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Sulfur Hexafluoride

October 16, 2023

My laboratory was sited in a research center that was also home to many high powered lasers. Q-switching is one technique for producing megawatt laser pulses, one consequence of which is the generation of high electric fields in the electromagnetic light pulse. The maximum electric field strength, called the dielectric strength, is about three megavolts per meter in air. A multi-megawatt laser pulse of a centimeter diameter will produce electric fields of this strength to ionize the air and create a spark. The tick-tick of such sparks would often reverberate through the research center hallways.

Nikola Tesla pictured with his high voltage spark generator

Nikola Tesla (1856-1943) pictured in December, 1899, in his laboratory in Colorado Springs,Colorado, with his high voltage spark generator. This photograph was a double exposure, and Tesla was not actually in the room.

The photograph without Tesla appears as fig. 8 in the article, "The Problem of Increasing Human Energy," in the June, 1900, issue of The Century Magazine.

(Wikimedia Commons image of an original by Dickenson V. Alley, restored by Lošmi. Click for larger image.)

Electric relays, and their high current cousins known as contactors, will also produce a spark when making or breaking a circuit. Although the voltages involved are less than a million volts, the spacing between the switched contacts at make or break is so small that the field strength in volts per meter is very large; so, a spark is produced and the switched contacts are eroded. This is like the economic imperative to increase return on assets (ROA). You can either increase the return, or reduce the assets.

The air spark erosion of switched contacts caused by making or breaking current is typically eliminated by the addition of a series combination of a resistor and a small value capacitor in parallel with the contacts. The charging of the capacitor when the contacts open reduces the voltage for the short interval when arcing happens. The resistor acts as a current limiter when the contacts close, thereby preventing arcing at closure.

While this palliative works well for current switching in homes and small industrial settings, much larger switches are needed for the high voltage transmission lines that criss-cross the countryside to link generators to users of electric power. In order to reduce resistive heating losses, these transmission lines are designed for high voltage that requires less current for the same power. However, the currents are still large enough for arcing to be a problem.

A method for arc suppression in high voltage switchgear is placing the contacts in sulfur hexafluoride (SF6) rather than air. Sulfur hexafluoride has about three times the dielectric strength of air, between 8.5-9.8 MV/m. Sulfur hexafluoride has been used in this application since the 1950s, but it's recently become problematic with our increased sensitivity to global warming. Sulfur hexafluoride is a potent greenhouse gas with a global warming potential by weight that's 23,900 times that of carbon dioxide CO2 and an estimated atmospheric residence time of 3,200 years.[1-3]

High voltage load switches at the Krümmel Nuclear Power Plant

High voltage (380 kilovolt) switchgear at the Krümmel Nuclear Power Plant. sulfur hexafluoride is contained in the contactor region at the junction of the "T" elements.

The nuclear power plant is of the boiling water reactor type with an output power of about 1,400 megawatts.

(Wikimedia Commons image by Jens Bender. Click for larger image.)

SF6 is used in many electrical devices, but its use was advantageous in some consumer products as well. From 1989 and 2006, Nike's Air shoes used SF6 because it's non-reactive and its large molecule has a slow leak rate.[4] A 1978 patent proposes filling tennis balls with SF6, the advantage being that they would remain pressurized longer than air-filled balls.[5] However, the largest use of SF6 is for high-voltage circuit breakers.[3] Older versions of such circuit breakers might contain 2,000 pounds of SF6, but modern breakers will typically employ less than a hundred pounds (45,359 grams). Since the density of SF6 is 6.17 grams/liter, a hundred pounds is a substantial 7,350 liters.

Tennis ball

The ubiquitous tennis ball. Older balls are useful in applications other than tennis.

As stated in the abstract of the 1978 patent, "...the inflation gas includes predetermined mixed amounts of air and sulfur hexafluoride (SF6) gas which effectively enables the ball to retain its pressurized state within a desired range of pressures for a period of time significantly longer than the ball would remain pressurized if the inflation gas were air alone."[5]

Not surprisingly, there are strict standards for the mechanical properties of balls used in professional sports. The International Tennis Federation's specification for a tennis ball requires a mass between 56.0 and 59.4 grams, a diameter between 6.541 and 6.858 cm, and a bounce, onto a concrete surface from a height of 254 cm, between 135 and 147 cm.[6]

Wikimedia Commons image (modified) by Noah Wulf.

According to the 2021 Inventory of U.S. Greenhouse Gas Emissions and Sinks, electrical transmission and distribution is responsible for about 75% of all SF6 emissions in the United States.[3] Gas leaks can occur from aging equipment, during maintenance, removal from service, and during equipment manufacturing and installation.[3] In 1999, the United States Environmental Protection Agency started a voluntary tracking and reduction effort for SF6, inviting companies to share data on finding and fixing leaks, gas recycling, and the reduction in the amount of gas used.[2]

California and Massachusetts have mandated the reduction of SF6 in electrical equipment.[2] One of the largest electric utilities in the country, Pacific Gas and Electric (PG&E), discovered in 1999 that it was losing between 20,000 and 30,000 pounds of SF6 per year, which is the equivalent global warming effect of about 57,000 automobiles.[2] By 2004, PG&E had reduced this annual loss rate to 11,000 pounds.[2] According to the EPA, SF6 emissions decreased from 4.3 million metric tons of CO2 equivalent to 2.4 million metric tons between 2011 and 2021.[2]

Of course, the best solution to the SF6 global warming problem is its elimination from such electrical equipment. However, when things are working well, there is resistance to such change. It's estimated that SF6-free circuit breakers will not be widely available for at least a decade; and, in that decade, there will be increased need for such devices in the shift to renewable energy sources.[2] When SF6 equipment is finally removed from electrical systems, millions of pounds of SF6 in existing equipment will need to be removed.[2] Hitachi and Mitsubishi presently offer decommissioning services with SF6 removal.[2]

Hitachi and Mitsubishi are presently manufacturing alternative equipment that's free of SF6.[2] One alternative is the use of hermetically sealed dry air, a technology now used in lower voltage equipment.[2] Another alternative is replacing SF6 with a mixture of carbon dioxide, oxygen, and C4-fluoronitrile.[2] While C4-fluoronitrile is itself a greenhouse gas, it is far less harmful than SF6, and it has about twice the dielectric strength of SF6.


  1. A. R. Ravishankara, S. Solomon, A. A. Turnipseed, and R. F. Warren, "Atmospheric Lifetimes of Long-Lived Halogenated Species," Science, vol. 259, no. 5092 (January 8, 1993), pp. 194-199, DOI: 10.1126/science.259.5092.194
  2. Anya Litvak, "A greener way to stop the current: Phasing out a climate super-menace in the grid," Pittsburgh Post-Gazette, June 20, 2023.
  3. Electric Power Systems Partnership, Sulfur Hexafluoride (SF6) Basics, United States Environmental Protection Agency, April 14, 2023.
  4. Hayley Bennett, "Magnificent molecules - Sulfur hexafluoride," Royal Society of Chemistry.
  5. Dale Lee Koziol and Thomas Freeman Reed, "Tennis Ball," US Patent no. 4,098,504, July 4, 1978.
  6. Howard Brody, "The tennis‐ball bounce test," Phys. Teach., vol. 28, no. 6 (September, 1990) pp. 407ff., http://dx.doi.org/10.1119/1.2343088.

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