Vacuum Circuit Breaker

Vacuum Circuit Breaker
Image Credit- Siemens Energy

In Vacuum circuit breakers, vacuum (degree of vacuum being in the range 10-7 to 10-5 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 obtained with other circuit breaker.

Principle:-

The production of a arc in vacuum circuit breaker and its extinction can be explained as follows;-

When the contacts of the breaker are opened in vacuum (10-7 to 10-5 torr), an arc is quickly extinguished because the metallic vapor, electrons and ions produced during arc rapidly condense on the surface of the circuit breaker contacts resulting in quick recovery of dielectric strength. 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:-

Above figure shows the parts of a typical vacuum circuit breaker. It consist of fixed 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 vapors falling on the surface of the outer insulating cover.

Working:-

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 quickly extinguished because the metallic vapors, electrons and ions produced during arc are diffused in a short time and seized by the surface of moving and fixed members and shield since vacuum has very fast rate of recovery dielectric strength. The arc extinction in a vacuum breaker occurs with a short contact separation say 0.625 cm.

Advantages:-

1. They are compact, reliable and have longer life.
2. There are no fire hazards.
3. There is no generation of gas during and after operation.
4. They require little maintenance and arc quiet in operation.
5. They can successfully withstand lightning surges.
6. They have low arc energy.

SF6 Circuit Breaker

SF₆ circuit breaker

In such circuit breaker SF₆ gas is used as the arc quenching medium. SF₆ is an electro-negative gas and has strong tendency to absorb free electrons.The contacts of the breaker are open in the high pressure. Flow of SF₆ gas and arc is struck between them. The conducting free electrons in the arc are rapidly captured by the gap to form relatively immobile ions.The loss of conducting electron in the arc quickly builds up enough insulating strength to extinguish the arc. SF₆ circuit breaker has been found to be very effective for high power and high voltage service.

Construction:-

Constructional details of SF₆ circuit breaker

Above figure shows the parts of typical SF₆ circuit breaker. It consists of fixed and moving contacts enclosed in a chamber(called arc interruption chamber) containing SF₆ gas. This chamber is connected to SF₆ gas reservoir. When the contact of breaker are opened, the valve mechanism permits a high pressure SF₆ gas from the reservoir to flow towards the arc interruption chamber. The moving contact is a hollow cylindrical current carrying contact fitted with an arc horn. The moving contact is also a hollow cylinder with rectangular holes in the sides to permit the SF₆ gas to let out through these holes after flowing along and across the arc. The tips of the fixed contact and arcing horn are coated with copper-tungsten arc resistant material. Since SF₆ gas is costly, it is reconditioned and reclaimed by suitable auxiliary system after each operation of the breaker.

Working:-

In the closed position of the breaker, the contacts remain surrounded by SF₆ gas at a pressure of about 2.8 Kg/cm². When the breaker operates, the moving contact is pulled apart and an arc is struck between the contacts. The movement of the moving contact is synchronized with the operating of a valve which permits SF₆ gas at 14 Kg/cm² pressure from the reservoir to the arc interruption chamber. The high pressure flow of SF₆ rapidly absorb the free electrons to form immobile negative ions which are ineffective as charge carriers. The result is that the medium between the contacts quickly build up high dielectric strength and causes the extinction of the arc. After the breaker operation i.e., after arc extinction valve is closed by the action of a set of spring.

Advantages :-

1. Due to the superior quenching property of SF₆ such circuit breaker have very short arcing time.
2. Since the dielectric strength of SF₆ gas is 2 to 3 times that of air, such breakers can interrupt much larger currents.
3. The SF6 circuit breaker gives noiseless operation due to its closed gas circuit and no exhaust to atmosphere unlike air blast circuit breaker.
4. The closed gas enclosure keep the interior dry so that there is no moisture problem.

Disadvantages:-

1. SF₆ circuit breaker are costly due to high cost of SF₆ gas.
2. SF₆ gas has to be reconditioned after every operation of the breaker. Hence additional equipment is required for this purpose.

There are many advantages of using SF₆ circuit breaker, only few have listed above.