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Sulphur Hexafluoride (SF6) Circuit Breakers - Construction, Working and Advantages

In such breakers, sulphur hexaflouride gas (SF6) is used as the arc quenching medium. 

The sulphur hexaflouride gas (SF6) is an electronegative gas and has a strong tendency to absorb free electrons. The contacts of the breaker are opened in a high pressure flow of sulphur hexaflouride (SF6) gas and an arc is struck between them. The gas capture the conducting free electrons in the arc 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 sulphur hexaflouride (SF6) circuit breakers have been found to be very effective for high power and high voltage service.

Construction of SF6 Circuit Breaker

An sulphur hexaflouride (SF6) circuit breaker consists of fixed and moving contacts enclosed in a chamber. The chamber is called arc interruption chamber which contains the sulphur hexaflouride (SF6) gas. This chamber is connected to sulphur hexaflouride (SF6) gas reservoir. A valve mechanism is there to permit the gas to the arc interruption chamber. When the contacts of breaker are opened, the valve mechanism permits a high pressure sulphur hexaflouride (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. The holes permit the sulphur hexaflouride gas (SF6) gas to let out through them 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 sulphur hexaflouride gas (SF6) gas is costly, it is reconditioned and reclaimed using suitable auxiliary system after each operation of breaker.

Working of SF6 CB

In the closed position of the breaker, the contacts remain surrounded by sulphur hexaflouride gas (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 synchronised with the opening of a valve which permits sulphur hexaflouride gas (SF6) gas at 14 kg/cm2 pressure from the reservoir to the arc interruption chamber. 
The high pressure flow of sulphur hexaflouride gas (SF6) rapidly absorbs the free electrons in the arc path to form immobile negative ions which are ineffective as charge 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 (i.e. after arc extinction), the valve is closed by the action of a set of springs.

Advantages of SF6

Due, to the superior arc quenching properties of sulphur hexaflouride gas (SF6) gas, the sulphur hexaflouride gas (SF6) circuit breakers have many advantages over oil or air circuit breakers. Some of them are listed below : 
  1. Due to the superior arc quenching property of sulphur hexaflouride gas (SF6), such circuit breakers have very short arcing time.
  2. Since the dielectric strength of sulphur hexaflouride (SF6) gas is 2 to 3 times that operation due unlike of air, such breakers can interrupt much larger currents.
  3. The sulphur hexaflouride gas (SF6) circuit breaker gives noiseless operation due its closed gas circuit and no exhaust to atmosphere unlike the air blast circuit breaker.