Page 342 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Construction of SF6 circuit breaker

A sulphur hexafluoride (SF6) circuit breaker consists of fixed and moving contacts enclosed in a chamber as in
Fig 7. The chamber is called arc interruption chamber which contains the sulphur hexafluoride (SF6) gas and it is
connected to sulphur hexafluoride (SF6) gas reservoir.
When the contacts of breaker are opened, the valve mechanism permits a high pressure sulphur hexafluoride
(SF6) gas from the reservoir to flow towards the arc interruption chamber.

The fixed contact is a hollow cylindrical 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 hexafluoride gas (SF6) 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 hexafluoride gas is costly, it is reconditioned and reclaimed using suitable auxiliary system after
each operation of breaker.

Fig 6 Fig 7

Working of SF6 circuit breaker

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 opens, 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 14kg /cm²
pressure from the reservoir to the arc interruption chamber.

The high pressure flow of SF6 gas rapidly absorbs the free electrons in the arc path to form immovable negative
ions which are ineffective as charge carriers. The result is that the medium between the contacts rapidly improve
the dielectric strength and causes the extinction of the arc. After the breaker operation (i.e. after arc extinction),
the valve mechanism is closed by a set of springs.
Advantage of SF6 circuit breaker
Due to the superior arc quenching properties of SF6 gas, the sulphur hexafluoride gas circuit breakers have
many advantages over oil or air circuit breakers. Some of them are listed below.
1 Such circuit breakers have very short arcing time.
2 Since the dielectric strength of SF6 gas is 2 to 3 times more than the air, such breakers can interrupt much
larger currents.
3 SF6 circuit breaker gives noiseless operation due to its closed gas circuit and no exhaust to the atmosphere
unlike the air blast circuit breaker.

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CITS : Power - Electrician & Wireman - Lesson 106-116 CITS : Power - Electrician & Wireman - Lesson 106-116