1-26 in a high-temperature environment are FEP, extruded polytetrafluoroethylene, and silicone rubber. The ambient (surrounding) temperature of a conductor is an important part of total conductor heating. Copper-versus-Aluminum Conductors—The two most common metals used for electrical conductors are copper and aluminum. Some advantages of copper over aluminum as a conductor are that copper has higher conductivity, is more ductile, has a higher tensile strength, and can be easily soldered. Two advantages of aluminum wire for carrying electricity over long distances are its lightness and it reduces corona (the discharge of electricity from a wire at high potential). Temperature Coefficient of Resistance—The temperature coefficient of resistance is the amount of increase in the resistance of a 1-ohm sample of a conductor per degree of temperature rise above 0ºC. The resistance of copper and other pure metals increases with an increase in temperature. Conductor Insulation—Insulators have a resistance that is so great that, for all practical purposes, they are nonconductors. Two fundamental properties of insulating materials are (1) insulation resistance and (2) the resistance to current leakage through the insulation. Dielectric strength is the ability of the insulation material to withstand potential difference. The dielectric strength of an insulator is determined by raising the voltage on a test sample until it breaks down. Insulating Materials—Some common insulating materials have properties and safety precautions that should be remembered. These are: The purpose of coating a copper conductor with tin when rubber insulation is used is to prevent the insulation from deteriorating due to chemical action. When extruded polytetrafluoroethylene insulation is heated, caution should be observed not to breathe the vapors. The most commonly used insulating materials for extremely high-voltage conductors are varnished cambric and oil-impregnated paper. Magnet wire is the common name for enamel-insulated wire used in meters, relays, small transformers, motor windings, and so forth. The Navy is getting away from using asbestos insulation because asbestos fibers can cause lung disease and/or cancer. Asbestos insulation becomes a conductor when it gets wet. Conductor Protection—There are several types of conductor protection in use. The type commonly used aboard Navy ships is wire-braid armor.
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