1. Overview Low-voltage current-limiting circuit breakers are among the most widely used electrical products in low-voltage power distribution systems. They can limit fault current while interrupting it, thus possessing a high breaking capacity. Unlike the arc-extinguishing chamber of a general circuit breaker, the arc-extinguishing chamber of a low-voltage current-limiting circuit breaker uses multiple arc-extinguishing grids. During the interruption process, the moving and stationary contacts first separate to generate an arc, which moves to the arc-extinguishing grids under the influence of electromagnetic, thermal, and flow fields. When the arc enters the grids, the near-polar voltage drop of the multiple short arcs causes the arc voltage to rise rapidly, thereby achieving the purpose of current limiting. To achieve a higher arc voltage, the arc-extinguishing chamber of a current-limiting circuit breaker has more grids than that of a general circuit breaker, and they are arranged more closely. This results in greater resistance for the arc when entering the arc-extinguishing chamber, and a longer dwell time at the grid inlet. Consequently, more metal vapor is generated at the grid inlet. Recent studies on low-voltage current-limiting circuit breakers have shown that the arc repeatedly appears both inside and outside the grid at the grid inlet, causing repeated drops in arc voltage, as shown in Figure 1 (1.25 ms/division). This reduces the current-limiting characteristics of the circuit breaker and increases the arcing time. Figure 1 shows the back-breakdown phenomenon during the breaking of a typical current-limiting circuit breaker. In 1988, Yoshiyuki Ikuma et al. from Nagoya University in Japan first observed the back-breakdown phenomenon of the arc using a fast camera. They also used microwave penetration technology to discover that during the breaking process of a low-voltage circuit breaker, before the sudden drop in arc voltage, the gap where back-breakdown is about to occur experiences a temperature rise. This is due to the reflection of the hot gas flow from the arc through the rear wall of the arc-extinguishing chamber into the corresponding area. The entry of ionized gas and the temperature rise reduce the critical electric field strength in the corresponding area, which can lead to back-breakdown. Currently, the short-gap, high-current arc generated during the breaking process of low-voltage current-limiting circuit breakers is a metal vapor-dominated arc. Therefore, to improve the arc voltage of this type of arc, it is necessary to control the metal vapor. In response to this situation, metal vapor jet control technology for current-limiting interruption of low-voltage circuit breakers has been developed in recent years. This paper introduces an insulating material inserted between the grid plates of the arc-extinguishing chamber to form a gap, allowing for more direct cooling of the arc within the grid-type arc-extinguishing chamber. Its effect is equivalent to the cooling effect of the solid wall of a narrow-slit arc-extinguishing chamber, but unlike that, it primarily relies on the gas-generating effect of the wall material. Therefore, we call this new type of arc-extinguishing chamber a hybrid arc-extinguishing chamber. Experiments have shown that using this technology in the arc-extinguishing chamber of a current-limiting circuit breaker can effectively reduce metal vapor jetting. Combined with an arc-isolating plate, it not only eliminates back-side breakdown but also ensures that the arc voltage entering the arc-extinguishing chamber remains consistently high, significantly shortening the arcing time, reducing the allowable energy, and greatly improving the breaking performance of the current-limiting circuit breaker. 2 Experimental Study of the Hybrid Arc-Extinguishing System To observe the arc movement, a two-dimensional fiber optic testing system developed by the Electrical Engineering Department of Xi'an Jiaotong University was used. The detection part of the system consists of 32 optical fibers arranged throughout the arcing area. Because the arc light is very strong, a threshold value is used for easy judgment; any light signal exceeding this value is considered a reflection of the arc. The system converts the optical signal transmitted through the optical fiber into an electrical signal, which is then stored in the computer. The imaging speed can reach 0.83μs, allowing for detailed observation of the arc movement. The experiment uses an LC single-frequency oscillating circuit to supply a 50Hz current. The entire experimental circuit and the current-limiting circuit breaker model are shown in Figure 2. Figure 2: Current-limiting circuit breaker model and experimental circuit. In the experiment, the fiber array of the two-dimensional fiber arc testing system was placed on the wall of the current-limiting circuit breaker. The arc movement observed during the opening process of the current-limiting circuit breaker is shown in Figure 3. Figure 3: Opening process of the current-limiting circuit breaker observed using the two-dimensional fiber arc testing system. From the observation results of the arc movement during the opening process of the current-limiting circuit breaker, it can be seen that the two-dimensional fiber array arc testing system can observe the arc movement during the opening process. In the above figure, each circle represents an optical fiber; if an arc is observed in the optical fiber, the corresponding small circle is filled. The results above show that the arc is generated at 1.5 ms, moves forward at 2.0 ms, begins to enter the arc-extinguishing grid at 3.2 ms, is completely inside the grid at 3.5 ms, and exits the grid at 4.2 ms. This indicates that conventional current-limiting circuit breakers suffer from back-side breakdown during the breaking process, reducing their breaking performance. In this paper, a gas-generating insulating material is directly inserted between the grid plates of the arc-extinguishing chamber, as shown in Figure 4. Under the high temperature of the arc, a large amount of insulating vapor is emitted. This restricts the arc root and, with the help of the vapor generated by the insulating material, further increases the pressure around the arc root, controlling the direction of the jet of metal