Understanding the Benefits and Risks of SF6

What are the Characteristics of SF6?

Sulfur Hexafluoride (SF6) is colorless, odorless, non-flammable, and chemically stable gas. At room temperature it does not react with any other substance. This stability is the result of a symmetrical arrangement of six fluorine atoms around a central sulfur atom. It makes the gas useful as an effective electrical insulator that extinguishes arcs; making it ideal for high and medium voltage electrical apparatus.

SF6 is formed by a chemical reaction between molten sulfur and fluorine which is obtained via electrolysis of hydrofluoric acid (HF). Pure SF6 is not poisonous. nor dangerous to inhale, provided that the oxygen content is high enough. In principle, one can inhale a mixture of 20 percent oxygen and 80 percent SF6 without harm; and. since SF6 is approximately 6 times heavier than air, it will tend to collect in cable ducts or at the bottom of tanks. Because of this, though not dangerous to inhale, there is a risk of suffocation due to lack of oxygen should it accumulate where people work.

What are the Benefits of SF6?

There are two reasons for using SF6 in electrical equipment:

• SF6 provides extremely good electrical insulation, and
• SF6 is effective in quenching electric arcs.

These properties make it possible to build electrical equipment and apparatus that are:

• Compact,
• Efficient in the amount of material used,
• Safe, and
• Durable (extended life cycles).

At normal atmospheric pressure, SF6 has a dielectric withstand capability that is 2.5 times stronger than air, and the gas normally operates at 3-5 times atmospheric pressure resulting in dielectric properties that are ten times better than for air.

SF6 insulated substation equipment are used where the space is restricted or the environment is severe; so the fact over a 40 year period, it has proven to be reliable (virtually maintenance-free) and has operated safely are significant.

What are the Relative Risks Associated with SF6?

The term “greenhouse effect” relates to how the atmosphere may be slowly warming due to man-made gas emissions. Some gas molecules in the atmosphere, principally carbon dioxide (CO2) and methane (CH4), reflect long wave-length heat radiation from the earth and causes heat to remain trapped in the atmosphere instead of disappearing into outer space. The SF6 molecule is very reflective and contributes to this effect; but its concentration in gaseous form is extremely low (0.000´000´000´003 by volume). Thus, the contribution of SF6 to the man-made greenhouse effect is very small, less than 0.1 percent of the total. As a point of comparison, carbon dioxide CO2 contributes about 60 percent.

The other concern are gases that, when released, are known to cause thinning of the ozone layer; meaning that more ultraviolet light can get to the ground, thus increasing the risk of skin cancer. These gases contain chlorine and SF6 does not contain chlorine and thus, does not affect the ozone layer.