what is the function of cable inner sheath and outer sheath?

Introduction and function of cable inner sheath and outer sheath
   In order to protect the cable insulation from damage and meet the requirements of various service conditions and environment, a protective layer is covered on the cable insulation layer, which is called the cable protective layer. The cable sheath is divided into inner sheath and outer sheath.
Introduction to cable inner sheath and outer sheath。

(a) Inner protective layer

The inner protective layer is a protective covering layer covering the cable insulation layer, which is used to prevent the insulation layer from being affected by moisture, mechanical damage, light and chemical aggressive media, and can also flow through short-circuit current. The inner sheath includes metal staggered sheath, flat aluminum sheath, corrugated aluminum sheath, copper sheath, comprehensive sheath, non-metallic plastic sheath and rubber sheath. Metal sheath is mainly used for oil immersed paper insulated cables and cross-linked polyethylene insulated power cables of 110 kV and above. Plastic sheath (especially PVC sheath) can be used for all kinds of plastic insulated cables; Rubber sheath is usually used for rubber insulated cables.
The specific gravity of aluminum is only 23. The thickness of aluminum sheath is much thinner than that of lead sheath, so aluminum sheath cable is much lighter than marine sheath cable. In addition, the resistance coefficient of aluminum is much less than that of lead, and the short-circuit heat capacity of aluminum bushing is large. In the system where the short-circuit current lasts a little longer, the aluminum bushing with general standard thickness can meet the technical requirements. For example, if the thermal stability is not enough in the calculation, the aluminum sleeve can be slightly thickened to meet the technical requirements without adding copper wire (or copper strip) shielding. Therefore, the aluminum sleeve cable is economical and practical. Laying with corrugated aluminum sheath is labor-saving: the outer diameter of the cable is large, which makes the size of the cable drum relatively large, so the laying construction is also difficult. In contrast, lead casing is much heavier than aluminum casing. In order to meet the technical requirements of short-circuit thermal stability, the cross-section of lead and gold must be much larger than that of aluminum. However, due to its compact structure, good chemical stability and corrosion resistance, compared with aluminum, lead casing will never replace aluminum casing. All kinds of cables used on land have their own characteristics, advantages and disadvantages. When straightening and laying pipelines, lead sheathed cables shall be preferentially used, and lead sheathed cables must be used for river crossing and submarine cables. Aluminum casing is not as durable as lead casing because it will be perforated soon after the outer sheath is damaged in one day.
According to the stress of the cable in use and the structure of the outer sheath, the original lead sheath base is divided into three categories, and the fork of each category becomes thicker with the increase of conductor section.

Class I: the cable is protected by armor layer (or hemp quilt). When in use, there is only external mechanical force and no tension. The thickness of lead sheath is 1.2 ~ 2.0mm

The second category is all kinds of split phase lead sheathed cables. The thickness of staggered sheath is 1.2-2mm
The third category is bare sheathed cables without outer sheath. And steel wire armored cables for underwater teaching. The thickness of lead sheath is 1.4 ~ 2.9 mm
Internal pressure, laying operation conditions and other factors are also considered for oil filled power cables, so the lead sheath is thicker.
The mechanical strength of aluminum sheath is much higher than that of lead sheath. Therefore, the aluminum sheath of various types of cables is unified again, with a thickness of 1.1 ~ 2.0mm
There is also a new coat structure, called comprehensive coat, which is made of aluminum letter polyethylene composite material, rolled up with hot air, extruded and combined with the coat. The cable with comprehensive sheath has the advantages of light weight and small volume. It can reduce the cost when used in the system with small zero sequence knowledge capacity. The system with the largest zero sequence knowledge capacity needs to be shielded with wire. As a radial water resisting layer, the metal foil with integrated sheath is an effective water resisting layer, but its real-time effect of resisting external force damage and perforation of outer sheath is fragile.
The inner protective layer of PVC insulated cables and XLPE insulated cables below 35KV is made of polyethylene nitrogen sheath or polyethylene sheath.
The outer sheath is the protective coating covering the cable sheath (inner sheath), which mainly plays the role of mechanical strengthening and anti-corrosion. Commonly used cables include metal inner sheath, outer sheath and plastic inner sheath. The outer sheath of metal sheath is generally composed of lining layer, armor layer and outer sheath layer. The lining layer is located between the metal sheath and the armor layer, which plays the role of installing the lining and the metal sheath to prevent corrosion. The armor layer is metal belt or wire, which mainly plays the role of mechanical protection, and the wire can bear tensile force. The outer coating is outside the armor layer and plays an anti-corrosion role in metal armor. The village mat and cover are composed of asphalt, polyethylene tape, impregnated paper, PVC or polyethylene sheath and other materials. The composition and structure of the outer sheath are also different according to the different environments and conditions of various cables. The outer sheath structure of common power cables is shown in the relevant cable model table.
There are two structures, the inner protective layer is a plastic sheath and the outer protective layer is a plastic sheath. One is that there is no outer sheath, only PVC or polyethylene sheath, and the other is that PVC or polyethylene sheath is extruded out of the armor layer, with the same thickness as the inner sheath. Due to the low working temperature of PVC, the traditional PVC outer sheath is not suitable for high-voltage XLPE cables with high working temperature and high insulation requirements. Therefore, it is very common to use high density polyethylene (chdpe) or low density polyethylene (LLDPE) as outer sheath, but it is not flame retardant. Therefore, fire prevention measures or flame retardant cables shall be taken during laying. Using high density polyethylene as outer sheath can improve the insulation level of sheath. Adhesive shall be provided for the outer sheath and corrugated metal sheath.

cable inner sheath manufacturer