One Article to Understand How to Select Mechanical Parameters of Vacuum Circuit Breakers

1. Rated Contact Gap

When a vacuum circuit breaker is in the open position, the distance between the moving and fixed contacts inside the vacuum interrupter is known as the rated contact gap. This parameter is influenced by several factors, including the breaker’s rated voltage, operating conditions, nature of the interrupting current, contact material, and dielectric strength of the vacuum gap. It primarily depends on the rated voltage and contact material.

The rated contact gap significantly affects the insulation performance. As the gap increases from zero, the dielectric strength improves. However, beyond a certain point, further increasing the gap yields diminishing returns in insulation performance and may severely reduce the mechanical life of the interrupter.

Based on installation, operation, and maintenance experience, typical rated contact gap ranges are:

• 6kV and below: 4–8 mm

• 10kV and below: 8–12 mm

• 35kV: 20–40 mm

2. Contact Travel (Overtravel)

Contact travel must be selected to ensure that sufficient contact pressure is maintained even after contact wear. It also provides the moving contact with initial kinetic energy during opening, increasing initial opening speed to break welded joints, reduce arcing time, and accelerate dielectric recovery. During closing, it allows the contact spring to provide smooth buffering, minimizing contact bounce.

If contact travel is too small:

• Insufficient contact pressure after wear

• Low initial opening speed, affecting breaking capacity and thermal stability

• Severe closing bounce and vibration

If contact travel is too large:

• Increased closing energy required

• Reduced reliability of closing operation

Typically, contact travel is 20%–40% of the rated contact gap. For 10kV vacuum circuit breakers, this is generally 3–4 mm.

3. Contact Operating Pressure

The operating pressure of a vacuum circuit breaker’s contacts has a significant impact on performance. It is the sum of the vacuum interrupter’s inherent self-closing force and the contact spring force. Proper selection must meet four requirements:

• Maintain contact resistance within specified limits

• Meet dynamic stability test requirements

• Suppress closing bounce

• Reduce opening vibration

Closing under short-circuit current is the most demanding condition: pre-arc currents generate electromagnetic repulsion, causing contact bounce, while closing speed is at its lowest. This scenario critically tests whether the contact pressure is sufficient.

If contact pressure is too low:

• Increased closing bounce time

• Higher main circuit resistance, leading to excessive temperature rise during continuous operation

If contact pressure is too high:

• Increased spring force (since self-closing force is constant)

• Higher closing energy requirement

• Greater impact and vibration on the vacuum interrupter, risking damage

In practice, contact electromagnetic force depends not only on peak short-circuit current but also on contact structure, size, hardness, and opening speed. A comprehensive approach is essential.

Empirical data for contact pressure based on interrupting current:

• 12.5 kA: 50 kg

• 16 kA: 70 kg

• 20 kA: 90–120 kg

• 31.5 kA: 140–180 kg

• 40 kA: 230–250 kg

4. Opening Speed

Opening speed directly affects the rate at which dielectric strength recovers after current zero. If the recovery of dielectric strength is slower than the rising recovery voltage, arc re-ignition may occur. To prevent re-ignition and minimize arcing time, adequate opening speed is essential.

Opening speed depends primarily on rated voltage. For fixed voltage and contact gap, the required speed varies with interrupting current, load type, and recovery voltage. Higher interrupting currents and capacitive currents (with high recovery voltage) require higher opening speeds.

Typical opening speed for 10kV vacuum breakers: 0.8–1.2 m/s, sometimes exceeding 1.5 m/s.

In practice, initial opening speed (measured over the first few millimeters) has a greater impact on breaking performance than average speed. High-performance and 35kV vacuum breakers often specify this initial speed.

While higher speed seems beneficial, excessive speed increases opening vibration and over-travel, intensifying stress on the bellows and leading to premature fatigue and leakage. It also increases mechanical stress on the mechanism, risking component failure.

5. Closing Speed

Due to the high static dielectric strength of vacuum interrupters at rated gap, the required closing speed is significantly lower than opening speed. Adequate closing speed is necessary to minimize pre-arc electrical erosion and prevent contact welding. However, excessive closing speed increases closing energy and subjects the interrupter to greater impact, reducing service life.

Typical closing speed for 10kV vacuum breakers: 0.4–0.7 m/s, up to 0.8–1.2 m/s if required.

6. Closing Bounce Time

Closing bounce time is a key indicator of vacuum circuit breaker performance. It is influenced by contact pressure, closing speed, contact gap, contact material, interrupter design, breaker structure, and installation/adjustment quality.

Shorter bounce time indicates better performance. Excessive bounce causes severe electrical erosion, increases risk of overvoltage, and may lead to contact welding during short-circuit or capacitor switching operations, as well as thermal stability tests. Prolonged bounce also accelerates bellows fatigue.

For 10kV vacuum breakers with copper-chromium contacts, closing bounce should not exceed 2 ms. For other materials, it may be slightly higher but should not exceed 5 ms.

7. Three-Pole Synchronism

Three-pole synchronism measures the degree of simultaneity in closing or opening of the three poles. Since opening and closing synchronism values are similar, only closing synchronism is typically specified.

Poor synchronism severely affects breaking capacity and prolongs arcing time. Due to fast operating speeds and small gaps, precise adjustment can easily meet requirements. Closing synchronism is generally required to be within 1 ms.

8. Alignment of Moving and Fixed Contacts (Coaxiality)

Proper coaxial alignment of moving and fixed contacts is critical for vacuum interrupter performance and is ensured through manufacturing precision. Whether this alignment is maintained after installation depends on the operating mechanism type and assembly process.

For suspended mechanisms, alignment is primarily determined by the mechanism itself. For floor-mounted types, mechanical alignment is equally important. During installation, avoid applying shear or lateral forces to the interrupter.

Typical coaxiality tolerance: ≤2 mm.