ABSTRACT
Today, unlike paper cables, most low-tension polymer-insulated cables need not be protected by a metal sheath, unless specifically required. It is very risky to bring a lead or aluminium sheath by the extrusion process on a polymeric cable, as the high heat would damage the cable. It is found that polymer-insulated cables having an extruded inner and outer sheath can sufficiently be relied upon against ingress of moisture and the surrounding affluent since they are not chemically very reactive. Also, polymers do not get corroded under normal conditions. In some cases, as in Germany, the United States and Japan, a corrugated steel sheath was applied by a seam welding process. Notwithstanding, it is essential that the cable have a mechanical protection to withstand severe laying and installation conditions, when buried underground or laid in any other manner. The environmental conditions, impact of a pick axe during excavation, abrasive forces during laying and soil subsidence could all inflict mechanical damage. Furthermore, to protect the cable electrically, particularly for mining cables, a part of the short-circuit current has to be diverted through armouring. It is mandatory for mining cables to have a minimum armour resistance of 75% of a phase conductor.
