One Article to Understand the Contact Separation Stages of a Vacuum Circuit Breaker
Vacuum Circuit Breaker Contact Separation Stages: Arc Initiation, Arc Extinction, and Oscillation
Stage 1: Initial Opening (Arc Initiation Phase, 0–3 mm)
Modern theory confirms that the initial contact separation phase (0–3 mm) is critical to the interrupting performance of vacuum circuit breakers. At the beginning of contact separation, the arc current always transitions from a constricted mode to a diffused mode—the faster this transition, the better the interruption performance.
Three measures can accelerate the transition from a constricted to a diffused arc:
Reduce the mass of moving components: During the development of vacuum circuit breakers, reducing the mass of the conductive clamp helps decrease the inertia of moving parts. Comparative tests show that this approach improves the initial opening speed to varying degrees.
Increase the force of the opening spring, ensuring it becomes effective during the early opening phase (0–3 mm).
Minimize the contact compression travel (ideally 2–3 mm), allowing the opening spring to engage in the separation process as early as possible.
Traditional circuit breakers typically use a plug-in contact design. Under short-circuit current, electromagnetic forces cause the finger contacts to grip the conductive rod tightly, resulting in zero force component in the direction of motion. In contrast, vacuum circuit breakers use a flat contact interface. When a short-circuit current occurs, the strong electromagnetic force acts as a repulsive force on the contacts.
This means contact separation does not need to wait for the full release of the contact compression spring—the separation occurs almost simultaneously with the movement of the main shaft (with negligible or minimal lag). Therefore, with minimal compression travel, the opening spring can act earlier, enhancing the initial opening speed. Since the initial driving force in this phase is the electromagnetic repulsion, the mass to be minimized includes all moving components. Thus, structural designs such as split-type or assembled mechanisms—often involving long and numerous linkages—are unsuitable for vacuum circuit breakers, as they hinder the achievement of high initial opening speeds.
Stage 2: Arc Extinction (3–8 mm)
When the contacts separate to 3–4 mm, the arc transition to a diffused mode is typically complete—this is the optimal window for arc extinction. Extensive testing has confirmed that the ideal arcing gap for interruption is 3–4 mm. If current zero occurs at this point, the density of metal vapor decays rapidly, and the dielectric strength across the gap recovers quickly, resulting in successful interruption. The driving force in this second stage is the opening spring.
In a three-phase system, if arc extinction occurs at the first current zero, the arcing time is approximately 3 ms (assuming contacts separate midway between two current zeros, by which time the gap is sufficiently large). To achieve extinction at a 3–4 mm gap, the average opening speed during this phase should be 0.8–1.1 m/s. When converted to the commonly used 6 mm measurement, the equivalent average opening speed is about 1.1–1.3 m/s—a range widely adopted by vacuum circuit breakers worldwide. However, this data is obtained from mechanical operation tests under no-load conditions. During high-current interruption, the actual opening speed is significantly higher due to the additional electromagnetic repulsive force contributing to contact motion. As a result, within the same time frame, the moving contact may travel 6–8 mm.
To minimize arcing time, special damping measures should be applied in the second stage to rapidly reduce the speed of the conductive rod. The timing of oil buffer engagement must be carefully controlled. The first stage requires fast separation, but the opening spring has not yet fully engaged. In the second stage, speed should be reduced—the opening spring must not be too strong, or it will prevent speed reduction, prolong arcing time, and complicate the third stage.
Stage 3: Oscillation (8–11 mm)
Due to the small contact gap and short opening duration in vacuum circuit breakers, the rapidly moving contacts must be stopped within an extremely short time. Regardless of the damping method used, the rate of velocity change remains high, making strong mechanical shock unavoidable. Residual vibration typically persists for about 30 ms. Currently, both domestic and international vacuum circuit breakers take approximately 10–12 ms for the moving contact to separate and enter the vibration zone, while arcing duration is typically 12–15 ms. Clearly, the locally molten contact surface begins to cool and solidify only after entering the vibration zone. This intense vibration inevitably splashes molten metal, forming sharp protrusions on the contact surface and leaving suspended metallic particles between the contacts—key external factors contributing to restrikes. Such design flaws are often not fully revealed in limited type tests, leading to insufficient awareness of this issue for a long time.
Conclusion
Designers of vacuum circuit breakers must pay close attention to the entire contact separation process. Key strategies include: reducing moving mass, increasing initial opening speed, promptly reducing speed in the second stage, and minimizing arcing time so that the arc extinguishes before the contacts enter the vibration zone. This provides sufficient cooling time for the contact surface and reduces vibration intensity. A well-designed separation profile—aligned with these mechanical and electrical principles—significantly enhances both mechanical and electrical service life, improving overall reliability and performance.
