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Reply To: Wiring Riddle No.8 – Armour for power cables


    Various metals may be used as the sheath of a cable such as lead, copper, aluminum, bronze, steel, etc. A sheath provides a barrier to moisture vapor or water ingress into the cable insulation. It is necessary to use such a sheath over paper insulation, but it also has a value over extruded materials because of water ingress. The thickness of the metal sheath is covered by ICEA and AEIC standards and specifications, but there are some constructions that are not covered. The thickness is dependent on the forces that can be anticipated during the installation and operation of the cable. Designs range from a standard tube to ones that are longitudinally corrugated. The bending radius of the finished cable is dependent on such configurations. To fully utilize the metal chosen, one should consider first cost, ampacity requirements especially during fault conditions, and corrosion. Interlocked Armor consists of a single metal tape whose turns are shaped to interlock during the manufacturing process. Mechanical protection is therefore provided along the entire cable length. Galvanized steel is the most common metal provided. Aluminum and bronze are used where magnetic effects or weight must be considered. Other metals, such as stainless steel or copper, are used for special applications. Interlocked-armor cables are frequently specified for use in cable trays and for aerial applications so that conduit and duct systems can be eliminated. The rounded surface of the armor withstands impact somewhat better than flat steel tapes. The interlocked construction produces a relatively flexible cable that can be moved and repositioned to avoid obstacles during and after installation. An overall jacket is often specified in industrial and power plants for corrosion protection and circuit identification. Neither flat-taped armor or interlocked armor is designed to withstand longitudinal stress, so long vertical runs should be avoided.