Important requirements of good insulating materials:-
The requirement of good insulating materials can be classified as electrical, mechanical, thermal and chemical. Electrically the insulating materials should have high resistivity to reduce the leakage current and high dielectric strength to enable it to with stand higher voltage with out being punctured or broken down. Also the insulator should have small dielectric loss. Insulators should have low density, a uniform viscosity for liquid insulators ensures uniform thermal and electrical properties .Liquid and gaseous insulators are used also as coolants. For example, transformer oil, hydrogen and helium are used for both insulation and cooling purposes. For such materials, good thermal conductivity is desirable. The insulators should also have small thermal expansion to prevent mechanical damage. It should be non ignitable or if ignitable, it should be self-extinguishable.
Chemically, the insulators should be resistance to oils, liquids, gas fumes, acids and alkalies. It should not deteriorate by the action of chemicals in soils or by contact with other metals. The insulators should not absorb water particles, since water lowers the insulation resistance and the dielectric strength. Insulating materials should have certain mechanical properties depending on the use to which they are put. Thus when used for electric machine insulation the insulator should have sufficient mechanical strength to withstand vibration. Good heat conducting property is also desirable in such cases Example of insulating materials are mica & porcelain. Mica sheets are used for the insulating leaves between commutator segments. Porcelain insulators are used for transmission line insulators, conductor, rail support on railways etc.