When mechanical stress or external force causes damage to the cable sheath and insulation, and joint damage, moisture or moisture will immerse along the longitudinal and radial gaps of the cable, reducing the electrical strength of the insulation. Therefore, most high-pressure submarine cables have a longitudinal direction to prevent moisture intrusion. Radial waterproof measures. The radial measure is mainly to wrap the semi-conductive water-swelling band outside the insulating shielding and metal shielding layer, and to add a metal waterproof layer or metal sheath outside the metal shielding layer. The electric field strength of the medium-voltage cable is relatively low, generally using aluminum-plastic composite The sheath is also made of polymer sheath only, and the high voltage cable is made of lead, aluminum and stainless steel. The polymer sheath is water-repellent, but has a certain water absorption rate, because its structure is mainly a semi-crystalline polymer composed of a crystalline phase and an amorphous phase. The crystal phase is compact in structure, the molecules in the amorphous phase are loosely arranged, and there are large gaps between the molecules. Under the action of the alternating electric field, the polar water molecules are continuously turned back and forth, and can penetrate into the insulating material through the gap and the grain boundary defects. When a polymer sheath is used, a water-blocking agent having a water absorbing effect is added to the sheath.
Longitudinal water blocking mainly adopts 1 compression type core; 2 Add water-blocking substance between the wires and the cable shielding area to block the diffusion channel of moisture in the cable core. Longitudinal water blocking is better with water-blocking powder. Its water absorption is several tens of times or even several thousand times. It has high water absorption strength and high expansion rate. It can rapidly expand to form gelatinous substances after water absorption, block the water seepage channel and terminate the water. And the further diffusion and extension of moisture to minimize the length of the damp cable.