The cable that can be used in water is called water blocking (waterproof) power cable. When the cable is laid under water, it is required that the cable should have the function of preventing (resisting) water, that is, it should have the function of full water resistance, so as to prevent water from entering into the cable and causing damage to the cable, so as to ensure the long-term stable operation of the cable under water. The difference between waterproof cable and ordinary cable is that ordinary cable cannot be used in water.

There are three types of waterproof cables used in China

An oil paper insulated cable is the most typical water resistant cable. Its insulation and conductor are filled with cable oil, and its insulation is covered with metal sheath (lead sheath or aluminum sheath). It is the best water resistant cable. In the past, oil paper insulated cables were mostly used for submarine (or underwater) cables. However, oil paper insulated cables are limited by the drop, causing oil leakage and inconvenient maintenance. Now they are used less and less.

Second, the EPDM insulated cable is widely used in medium and low voltage underwater transmission lines because of its superior insulation performance without the worry of "water tree". The waterproof rubber sheathed cable (JHS type) can operate safely in shallow water for a long time.

Because of its excellent electrical, mechanical and physical properties, simple production process, light structure, large transmission capacity, convenient installation and maintenance, and not limited by drop, tri cross linked polyethylene (XLPE) insulated power cable has become the most widely used insulating material. However, it is particularly sensitive to moisture. If the insulation is stained with water in the process of manufacturing and operation, It is prone to "water tree" breakdown, which greatly shortens the service life of the cable. Therefore, XLPE insulated cables, especially medium and high voltage cables under AC voltage, must have "water blocking structure" when used in water environment or humid environment. The so-called "water blocking structure" includes radial water blocking structure and longitudinal water blocking structure.