What is the purpose of the vacuum bulb in a vacuum circuit breaker?

The vacuum bubble(vacuum interrupter) in a vacuum circuit breaker is its most crucial component, playing a vital role. Simply put, it's a device that reliably interrupts and insulates circuits in a high-vacuum environment.

Its main functions and principles can be explained in detail from the following aspects:

1. Core Function: Arc Extinguishing

When a circuit breaker needs to interrupt current (especially high current or fault current), the separation of the moving and stationary contacts generates a high-temperature, high-pressure electric arc. If not extinguished promptly, the arc will continue to conduct electricity, burning out the equipment.

A vacuum bubble provides a near-ideal gas-free environment (internal pressure below 10^-4 Pa). In this high vacuum, gas molecules are extremely rare, and the ionized matter required for arc combustion is very scarce.

When an arc is generated between the contacts, it is actually maintained by a plasma of metal vapor produced by the evaporation of the contact material itself.

Because the vacuum environment is extremely favorable for the diffusion of the arc plasma, when the current crosses zero, the metal vapor diffuses, condenses, and recombines into a solid at an extremely high speed into the surrounding vacuum region, causing the dielectric strength of the contact gap to recover rapidly. The arc can therefore not be sustained and is "extinguished."

2. Key Advantage: Insulation

After a circuit breaker trips, the contacts must withstand the system voltage without breaking down.

Vacuum is an excellent insulating medium, with insulation strength far exceeding that of air, and even exceeding that of transformer oil and SF6 gas (at high pressure). A small vacuum bulb can provide sufficient insulation distance, allowing vacuum circuit breakers to be designed very compactly.

3. Structural Composition (to aid in understanding its function)

A vacuum bubble mainly consists of:

Moving/stationary contacts: usually made of special alloys (such as copper-chromium alloys), which are arc resistant, weld resistant, and have good conductivity.

The moving and stationary conductive rods are internal contacts, externally connected to the main circuit.

(1) One end of the stationary conductive rod is brazed to the stationary contact, and the other end is brazed to the fixing plate. The fixing plate is used to fix the vacuum interrupter to the vacuum circuit breaker and connect the stationary conductive rod of the vacuum interrupter to the main circuit.

(2) One end of the moving conductive rod is brazed to the moving contact. The moving conductive rod passes through the bellows and guide sleeve and extends out of the outer casing. The diameter of the extended part of the moving conductive rod is small, which is used to install the conductive clamp.

There is a screw hole at the center of the end. This screw hole is used to connect to the mechanical operating system. The moving conductive rod drives the moving contact to move axially to complete the closing and opening actions. The guide sleeve mainly ensures the linear movement of the moving conductive rod and prevents the moving conductive rod from twisting, which would damage the bellows and cause vacuum leakage in the interrupter.

Shielding Covers:

Main Shielding Cover: Surrounds the contacts, absorbs the metallic vapor generated by the electric arc, and prevents it from condensing on the inner wall of the ceramic shell, thus preventing insulation degradation.

Equalizing Shielding Cover: Optimizes the internal electric field distribution.

The primary function of the shielding cover is to shield against the metal vapor and molten metal droplets generated during the current interruption process. During current interruption, the vacuum arc between the contacts causes metal vapor and droplets (arc products) to be ejected from the contact material and dispersed in all directions. These arc products mainly deposit on the inner surface of the shielding cover.

Without the shielding cover, the arc products would deposit on the insulating outer shell, damaging its insulation. The shielding cover also prevents arc products from reflecting back into the contact gap, causing after-arc reignition and re-breakdown. Furthermore, the shielding cover helps to distribute the electric field more uniformly within the vacuum interrupter, thereby improving the insulation level of the vacuum interrupter.

Bellows: A sealed, expandable metal component. It connects the moving contact rod and the housing, allowing the moving contact to move under the drive of an external operating mechanism while maintaining the complete seal of the vacuum bubble. This is a key component for ensuring the vacuum life.

A bellows is an alloy elastic element with a corrugated wall. Under external force, it can lengthen or shorten along its axis. In a vacuum interrupter, one end of the bellows is welded to the flange on the center hole of the moving flange, and the other end is welded to the moving conductive rod.

Through the axial expansion and contraction of the bellows, the moving conductive rod can move axially within a certain range without compromising the airtightness of the outer casing. In the vacuum interrupter, the outside of the bellows is a vacuum, while the inside is atmospheric. An atmospheric pressure difference exists between the inside and outside of the bellows. This pressure presses the moving and stationary contacts tightly together, creating a self-closing force.

Ceramic housing: Provides insulation and support, and forms a permanent high-vacuum sealed cavity with the metal end caps at both ends through special processes (such as brazing).

The outer shell of the vacuum interrupter is both a vacuum container and an insulator between the moving and stationary contacts.

The core advantages of vacuum extinguishing chambers 

Strong Arc Extinguishing Capacity:Rapid and reliable fault current interruption, particularly suitable for frequent operation.

Long Lifespan:Minimal contact erosion, resulting in long electrical and mechanical lifespans.

High Safety:Oil-free, SF6 gas-free, eliminating the risk of fire, explosion, or greenhouse gas leakage.

Small Size and Light Weight:Due to the high strength of vacuum insulation, the required arc-extinguishing chamber is small.

Simple Maintenance:Vacuum extinguishing chambers are fully sealed, requiring no internal inspection; they are typically designed for "maintenance-free" operation (although periodic checks of vacuum level and mechanical properties are necessary).

Important Notes

The vacuum level of the vacuum circuit breaker is its lifeline. Once the vacuum level is lost (leaking), its insulation and arc-extinguishing capabilities will completely fail. Therefore, during operation and maintenance, its vacuum status needs to be indirectly determined through withstand voltage tests or specialized vacuum level detectors.

The vacuum circuit breaker is a non-repairable component; once damaged or with an unacceptable vacuum level, it must be replaced entirely.

In short, the vacuum circuit breaker is the "heart" of the vacuum circuit breaker. It utilizes a high vacuum environment to achieve rapid, safe, and efficient arc extinguishing and reliable electrical isolation, making it the most mainstream arc-extinguishing technology in modern medium-voltage power distribution systems.

Every arc-extinguishing chamber manufactured by Shaanxi Huadian undergoes rigorous testing of all parameters (power frequency withstand voltage, lightning withstand voltage, vacuum degree measurement, etc.) to ensure 100% performance compliance and provides a complete traceability system.We sincerely invite cooperation; please contact us if you are interested.

Email:pannie@hdswitchgear.com.

Whatsapp/Wechat:+8618789455087