Firstly, we will understand the electrical insulator definition.
An electrical insulator is a material whose internal electric charges does not flow freely, and therefore make it very hard to conduct an electric current under the influence of an electric field. Electrical insulator includes porcelain pin post insulators, line post insulators and so on.

After learning the electrical insulator definition, we will understand the electrical insulator properties.
A perfect electrical insulator does not exist, because a portion of the insulator could become electrically conductive if the voltage applied across it exceeds the breakdown voltage. This is known as breakdown voltage of an insulator. Such as porcelain pin post insulator is puncture insulator.

Last, understand electrical insulator materials:
Some materials such as glass, paper and Teflon, porcelain, composite, which have high resistivity, are very good electrical insulators, also called porcelain electrical insulators. A much larger class of materials, even though they may have lower bulk resistivity, are still good enough to insulate electrical wiring and cables. Examples include rubber-like polymers and most plastics. Such materials can serve as practical and safe insulators for low to moderate voltages (hundreds, or even thousands, of volts).

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