Over 10,000 tons of sulfur hexafluoride (SF6) are produced annually, most for use as a gaseous dielectric material. It is a high-value alternative to oil and air in circuit breakers and switchgear.
SF6 is an extremely robust insulator. It consists of six fluorine atoms bonded in an octahedral configuration to a central sulfur atom. The gas is not soluble in water. Moreover, it has little tendency to react with other materials except for lithium. Because SF6 has much higher dielectric strength than air or dry nitrogen, it serves extensively as the dielectric medium for high-voltage circuit breakers, switchgear and other electric equipment where appropriate.
When space is limited such as in an indoor electrical room, SF6-filled equipment is useful since it can be manufactured in a smaller size for a given voltage. SF6 usage is now strongly restricted by international treaty because it is a notorious greenhouse gas (more than twice as bad as CFC-12 Freon).
The working principles of gas circuit breakers vary depending on the manufacturer. But puffer type gas breakers of single pressure type are the most common. Here a cylinder is tied to the moving contact. When the breaker opens, the cylinder moves with the contact away from the fixed contact. Because of the fixed piston the gas inside the cylinder compresses. The compressed gas flows through the nozzle and over the electric arc. The dielectric strength of the medium between the separated contacts rises rapidly and restores quickly as fresh gas fills the space.
Exposing SF6 to a powerful arc causes chemical decomposition, but SF6 tends to reconstitute immediately, so it is known in the electrical trade as “self-healing.” SF6 is not considered toxic, but breathing it for long periods isn’t a good practice because it displaces oxygen in the lungs. Because SF6 is quite dense, it tends to settle in trenches and can be hazardous to workers.
SF6 is also the dielectric medium of choice in that most exotic of electrical wiring methods, Integrated Gas Spacer Cable (Type IGS), covered in 2014 National Electrical Code Article 326.
IGS was designed for high-voltage exterior feeders. Type IGS is a factory assembly of one or more conductors, each individually insulated and enclosed in a loose fit nonmetallic flexible conduit. Terminations and splices are capable of maintaining a nominal gas pressure of 20 lb/in2. A valve and cap are provided for each length of cable and conduit to check the gas pressure and to inject gas into the conduit. The conductors are one-to-twenty solid ½-in-diameter aluminum rods. The minimum conductor size is 250 kcmil and the maximum size is 4,750 kcmil, so this is high ampacity wire!
Type IGS is permitted for use underground as service entrance, feeder or branch-circuit conductors. It is not to be installed as interior wiring or exposed in contact with buildings.
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