A sulfur hexafluoride (SF6) circuit breaker uses gas as part of its current-interruption system. The National Instructional Media Institute (NIMI) training text describes one design in which fixed and moving contacts are housed in an interruption chamber containing SF6. The chamber in that example is connected to a gas reservoir.
What happens inside the interruption chamber?
When the contacts separate, an electric arc forms between them. In the design described by NIMI, gas flows into the interruption chamber during the opening operation. The gas captures free electrons in the arc path, helping the medium regain insulating strength as the arc is extinguished.
This is an explanation of the example in the training text. Gas arrangements and interruption mechanisms vary by equipment design, so the chamber layout in one breaker should not be assumed for another.
Why does the gas require care?
NIMI notes that intense or sustained arcing can produce hazardous SF6 decomposition products. The condition of gas or equipment cannot be established from appearance or smell. Work involving the breaker and its gas system belongs to qualified personnel following the applicable safety requirements and the manufacturer's documentation.
This overview does not provide instructions for opening equipment, handling gas, or working on pressurized or energized apparatus.
Source: NIMI, Electrician (Power Distribution), 2nd Year Trade Theory, PDF page 90.