IEC 60480 GUIDELINES FOR THE CHECKING AND TREATMENT OF SULFUR HEXAFLUORIDE (SF6)
IEC 60480 GUIDELINES FOR THE CHECKING AND TREATMENT OF SULFUR HEXAFLUORIDE (SF6) TAKEN FROM ELECTRICAL EQUIPMENT AND SPECIFICATION FOR ITS RE-USE
7 Health and safety
7.1 Introductory remark
SF6 is odourless, tasteless and colourless. It is chemically stable at room temperature and is non-combustible.
In principle, a mixture of 20 % of oxygen and 80 % SF6 can be inhaled without adverse effect. However, it is generally recommended that the maximum concentration of SF6 in the working environment should be kept lower than 1 000 µl/l. This is the value accepted for a full time (8 h/day, 5 day/week) work schedule. This is not related to toxicity, but is an established limit for all non-toxic gases which are not normally present in the atmosphere.
7.2 Precautions necessary with SF6
In addition to the information given above, several simple precautions should be observed when SF6 gas is employed. The following are the main areas of importance.
7.2.1 Oxygen depletion
SF6 gas is about 5 times heavier than air and if released in large quantities into the working environment may accumulate in low-lying areas.
In doing so the air is displaced and consequently the quantity of available oxygen will fall.
If the oxygen concentration falls below 16 % (see IEC 61634), a danger of asphyxiation will exist for any personnel working in these areas. Particularly sensitive areas are those below ground level, poorly or not ventilated, such as cable ducts, trenches, inspection pits and drainage systems.
However, after a period of time and depending upon air movement and ventilation, the SF6 will become mixed with the working environment air and its local concentration will fall to acceptable levels.
7.2.2 Mechanical
In many applications the pressure of SF6 gas employed is above atmospheric pressure. This implies that special precautions have to be taken when handling the equipment, to avoid exposing workers to the risks associated with mechanical failure of the enclosure walls.
7.2.3 Freezing
If compressed SF6 is released rapidly, the sudden expansion reduces its temperature. The gas temperature may fall to well below 0 °C.
A worker accidentally subjected to a jet of gas, during equipment filling for example, runs a risk of serious freezing burns, if he is not equipped with protective clothing and eye protection.
7.3 Necessary precautions with used SF6
Under certain conditions, the SF6 gas used in electrical equipment can contain decomposition products with potential toxic properties.
Decomposition can occur in two ways: electrical discharges and heating.
7.3.1 Electrical discharge decomposition
Any form of electrical sparking in SF6 gas will tend to break the molecule down into its constituent parts. Following this chemical breakdown however, the vast majority of the breakdown products recombine back to SF6.
As mentioned in Clause 5, the presence of oxygen and water vapour (and metal surfaces), render other reaction paths possible. Some of these paths lead to the appearance of small quantities of by-products, which may have toxic properties.
7.3.2 Thermal decomposition
Above about 500 °C SF6 gas begins to break down into its constituent elements.
Such temperatures may result, for example, from fires, heating elements, cigarette smoking, etc.
The presence of metallic surfaces can enhance this effect through catalytic reactions. Decomposition products can appear by similar mechanisms to those cited in 7.3.1.
7.4 Health considerations
During normal service, the SF6 gas remains inside the electrical equipment and the stable toxic decomposition products are trapped by adsorbents or on the internal surfaces of the enclosure.
SF6 can escape from the electrical equipment in three ways:
• leakage;
• inadequate handling procedures;
• enclosure failure.
A risk to health exists only if a sufficient quantity of the decomposition products are present in the working environment coupled with the exposure time. These situations are treated in detail in IEC 61634.
