Vacuum Circuit Breaker Construction Working Advantages & Applications (VCB)

Vacuum Circuit Breakers (VCB) :
Q. Explain, construction, working principle of vacuum circuit breaker with neat diagram.
Q. Explain the principle and working of vacuum circuit breaker.
Q. Explain working of vacuum circuit breaker with neat diagram.
Q. Draw neat diagram of vacuum circuit breaker and explain its operation and state its applications.
Vacuum Circuit Breaker
  • Since vacuum offers the highest insulating strength it can be used as are quenching medium.
  • It posses far superior arc quenching properties than any other medium.
  • Thus the vacuum of the range of 10^ -7 to 10^ -5 bar is used as the arc-quenching medium in vacuum circuit breakers.
Construction of Vacuum Circuit Breakers :
  • Figure (a) shows the cross section of the 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 prevent the deterioration of the internal dielectric strength by preventing metallic vapours falling on the inside surface of the insulating cover.
Working of Vacuum Circuit Breakers :
  • When the fault occurs on the system breaker operates and the moving contact separates from the fixed contact.
  • While separating the contacts an arc is struck between them.
  • The arc is produced due to the ionisation of metal ions, which depends upon the materials of contacts.
  • The arc is quickly extinguished because the metallic vapour electrons and ions produced during arc are diffused in a short time and seized by the surface of moving and fixed members and shields.
  • Since vacuum has very fast rate of recovery of dielectric strength, the arc extinction in a vacuum occurs with a short contact separation (say 0.625 cm).
Vacuum Circuit Breaker
Figure (a)
Advantages of Vacuum Circuit Breakers :
Q. Write any two advantages of vacuum circuit breakers.
Vacuum circuit breaker have the following advantages :
  1. There are on file hazards.
  2. They are compact in size.
  3. They are reliable and have longer life.
  4. There is no generation of any type of gas during and after operation.
  5. They can interrupt any fault current. The outstanding feature of a VCB is that it can break any heavy fault current perfectly just before the contacts reach the definite open position.
  6. Maintenance required is less.
  7. Operation is quiet.
  8. They can successfully withstand lightning surges.
  9. They have low inertia and hence require smaller power for control mechanism.
Applications of Vacuum Circuit Breakers :
Q. write any two applications of vacuum circuit breakers.
        Vacuum circuit breakers are used where maintenance free circuit breakers should prove a definite advantage like a remote area. These circuit breakers are being employed for outdoor applications ranging from 22kV to 66KV and even for limited rating of say 60 to 100 MVA.

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