SF6 gas, used as a dielectric medium in circuit breakers and switchgear, has a global warming potential 24 000 times greater than CO2 and some of its by-products were used as chemical weapons in World War I.
In life, in business, in the ordinary course of doing things, there is a certain level of risk that comes with just existing. Granted, as individuals or as business people, we will do a calculation of sorts before undertaking riskier risks. For example, we weigh likelihoods against less risky alternatives against convenience against cost. And, at the end of the analysis, we often resign ourselves to certain things that we don’t like, but simply accept because there are no better, cost-effective alternatives available at the present time. These are the “necessary evils” in life.
That is the category into which SF6 might be placed – until now.
Sulphur hexafluoride (SF6) is an inorganic, colourless, odourless, non-flammable, heavy gas that has been widely-regarded as an excellent electrical insulator [1]. However, although SF6 is inert during normal use, highly toxic by-products are produced that pose a serious health threat to the workers who come into contact with them when electrical discharges occur through everyday usage within SF6-filled equipment [2].
SF6 has been identified as the most potent and persistent greenhouse gas in existence. In fact, it has a global warming potential approximately 24 000 times greater than carbon dioxide [3].
With these very real health and environmental risks, why are we still installing switchgear containing SF6 gas in applications below 35 000 V when much safer alternatives actually exist? Are we continuing to treat SF6 as a “necessary” evil when it is indeed entirely unnecessary at certain voltages?
SF6 gas in switchgear
SF6 was discovered around the turn of the twentieth century and, by the 1950s, the electric power industry in the USA widely praised SF6 as a highly favourable gaseous dielectric medium in high and medium-voltage circuit breakers, switchgear, gas-insulated substations, and other electrical equipment.
SF6 presented some very significant advantages at the time. For one, it was an excellent replacement for oil-filled circuit breakers and switches that contained what we discovered were harmful polychlorinated biphenyls (PCBs). In addition, SF6 is not flammable, and so took the threat of explosions and fire off the table as well.
Moreover, SF6 gas under pressure has a much higher dielectric strength than air or dry nitrogen, and this attribute made it possible to reduce the size of switchgear significantly, particularly in the underground environment. Sounds like the perfect solution – that is, until research decades later would prove otherwise.
In time, it would be discovered that SF6 chemically decomposes into by-products of varying mixtures and concentrations when exposed to electric discharges, including switching arcs, failure arcs and spark discharges [4]. Those by-products include, among other things, disulphur decafluoride (S2F10), which is also referred to as sulphur pentafluoride (SF5). Disulphur decafluoride is a highly toxic gas. Its toxicity in some animal species is on par with phosgene, the infamous chemical warfare pulmonary agent used in World War I [5]. S2F10 was considered at one time for use as a chemical warfare pulmonary agent during World War II, precisely for its insidious nature, as it provided little warning of exposure to the victim [6].
The toxicity of both SF6 and S2F10 would be of less concern but for the documented fact of leakage, as well as uncontrolled releases or discharges that occur during routine development, testing, commissioning, maintenance and repair and decommissioning of SF6 -filled equipment. These discharges and leakages are the causal connection between SF6 and legitimate concerns over the health and welfare of utility employees, the community, and the environment.
Leakage of SF6 and its by-products, including S2F10 from equipment in service is the unplanned [3], usually continuous, emission of gas from a sealed or closed system [7]. This generally occurs at seals and joints, and sometimes by molecular diffusion through certain enclosure materials [8].
In their book, Gaseous Dielectrics X, Blackman and Kantamaneni write that “under ideal conditions, SF6 would remain contained within . . . [the] equipment. In reality, however, SF6 is inadvertently emitted into the atmosphere as leaks during various stages of the equipment’s lifecycle. SF6 can also be released accidentally during equipment installation, servicing, or de-commissioning” [9].
Fig. 1: Gas-insulated switchgear.
SF6 and by-products are difficult to detect
The US Environmental Protection Agency (EPA) has referred to S2F10 as “the by-product of greatest concern due to its relatively high toxicity,” [10] and has concluded that “SF6 by-products are difficult to detect chemically under normal working conditions…” SF6 by-products such as SOF3 and SF4 (sulphur tetrafluoride) have a strong “rotten egg” odour at low concentrations, and, at high concentrations, are irritating to the eyes, nose, throat, and lungs. Solid by-products (i.e. metal fluoride by-products) are white, grey, or tan powders that can often be observed when present and are irritating to exposed skin.
