How Do Vacuum Circuit Breakers Work?

Vacuum circuit breakers are often used in high voltage electrical systems to perform the same function as breakers in a home’s electrical panel. The main function of a vacuum circuit breaker is to cut off power to electrical outlets in case of a short circuit or electrical malfunction to allow manual disconnection of circuits so that they can be repaired. A notable feature of vacuum circuit breakers is that they contain evacuated spaces.

 

Breaker Basics

To understand how vacuum circuit breakers work, it is important to first understand how normal circuit breakers work. Basic circuit breakers are made of two metal contacts linked together by a spring-loaded mechanism that acts as trip.

The spring-loaded trip mechanism contains a mechanical heat sensor that monitors levels of electricity passing through electrical outlets. The mechanical heat sensor trips off the breaker when temperature levels increase due to overloading electrical conductors.

A magnetic sensor on the spring loaded trip mechanism trips off the breaker in case of a short circuit. The trip mechanism helps to control overload and short circuits by pulling the contacts apart to stop the flow of electricity in the circuit.

 

VS1-Series vacuum circuit breakers

 

Vacuum Power

As the name suggests, vacuum circuit breakers contain a vacuum between the contacts. The vacuum is just an empty space that does not contain any solid, liquid, or gas. A vacuum is one of the best insulators available. It does not provide room for arcing and electricity cannot pass through a vacuum. Therefore, a vacuum is a suitable arc-extinguishing medium. Some high voltage breakers use a blast of high-pressure air to blow arcs apart. Others are filled with special oil that prevents the arcing. A considerable drawback of the air blast breakers is that they are highly complicated. On the other hand, oil-filled breakers are flammable and are highly toxic in case the oil leaks.

 

Arcing Problem

Circuit breakers often arc between the contacts for a few microseconds when they trip off pulling the electrical contacts apart. The arcing of the circuit breaker may ionize air and make it conduct electricity or provide a path for the electricity to go past the breaker. This is one of the main reasons why high voltage systems use vacuum circuit breakers because they cannot be ionized providing a path for electricity to go past the breaker. Although vacuum circuit breakers also experience arcing, electricity cannot go past the breaker before the contacts pull away from each other.

 

Vacuum Circuit Breaker Working Principle

It has a very simple construction in comparison to an air circuit breaker or oil circuit breaker. In a vacuum circuit breaker (VCB) current carrying contacts are separated in the vacuum. The separation of contacts and interruption of arc occurs in a vacuum chamber in the VCB, which is known as vacuum interrupter.

 

 

The outer envelope of vacuum interrupter is normally made of glass because the glass envelope facilitates the examination of the breaker from outside. A sputter shield, usually made of stainless steel is placed between the contacts and the envelope to prevent the metal vapours reaching the envelop because it reduces the breakdown strength between contacts.

Inside the sputter shield, VCB has one fixed and one moving contact. The distance between contacts is kept between 5 to 10 mm depending upon operating voltage.

The metallic bellows made of stainless steel is used for moving the lower contact. Copper-bismuth, copper-lead and copper-chromium are some of the alloys used as contact material.

The lower end of the breaker is fixed to a spring-operated or solenoid operated mechanism so that the metallic bellows inside the vacuum interrupter chamber can be moved upward and downward during closing and opening operations respectively.

It is very necessary to understand the arc of VCB to understand the working principle of a vacuum circuit breaker. In other circuit breakers arc is formed due to ionization of particles in the medium between the contacts. But a vacuum arc is formed by evaporating its surface material and continued to until the next current zero instant.

At current zero instant, the arc is quenched, and contact metal vapours are condensed again on the contact surface. Due to the high dielectric strength of vacuum, vacuum arc can be quenched within very small contact gap.

The current must be interrupted at zero current instant; otherwise, high voltage transients will be induced due to current chopping. Therefore, for successful arc interruption, it should be stable before current zero instant.

The arc stability depends upon the contact material very much. Thus arc extinction process in a VCB is related to the material and shape of the contacts and the method used in condensing the metal vapour. The contact surface is so designed that the arc root keeps on moving so that temperature at one point on the contact does not attain a very high value.

 

Applications

Vacuum circuit breakers, air blast breakers, and oil-filled breakers all perform exceptionally well in transmission lines because these electrical systems do not trip very often. In addition, the three types of breakers require minimal maintenance when used in transmission line applications. However, vacuum circuit breakers perform better than the other two types of breakers in handling line spikes and surges. In addition, vacuum breakers perform much better in high voltage applications that require frequent connection and disconnection of electrical systems.

Circuit breakers are essential electrical components because they help to minimize the effects of temperature and power surges on electrical fixtures.