Vacuum Circuit Breakers Are Wonderful From Many Perspectives

Vacuum circuit breakers have a lower maintenance requirement and are a better choice for low-to-medium voltage systems. They provide a more effective and reliable arc extinguishment than other types of breaker.

The arc is quickly extinguished in the vacuum due to the ionization of metal vapors. These vapors, along with electrons and ions produced during arcing, rapidly condenses on the surface of the CB contacts, restoring dielectric strength.

Insulation

Vacuum circuit breakers are used in high & medium voltage distribution system for switching load & protecting equipment from short-circuit currents. They are more reliable than other types of circuit breakers & have faster arc quenching & recovery. They are also easy to maintain & don’t require any auxiliary oil system.

Arc interruption in these breaker takes place inside the vacuum chamber of the breaker which consists of a steel arc chamber at the center of symmetrically placed ceramic insulators. The movable contact is housed inside this shield & connected to the control mechanism via stainless steel bellows. The vacuum pressure in this chamber can be maintained up to 10-6 bar. When the air circuit breakers operates, the moving contacts separate from each other in vacuum which results in ionization of metal oil on both of them. This vapor then forms an arc across the gap and interrupts the current at first zero of alternating current wave. This is due to very high insulating strength of the vacuum.

Arc Shield

Vacuum circuit breakers are extremely simple to construct. They mainly consist of fixed contacts, moving contact and an arc shield positioned inside the arc interrupting chamber. The outer envelope of a vacuum circuit breakers is made up of glass. This is important because it aids in evaluating the VCB from outside after its operation. If the glass becomes milky from its original silvery mirror shine, it means that the VCB is losing vacuum.

When the current-carrying contacts separate in a vacuum, it leads to the formation of an electrical arc between them. This arc is produced due to ionization of metal ions and its density depends on the contact material used. This arc is quickly extinguished by the use of vacuum as an insulating and quenching medium. Vacuum offers a high insulating strength as compared to other insulating mediums like air and SF6. Moreover, it has exceptional arc quenching abilities. This makes it an ideal arc interrupter for capacitor switching.

Moving Contacts

Vacuum circuit breakers, also called VCBs, use stainless steel metallic bellows to move the movable contact and break its connection with the fixed one. The broken connection ionizes metal vapors, which then create an electrical arc between the contacts. This arc must be extinguished quickly to prevent damage to equipment. VCBs are used in high-voltage applications, such as transformers and sizable motors, and are often paired with static overcurrent relays.

When the VCB’s current carrying contacts are separated, the temperature of their connecting parts becomes extremely high due to ionization. This ionization fills the space between the contacts with positive ions, which then discharge from the contact material. The arc is then interrupted at the first current zero point, and its dielectric strength recovers rapidly. This is the main reason why vacuum circuit breakers have a higher efficiency than other types of breaker. Their life span is also longer than other breaker types.