Cable protective layer has inner sheath and outer protecting layer. Inner sheath main function is to protect the core wire insulation layer from external mechanical damage, for oil impregnated paper insulated cables can also prevent the water immersion and impregnant outflow. Outer protective layer function is to corrosion, mechanical protection and shielding.
Inner sheath of cables divided into metal (aluminum, lead, steel) and non-metal (rubber, plastic). Metal sheath is used for oil impregnated paper insulated cables, rubber, plastic sheath for rubber insulated cables. Due to the rubber, plastic sheathed power cable usage is less, steel sheath is mainly used for ultra high voltage power cable.
(a) lead sheath
(1) the lead sheath has not completely permeable and can reliably prevent the intrusion of water and moisture, good sealing performance.
(2) the melting point of lead low (327.5 DEG C), in manufacturing lead sheath does not make paper insulation overheat and damage. Lead in heated to 200 DEG C around into a semi liquid state, in the lower Ju can squeeze into a closed sheath, so that lead pressing process is relatively simple, at the same time, the sealing process is also very convenient.
(3) resistant corrosion. Lead to dilute sulfuric acid and hydrochloric acid does not work, only nitrate, acetic acid, carbolic acid and some organic putrefaction of lead can corrode.
(4) the toughness of the lead sheath is good, and it does not affect the curve of the cable.
(b) Aluminum sheath:
(1) the relative density is small, the weight is light, and the transportation is convenient,
(2) high mechanical strength,
(3) rich in resources,
(4) Aluminum crystal structure is stable, there is no as lead that coil change phenomenon. Aluminum fatigue strength is also very good, shock resistance than lead alloy (containing 5% antimony) up 100 times.
(5) High conductivity of aluminum, is seven times of the lead, so the electromagnetic interference shielding is better than lead. At the same time, its electrical conductivity is good.