Working Principle of Vacuum Circuit Breaker

• September 24, 2025
• Gaurav Joshi

Vacuum circuit breaker is one of the most reliable and efficient current interruption technologies available today. It dominates the medium voltage switchgear industry. Experts also believe it will soon lead the high and extra-high voltage industry. But why is this technology so reliable? And how does it actually work?

In this article, we will explain the working principle of vacuum circuit breaker step by step.

Table of Contents

1. Why Do We Need Circuit Breakers
2. What Causes the Arc
3. Role of Vacuum in Arc Quenching
4. Current Zero and Recovery
5. Why Vacuum Is More Reliable
6. Challenges in Vacuum Circuit Breakers
7. Sequence of Interruption in Vacuum Circuit Breaker
8. Components of a Vacuum Circuit Breaker
9. Summary of the Working Principle of Vacuum Circuit Breaker
10. Conclusion

Why Do We Need Circuit Breakers?

Electric current flows continuously through power systems. But there are times when the current must be interrupted. The simplest way is to open a switch or cut the conductor. Cutting a conductor is not practical or safe. So we use switches and circuit breakers.

When circuit breaker contacts are closed, they carry current. But the moment we open these contacts, an arc forms. The arc is unavoidable. It happens when current flows through the air between contacts.

In low voltage systems, the arc is weak. A normal switch can handle it. But in high and extra-high voltage systems, the arc is very strong. At levels like 12 kV, 36 kV, 145 kV, 245 kV, and higher, the arc intensity is huge. We need a special medium to quench it. Vacuum is one such medium.

What Causes the Arc?

The arc forms because of free electrons present around the contacts. When contacts open, these free electrons ionize due to voltage pressure. The ionization forms the arc.

Let’s imagine a simple case. Two contacts of a circuit breaker are placed inside a chamber filled with air. When the contacts close, current flows smoothly. But when a fault occurs, we must open them. At that moment, free electrons in the air around the contacts ionize. The voltage across the contacts pushes them, and the arc forms.

The presence of the arc means current is still flowing. The fault current is not interrupted. This is not a successful operation. True interruption happens only when the arc is extinguished.

So, the main reason the arc forms is the presence of free electrons near the contacts.

Role of Vacuum in Arc Quenching

If we can remove these free electrons, the arc cannot last. This is where vacuum plays an important role. Vacuum has no solids, liquids, or gases. It provides an empty medium.

When contacts open inside a vacuum chamber, the arc forms for a very short time. But there are not enough free electrons available. The arc dies out at the next current zero of the AC waveform.

This is the key working principle of vacuum circuit breaker. The absence of free electrons ensures the arc extinguishes quickly.

Current Zero and Recovery

During interruption, another factor comes into play. It is the rate of rise of recovery voltage, or RRRV. When current approaches zero, the recovery voltage tries to force current to continue.

If RRRV is greater than the dielectric strength around the contacts, the arc reignites. Interruption fails. Current still flows in the system. This is the worst case.

So, as design engineers, the goal is clear. The dielectric strength around the contacts must rise faster than RRRV. Only then is the interruption successful.

Vacuum ensures this condition. It allows dielectric strength to grow very fast. As a result, the arc does not reignite. The interruption is reliable.

Why Vacuum Is More Reliable

Vacuum circuit breakers score higher than other technologies. In SF6 breakers, the gas has environmental impacts. Vacuum, on the other hand, is clean and safe.

In vacuum, dielectric strength always outpaces recovery voltage. That ensures reliable operation. This makes the working principle of vacuum circuit breaker simple, efficient, and environment-friendly.

Challenges in Vacuum Circuit Breakers

One issue does exist. When the arc forms, it heats the contacts inside the vacuum interrupter. Heated contacts can cause problems. In some cases, they may even reignite the arc.

This issue is critical. It can lead to failure of the vacuum circuit breaker. But it needs detailed study. Engineers often explore this in advanced discussions.

Sequence of Interruption in Vacuum Circuit Breaker

The interruption process follows a clear sequence:

1. The system detects a fault.
   
2. The relay sends a command to the circuit breaker.
   
3. The circuit breaker trips.
   
4. Contacts open.
   
5. An arc forms for a brief time.
   
6. In vacuum, there are no free electrons.
   
7. The arc extinguishes quickly at current zero.
   
8. Current stops flowing.
   
9. The circuit is safe.
   

This entire process happens within milliseconds. It is fast, efficient, and highly reliable.

Components of a Vacuum Circuit Breaker

The interrupter, or vacuum bottle, is the core part. But it is not enough alone. Other parts are necessary.

Inside the bottle, there are two contacts:

• Fixed contact
  
• Moving contact
  

There is also a bellow, which allows the moving contact to shift.

To move the contacts, an operating mechanism is required. We cannot do this by hand. The most common mechanism is spring-operated. The springs open the contacts at the right speed and pressure.

The interrupter and mechanism are assembled inside insulating materials. These materials protect the vacuum bottle and mechanism from the outside environment.

This complete assembly is what we call a vacuum circuit breaker.

Summary of the Working Principle of Vacuum Circuit Breaker

• Current flows when contacts are closed.
  
• A fault occurs. Relay sends a trip signal.
  
• Contacts open.
  
• Arc forms briefly due to ionization.
  
• Vacuum has no free electrons.
  
• Arc extinguishes quickly at current zero.
  
• Recovery voltage cannot reignite the arc.
  
• Current interruption is successful.
  
• Circuit is protected.
  

Conclusion

The working principle of vacuum circuit breaker is simple and effective. It relies on the absence of free electrons in vacuum to extinguish the arc. The dielectric strength rises faster than recovery voltage, ensuring no reignition. The entire process is fast, clean, and reliable.

For better clarity and visual understanding, it is recommended to watch the video demonstration of this concept.