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Why SF6 gas is used in circuit breaker?
Mainly in a circuit breaker the medium is used to extinguish the arc and provide adequate insulation between the contacts and from each contact to earth. Normally used insulation mediums are air, oil and SF6.
Preferably, SF6 gas is used in circuit breaker because of the following reasons,
1) Very high dielectric strength
Dielectric strength of an insulating material is determined by the maximum electric field it can withstand intrinsically without losing its insulating capability. The dielectric strength of SF6 at normal pressure and temperature is 2 -3 times compared to that of air.
2) Very good arc quenching ability
As SF6is an electromagnetic gas, it has an affinity for the free electrons. During the operation of circuit breaker there is a huge amount of free electrons present in the arcing region, the main aim of the circuit breaker is to extinguish the arc. SF6 at high pressure absorbs the electrons and forms SF6. The reaction may occur in the following ways
SF6 + e ⇒ SF6-
SF6 + e ⇒ SF5- + F
3) Excellent thermal stability
SF6 gas decomposed thermally at a temperature of about 500 degree centigrade, which is pretty high. The gas also works as a cooling agent during the operation as it absorbs energy during the absorption of electrons. The thermal time constant of SF6 is low and for this the pressure at which it can be stored are lower compared to that of air. This also ensures the small size of the circuit breaker.
4) Great thermal conductivity
Excellent heat transfer properties are seen in SF6 gases because of its high molecular weight and its low gaseous viscosity, which enables it to transfer heat by convection more effectively than other gases.
