1. Thermal Properties :
The Insulating Materials used must be stable within the allowed Temperature limits. Certain materials like, waxes, plastics, bitumen compounds and resins are softened at low moderate temperatures. This may cause the deterioration of mechanical properties and the insulation may melt out of the coil windings etc., causing internal short circuits. The chemical changes are accelerated at high temperatures and hence a maximum working temperature is fixed for any type of materials used as insulators. Some organic compounds are damaged by exposure to ultraviolet rays and corona. Ability of temperature to affect such diverse and important properties like electrical properties, mechanical strength, hardness, viscosity, solubility etc., has made the thermal properties all the more important. Hence all the classification of insulating materials are made on the basis of their operating temperature. Some of the thermal properties have been discussed in the succeeding paragraphs.
2. Melting Point :
The melting point of liquid dielectric used in solids state should be sufficiently high. Melting point is important in specific cases like non-draining compound paper insulating cable. It is desired that in the entire operating temperature of the cable the impregnating compound must not melt to avoid migration of oil. The insulating materials for such purpose should be chosen with a melting point much above that of the operating temperature.
3. Flash Point:
In liquid insulators flash point is an important property to be observed, the insulating material, usually oil, used in the transformer and switchgear should not catch fire at the operating temperature. The materials which are required to withstand high temperature of about 1000°C or so should retain the electrical properties and chemical structures, over the entire range of temperature. Flash point of liquid dielectric is that temperature at which the liquid begins to ignite. Insulating materials which are exposed to arcing should have self extinguishing resistant to cracking or carbonisation of the material.
4. Volatality :
A volatile material cannot act as a good insulator. When a trapped, 11,0 s is evolved from a volatile insulating material subjected to volatile stress, the breakdown of insulator is accerted. Hence material used for insulation should not exhibit volatile property.
5. Thermal Conductivity :
During the electrical operation heat is generated due to the copper losses within the conductor and its dielectric losses within the insulator. The entire heat produced should be transferred to the outside atmosphere so as to maintain the operating temperature within limits. Hence, an insulator that is used should not have very low thermal conductivity especially in high voltage apparatus where the thickness of insulation is more. Normally most of the insulating materials are poor conductors of heat.
6. Thermal Expansion:
Thermal expansion is important because of the mechanical effects caused by thermal expansion due to temperature changes. Due to repeated and rapid load cycles of an equipment corresponding expansion and contraction of the insulator occurs leading to the possibility of formation of voids. When two insulating materials are combined to form and insulating system due to two different coefficients of thermal expansion, formation of voids is inevitable. This void formation plays a major role in the breakdown of the insulation. Thermal expansion is of significant importance where heavy currents are involved.
7. Heat Resistance :
An insulating material should be able to withstand temperature variations within the specified limits without damaging its other properties. An insulating material which retains its properties at high temperatures has got the advantage that current loading can be increased and thus the apparatus can handle more power.