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pressure valves and filters. As soon as the pressure in the
interrupting chamber falls below a pre-set value, the valve
in the reservoir opens and builds up the lost pressure.
Due to the very high pressure in the reservoir, compared
to only almost three times the atmosphere in the chamber,
it is possible that the pressure inside the interrupting
chamber may sometimes exceed the required value. In
the interrupting chamber, therefore, are also provided
high-pressure release valves to pump the excess gas back
to the reservoir through a compressor and a filter. The
total gas circuit is a closed cycle without any venting to
the atmosphere.
This gas is electronegative and its molecules quickly
absorb the free electrons in the arc path between the
contacts to form negatively charged ions. This apparent
trapping of the electrons results in a rapid build-up of
dielectric strength after a current zero. The detailed
sequence of arc extinction may be summarized as follows.
The contacts begin to compress a quantity of SF6 gas
as soon as they start opening. This opening also causes
arc plasma between the contacts. The temperature of the
arc plasma ionizes the gas into sulphur and fluorine atoms
and quickly becomes quenched through the turbulence of
the compressed gas through a very strange process of
negative ion formation. At higher temperatures, the S
atoms become ionized into S+ protons and SN neutrons.
The S" electrons of the neutrons are immediately absorbed
by the fluorine atoms to form fluorine ions (F) which are
Figure 19.14 11 kV SF6 circuit breaker (Courtesy Voltas) heavy and are sluggish. They contribute little to maintaining

(a) Exploded view of a pole (b) Breaker in a draw-out position
Figure 19.15 General arrangement of a 3-12 kV, SF, circuit breaker in a housing (Courtesy: Voltas)