In such vacuum circuit breaker, the arc quenching medium is vacuum (degree of vacuum being in the range from 10-7 to 10-5 torr ). As vacuum is height insulating strength, so the vacuum has far superior arc quenching than others medium. When contacts of breaker are opened in vacuum, the interruption occurs at first zero with dielectric strength between the contacts building up at a rate thousands of times higher that obtained with other circuit breakers.
From the figure, we can see that, Vacuum circuit breaker consists of fixed contact, moving a contact and vacuum interrupter. The movable member is connected to the control mechanism by operating rod. A glass vessel or ceramic vessel is used as the outer insulating body.
Sectional view of vacuum circuit breaker is shown in fig, When the contact separate, the current to be interrupted initiates a metal vapor arc discharge and flows through this plasma until the next current zero. The arc is then extinguished and the conductive metal vapor condenses on the metal surface within a matter of microseconds. As a result, the dielectric strength in the break builds up very rapidly.
The self-generated field causes the arc causes the arc root to travel, thereby preventing local overheating when large currents are being interrupted. The rapid buildup of the dielectric strength in the break enables the arc to be safety extinguished even if contact separation occurs immediately prior to current zero. The maximum arcing time for the last pole to clear is stated to be 15 ms.
Further, the arc voltage developed in interrupter is low ( say between 20 to 200 V )due to the high conductivity of metal vapor plasma. For these reasons the arc energy developed in the vacuum circuit break is very small. The voltage rating of vacuum circuit Breakers is up to rated voltage of 66 KV. This type of breaker is especially suited for industrial application, where the switching frequency is high combined with a high degree of pollution.
Advantage of Vacuum circuit Breaker:
- The vacuum circuit breaker is compact, reliable and longer life.
- The vacuum circuit breaker has no fire hazards.
- It is no generation of gas during and after the operation.
- It can interrupt any fault current.
- This requires little maintenance and arc quiet in operation.
- It can successfully withstand lightning surge.
- The vacuum circuit breaker has low arc energy.
- It has low inertia and hence requires smaller power for the control mechanism.