Among all failure types of vacuum circuit breaker(VCB), the most serious and dangerous hidden fault is the decrease in vacuum degree in the vacuum interrupter, which leads to vacuum loss. It carries a higher risk priority than mechanism jamming, contact overheating, spring fatigue, secondary circuit faults and other issues. As the core component of a VCB, the vacuum interrupter relies on its internal high-vacuum environment to extinguish electric arcs and provide insulation isolation. This fault is highly concealed and can hardly be predicted in advance through visual inspection or routine live patrol checks. This article is a full review of this hazard from five dimensions: fault phenomenon, cause analysis, fault hazards, handleing method and preventive measures.
1. Fault Phenomenon
Current interruption and arc extinction take place inside the vacuum bottle of a VCB. Since VCB are not equipped with built-in devices for qualitative or quantitative vacuum level monitoring, vacuum degradation is categorized as a latent fault, which is far more dangerous than evident faults.
2. Cause Analysis
The primary causes of reduced vacuum degree are:
Defects in the material or manufacturing process of the vacuum bottle, resulting in micro-leakage points on the vacuum bottle itself;
(2) Defects in the material or fabrication of the bellows inside the vacuum bottle, which develop leakage points after repeated operations;
(3) For split-type vacuum circuit breaker (e.g., VCB fitted with electromagnetic operating mechanisms), the long operating connecting rod adversely affects the breaker's synchronism, contact bounce and over-travel characteristics during operation, accelerating the rate of vacuum degree reduction.
3. Fault Hazards
The decrease in Vacuum level seriously impairs the VCB's overcurrent interruption capacity and sharply shortens its service life. In extreme cases, it may lead to breaker explosion.
4. Handling method
(1) During scheduled power-outage maintenance of circuit breakers, a vacuum tester must be used to perform a qualitative vacuum degree test on the vacuum bulb to ensure it maintains a sufficient vacuum degree;
(2) Once the decrease of vacuum degree is confirmed, the vacuum bottle shall be replaced, followed by characteristic tests covering stroke, synchronism and contact bounce.
5. Preventive Measures
(1) Select mature VCB products supplied by reputable manufacturers;
(2) Select a vacuum circuit breaker with an integrated main body and operating mechanism;
(3) During routine inspections, operating personnel should check for any signs of discharge on the exterior of the circuit breaker's vacuum bulb. If discharge is observed, the vacuum level test results for the vacuum bulb will generally be deemed unsatisfactory, and the vacuum bulb must be replaced promptly after de-energizing the circuit;
(4) Maintenance personnel shall perform characteristic tests for synchronism, contact bounce, stroke and over-travel during power-outage maintenance to keep the circuit breaker in sound operating condition.
Conclusion
The fundamental performance of a vacuum circuit breaker hinges on qualified vacuum inside its interrupter. Vacuum failure means the loss of core capabilities for arc extinction and short-circuit protection. For switchgear manufacturers, EPC contractors and power station operation & maintenance teams, including vacuum interrupter integrity inspection in regular maintenance is the top priority to ensure long-term safe and reliable operation of vacuum circuit breaker.