Abstract: Based on the principle of thermal breakdown, a dynamic arc model was established to simulate the back-breakdown phenomenon. The influence of various factors on the back-breakdown phenomenon was analyzed, and a novel hybrid arc extinguishing system that can eliminate the back-breakdown phenomenon was proposed. Keywords: Low-voltage circuit breaker, back-breakdown, arc 1 Introduction Low-voltage circuit breakers are one of the most widely used electrical products in low-voltage power distribution systems. In order to obtain a higher arc voltage, the grid plates of the circuit breaker's arc extinguishing chamber are arranged closely. This results in greater resistance to the arc when entering the arc extinguishing chamber, and a longer dwell time at the grid plate entrance. Recent studies on low-voltage circuit breakers have shown that the arc repeatedly appears inside and outside the grid plate at the grid plate entrance, leading to repeated drops in arc voltage, which is the back-breakdown phenomenon. It reduces the breaking performance of the circuit breaker and increases the arcing time. In 1988, Yoshiyuki Ikuma et al. of Nagoya University, Japan, first observed this back-breakdown phenomenon using a high-speed camera. They also used microwave penetration technology to find that during the breaking process of low-voltage circuit breakers, before the arc voltage suddenly drops, the temperature rises in the contact gap. This is because the hot gas flow of the arc enters the corresponding area through reflection from the rear wall of the arc-extinguishing chamber. The entry of free gas and the temperature rise reduce the critical electric field strength in the corresponding area, which is one of the reasons for back-side breakdown. C. Fievet et al. in France also found that the temperature is still high in the area where the arc passes, and there is residual current, which will lead to back-side breakdown in the form of thermal breakdown [1]. Professor Manfred Lindmayer in Germany initially proposed a back-side breakdown model based on thermal breakdown [2]. Figure 1 shows the typical waveform of back-side breakdown. Figure 1 Typical waveform of back-side breakdown Through the analysis of back-side breakdown, based on the principle of thermal breakdown, an arc dynamic model based on magnetohydrodynamics was established, and the mechanism of back-side breakdown phenomenon was simulated. Using advanced high-speed optical testing equipment and a multi-channel oscilloscope, numerous experiments were conducted on a low-voltage circuit breaker model, revealing that electromagnetic fields suppress back-breakdown in low-voltage circuit breakers. By altering the structure of the arc-running zone before the arc-extinguishing chamber, different gas flow conditions were created. Experiments demonstrated that a suitable gas flow condition facilitates rapid arc entry into the arc-extinguishing chamber, causing a rapid rise in arc voltage, which suppresses or even eliminates back-breakdown, improving the breaking characteristics of the current limiter. Based on this, a novel arc-extinguishing chamber structure capable of eliminating back-breakdown is proposed. 2. Research and Analysis of the Mechanism of Back-Breakdown In recent years, modern testing techniques have revealed the instability of arc movement during low-voltage circuit breaker breaking. During arc extinguishing, the arc transfers multiple times inside and outside the arc-extinguishing chamber, leading to a drop in arc voltage, i.e., back-breakdown. The reignited arc repeatedly enters the arc-extinguishing chamber until extinguishing. Numerous experiments have shown that during the breaking process of low-voltage circuit breakers, before the occurrence of back-breakdown, a temperature rise occurs outside the arc-extinguishing chamber. This is because the hot gas flow of the arc is reflected by the back wall of the arc-extinguishing chamber, resulting in a backflow. The conductivity of the corresponding area increases, the critical field strength decreases, and back-breakdown is more likely to occur. C. Fievet et al. in France found [1] that when the arc enters the arc-extinguishing chamber, due to the near-pole voltage drop of multiple short arcs and the large conductivity of the hot gas outside the grid, the internal and external currents are redistributed inside and outside the arc-extinguishing chamber of the circuit breaker. By measuring the current with a Rogowski coil, it was found that even a few milliseconds after the arc has left the arc-starting point, there is still a current of several amperes in the initial region of the arc. This indicates that the back-breakdown phenomenon is related to the gas temperature outside the arc-extinguishing chamber, the critical electric field strength, and the conductivity. Professor Manfred Lindmayer in Germany initially proposed a back-breakdown model based on thermal breakdown [2]. Based on this model, we conducted in-depth research and established an arc dynamic model based on magnetohydrodynamics according to the principle of thermal breakdown. The calculation results show that the model established based on this current redistribution principle is consistent with the actual situation. Especially when the temperature outside the arc-extinguishing chamber is high and the residual current is large, back-breakdown is prone to occur. This is consistent with the experimental results of C. Fievet. In Figure 2, the arc has entered the arc-extinguishing grid at 1.92 ms, the arc voltage rises rapidly, and the equivalent resistance of the arc remains relatively high due to the near-electrode voltage drop, while the resistance of the back-breakdown region continuously decreases. As the resistance of the back-breakdown region gradually decreases, the current is gradually diverted by this conductive channel, causing the temperature of this region to rise rapidly, the resistance to decrease rapidly, and the arc voltage to drop suddenly, resulting in back-breakdown. The arc has exited the arc-extinguishing grid at 2.16 ms. This shows that thermal breakdown is one of the causes of back-breakdown. Figure 2 Simulated arc back-breakdown phenomenon 3 Measures to eliminate back-breakdown phenomenon We studied various factors that may eliminate back-breakdown phenomenon. 