LGE3630NN | Eaton molded case circuit breaker accessory frame | Eaton

Vacuum circuit breakers use a vacuum as an arc extinguishing medium. This allows the breaker to interrupt current at the first current zero and offers high dielectric strength recovery.

VCBs have an outer envelope made of glass joined with end caps. This helps in examination of the vacuum circuit breakers after its operation.


Vacuum circuit breakers offer a high insulating strength as compared to other interrupting mediums like air, SF6, etc. They are primarily used for power distribution applications.

When the moving and fixed contacts are separated within a vacuum environment, typically ranging from 10-7 to 10-5 torr, an arc is formed through the ionization of metal vapors emitted by the contact surfaces. Unlike other arc quenching media, such as oil, which requires a large quantity of arc-generating heat to vaporize the liquid, the arc in a vacuum is rapidly extinguished. Stainless steel metallic bellows are employed to actuate the movement of the moving contacts. The design of these bellows is critical to the life of a molded breakers. It ensures that they can operate repeatedly under heavy load conditions, thus allowing them to serve a variety of switching tasks. They are especially suited to interrupting double earth faults and phase currents in railway applications. They are also used in industrial applications, where they manage electric supply switching and traction current.


Vacuum circuit breakers use a vacuum as their primary arc quenching medium. This allows the VCB to quickly extinguish an electrical arc between the fixed contact and moving contact. This is done using a mechanism called the actuator mechanism, which forces the movable contact to break its connection with the fixed contact. This ionizes the metal ions and causes the arc to disappear in seconds.

The arc extinguishing process is dependent on the shape and material of the contacts, but in general a VCB’s moving and fixed contact are made from copper-chrome or copper bismuth, which transfer electricity well. The insulating vessel is also made from stainless steel metallic bellows, which are designed to be maintenance free and perform without vibration or noise when switching on and off. The arc interrupting chamber is housed within an outer insulating envelope, which is typically glass. This allows the VCB to be examined from outside after its operation, if needed. The outer envelope should be shiny and smooth, but if it turns milky from its original silvery mirror finish then it indicates that it has lost its vacuum.



Vacuum circuit breakers are used in medium voltage switchgear applications as they have a high fault interrupting capacity and excellent recovery. This is due to the fact that they interrupt current by pushing the contacts apart in a vacuum, which increases the insulating strength hundreds of times compared to air and SF6.

When the movable contact of a vacuum circuit breaker is broken, arcs are produced by the ionisation of metal ions in the contacts. These arcs are quickly extinguished because metallic vapours, electrons and ions created in the arc are diffused within short time and seized by the surfaces of moving and fixed contacts and shields. The moving contact of a vacuum circuit breaker is moved by stainless steel metallic bellows. The outside envelope is constructed of glass, which helps to examine the VCB after it has been operated. The arc quenching chamber, current terminals and traction insulator make up the rest of the construction.