Sulfur Hexafluoride Circuit Breaker

Sulphur hexafluoride (SF6) is an electronegative halogen gas having good dielectric properties. Uncharged particles of an electronegative gas have an affinity to attach themselves to free electrons producing less mobile and heavy negative ions. The contribution of the latter to the ionization process creating electron avalanches is much less as compared to an electron. Hence, the electric field stress required to cause breakdown of an electronegative gas is high.

SF₆ has a high dielectric strength, which is about 2.35 times that of air at atmospheric pressure (about 70 kV/cm). At higher pressures, the dielectric strength increases and at a pressure of about 3 kgf/cm^-, the dielectric strength is twice of that at atmospheric pressure. An addition of 30% of SF₆ to air (by volume) doubles the dielectric strength of air. It is because of the high dielectric strength that SF₆ is being widely used in electrical h.v. equipments.

SF₆ is non-toxic and non-inflammable. It is stable upto 500°C and inert up to 150^CC. At room temperature, therefore, it does not attack metals, plastic and other materials used in high voltage equipment construction. However, at high temperatures caused by arcs, it decomposes into various components—principally SF₄ and SF_: together with small amounts of S₂, F₂, S, F, etc. But decomposed components SF_: and SF₄ recombine. The products of decomposition are non-inflammable.

SF₆ is a very good coolant, much better than air. The thermal time constant is also low. Hence, when the arc current is interrupted in a SF₆ medium, most of the heat will be taken away from the arc channel quickly. This helps in speedy recovery of dielectric strength in the arcing gap. The arc time constant, defined as the time between current zero and the instant the conductance of the space between the contacts reaches zero value, is very low in SF₆. It is of the order of 1 μs.

So, in addition to providing extremely good electrical insulation, it also quenches electric arcs very effectively. Circuit-breakers using SF₆ as the insulating and quenching medium are compact in size, safe and will have longer endurance. Usually the gas is used at 3-5 times the atmospheric pressure. SF₆ at atmosphere pressure has a low liquefaction temperature (about -60°C) and remains in gaseous state at room temperature.

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