An insulator is a material that has such low conductivity that the flow of electricity through the material is negligible. The ability of material to insulate is measured by resistivity, which is the resistance to electrical flow. Insulators have high resistivity compared to conductors or semiconductors. Materials that make good insulators include rubber, plastic, glass, porcelain, and ceramics. In the early years of the electric industry, rubber and glass were commonly used as insulators. But since glass and rubber are not as durable, it is more common today to find plastic, porcelain, or ceramic material used as insulators.
In the electric grid, insulators are used to prevent electricity from flowing where it isn’t wanted. This includes insulation on wires that otherwise would come in contact with a conductor of electricity and insulators on various types of electrical equipment. In overhead systems, line conductors must be separated from poles and other line conductors by insulators to prevent short circuits. In underground systems, they must be separated from each other and the ground by insulation. Overhead insulators are typically made of porcelain or plastic (which is often called polymer). Insulation for underground cable is usually made of either plastic or oil-impregnated paper tape.