All electrical systems require some kind of insulation to prevent short circuits and leaking currents. 3 forms of insulators: solid, liquid and gaseous Performance of these insulators depend on the temperature Classification according to their temperature rating. Impregnation: Letting the solid material absorb some liquid With the advent of new materials, the classification has been modified by International Electrotechnical Commission: The transformer insulation: (a) conductor or turn-to-turn insulation, (b) coil-to-coil insulation, (c) low voltage coil-to-earth insulation, (d) high voltage coil-to-low voltage coil insulation, and (e) high voltage coil-to-ground insulation. Transformer oil -- provides the required dielectric strength and insulation -- cools the transformer by circulating itself through the core and the coil structure. -- should be in the liquid state over the complete operating range of temperatures between -40°C and+50°C. -- gets o