SF6
circuit breaker
Sulphur hexafluoride (SF6) is an inert, heavy gas having good dielectric and
arc extinguishing properties. The dielectric strength of the gas increases with
pressure and is more than of dielectric strength of oil at 3 kg/cm2. SF6 is now
being widely used in electrical equipment like high voltage metal enclosed cables;
high voltage metal clad switchgear, capacitors, circuit breakers, current transformers,
bushings, etc. The gas is liquefied at certain low temperature, liquefaction
temperature increases with pressure.
Sulphur hexafluoride gas is prepared by burning coarsely crushed roll sulphur
in the fluorine gas, in a steel box, provided with staggered horizontal shelves,
each bearing about 4 kg of sulphur. The steel box is made gas tight. The gas
thus obtained contains other fluorides such as S2F10, SF4 and must be purified
further SF6 gas generally supplier by chemical firms. The cost of gas is low
if manufactured in large scale.
During the arcing period SF6 gas is blown axially along the arc. The gas removes
the heat from the arc by axial convection and radial dissipation. As a result,
the arc diameter reduces during the decreasing mode of the current wave. The
diameter becomes small during the current zero and the arc is extinguished. Due
to its electronegativity, and low arc time constant, the SF6 gas regains its
dielectric strength rapidly after the current zero, the rate of rise of dielectric
strength is very high and the time constant is very small.
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