Materials which are poor conductors of heat and electricity are called Insulators. The high voltage supply through an electrical circuit can be extremely dangerous but the insulators around the wires will reduce the danger by not letting the electric charge flow through it. Some of the examples of insulators are plastic, wood, glass, rubber and air.
Impurities can convert an insulator into a conductor. For example, pure water is an insulator, dirty water is a weak conductor, when salt is added to water because of the free-floating ions, it conducts well. A number of properties limit the insulators ability to conduct electricity which is listed below.
Property 1: In an insulator, the valence electrons are tightly held together. They do not have free electrons to conduct electricity.
Property 2: The ability of the material to not allow the electric current to pass through it is called electrical resistance. The resistance of an insulator per unit cross-sectional area per unit length is called resistivity. Insulators have very high resistivity. For example, insulators like glass have a resistivity value as high as 1012 Ωm. The resistance of the insulator is considerably reduced in the presence of moisture and when there is an increase in temperature.
Property 3: Insulators have large dielectric strength. The dielectric strength is the maximum electric field that the insulator can withstand without undergoing electrical breakdown and becoming electrically conductive.
Property 4: Good insulators have a high air permeability (the ability of the material to allow air to flow through its pores) since air itself is an insulator.