vapor emitted from the electrodes. Furthermore, the gas generated by the insulating material cools the arc column, increasing the arc resistance and arc voltage. Figure 4 compares the arc-extinguishing chamber grids using VJC technology with those of a conventional arc-extinguishing chamber. Through observation of the arc movement during interruption using a two-dimensional fiber optic arc testing system, this paper finds that in conventional current-limiting circuit breakers, the arc repeatedly experiences back-breakdown during interruption, which corresponds to the drop in arc voltage during the interruption process. After inserting gas-generating insulating material between the arc-extinguishing chamber grids, the insulating plate between the grids restricts the diffusion of hot airflow and generates a large amount of gas, providing more direct cooling of the arc. Therefore, the arc voltage is more stable, and the number of times the arc repeatedly appears outside the arc-extinguishing chamber during the entire interruption process is greatly reduced. Using an arc-isolating plate, due to its optimal ventilation, can suppress the backflow of hot airflow during interruption, control the backward diffusion of metal vapor, and allow a large amount of energy to be discharged. It has good interruption characteristics and can very effectively suppress the occurrence of back-breakdown. From the observation results of the two-dimensional fiber optic arc testing system and the waveform diagram of the arc voltage and current during interruption of the current-limiting circuit breaker, it can be seen that back-breakdown almost never occurs. However, during the interruption, due to the diffusion of a large amount of metal vapor, the arc resistance decreases and the arc voltage continues to drop, with the highest and lowest values ​​sometimes differing by 150V, which is not conducive to improving the interruption characteristics. Considering the advantages and disadvantages of the above structure, a hybrid arc-extinguishing chamber with a narrow slot arc-extinguishing chamber, a grid arc-extinguishing chamber, and an arc-isolating plate is adopted. The arc voltage and current waveforms obtained from multiple experiments of the current-limiting circuit breaker and the arc motion images observed by the two-dimensional fiber array arc test system clearly show that this structure completely suppresses the occurrence of back-side breakdown, and once the arc enters the grid arc-extinguishing chamber, the arc voltage always maintains a high value, and the arcing time and charging energy are minimized. Through the comparison of various structures in the experiment. In Figure 4 above, structures (1) and (2) adopt a narrow slot and grid plate combination. The arc-extinguishing chamber in structure (1) is equipped with an insulating plate with an opening area of ​​50% to prevent flying arc. However, back-side breakdown sometimes occurs during interruption. In structure (2), in addition to using a narrow slit and grid plate, an insulating isolation plate is connected after the grid plate in the arc-extinguishing chamber. This ensures the flow of gas in the arc-extinguishing chamber, weakens the backflow of hot gas, and prevents the arc from running out of the grid plate at the rear of the arc-extinguishing chamber, thus preventing a sudden drop in voltage. Structure (3) also uses this method to weaken the backflow of hot gas, but does not use a narrow slit and grid plate. Figure 4 shows (4) and (5) as side views of (2) and (3), respectively. The arc-breaking voltage and current waveforms of these structures are different. As shown in Figure 5. Figure 5 Breaking characteristics of various arc-extinguishing systems In Figure 5, (1) and (2) are 1.25 ms/division, and (3) is 0.625 ms/division. Based on the experimental results in Figure 5, the comparison shown in Table 1 is as follows: Table 1 Comparison of Breaking Characteristics of Various Arc-Extinguishing Chambers a: Breaking characteristics under general conditions b: Breaking characteristics when using a narrow slot and grid plate combination c: Breaking characteristics when using an arc-isolating plate alone d: Breaking characteristics of a novel hybrid arc-extinguishing system As can be seen from Table 1, in current-limiting circuit breakers with general structures, the average arcing time is relatively long, and the average number of back-breakdowns is also relatively high. The current-limiting circuit breaker model using a narrow slot and grid plate combination generally has no more than one back-breakdown during breaking, but this still affects the improvement of breaking characteristics. The best breaking condition is the current-limiting circuit breaker using a hybrid arc-extinguishing chamber with a narrow slot arc-extinguishing chamber, a grid plate arc-extinguishing chamber, and an arc-isolating plate combination. It not only completely eliminates back-breakdown, but also, during breaking, once the arc enters the grid plate arc-extinguishing chamber, the arc rises rapidly due to the near-electrode voltage drop and maintains this high value thereafter. Therefore, the average arcing time is greatly reduced, by 20% compared to the general structure, significantly improving the breaking characteristics. 3. Conclusion Experiments have shown that the short-gap, high-current arc generated during the breaking process of current-limiting circuit breakers is different from traditional air arcs; this type of arc is dominated by metal vapor. Therefore, to improve the arc voltage of this type of arc, it is necessary to control the metal vapor. This paper studies a novel hybrid arc-extinguishing system that combines a narrow-slit arc-extinguishing chamber, a grid arc-extinguishing chamber, and an arc-isolating plate in a current-limiting circuit breaker. This system controls the metal vapor jet of the short metal arc between the grids while simultaneously reducing the backflow of hot gas in the arc-extinguishing chamber. Experiments demonstrate that the novel hybrid arc-extinguishing system not only effectively suppresses back-side breakdown but also ensures that the arc entering the arc-extinguishing chamber maintains a consistently high arc voltage, effectively improving the breaking performance of the current-limiting circuit breaker.