An electrical insulator is a material whose internal electric charges do not flow freely; very little electric current will flow through it under the influence of an electric field. This contrasts with other materials, semiconductors and conductors, which conduct electric current more easily.
Why is electrical insulation needed?
The insulation limits the flow of current between the different conductors and between the conductors to ground. It is therefore very important that the insulation has the opposite transfer properties of the conductor. Conductors are usually metallic, and the most common are copper or aluminium, both of which are known to be very good conductors of electric current due to the materials high current carrying capacity and constant thermal properties.
How insulation material is made?
The insulation is usually made from a non-metallic material. The majority of electrical insulation is made of PVC, plastic, rubber, Styrofoam, paper, glass and dry air. It should resist current and keep it within the path alongside the conductor. Many of the best insulators are man-made.
In theory, insulation should not - and does not - carry electrical current. However, in reality there is not an ideal insulation material. Every kind of insulation has some resistance. The resistance of the insulation material needs to be very high but it is not infinite. Obviously the higher the resistance, the better the insulation capacity it has.