However, these gross physical indicators of the presence of by-products should not be relied upon as safety mechanisms due to the possibility of severe injury, especially given that the most toxic by-product, S2F10, is generally odourless in pure form at typical environmental temperature [11].
Health hazards of SF6 and S2F10
While SF6 is regarded as a non-toxic gas, experts agree that it can displace oxygen in the lungs, and therefore cause asphyxia if too much is inhaled. SF6 gas is approximately five times heavier than air and, if released or leaked in sufficient quantity, tends to accumulate, initially in low-lying areas where there is no natural ventilation, and may cause asphyxiation [12].
Under normal circumstances, it might be considered rare to be exposed to SF6 without sufficient oxygen dilution [4]. However, for utility employees, this type of exposure is well within the normal course of duties when installing, servicing, recycling, monitoring, and de-commissioning SF6 -filled switchgear in vaults, basements, buildings and other enclosed spaces. Indeed, if a substantial quantity of SF6 gas leaks in an enclosed area, it can pose real danger of asphyxiation to personnel, due to oxygen deficiency [13].
In addition, toxic solid by-products of SF6 in the form of fine powders such as aluminium fluoride (AlF3), copper fluoride (CuF2), and wolfram (tungsten) oxide (WO3) can be present as a result of interaction with teflon, copper and tungsten contacts, and aluminium from shields. These are toxic if ingested or inhaled, causing eye, nose and throat irritation, pulmonary edema (fluid in the lungs), and other lung damage, skin and eye burns, nasal congestion, bronchitis, and rashes [14].
With respect to S2F10, as noted here, it is routinely referred to in the literature as being highly toxic. The Centre for Disease Control’s National Institute for Occupational Safety and Health Pocket Guide (NIOSH) indicates that one can be exposed to S2F10 by inhalation, ingestion, skin and/or eye contact. The symptoms include irritation of the eyes, skin, and respiratory system, and the target organs are the eyes, skin, respiratory system and central nervous system. Cell culture toxicity tests indicate that S2F10 is “literally orders of magnitude more toxic than other SF6 breakdown products in our cell culture systems” [15].
Extensive animal toxicology studies were conducted to evaluate S2F10 as a candidate warfare agent. Exposure to 5% concentrations resulted in animal death within a few minutes [16]. Exposure to lower concentrations produced death within an hour of exposure, and exposure to various low concentrations still resulted in lung damage.
Longer term exposure (18 hours or more) at various concentrations were studied. Those exposed to higher concentrations died within 16 hours; those at lower concentrations survived. All were autopsied, revealing lung damage ranging from significant to generalised lung irritation, as well as lung lesions, edema, and lung hemorrhages [17].
Once again, the subjects gave no initial indication of exposure. Once symptoms did appear, however, they included respiratory distress, which progressed to convulsions and death. Death was determined to result from anoxia (lack of oxygen) due to a vigorous pulmonary edema (lungs filled with liquid) and hyperemia (blood in the lungs) [18], [24].
Finally, other by-products of SF6, including thionyl sulphide (SOF2) or sulphur tetrafluoride (SF4), silicon tetrafluoride (SiF4), sulphuryl fluoride (SO2F2), and hydrogen fluoride (HF) are extremely irritating to the eyes, nose and throat. Other health effects include pulmonary edema, skin and eye burns, nasal congestion and bronchitis due to corrosive properties [19].
“Sulphur tetrafluoride exposure in an underground enclosed space for six hours causes shortness of breath, chest tightness, productive cough, nose and eye irritation, headache, fatigue, nausea, and vomiting. Physical abnormalities in tissues of the lung were observed, and tests showed obstruction of lung function” [20].
SF6 is a greenhouse gas
In1997, SF6 was identified by the Intergovernmental Panel on Climate Change (IPCC) as a highly potent greenhouse gas which contributes to climate change. SF6 is 22 000 times more effective at trapping infrared radiation than an equivalent amount of carbon dioxide over a 100-year period [21]. It also has an atmospheric life of 3 200 years, and its accumulation in the atmosphere is virtually irreversible [22].
SF6 world production was at 7000 tonnes in 1993, and was expected to reach 10 000 metric tonnes per year by 2010. The electrical industry uses approximately 80% of that amount. In 2002, SF6 emissions from the US electric power industry totaled 589 metric tonnes. From a greenhouse gas perspective, that equates to 14,1-million tons of CO2, or 5% of total CO2 and non-CO2 greenhouse gas emissions from US industrial processes [23]. Although SF6 is emitted in smaller quantities than other greenhouse gases, it has a significant long term impact on global climate change [24].
