Other than bare copper wire used to ground electrical systems, almost every electrical wire used in homes, vehicles and electrical equipment is shielded by some sort of plastic, rubber or other insulating material. The insulation's main purpose is for safety and to keep the wires conducting the electricity from contacting other wires or surfaces. There are many kinds of insulating materials and many reasons for selecting specific types for your needs.
The most common material used to insulate electric wire in modern times is plastic. Before the invention of plastic, both cloth and paper, often coated with paint, waxes or tar-like compounds was used. Some wire is still insulated with these old-fashioned materials but used only in specific and unusual circumstances.
Polymers and Magnesium Oxide
Heat is the major drawback to using plastic as a wire insulation material. Some wires carry current through naturally hot environments such in electric stoves and heaters and some wires tend to heat up when electricity is flowing through the wire. In these circumstances, thermoplastic insulation made of heat-resistant polymers is available. In very high heat situations, wire insulation made of non-conductive inorganic minerals such as magnesium oxide can be used.
There are dozens of specific insulating plastics available to make the wire resistant to specific chemicals such as petroleum products, chlorine and others. Wires for outdoor use need to be U.V. resistant to keep sunlight from degrading the insulation. Wires intended to be buried underground have special insulation to better protect the wires in that situation.
Though the colour of the insulation on wire has no effect on how well it insulates, many wires are selected and used in specific ways depending on their colour. In most home wiring situations, hot wires are either red or black and the neutral wire is white. Ground wires are either green or bare. In 12-volt systems such as in cars and boats, black wires are negative or ground wires while red or other colours are positive or hot wires.
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