A true Glass Insulators is a material that does not respond to an electric field and completely resists the flow of electric charge. In practice, however, perfect insulators do not exist. Therefore, dielectric materials with high dielectric constants are considered insulators. In insulating materials valence electrons are tightly bonded to their atoms. These materials are used in electrical equipment as insulators or insulation. Their function is to support or separate electrical conductors without allowing current through themselves. The term also refers to insulating supports that attach electric power transmission wires to utility poles or pylons.
Some materials such as glass, paper or Teflon are very good electrical insulators. Even though they may have lower bulk resistivity, a much larger class of materials 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)
From:wikipedia