First of all what is a Circuit Breaker?
A CB is an element which automatically opens circuit under fault conditions in order to prevent damage to the equipment.
Principle of CB:
It consists of two electrodes which are fixed and movable. Whenever Fault occurs the trip coils of the CB get energised and the movable contacts are pulled apart, which will open the circuit. But under normal conditions these contacts remain closed.
SF6 Circuit Breaker
It is basically a suplhur Hexaflouride Circuit Breaker and this SF6 gas is used for arc quenching mechanism. SF6 is electronegative gas and has high tendency to absorb electrons. (arc mechanism will be covered later)
Due to its electronegativity the low arc time constant , the SF6 gas regains its dielectric strength rapidly after final current zero, the rate of rise of dielectric strength is very high and time constant is very small.
In SF6 CB, the gas is made to flow from high pressure zone to low pressure zone through a convergent-divergent nozzle.The mass flow is a function of nozzle throat diameter, the pressure ratio and time of flow. The nozzle is located such that flow of gas covers the arc. The gas flow attains almost supersonic speed in the divergent portion of the nozzle, thereby the gas takes away the heat from the periphery of the arc, causing reduction in the diameter of arc. Finally the arc diameter becomes almost zero at the current zero and the arc is extinguished. The arc space is filled with fresh SF6 gas and the dielectric strength of the constant space is rapidly recovered due to the electronegativity of the gas.
Image Source: Google Images
