Working principle of Vacuum Circuit Breaker | Explained

• January 9, 2025
• Gaurav Joshi

Before we understand what is the working principle of vacuum circuit breaker, it is important to know why we need circuit breakers. Let’s start with that. In this blog, you’ll learn –

• Why do we need circuit breaker
• Working principle of vacuum circuit breaker
• The famous race between RRRV & RRDS
• Sequence of current interruption
• Construction of Vacuum circuit breaker
• FAQs of Vacuum circuit breaker

Why Do We Need Circuit Breakers?

The function of the circuit breaker is to help us in breaking the current. It could be normal current or it could be an abnormal (fault) current. The fact is, the moment we break the flow of current using a switch or circuit breaker, an arc is established between the separated contacts. Arc is nothing but the flow of current happening even if the contacts are separated. Arc is inevitable. The intensity of the arc depends up on the current level. For the normal currents, the intensity will be small. However, for fault currents where the current values are huge, the intensity of the arc would be very high.

For smaller voltage level, we do not need any special arrangement to break the current and extinguish the arc. However, once we go up in the voltage level lets say above 1000V like 12kV or 36kV – we need to provide special arrangements for current and arc interruption. And hence, circuit breaker is required. Vacuum is one of the special arrangement that can help us break the current and the arc in a very efficient way.

What is the Vacuum Circuit Breaker working principle?

First lets understand how we can interrupt the arc. This is important because once the arc is interrupted the flow of current will completely stop. So, why there is an arc? –

Arc is due to the free electrons that are present in the surroundings of the separated contacts.

Imagine, I have an enclosure with two contacts in it which is surrounded by the air. When the fault occurs, I will separate this contacts to break the current and protect the other equipment from getting damaged. The moment contact separates, the surrounding air gets ionized and since the voltage is also present across the two separated contacts, the arc is established. This will allow the fault current to flow. The current is not yet interrupted.

The major reason for the arc establishment is the free electrons in the air. And hence, if we can some how remove these free electrons, we will be able the extinguish the arc. One way to achieve this is to place the current carrying contacts inside an empty space (vacuum), where there are no or very less free electrons present. In such scenario, if we open the contacts under the fault condition the arc will stuck but, since there are no free electrons available the arc will be extinguished during the next current zero (Alternating current sinusoidal wave).

The Famous Race Between RRRV and RRDS

At the current zero, the rate of rise of recovery voltage (RRRV) will come into picture. If the RRRV is stronger than the rate of rise of dielectric strength (RRDS) around the contacts, it will reignite the arc. And the fault current will keep on flowing. Therefore, we have to make sure that, the dielectric strength around the contact is stronger than that of the RRRV. Only then a successful interruption will happen.

In the race between RRRV & rate of rise of dielectric strength, the rate of rise of dielectric strength must win to achieve the successful interruption.

Vacuum helps in maintaining the higher dielectric strength which supports in clearing the arc. So, we can say, the vacuum is a dielectric medium and not the interruption medium. This is the working principle of vacuum circuit breaker.

Sequence of Current Interruption in vacuum circuit breaker

Let us summarise the complete sequence of current interruption in vacuum circuit breakers.

1. Fault is detected by the relays and command is given to circuit breaker to trip.
2. Contacts separates in the vacuum bottle.
3. Arc is stuck due to the ionisation.
4. Since, the contacts are in vacuum, where there are no sufficient free electrons available for arc to continue the arc extinguishes.
5. As soon as the arc is quenched, no current will flow through the circuit breakers, preventing the circuit from being damaged.

Want to master the circuit breaker control schematics? Here is the expert guidance on the same – “Circuit Breaker Control Schematics Masterclass“

Construction of the Vacuum Circuit Breaker

VCB has limited parts when compared to the other types of breaker like SF6. This makes the VCB rigid and reliable option.

1. Fixed & Moving contacts – As we seen initially, we need two contacts which will help us in breaking the current and arc. In VCB, the design of the contact is crucial.
2. Vacuum Interrupter – The contacts must be kept inside a hermetically sealed bottle. This is done with the help of vacuum bottles. This is where the current interruption happens. These bottles are sealed for life.
3. Operating Mechanism – We know that, to break the current, we must separate the contacts. To do this, we need an operating mechanism. In vacuum circuit breaker, spring mechanism is used for open and close operation of the breaker with the right amount of force.
4. Insulating Enclosure – The whole construction is built within a closed insulating structure to shield it from exterior conditions.

Conclusion

Vacuum circuit breakers represent a significant advancement in electrical safety and reliability. By utilizing vacuum as a dielectric medium, they efficiently manage and extinguish arcs, ensuring the integrity of electrical systems even under extreme conditions. Their compact design, robustness, and eco-friendly operation make them a preferred choice in high-voltage applications. As the demand for safe and sustainable solutions grows, vacuum circuit breakers will continue to play a pivotal role in the future of electrical systems.

Frequently asked questions

Watch the video on “How Vacuum Circuit Breaker works” –