An insulator, also called a ‘dielectric’, is a material that resists the flow of electric charge. In insulating materials valence electrons(outermost electrons in the atoms) 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. Some materials such as glass, paper or Teflon are very good electrical insulators and are "good enough" to insulate electrical wiring and cables. Most plastics can serve as practical and safe insulators for low to moderate voltages (hundreds, or even thousands, of volts). Electrical insulation is the absence of electrical conduction. Electronic band theory (a branch of physics) says that a charge will flow if states are available into which electrons can be excited. This allows electrons to gain energy and there by move through a conductor such as a metal. If no such states are available, the material is an insulator.