Cable sheath is the protective layer covering the cable insulation layer. It, conductor and insulation layer are collectively referred to as the three components of the cable. Typical sheath structure, including sheath (inner sheath) and outer sheath. Do you know the different functions of sheath and outer sheath?

1、 Function of cable inner sheath

The inner protective layer is a protective covering layer coated on the cable insulation to prevent the insulation layer from being affected by moisture, mechanical damage and optical and chemical corrosive media. At the same time, it can also flow through short-circuit current.

The inner protective layer includes metal staggered sheath, flat aluminum sheath, wrinkled aluminum sheath, copper sheath and comprehensive sheath, as well as non-metallic plastic sheath, rubber sheath, etc. Metal sheath is mostly used for oil immersed paper insulated cables and cross-linked polyethylene insulated power cables of 110kV and above; Plastic sheath can be used for all kinds of plastic insulated cables; Rubber sheath is generally used for rubber insulated cables.

2、 Function of cable outer sheath

The outer protective layer is the protective covering layer wrapped outside the cable sheath (inner protective layer), which mainly plays the role of mechanical reinforcement and anti-corrosion. The commonly used cables include the outer protective layer whose inner protective layer is metal protective layer and the outer protective layer whose inner protective layer is plastic sheath.

The outer protective layer of metal sheath is generally composed of liner layer, armor layer and outer cover layer. The liner layer is located between the metal sheath and the armor layer, which plays the role of anti-corrosion of armor liner and metal sheath. The armor layer is metal belt or metal wire, which mainly plays the role of mechanical protection, and the metal wire can bear tensile force. The outer covering layer is outside the armor layer and plays an anti-corrosion role on the metal armor. The backing layer and outer covering layer are composed of asphalt, PVC tape, impregnated paper, PVC or polyethylene sheath and other materials. According to the different environment and conditions of various cables, the composition and structure of their outer sheath are also different.

The inner protective layer is a plastic sheath, and the outer protective layer has two structures. One is that there is no outer protective layer but only polyvinyl chloride (PVC) or polyethylene sheath. The other is that PVC sheath or polyethylene sheath is extruded outside the armor layer, and its thickness is the same as that of the inner sheath. Due to the low working temperature of PVC, the traditional PVC outer sheath is not suitable for high-voltage cross-linked polyethylene (XLPE) cables with high operating temperature and protective layer insulation requirements. Therefore, it is common to use high-density polyethylene (CHDPE) or low-density polyethylene (LLDPE) as outer sheath, but it has no flame retardancy. Fire prevention measures or flame retardant cables shall be considered during laying. Using HDPE as the outer protective layer can improve the insulation level of the protective layer. There shall be adhesive between the outer sheath and the corrugated metal sleeve.