Vacuum Circuit Breaker (VCB) – Principle, Construction and Working

What is VCB?

VCB stands for Vacuum Circuit Breaker. In vacuum circuit breakers, the vacuum is used as the arc quenching medium.

Vacuum offers the highest insulating strength. So it has far superior arc quenching properties than any other medium (oil in oil CB, SF6 in SF6 circuit breaker).

For example, when contacts of a breaker are opened in the vacuum, the interruption occurs at first current zero with dielectric strength between the contacts building up at a rate thousands of times higher than that obtained with other types of circuit breakers. 

The degree of vacuum is in the range from 10^-7 to 10^-5 torr.

The technology is suitable for mainly medium voltage switchgear application. For higher voltage vacuum technology has been developed but not commercially viable. 

Principle of Vacuum Circuit Breaker

When the contacts of the breaker are opened in the vacuum (10^-7 to 10^-5 torr), an arc is produced between the contacts by the ionization of metal vapors of contacts.

However, the arc is quickly extinguished because the metallic vapors, electrons, and ions produced during arc rapidly condense on the surfaces of the circuit breaker contacts, resulting in a quick recovery of dielectric strength. 

The salient feature of vacuum as an arc quenching medium is that as soon as the arc is produced in the vacuum, it is quickly extinguished due to the fast rate of recovery of dielectric strength in the vacuum. 

Construction of Vacuum Circuit Breaker

Typical parts of the vacuum circuit breaker are shown in the figure. 

It consists of fixed contact, moving contact and arc shield mounted inside a vacuum chamber (vacuum interrupter). It is shown as an insulating vessel in the given figure.

The movable member is connected to the control mechanism by stainless steel bellows. This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of the leak.

A glass vessel or ceramic vessel is used as the outer insulating body. 

In principle, 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.

Interrupter contacts as a key part made of copper – chromium (CuCr) material with spiral shape have low contact wear characteristics and withstand voltage is excellent.

Spiral contacts make the arc generated between the surfaces of contacts rotated around the surface of contact by the induced magnetic field generated due to the spiral contact structure. This results in preventing local heating, thereby corruption and interrupting instantaneously.

Vacuum circuit breakers are employed for outdoor applications ranging from 22kV to 66kV. Even with limited rating say 60 to 100MVA, they are suitable for the majority of applications in rural areas.

The working of Vacuum circuit breakers is briefly explained below,

• 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).