One of the functions of the insulation is hest transfer. Consider an underground cable under operation. This cable is recommended for operations. This cable is recommended for operation with certain limitations of voltage and current. Suppose if the voltage is increased insulating material is able to withstand the higher voltage stress, the voltage will cause increase of dielectric loss that will increase heat generation. Ultimately the temperature of the insulating will further increase. If the insulator starts losing its insulating properties, ultimately breakdown will occur. Secondly if the load current in the cable is increased, P losses will increase, resulting once again in increased heat generation and failure as mentioned. Temperature affects such diverse and important properties as electrical properties, mechanical strength, hardness, viscosity, solubility etc. This makes thermal properties more important. It will now be appreciated why a complete classification of insulating materials on their operating temperature has been done. Various thermal properties are discussed below.
(1) Melting point: It assumes importance in specific cases like non draining compound, impregnated paper, insulated cable etc. It is desired in the entire of operating temperature range of cables the impregnating compound must not melt avoid to migration of oil.
(2) Flash Point: It will impose restriction in manufacturing process to avoid possible hazard of apparatus catching fire.
(3) Volatility: IT assumes importance from the fact that when a trapped gas is evolved from a volatile insulating material subjected to voltage stress. The breakdown is very rapid. A volatile material cannot be a good insulator.
(4) Thermal conductivity: Heat generated due to power losses and dielectric losses will be dissipated through the insulator itself. However effectively this flow of heat takes place, depends on the thermal conductivity of the insulator. An insulator with better thermal conductivity will not allow temperature rise because of heat transfer through it to the from the
Atoms sphere. This property assumes great importance in high voltage apparatus where thickness of insulation is more.
Thermal expansion: An insulator with a high coefficient of expansion poses problems. Repeated load cycles of an apparatus cause corresponding and contraction of the insulator leading to the possibility of the formation of voids in it. When there are insulating materials involved to form insulation system different coefficients of expansion of the two will further increase the formation of voids. These voids have been found to be the major causes of insulation breakdown. Be it understood that thermal expansion is of significant importance where heavy currents are involved. In communication and electronic system this obviously is not significant.
(5) Heat Resistance: This is a general property which desires that a dielectric should withstand temperature variations within desirable limit, without damaging its other important properties .If an insulator has favourable properties at ambient temperature but it not able to retain these properties to desirable extent at higher temperatures but is not able to which it has to operate, it is not good insulator. On the contrary an insulation which is capable of withstanding higher temperatures up to which it has to operate. This means that the current loading can be increased. This making apparatus capable of handling more power. This favours economy.