What Is Vacuum Circuit Breaker And Its Construction And Working
Vacuum Circuit Breaker (VCB) is a Circuit breaker where vacuum of the order of 10-6 to 10-10 torr is used as an arc quenching medium. 1 torr is equivalent to a pressure represented by a barometric head of 1 mm mercury. Vacuum Circuit Breaker is used for low and medium voltage applications.
Construction of Vacuum Circuit Breaker
Vacuum Circuit Breaker consists of Vacuum interrupter, Enclosure, Contacts, Vapor Condensing Shield, Metallic Bellows and Seal.
Vacuum Interrupter: A vacuum interrupter has a steel arc chamber in the center and symmetrically arranged ceramic insulators.
Refer the figures below showing the main parts of a typical vacuum interrupter. Modern constructions of this interrupter have a metal shield surrounding the arcing contacts.
The diameters of the contacts and their stems are matched with those of the arc chamber and insulators.The moving contacts are made movable by the use of metallic bellows.
The arc chamber is welded to the housing flange, which in turn are brazed to metallised ceramic insulators, thus giving a hermetically sealed interrupter. The vacuum pressure is generally 10-6 bar.
Enclosure: The enclosure is made of impermeable insulating material like glass. The enclosure must not be porous and should retain high vacuum of the order of 10-7 torr.
Contacts:There are two types of contacts, moving and fixed. The moving contact is connected with large stem connected to operating mechanism of breaker. Contacts of Vacuum Circuit Breaker have generally disc shaped faces. The disc is provided with symmetrical grooves in such a way that the segments of the two contacts are not in the same line. The magnetic field set-up by the components of currents with such geometry causes the plasma of the arc to move rapidly over the contacts instead of remaining stable at one point. The concentration of the arc is thus prevented and the arc remains in diffused state. The sintered material used for contact tip are generally copper-chromium or copper bismuth alloy.
Vapor Condensing Shield: These metallic shields are supported on insulating housing such that they cover the contact region. The metal vapor released from the contact surface during arcing is condensed on these shields and is prevented from condensing on the insulting enclosure.
Metallic Bellows: One end of the bellows is welded to the enclosure. The other end is welded to the moving contact. The bellows permit the sealed construction of the interrupter and yet permit movement of the contact. Stainless steel bellows are generally used in vacuum interrupters. Carefully observe every component of Vacuum Circuit Breaker as shown in figure below.
Working Of Vacuum Circuit Breaker :
- When the breaker operates, the moving contact separates from the fixed contact and an arc is struck between the contacts. The production of the arc is due to the ionization of metal ions and depends very much upon the material of contacts.
- The arc is quickly extinguished because the metallic vapors, electrons, and ions produced during arc are diffused in a short time and seized by the surfaces of moving and fixed members and shields.
- Since vacuum has a very fast rate of recovery of dielectric strength, the arc extinction in a vacuum breaker occurs with a short contact separation (say 0.625 cm).