Working of Vacuum Circuit Breaker, Construction, Advantages & Application:
Introduction:
A vacuum circuit breaker is used for switching as well as protection of the connected equipment, so they have to be very fast-acting to ensure that during any fault the breaker automatically trips before the fault current can cause damage to the equipment. In this article lets discuss in detail Working of Vacuum Circuit Breaker, its Construction, Advantages & at last its advantages.
Circuit Breaker
Circuit breakers provide a manual means of energizing and de-energizing a circuit and automatic over current protection. Unlike fuses, which must be replaced when they open, a circuit breaker can be reset once the over current condition has been corrected. Pushing the handle to the “OFF” position then back to the “ON” position restores the circuit. Circuit breaker usually operate under transient state of short-circuit current.
Circuit Breaker Operation
In the following illustration, an AC motor is connected through a circuit breaker to a voltage source. When the circuit breaker is closed, a complete path for current exists between the voltage source and the motor allowing the motor to run. Opening the circuit breaker breaks the path of current flow and the motor stops. The circuit breaker automatically opens when it senses a fault. After the fault has been cleared, the breaker can be closed, allowing the motor to operate.
VCB full form – Vacuum Circuit Breaker
Vacuum Circuit Breaker:
A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in vacuum. The technology is suitable for mainly medium voltage application. In such breakers, vacuum (degree of vacuum being in the range from 10 to 10 torr) is used as the arc quenching medium. Since vacuum offers the highest insulating strength, it has far superior arc quenching properties than any other medium. For example, when contacts of a breaker are opened in 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 circuit breakers.
Vacuum Circuit Breaker Diagram
Working Principle of Vacuum Circuit Breaker
The production of arc in a vacuum circuit breaker and its extinction can be explained as follows: When the contacts of the breaker are opened in vacuum (10 to 10 torr), an arc is produced between the contacts by the ionization of metal vapours of contacts. However, the arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc rapidly condense on the surfaces of the circuit breaker contacts, resulting in quick recovery of dielectric strength. The reader may note the salient feature of vacuum as an arc quenching medium. As soon as the arc is produced in vacuum, it is quickly extinguished due to the fast rate of recovery of dielectric strength in vacuum.
Construction:
VCB Breaker Parts Name
Figure shows the parts of a typical vacuum circuit breaker. It consists of fixed contact, moving contact and arc shield mounted inside a vacuum chamber. 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 leak. A glass vessel or ceramic vessel is used as the outer insulating body. The arc shield prevents the deterioration of the internal dielectric strength by preventing metallic vapours falling on the inside surface of the outer insulating cover.
Working of Vacuum Circuit Breaker
When the breaker operates, the moving contact separates from the fixed contact and an arc is struck between the contacts. The production of 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 vapours, 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 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).
Advantages of the Vacuum Circuit Breaker
Vacuum circuit breakers have the following advantages:
- They are compact, reliable and have longer life.
- There is no generation of gas during and after operation.
- They can interrupt any fault current. The outstanding feature of a V C B is that it can break any heavy fault current perfectly just before the contacts reach the definite open position.
- They require little maintenance and are quiet in operation.
- They have low arc energy.
- They have low inertia and hence require smaller power for control mechanism.
Application of Vacuum Circuit Breaker
- For a country like India, where distances are quite large and accessibility to remote areas difficult, the installation of such outdoor, maintenance free circuit breakers should prove a definite advantage.
- Vacuum circuit breakers are being employed for outdoor applications ranging from 22 kV to 66 kV. Even with limited rating of say 60 to 100 MVA, they are suitable for a majority of applications in rural areas.
Vacuum Circuit Breaker FAQ
Where are vacuum circuit breakers used?
Circuit-breaker vacuum interrupters are used primarily in the power sector in substation and power-generation facilities, and load-switching vacuum interrupters are used for power-grid end users.Vacuum circuit breaker`s are being employed for outdoor applications ranging from 22 kV to 66 kV. Even with limited rating of say 60 to 100 MVA, they are suitable for a majority of applications in rural areas.
Why is vacuum used in VCB?
VCB uses vacuum media for quenching the arc.When the contacts of the breaker are opened in vacuum (10 to 10 torr), an arc is produced between the contacts by the ionization of metal vapours of contacts. However, the arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc rapidly condense on the surfaces of the circuit breaker contacts, resulting in quick recovery of dielectric strength
What is vacuum circuit breaker used for?
A vacuum circuit breaker is used for switching as well as protection of the connected equipment, so they have to be very fast-acting to ensure that during any fault the breaker automatically trips before the fault current can cause damage to the equipment. They interrupt an electric circuit to prevent unwarranted current, caused by a short-circuit, typically resulting from an overload
What is difference between ACB and VCB?
VCB are suitable for HV circuit breaker. ACB are suitable for medium voltage use.
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