1. Introduction Low-voltage circuit breakers are among the most widely used electrical products in low-voltage power distribution systems. To obtain a higher arc voltage, the plates of the circuit breaker's arc-extinguishing chamber are arranged closely. This results in greater resistance for the arc entering the arc-extinguishing chamber and a longer dwell time at the plate inlet. Recent research on low-voltage circuit breakers has shown that the arc repeatedly appears inside and outside the plate at the plate inlet, leading to repeated drops in arc voltage—this is the phenomenon of back-side breakdown. It reduces the breaking performance of the circuit breaker and increases the arcing time. In 1988, Yoshiyuki Ikuma et al. from Nagoya University in Japan first observed this back-side breakdown phenomenon using a high-speed 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, a temperature rise occurs in the contact gap. 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 is one of the causes of back-side breakdown. French researchers, including C. Fievet, also discovered that the temperature remains high in the area traversed by the arc, resulting in residual current that can lead to back-side breakdown via thermal breakdown. Based on the analysis of back-side breakdown and the principle of thermal breakdown, they established a dynamic arc model based on magnetohydrodynamics to simulate the mechanism of back-side breakdown. Using advanced high-speed optical testing equipment and a multi-channel oscilloscope, they conducted extensive experiments on a low-voltage circuit breaker model, finding that electromagnetic fields suppress back-side 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-side breakdown, improving the breaking characteristics of the current limiter. Based on this, a novel arc-extinguishing chamber structure capable of eliminating back-side breakdown was proposed. 2 Research and Analysis of the Mechanism of Back Breakdown In recent years, people have discovered the instability of arc movement during the interruption of low-voltage circuit breakers through modern testing technology. During the arc extinguishing process, the arc moves multiple times inside and outside the arc extinguishing chamber, resulting in a drop in arc voltage, i.e., back breakdown. The reignited arc enters the arc extinguishing chamber multiple times until it is extinguished. A large number of experiments have found that during the interruption of low-voltage circuit breakers, before the back breakdown phenomenon occurs, there is a temperature rise outside the arc extinguishing chamber of the grid. This is because the hot gas flow of the arc is reflected by the back wall of the arc extinguishing chamber and generates backflow, the conductivity of the corresponding area increases, the critical field strength decreases, and it is easy to cause back breakdown. 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 when the arc has left the arc ignition point for several milliseconds, there is still a current of several amperes in the initial region of the arc. This indicates that back-breakdown is related to the gas temperature outside the arc extinguishing chamber, the critical electric field strength, and the conductivity. Based on this model, we conducted in-depth research and established an arc dynamic model based on magnetohydrodynamics, using the principle of thermal breakdown. Calculation results show that the model established based on this current redistribution principle is consistent with actual conditions. Back-breakdown is more likely to occur, especially when the temperature outside the arc extinguishing chamber is high and the residual current is large. This is consistent with C. Fievet's experimental results. Because the near-electrode voltage drop remains relatively high, 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 sharply, resulting in back-breakdown. The arc had already exited the arc extinguishing grid at 2.16 ms. This indicates that thermal breakdown is one of the causes of back-breakdown. 3. Measures to Eliminate Back-Breakdown We studied various factors that may eliminate back-breakdown. 3.1 Effect of External Magnetic Field A magnetic field can accelerate the movement speed of the arc, allowing it to enter the arc-extinguishing chamber more 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 using the current flowing through the circuit breaker. With an external two-turn coil, when the expected current was 2000A, the arc voltage drop was quite severe. 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 of times decreased. From the experimental results, increasing the arc-blowing magnetic field reduced the number of arc voltage drops, but the back-breakdown phenomenon still existed. 3.2 Effect of Airflow Field The airflow field has a very direct impact on the back-breakdown phenomenon of the circuit breaker. Because poor gas flow will cause hot air to flow back, and because it causes the arc to linger in front of the arc-extinguishing grid for a longer time, a hot area for back-breakdown is easily formed in the front of the arc-extinguishing chamber. Research indicates that adding an insulating arc-blocking plate behind the grid plates allows for the smooth exhaust of hot air within the arc-extinguishing chamber without arc flash. Experiments have shown that this significantly limits back-side breakdown and essentially eliminates voltage drops. However, the arc voltage gradually decreases to a relatively low value, reducing breaking performance. Therefore, further measures are needed. To address this, we directly insert gas-generating insulating material between the grid plates of the arc-extinguishing chamber and simultaneously add an arc-blocking plate at the rear of the chamber. 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. This type of narrow-slit arc-extinguishing chamber, a hybrid arc-extinguishing chamber combining grid plates and an arc-blocking plate, is employed. The waveforms of the arc voltage and current 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-side breakdown. Furthermore, once the arc enters the arc-extinguishing chamber, the arc voltage remains consistently high, and the arcing time and allowable energy are minimized. 4. Conclusion Back-side breakdown often occurs during the interruption of low-voltage circuit breakers, affecting their breaking performance. This paper analyzes back-side breakdown and, based on the principle of thermal breakdown, establishes an arc dynamic model based on magnetohydrodynamics to simulate the mechanism of back-side breakdown. Experiments show that increasing the arc-blowing magnetic field can suppress back-side breakdown to a certain extent, and the airflow conditions within the arc-extinguishing chamber have a direct impact on back-side breakdown. Improving the return flow of hot air and its retention within the arc-extinguishing chamber is beneficial for eliminating back-side 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.