ABB HPA 12kV Instruction For Installation, Service And Maintenance page 5

The main characteristics of Sf gas are:
The gas is neither combustible nor toxic.
It is chemically stable and will not age with time.
Breaking capacity of Sf6 gas is high even at relatively low pressure, because of
its superior dielectric and thermal properties.
The interruption in Sf gas is not forced and thus no over voltages are
generated. No damping resistor or surge arrestor is needed, not even when
controlling small motors.
The dielectric strength at the relevant pressure is about 3 times higher than air
and is roughly at par with oil.
Any leakage is easily detected. To provide an extra margin of safety, the
breaker is capable of interrupting its rated current at rated voltage even at
atmospheric pressure.
The Sf breaker type HPA operates on what is usually referred to as the puffer
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principle. This involves compressing the gas between a static piston and a movable
cylinder during the interruption action. The compressed gas is then blown through a
nozzle in which the arc is taking place. A combination of careful attention to design
and the use of proven components has meant that arcing times in the new HPA
breaker have been reduced to a minimum. As a result, the new breaker contacts will
normally not need replacement during the service life of the breaker. The breaker pole
operation is shown in fig. 2.
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Fig - 2
(The Puffer Principle)
(4)