Liquids for switchgear (switchgear oils) must have arc sup­pression properties and high dielectric strength. Arc suppres­sion properties are basically due to the high thermal conduc­tivity of hydrogen gas produced by the decomposition of switchgear oils. Thus it is desirable that liquids easily pro­duce hydrogen gas and that the amount of free carbon pro­duced by their decomposition be small. Good insulation, re­quires not only high dielectric strength, but also rapid insulation recovery after interruption of electric arcs.

Besides these properties, it is desirable that switchgear oils have chemical stability to maintain good dielectric prop­erties, and that they be compatible with the solids used. Insu­lating oils that have the above-mentioned properties are min­eral oils. Switchgear oils are specified in IEC 60296 and ASTM D387. They are classified in the same classes as trans­former oils.

The kinematic viscosities of insulating oils in these classes are relatively low: for insulating oils at 40°C classified in IEC 60296 as Class I and Class IA, Class II and Class IIA, and Class III and Class IIIA they are <16.5 X 10—6 m2/s, <11.0 X 10—6 m2/s, and <3.5 X 10—6 m2/s, respectively. The kine­matic viscosities of insulating oils at 40°C classified in ASTM D3487sa Type I and Type II are <12.0 X 10—6 m2/s. Low ki­netic viscosity allows mechanical parts of switchgears to per­form freely, and oil flows owing to hydrogen gas evolved by decomposition of switchgear oils to be easily produced and fa­cilitate arc suppression.

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