Vacuum Circuit Breaker is most reliable among other circuit breakers like oil circuit breaker or SF6 circuit breaker. Vacuum is used as arc quenching medium which offers highest insulating strength.
Now let’s dive in details and we will learn that What is vacuum circuit breaker and How it works? Also, The advantages and disadvantages of vacuum circuit breaker, along with the Construction ,working principle and Diagram of vacuum circuit breaker (VCB).
In a vacuum circuit breaker, the vacuum of the order of 10-5 to 10-7 torr is used as an arc quenching medium. (1 torr = 1mm of Hg). Basically, vacuum is a pressure below atmospheric pressure where no gas is present.
They are used for switching and protection purpose in power system.
Vacuum circuit breaker has replaced oil circuit breakers in 1970s and then SF6 circuit breakers in 1980s.
These breakers have highest insulating strength than any other medium. Vacuum circuit breaker does the interruption of current at the first current zero. It means it interrupts the current in the half cycle. You will get to know about this line in detail in later half of this post.
The schematic Diagram of Vacuum Circuit Breaker is shown below. It has an outer envelope which is made up of glass joined with end caps.
This glass shield is used to examine the breaker after operation, if the colour becomes milky white then it indicates that the breaker is losing its vacuum.
Two contacts named as fixed contact and moving contact are placed as shown in the figure. The main arcing process is done in between these two contacts.
The moving contact is connected with metallic bellows which are made up of stainless steel. These bellows are used to move the moving member up and down and a spring mechanism is connected with the bellows to operate them.
Sputter shield made up of stainless steel is present to prevent metal vapour from reaching outer envelope.
The ceramic envelope is also present to insulate the chamber.
During Normal operation, the fixed contact and moving contact both are connected to each other and the current flows through them. But as soon as any fault occurs in the system, the moving contact start separating from the fixed contact and arc phenomenon occurs between them.
Vacuum circuit breakers have a different way of arc generation. The arc is generated due to vaporisation of metal vapours from the contact surface.
A contact surface has large number of microscopic projections and when current reaches to these projections.
Now we know that, after disconnecting the contacts, the last point of current will be the contact surface which results in high current density at these points and due to which resistive heating occurs.
Due to excessive heating, the metal ions starts vaporising from the contact surface.
When these ions come in between of two contacts, then they provide a path to the current and current starts flowing which we see as an arc. This is how arc generates in these breakers.
As Alternating current is sinusoidal in nature. So, the current wave has two zeros in one cycle as shown below.
When this arcing current reaches its first zero (at half cycle) the dielectric strength of the vacuum rapidly builds up and it interrupts the current at first half cycle. The arc will not restrike because of very high insulating strength of vacuum.
After the current interruption at fist half cycle, the metallic ions will condense again on the contact surface.
In this way, the arc and extinguishment and current interruption is done in a vacuum circuit breaker.
Now, we will see the advantages, disadvantages and uses of vaccum circuit breaker.
As we said earlier that VCBs or Vacuum circuit Breaker has many advantages over other breakers. The advantages are listed below:
Frequently Asked Questions
Q. What is the Full form of VCB?
Answer: Vacuum Circuit Breaker
Q. Voltage Range for which VCBs are used?
Answer: 11kV to 33kV
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