SF6 gas is 5.135 times heavier than air, and its boiling point is -60°C at one atmosphere. Below 150°C, SF6 exhibits excellent chemical inertness and does not react chemically with the metals, plastics, and other materials commonly used in circuit breakers. When it decomposes into various components under the high temperatures caused by a high-power electric arc, it recombines within a very short time after the arc extinguishes. SF6 contains no carbon and no air, thus preventing contact oxidation. SF6 has a very high dielectric strength, which increases with pressure. At one atmosphere, the dielectric strength of SF6 is approximately 2 to 3 times that of air. At an absolute pressure of 3 atmospheres, the dielectric strength of SF6 can reach or exceed that of commonly used insulating oils. SF6 has excellent arc-extinguishing performance; in a simple interrupting arc-extinguishing chamber, its arc-extinguishing capacity is 100 times greater than that of air. In SF6, when the arc current is close to zero, there is a very high temperature only at the very small diameter core of the arc column, surrounded by a non-conductive layer. In this way, the dielectric strength of the arc gap will recover quickly after the current crosses zero.
- High dielectric strength (approximately 8 to 9 times that of air at 5 bar).
- It is inherently negatively charged, trapping free electrons to form heavy ions with low mobility, thus preventing avalanche breakdown.
- Good thermal conductivity, high ionization energy, and low dissociation temperature result in excellent arc-quenching performance.
- It exhibits very high conductivity at high temperatures, demonstrating low arc voltage. It is colorless, odorless, inert, and non-toxic.
- Some disadvantages of SF6 gas are its high cost, its tendency to form corrosive metal fluorides during arc discharge, and its status as a greenhouse gas.
A circuit breaker that uses sulfur hexafluoride (SF6) gas as both the arc-quenching and insulating medium. It is abbreviated as SF6 circuit breaker. The use of sulfur hexafluoride as an arc-quenching medium in circuit breakers began in the early 1950s. Due to the excellent properties of this gas, the voltage and current parameters of a single-break SF6 circuit breaker are significantly higher than those of compressed air circuit breakers and oil-limited circuit breakers, and it does not require high gas pressure or a large number of series breaks. In the 1960s and 1970s, SF6 circuit breakers were widely used in ultra-high voltage, high-capacity power systems. By the early 1980s, SF6 circuit breakers with 363 kV single-break, 550 kV double-break, and rated breaking currents of 80 and 100 kA had been successfully developed.
