero. Therefore, collisional ionization will not occur, leading to vacuum gap breakdown. The "V"-shaped curve of Python's theorem, derived experimentally under a uniform electric field, can be expressed as: Uj = KLa L — gap distance; a — gap coefficient (a = 1 for gap < 5 mm, a = 0.5 for gap > 5 mm). The "V"-shaped curve of Python's theorem shows an inflection point when the vacuum level reaches 10³ Torr, after which the curve flattens, and the breakdown voltage remains almost unchanged. When the vacuum level and gap distance are the same, the breakdown voltage varies with the electrode material. Higher mechanical strength and melting point of the electrode material result in a higher vacuum gap breakdown voltage. Mechanism of Vacuum Insulation Failure As mentioned earlier, in a high-vacuum space like a vacuum interrupter, the free path of gas molecules is very large, preventing collisional separation and thus ensuring the vacuum gap will break down under high voltage. Therefore, there are several explanations for the failure of vacuum insulation: field emission causing breakdown, micro-particle-induced breakdown, and micro-discharge leading to breakdown. The field emission theory explains the breakdown of vacuum gaps by the concentration of energy in the gap's electric field. Electrons are emitted or evaporate at the protruding parts of the electrode's microscopic surface, impacting the anode and causing localized heating, further releasing ions or vapor. Positive ions then collide with the cathode, causing secondary emission, and this continuous accumulation eventually leads to gap breakdown. The famous Fowler and Noraheim field emission current expression I is: I = AE²e⁻B/E, where E is the electric field strength; A is a constant related to the area of ​​the emission point; and B is a constant related to the emission from the electrode surface. Under small gaps (<1mm) and short pulse voltage conditions, it is reasonable to assume that vacuum gap breakdown is caused by field emission. However, under long gaps and continuous pressure and long pulse voltage conditions, some scholars believe that there are other mechanisms for vacuum breakdown: (1) Cathode-induced breakdown: Under a strong electric field, due to the Joule heating effect of the field emission current, the temperature of the protrusions on the cathode surface rises. When the temperature reaches the critical point, the protrusions melt and generate steam, causing breakdown. (2) Anode-induced breakdown: Due to the electron beam emitted by the cathode, the anode is bombarded, causing a point to heat up and generate melting and steam, resulting in gap breakdown. The conditions for anode-induced breakdown are related to the electric field enhancement coefficient and the gap distance. Explanation of micro-particle-induced breakdown : Assuming that a relatively loose micro-particle is attached to the electrode surface, under the action of the electric field, the micro-particle falls off and accelerates. When this micro-particle hits the opposite electrode, the impact heating can cause it to melt and generate steam, resulting in breakdown. Explanation of micro-discharge leading to vacuum gap breakdown: The cathode surface of the electrode is contaminated, which will cause micro-discharge phenomenon. Micro-discharge is a small, self-extinguishing current pulse with a total discharge charge of 3107C and a duration ranging from 50ms to a fewms. Discharge typically occurs in gaps larger than 1mm. The breakdown mechanism of these vacuum gaps indicates that the material and surface condition of the vacuum electrodes are crucial factors for the insulation of the vacuum gap. The insulation withstand capability of the vacuum gap is related to the prior opening and closing operation conditions. The breakdown voltage of the vacuum circuit breaker contact gap varies depending on the opening and closing operation conditions before the withstand voltage test. An Italian engineer from Colombo discussed this issue at an equipment discussion meeting: the test object was a 24kV circuit breaker with copper-chromium contacts, a rated breaking current of 16kA, a rated current of 630A, a contact distance of 15.8mm, a contact opening speed of 1.1m/s, and a closing speed of 0.6m/s. The test procedure is listed in Table 1. The maximum breakdown voltage after the closing-opening operation (test series 2-5) was lower than the value given after the no-load cycle (test series 1), which means that the contact breakdown distance is reduced by the arc current. Meanwhile, the values ​​measured in series 2 and 5 are also lower than the test values ​​in series 3 and 4, while the current zero-crossing waveform and polarity seem to have no significant effect. The test results confirm that the form of opening and closing operation affects the insulation withstand capability between circuit breaker contacts. The breakdown voltage is in the range of 30-50kV, the breakdown distance is between 0.6-2mm, and the electric field strength of the contacts at breakdown is 25-44kV. The results of the above experiments by the engineers in Columbus, Italy, indicate that the decrease in vacuum insulation strength after interrupting a large current is a common phenomenon in vacuum switches. Therefore, in the early days of my country, the gap insulation of vacuum circuit breakers would decrease after breaking a fault, failing to meet the insulation level required by the product's technical specifications. Thus, the Ministry of Energy's ordering requirements for indoor high-voltage vacuum circuit breakers (Ministry Standard DL403-91) allow for testing after the electrical life test of the vacuum circuit breaker, where the inter-pole withstand voltage value is reduced to 80% of the original standard. If the test is passed, the circuit breaker is considered to have passed the type test. So, how do we explain why many vacuum circuit breaker manufacturers repeatedly emphasize in their product presentations that the gap insulation of their vacuum circuit breakers does not decrease after the electrical life test? Let's take a 10kV vacuum circuit breaker as an example: Through technological and process improvements, the inter-electrode insulation level of the vacuum interrupter is significantly improved compared to earlier products, reaching, for example, value A, which is much higher than the withstand voltage value C specified in the product standard (42kV power frequency, 75kV impulse). New products, tested at value C, will naturally not break down. After the electrical life test, the gap insulation level decreases from value A to value B, but since value B > value C, the insulation is checked according to value C, and breakdown will not occur during the test. However, the A' value of older products is greater than value C. New products, tested at value C, will naturally pass. After interrupting a fault, the value drops from A' to B'.