3.1 Influence of external magnetic field The magnetic field can accelerate the movement speed of the arc, making it enter the arc-extinguishing chamber quickly and reducing the dwell time in front of the arc-extinguishing grid. In the experiment, two magnetic plates were clamped on both sides of the arc-extinguishing chamber, and an external arc-blowing magnetic field was generated by the current flowing through the circuit breaker. With an external two-turn coil, the arc voltage drop was severe when the expected current was 2000A. When the expected current was increased to 3000A and 4000A respectively, the number of arc voltage drops decreased, and the drop amplitude also decreased. With an external multi-turn coil, the arc voltage rose rapidly, and the voltage drop phenomenon still existed, but the number decreased. The experimental results show that increasing the arc-blowing magnetic field reduced the number of arc voltage drops, but the back-breakdown phenomenon still existed. The results obtained in the experiment are shown in Table 1, with 3000A as an example. Table 1 Breaking characteristics under different arc-blowing magnetic fields. 3.2 Influence of airflow field The airflow field has a very direct influence on the back-breakdown phenomenon of the circuit breaker. Because poor gas flow causes hot air to flow back, and because it causes the arc to stagnate in front of the arc-extinguishing grid for a longer time, a hot area prone to back-breakdown is easily formed in the front of the arc-extinguishing chamber. According to research, adding an insulating arc-blocking plate behind the grid plates allows the hot gas flow inside the arc-extinguishing chamber to escape smoothly without arcing. Experiments showed that this greatly limits back-side breakdown and essentially eliminates voltage drops. However, the arc voltage gradually drops to a relatively low value, reducing breaking performance. Therefore, other measures are needed. The breaking characteristics of a fully open arc-extinguishing chamber are shown in Figure 3. To address this, we directly inserted gas-generating insulating material between the grid plates of the arc-extinguishing chamber and added an arc-blocking plate at the rear, as shown in Figure 4. Under the high temperature of the arc, a large amount of insulating vapor is emitted. This restricts the expansion of 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 raising the arc voltage. Source: http://www.tede.cn Figure 4 The new hybrid arc extinguishing system uses this narrow-slit arc extinguishing chamber, that is, a hybrid arc extinguishing chamber that combines grid plates and arc-blocking plates. The arc voltage and current waveforms obtained from multiple experiments of the current-limiting circuit breaker and the arc motion images observed using a two-dimensional fiber array arc testing system clearly show that this structure completely suppresses back-end breakdown, and once the arc enters the grid plate arc extinguishing chamber, the arc voltage always maintains a high value, with minimal arcing time and allowable energy. The breaking characteristics of the new hybrid arc extinguishing system are shown in Figure 5. Figure 5 Breaking characteristics of the new hybrid arc extinguishing system We also compared this new arc extinguishing system with several existing arc extinguishing systems. The comparison results are shown in Table 2 after multiple experiments. Table 2 Comparison of Breaking Characteristics of Various Arc-Extinguishing Chambers A – Breaking characteristics under general conditions B – Breaking characteristics when using narrow slots and grid plates C – Breaking characteristics using only arc-isolating plates (Figure 3) D – Breaking characteristics of the novel hybrid arc-extinguishing system (Figure 5) From the experimental results above, it can be seen that the novel arc-extinguishing system eliminates back-breakdown, reduces arcing time, and greatly improves the breaking performance of low-voltage current-limiting circuit breakers. 4. Conclusion Back-breakdown often occurs during the breaking of low-voltage circuit breakers, affecting their breaking performance. This paper analyzes back-breakdown and, based on the principle of thermal breakdown, establishes an arc dynamic model based on magnetohydrodynamics to conduct a mechanism simulation study of back-breakdown. Experiments show that increasing the arc-blowing magnetic field can suppress back-breakdown to a certain extent, and the airflow conditions within the arc-extinguishing chamber have a direct impact on back-breakdown. Improving the return flow of hot air and its retention within the arc-extinguishing chamber is beneficial for eliminating back-breakdown. Experiments have shown that the new arc-extinguishing system for current-limiting circuit breakers, which combines the arc-extinguishing chamber with the arc-isolating plate and incorporates an insulating gas-generating plate, not only effectively suppresses back-side breakdown but also ensures that the arc entering the arc-extinguishing chamber always has a stable and high arc voltage, thus effectively improving the breaking performance of the current-limiting circuit breaker. Authors' Affiliations: Chen Xu (Xi'an Jiaotong University 710049) Wei Qiang (Xi'an Jiaotong University 710049) Chen Degui (Xi'an Jiaotong University 710049) Reference Source: http://tede.cn [1] Fievet C, Petit P, et al. Residual conduction in low voltage circuitbreaker. The Eleventh International Conference on Gas Discharges and Their Applications, Chuo University, Tokyo, 1995 [2] Manferd Lindmayer. Simulation of stationary current-voltage characteristics and of back-commutation in rectangular arc channels. [3] Niemeyer L. Evaporation dominated high current arcs in narrow channels, IEEE Trans. on Plasma Science, 1978, 97(3) [4] Yuan Haiwen. Research on arc motion and its two-dimensional fiber optic array digital test system in low voltage current limiting circuit breaker: [Doctoral dissertation]. Xi'an Jiaotong University, 1997