An insulator is a material that resists the flow of electric charge. In insulating materials valence electrons are tightly bonded to their atoms. These materials are used in electrical equipment to support or separate electrical conductors without allowing current through themselves.
Insulators are commonly used as a flexible coating on electric wire and cable. But for high-voltage power lines commonly use just air, since a solid (e.g., plastic) coating would be impractical while air is sufficient. However, wires which touch each other will produce cross connections, short circuits, and fire hazards.