Conclusion
For the protection of human health and for the environmental, it is imperative to mitigate and control – if not entirely eliminate – the discharges of these toxic and corrosive decomposition substances which are the result of using SF6 in high and medium-voltage circuit breakers, switchgear and other electrical equipment. From a corporate utility and risk management perspective, SF6-filled equipment is no longer a necessary evil. Indeed, it is entirely unnecessary, and jeopardises both human health and the environment.
References
[1] WT Tsai: “The Decomposition Products of Sulfur Hexafluoride (SF6): Review of Environmental and Health Risk Analysis”, Journal of Flourine Chemistry 128 (2007), 1345-1352, at 1346-47.
[2] Id, at 1347; US Environmental Protection Agency, Office of Air and Radiation, By-products of sulphur Hexaflouride (SF6) use in the electric power industry (2002), at 1.
[3] Id.
[4] US Environmental Protection Agency, Office of Air and Radiation: “By-products of sulphur hexaflouride (SF6) use in the electric power industry” (2002), at 1.
[5] GD Griffin et al.: “Disulphur decafluoride (S2F10): A review of the biological properties and our experimental studies of this breakdown product of SF6”, Health and Safety Research Division, Oak Ridge National Laboratory (1991), 545-552, at 547.
[6] Id. at 546.
[7] Unipede/Eurelectric: “Guide to safe use of SF6 in gas insulated electrical equipment”, 1998.
[8] Id.
[9] J Blackman, R Kantamaneni: “EPA’s SF6 emission reduction partnership: maximising the benefits of SF6 emission reduction for electric utilities”, Gaseous dielectrics X, edited by Christophorou, et al. (2002), Springer, New York.
[10] US Environmental Protection Agency, Office of Air and Radiation: “By-products of sulphur hexaflouride (SF6) use in the electric power industry” (2002), at 2.
[11] US Environmental Protection Agency, Office of Air and Radiation: “By-products of sulphur hexaflouride (SF6) use in the electric power industry” (2002), at 4; GD Griffin et al.: “Disulphur Decafluoride (S2F10): A review of the biological properties and our experimental studies of this breakdown product of SF6”, Health and Safety Research Division, Oak Ridge National Laboratory (1991), at 546.
[12] Unipede/Eurelectric: “Guide to safe use of SF6 in gas insulated electrical equipment”, (1998), at 3, 5; retrieved on 1 September, 2015.
[13] Id.
[14] Mollie Averyt, US EPA’s International Conference on SF6 and the Environment: “SF6 by-products: Safety, cleaning, and disposal concerns (November 29, 2006), at slide 5.
[15] GD Griffin, et al.: “Disulphur decafluoride (S2F10): A review of the biological properties and our experimental studies of this breakdown product of SF6”, Health and Safety Research Division, Oak Ridge National Laboratory (1991), at 548.
[16] Id.
[17] US Environmental Protection Agency, Office of Air and Radiation: “By-products of sulphur hexaflouride (SF6) use in the electric power industry” (2002), at 3, citing Dervos and Vassiliou 2000, Hazard Substances Data Bank (HSDB) 2001; GD Griffin et al., “Disulphur decafluoride (S2F10): A review of the biological properties and our experimental studies of this breakdown product of SF6”, Health and Safety Research Division, Oak Ridge National Laboratory (1991), at 548.
[18] Id. at 546.
[19] US Environmental Protection Agency, Office of Air and Radiation: “By-products of sulphur hexaflouride (SF6) use in the electric power industry” (2002), at 2.
[20] A Kraut, R Lilis: “Pulmonary effects of acute exposure to degradation products of sulphur hexafluoride during electrical cable repair work”, British Journal of Industrial Medicine (1990), 47:829 – 832 at 830.
[21] IPCC Fourth Assessment Report: Climate Change 2007, Work Group I: “The physical science basis”, Section 2.10.1 “Direct global warming potentials”, Table 2.14; retrieved 2 September 2015.
[22] J Blackman, R Kantamaneni: “EPA’s SF6 Emission Reduction Partnership: Maximising the benefits of SF6 emission reduction for electric utilities”, Gaseous dielectrics X, edited by Christophorou, et al. (2002), Springer, New York.
[23] Id.
[24] Id.
Contact Lisa Blackburn, Innovative Switchgear, lblackburn@innovative-switchgear.com
