What are the important requirements of a good insulating material?
Important requirements of good insulating materials:-
The necessity of good insulating materials can be classified as mechanical, electrical, chemical and thermal. Electrically the insulating materials must have high resistivity to decrease the leakage current and high dielectric strength to enable this to with stand higher voltage without being broken down or punctured. Also the insulator must have small dielectric loss. Insulators must have low density; a uniform viscosity for liquid insulators makes sure uniform thermal and electrical properties. Gaseous and Liquid insulators are used also as coolants. For illustration, transformer oil, hydrogen and helium are used for both insulation and cooling purposes. For such materials, good thermal conductivity is desirable. The insulators must also have small thermal expansion to prevent mechanical damage. This must be non ignitable or if ignitable, this must be self-extinguishable.
Chemically, the insulators must be resistance to liquids, oils, gas fumes, alkalies and acids. This must not deteriorate by the action of chemicals in soils or with contact with other metals. The insulators must not absorb water particles, because water lowers the insulation resistance and the dielectric strength. Insulating materials must have definite mechanical properties depending upon the use to that they are put. Therefore when used for electric machine insulation the insulator must have sufficient mechanical strength to withstand vibration. Fine heat conducting property is also desirable in this case. Illustration of insulating materials is mica and porcelain. Mica sheets are employed for the insulating leaves in between commutator segments. For transmission line insulators, rail support on railways and conductor, porcelain insulators are used.