The raw materials used for cable insulation and sheath are mainly plastic and rubber materials, and many kinds of materials with special functions are derived from them. When the material manufacturer fabricates the material, it will go through the processes of Compound mixing, granulating, cooling and drying, as well as during the transportation and storage of the material, which often causes the material to be damp to varying degrees, so that the material contains moisture to varying degrees. Therefore, before the cable manufacturer extrude the material onto the cable conductor, it must dry the material. The extruder unit is equipped with a material drying device to prevent defects such as bubbles and sand holes in the extruded insulation layer and sheath, and blistering on the surface. This is a rigid process requirement of the cable manufacturer, otherwise the finished cable will not pass the factory withstand voltage test.
2. The cable manufacturing process is damp
During the insulation extrusion process, the insulation layer is scratched, causing holes or debonding in the insulation layer, and water enters the insulation core in the cooling water tank, resulting in a decrease in insulation resistance. Or when the sheath is extruded, the sheath is damaged and water enters, causing the insulation layer to become damp and the insulation resistance to decrease. When manufacturing multi-core cables, even if the insulation layer is extruded intact, it may be damaged and water may enter when the insulation cores are twisted into cables and when the sheath is extruded, so the finished cable cannot pass the factory withstand voltage test.
3. The cable is damp during construction
During the construction of direct buried cables, if the cable trench excavation, cable burial operation, cable intermediate joints and terminal joints are not made in a standardized manner, the cable sheath and insulation layer are likely to be damaged. If the soil is moist or water accumulates in the cable trench, water will definitely enter the cable. When the insulation is damp, the surface resistance of the cable insulation decreases and the surface leakage current increases, the insulation resistance decreases, and the electric field between the conductor and the insulation layer is distorted. The uneven distribution of the electric field in the insulation will cause free discharge inside the insulation and even cause cable breakdown. After-sales service practice has shown that more than 95% of the direct buried cable insulation resistance decrease accidents are caused by improper construction.
