In such vacuum circuit breaker, the arc quenching medium is vacuum. In a vacuum circuit breaker, the degree of vacuum being in the range from 10-7 to 10-5 torr . As the vacuum is height insulating strength, so the vacuum has far superior arc quenching than others medium for circuit breaker. When contacts of the circuit 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 than 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.
Multiple Choice Question (MCQ) of Electronics page-17:241. Which of the following statement is true? a) The saturation voltage VCF of silicon transistor is more than germanium transistor. b) The saturation voltage VCE for germanium transistor is more than silicon transistor. c) The saturation voltage VCE for silicon transistor is same as that for germanium. d) The saturation voltage VCE for silicon transistor is lower than germanium transistor.
Multiple Choice Question (MCQ) of Electronics page-16:226. Which of the following statement is correct? a) Inner electrons are always present in the semiconductor. b) Bound electrons are always present in the semiconductor. c) Free electrons are always present in the semiconductor. d) Inner and bound electrons are always present in the semiconductor.
Multiple Choice Question (MCQ) of Electronics page-15:211. The materials whose electrical conductivity is usually less than 1 × 106 mho/m are a) Semiconductors b) Conductors c) Insulators d) Alloys
Multiple Choice Question (MCQ) of Electronics page-14:196. In which of the following device the base resistors are not added in the package but added externally? a) UJT b) CUJT c) PUT d) None of the above
Multiple Choice Question (MCQ) of Electronics page-13:181. The conduction in JEFT is always by the a) Majority carriers b) Minority carriers c) Holes d) Electrons e) Holes and electrons simultaneously