Indoor Vacuum Circuit Breaker: Operation & Maintenance
Overvoltage Protection
Vacuum circuit breakers possess excellent current-breaking performance. However, when interrupting inductive loads, the rapid change in current can generate high overvoltages across the inductance, which requires attention.
When switching small-capacity motors, starting currents are relatively high; measures should be taken to reduce the inrush current.
For transformers, the need for protection varies with design. Oil-immersed transformers have high impulse voltage withstand capability and large stray capacitance, generally not requiring additional protective devices. However, for dry-type transformers with lower impulse withstand levels or furnace transformers subject to frequent switching and lagging currents, protective measures such as metal-oxide surge arresters, distributed cable capacitance, or added shunt capacitors are recommended.
For vacuum circuit breakers used in feeder protection, the long line length provides sufficient stray capacitance, and multiple connected devices help suppress high restriking overvoltages. Thus, special protective measures are typically unnecessary.
For capacitor banks, field tests show that overvoltages during closing operations generally do not exceed twice the system voltage. In China, shunt capacitors are typically used at voltages below 60 kV, where equipment insulation levels are sufficient to withstand such overvoltages without damage. However, vacuum circuit breakers with poor mechanical performance may exhibit prolonged contact vibration during operation, leading to high overvoltages—phenomena observed in both domestic and international tests and thus require attention.
Strict Control of Closing and Opening Speeds
If the closing speed of a vacuum circuit breaker is too low, pre-arc time increases, accelerating contact wear. Additionally, as vacuum interrupters typically use copper welding and high-temperature degassing processes, their mechanical strength is relatively low and they are sensitive to vibration. Excessively high closing speeds can cause significant mechanical shock, imposing strong forces on the bellows and reducing their service life. Typically, the closing speed of vacuum circuit breakers ranges from 0.6 to 2 m/s, with an optimal value depending on the specific design.
During interruption, the arcing time is very short—usually less than 1.5 power-frequency half-cycles. To ensure sufficient dielectric strength at the first current zero, it is generally required that the contact travel reaches 50%–80% of the total stroke within the first half-cycle. Therefore, the opening speed must be strictly controlled.
Furthermore, both the opening and closing dampers should have good performance characteristics to minimize mechanical impact during operation, thereby extending the service life of the vacuum interrupter.
Strict Control of Contact Travel
It is incorrect to assume that a larger contact gap benefits arc extinction and arbitrarily increase the contact travel. Vacuum circuit breakers have relatively short contact strokes. For rated voltages of 10–15 kV, the typical contact stroke is only 8–12 mm, with an over-travel of 2–3 mm. Excessively increasing the contact travel can impose excessive stress on the bellows after closing, potentially damaging the bellows and compromising the vacuum seal of the interrupter.
Strict Control of Load Current
Vacuum circuit breakers have limited overload capacity. Due to the vacuum between the contacts and the enclosure acting as a thermal insulator, heat from the contacts and conductive rods is mainly dissipated through conduction along the rod. To ensure the operating temperature remains within allowable limits, the working current must be strictly controlled and kept below the rated value.
Rigorous Acceptance Testing upon Commissioning
Although vacuum circuit breakers are thoroughly tested before factory shipment, after transportation and on-site installation, key parameters must be re-measured and verified to detect any changes due to handling or misalignment between the breaker and operating mechanism. Key parameters to verify include:
Closing bounce
Opening synchronization
Contact gap (opening distance)
Compression travel
Closing and opening speeds
Closing and opening times
DC contact resistance
Interrupter insulation level
Mechanical operation tests
All results must meet the manufacturer’s technical specifications before the breaker is put into service.
Maintenance Intervals for Vacuum Circuit Breakers
Maintenance intervals should follow established regulations and be adjusted based on actual operating conditions. It is a misconception that vacuum circuit breakers require no maintenance. Specific guidelines include:
Perform power-frequency withstand voltage tests across the interrupter poles during seasonal or annual preventive maintenance to assess vacuum integrity.
After 2,000 normal operation cycles (making/breaking load current) or 10 interruptions of rated short-circuit current, inspect all screws for looseness. Maintenance should follow the manufacturer’s instructions. If all parameters remain within acceptable limits, the breaker may continue in service.
If a vacuum circuit breaker has been out of service or in storage for 20 years, its vacuum level should be tested using the specified method for vacuum interrupters. If the vacuum does not meet requirements, the interrupter must be replaced.
Vacuum Interrupter
The vacuum interrupter is the core component of a vacuum circuit breaker. It uses glass or ceramic envelopes for structural support and hermetic sealing, containing moving and stationary contacts along with a shield. The interior is under high vacuum, typically 1.33 × 10⁻⁵ to 1 Pa, ensuring reliable arc interruption and insulation performance.
Once the vacuum level degrades, the interrupting capability deteriorates significantly. Therefore, the vacuum interrupter must be protected from any external impact—no knocking, tapping, or applying force during handling or maintenance. Never place objects on top of the circuit breaker to avoid accidental impact.
Manufacturers perform strict parallelism checks and precise assembly before delivery. During maintenance, all interrupter mounting bolts must be uniformly tightened to ensure even stress distribution and prevent damage.
