The protective layer of power cables can be divided into inner sheath and outer sheath. The main function of the inner sheath is to protect the insulation layer of the core from external mechanical damage. For oil immersed paper insulated cables, it can also prevent the infiltration of water and the outflow of immersion agents. The function of the outer protective layer is to prevent corrosion, mechanical protection, and shielding. 1、 Inner sheath

The inner protection of power cables is divided into metal (aluminum, lead, steel) and non-metal (rubber, plastic). Metal sheaths are commonly used for oil immersed paper insulated cables; Rubber and plastic sheaths are commonly used for rubber plastic insulated cables. Due to the limited use of rubber and plastic sheathed power cables, steel sheaths are mainly used in ultra-high voltage power cables. Therefore, only lead sheaths and aluminum sheaths are introduced here. （1） Lead sheath

The use of lead to manufacture the inner sheath of power cables has a long history, mainly due to its many technical characteristics. 1. Advantages

(1) The lead sheath has complete impermeability, which can reliably prevent the invasion of moisture and moisture, and has good sealing performance; (2) The melting point of lead is low (327.5 ° C), and it will not cause damage to the paper insulation due to overheating during the manufacturing of lead sheaths. When heated to around 200 ° C, lead forms a semi liquid state and can be extruded into a closed sheath at lower temperatures, making the lead pressing process simpler and the sealing and welding process more convenient,

(3) Good corrosion resistance. Lead has no effect on dilute sulfuric acid and hydrochloric acid, only the decaying substances of nitric acid, acetic acid, carbolic acid, and certain organic compounds can corrode lead.