Insulation Temperature Ratings

An insulation system is an assembly of insulating materials in association with the conductors and supporting structural parts of an electrical equipment and apparatus. Insulation systems for electrical equipment may be classified as solid, liquid, air and vacuum, and gases. The liquid insulation system comprise of mineral oil, silicone, and other less-flammable fluids. The gas that is primarily used for electrical insulation is SF6 gas known as sulfur hexafluoride gas.

The air and vacuum insulation system has been used from the very beginning and its characteristics are well documented and understood.

The ability of insulating materials or an insulation system to perform its intended function is impacted by other aging factors. The major aging factors are electrical stresses, mechanical stresses, environmental stresses, and thermal stresses. Mechanical stresses imposed upon the system and its supporting structure by vibration and differential thermal expansion may become of increasing importance as the size of the apparatus increases. Electrical stresses will be more significant with high-voltage apparatus or with equipment exposed to voltage transients. Environmental stresses will have an impact depending on the presence of moisture, dirt, chemicals, radiation, or other contaminants. Thermal stresses depend upon environmental conditions (high ambient), loading, and ability to dissipate heat. All such factors should be taken into account when selecting insulating materials and/or insulation systems. To help the user, IEEE Standard 1-2001, “IEEE recommended practice—General principles for temperature limits in the rating of electrical equipment and for the evaluation of electrical insulation,” has established the temperature rating for solid insulation systems of electrical equipment and apparatus. The IEEE Standard 1-2001 takes into account these factors in establishing the standards of temperature limits for particular classes of apparatus. Thus, for temperature rating purposes insulation systems are divided into classes according to the thermal endurance of the system.

According to IEEE Standard 1-2000, insulation system classes may be designated by letters and may be defined as assemblies of electrical insulating materials in association with equipment parts. These systems may be assigned temperature rating based on service experience or on an accepted test procedure that can demonstrate an equivalent life expectancy. The thermal classification of electrical insulating systems established by IEEE Standard 1-2000 is given in Table.