When mechanical stress or external force causes cable sheath and insulation damage and joint damage, moisture or moisture will be immersed along the longitudinal and radial gap of the cable, reducing the electrical strength of insulation. Therefore, most high voltage submarine cables have longitudinal and radial waterproofing measures to prevent moisture invasion. Radial measures are mainly on the outside of the insulation block and metal shielding layer winding and half resistance water expansion zone, adding metal waterproof layer on the outside of the metal shielding layer metal sheath, medium voltage cable electric field intensity is relatively low, the general use of lu: su composite sheath, there are only with polymer sheath, high voltage cable, the use of the lead, aluminium, stainless steel metal seal. Polymer sheath is waterproof, but has a certain water absorption rate, because its structure is mainly composed of crystalline phase and amorphous phase semi crystalline polymer. The structure of crystalline phase is compact, the arrangement of molecules in amorphous phase is loose, and there are large intermolecular gaps. Under the action of alternating electric field, polar water molecules constantly flip back and forth and can penetrate into the insulating material through the interstitial and grain boundary defects. When using polymer sheath, the sheath should be added with water-absorbing water-blocking agent.
Longitudinal water resistance mainly adopts compression type line center; Add water-repellent substances between the wires and the shielding area of the cable heart to block the diffusion channel of water in the cable heart. Longitudinal water resistance with water resistance powder filling effect is good, its water absorption is dozens of times or even thousands of times its own, water absorption strength is large, high expansion rate, water absorption can rapidly expand into gelatinous material, block water seepage channel, terminate the moisture and moisture further diffusion and extension, so that the length of the damp cable to the minimum.