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sulphur hexaflouride ( SF6 ) gas is used as the arc quenching medium. The sf6 gas is an electro negative gas and has a strong tendency to absorb free electrons. The contacts of the breaker are opened in a high pressure flow of sf6 gas and an arc is struck between them. The conducting free electrons in the arc are rapidly captured by the gas to form relatively immobile negative ions. This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc. The sf6 circuit breakers are very effective for high power and high voltage service.

Construction: Fig 15 shows the parts of a 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 contacts of breaker are opened the valve mechanism permits a high pressure sf6 gas from the reservoir to flow towards the arc interruption chamber. The fixed 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 fixed contact, moving contact and arcing horn are coated with copper-tungsten arc resistant material. Since sf6 gas is costly, its reconditioned and reclaimed by a suitable auxiliary system after each operation of the breaker.





Working: In the closed position of the breaker the contacts remained 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 opening 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 absorbs the free electrons in the arc path to form immobile negative ions which are ineffective as charge a carriers. The result is that the medium between the contacts quickly builds up high dielectric strength and causes the extinction of the arc. After the breaker operation the valve is closed by the action of a set of springs.


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http://electricalandelectronics2008/10/12/sulpher-hexa-flouride-sf6-circuit-breakers/