There is a gas called sulphur hexafluoride, or SF6, for short. Most housewives are fully knowledgeable about SF6 and buy it with the groceries. Or not really.
Okay, no housewives, then. But, listen to the tale. It so happens that if you take a domestic circuit breaker (or trip switch) of the type you find in a domestic switchboard, its purpose is to open to interrupt current when there is an overload.
When the overload occurs, the contact of the circuit breaker opens and there is a small arc. The arc goes out after a while, since it cools down. This all happens very quickly. In an 11 000 V circuit breaker, the contacts that open under an overload will also have an arc. However, if that arc was in air, it would not go out, owing to what is called the ‘plasma effect’. So, as a result, the contacts of an 11 000 V circuit breaker are immersed in a tank in insulating oil, which cools the arc down in an overload condition.
The downside of this is that you have to change the oil every two years or so – more often if the circuit breaker is used regularly. These so-called oil circuit breakers, or OCBs, have a very long life. If they are looked after properly, they will last 60-plus years. And changing the oil is no big deal.
But enter stage left, SF6, the miracle gas. An extract from Wikipedia: “SF6 is used in the electrical industry as a gaseous dielectric medium for high-voltage circuit breakers, switchgear . . . often replacing oil-filled circuit breakers that can contain harmful PCBs.” (Oops, that’s a lie; PCB’s vanished years ago.)
Wikipedia goes on: “SF6 gas, under pressure, is used as an insulator in gas-insulated switch- gear (GIS) because it has a much higher dielec- tric strength than air or dry nitrogen. This prop- erty makes it possible to significantly reduce the size of electrical gear. This makes GIS more suitable for certain purposes, such as indoor placement, as opposed to air-insulated electrical gear, which takes up considerably more room. Gas-insulated electrical gear is also more resistant to the effects of pollution and climate, as well as being more reliable in long-term operation because of its controlled operating environment . . .”
Oh, wow, ain’t it wonderful? It is for all these wonderful reasons that SF6 very largely displaced oil in 11 000 V circuit breakers and, in GIS switchgear, it continues to do so. There are even YouTube videos of people having fun with SF6 which, since it is six times heavier than air, makes your voice go very deep when you breathe it in. There is a minor downside – when it is exposed to sustained or intense electrical arcs, SF6 gas decomposes to form sulphur fluoride gases and toxic metal fluorides.
If moisture is present, the decomposition by-products can include hydrofluoric and sulphuric acids. Wait a minute . . . hydrofluoric and sulphuric acids? You use the first to etch glass and the second to dissolve rust. Right. Useful stuff.
But there is also a major downside. You may want to dispose of this gas or the gas may find its way into the atmosphere for some reason. Should we worry? Waaaal . . . according to the Intergovernmental Panel on Climate Change, SF6 is the most potent greenhouse gas that it has evaluated, with a global warming potential 22 800 times that of carbon dioxide (CO2) over a 100-year period.
Average global SF6 concentrations increased by about 7% each year during the 1980s and 1990s. But, comfortingly (again from Wikipedia) “given the low amounts of SF6 released, compared with CO2, its overall contribution to global warming is estimated to be less than 0.2%”.
So that would mean it is still in use, right? Is it 0.2 % a year or what? And being indestructible (unlike CO2), it is not going to go away. So, in 50 years, 10% of global warming will be by an indestructible gas? But there is a financial imperative by switchgear manu- factures to keep on with the stuff. Well, if you believe in global warming, it makes no sense. Fortunately, I don’t . . . but using the stuff really makes no sense.