A conductive cable is a wire or a group of wires that conducts electricity. Substances which do not allow the passage of current through them are known as insulators. An insulating material should possess a very good dielectric strength and fire proofing qualities. In addition to good mechanical strength, it should have high chemical inertness. Often the most suitable insulation for resisting dielectric stress may not have an outer surface that is suitable for the conditions which the cable must meet. Thus, for improvement in resistance to flame, oil, abrasion, and chemical environment, a suitable jacket is extruded around the insulated conductor.
Selection of insulating materials is based on a number of performance factors, like electrical, mechanical, thermal and chemical. The materials used for insulation are broadly classified as thermoplastic or thermosetting. Thermoplastic materials can be repeatedly melted or dissolved in various solvents. They are more elastic, less brittle and do not lose elasticity when subjected to prolonged heating. They have high insulating properties and are water repellent. Polyvinyl chloride, polyethylene and polypropylene are a few examples of thermoplastic insulations.
Polyvinyl Chloride is used for jacketing single and multi conductor cables, particularly when shielded. It provides toughness, resistance to moisture and oil, and has good low-temperature properties. It has high tensile strength, superior conductivity, better flexibility, ease of jointing with excellent fire retarding quality. It also safeguards against most forms of electrolytic and chemical corrosion. Cables with Polyvinyl Chloride jackets are suitable for installation in pipes, gutter or tray, underground ducts, direct-earth burial, and overhead on messengers.
Polyethylene has a 2% black pigment which prevents deterioration of the cable from ultra-violet rays of sunlight. It is mostly used in such circumstances where extreme resistance to moisture and corrosion is required. It has excellent chemical resistance, meaning that it is not attacked by strong acids or strong bases. One of the main disadvantages of polyethylene is that without special treatment it is not readily biodegradable, and thus accumulates.
Polypropylene possesses excellent resistance to degreasing agents and electrolytic attack. It has lower impact strength, but its working temperatures and tensile strength are superior to polyethylene. Polypropylene is light in weight, resistant to staining, and has a low moisture absorption rate. It is liable to degradation from exposure to heat and UV radiation such as that present in sunlight. It deteriorates in the presence of bacteria, mold or other elements. It has high compressive strength and is known for its smooth finish.