solid cable: lead (Pb) is used as the protective shell on the outside, paper soaked in insulating oil is used to cover the conductor inside, and jute and other fillers are filled in the gap. Because the gap in the lead layer is filled with filler, it is called solid cable. Three of them are introduced as follows: ribbon cable: the conductor is round, semi-circular or fan-shaped soft copper wire, which is twisted with insulating paper. Then fill the gap with filler. The exterior is wrapped with insulating paper, soaked in insulating oil, and then covered with lead layer. This kind of cable is used for transmission and incoming lines below 1kV. SL cable: each round core is covered with insulating paper and then covered with lead layer. The direction of electric field is perpendicular to the insulation layer, which increases insulation endurance and is suitable for 2kV ~ 3 kV power transmission. And the cooling effect is good. H cable: metallized paper is wound on the insulation core, twisted into a circle with the filler, and covered with lead layer. Lighter than SL cable, it has good heat dissipation and can be used for 3kV transmission. Oil-filled cable: Because the lead coating on the surface of solid cable is an inelastic rigid body. Therefore, when the temperature rises, the lead layer expands, and after cooling, it shrinks incompletely, which is easy to produce gaps in the insulation layer, causing local high-voltage discharge and leading to the deterioration of insulation. In view of this defect, insulating oil of more than one atmosphere can be filled in the cable, which can effectively inhibit the generation of voids, so the pressure resistance is improved. It is called oil filled cable, or OF cable for short. It can be used in the high voltage range of 66~154KV. Because it is often used for long-distance power transmission, an oil filling station should be set at a proper distance to maintain the oil pressure in the cable. Pressure cable: placing the insulated core wire in the steel pipe, and then filling it with nitrogen, hydrogen or insulating oil at 12~14 atmospheres, can inhibit the partial discharge caused by bubbles or gaps in the insulation body, resulting in the deterioration of the insulation material. Its breakdown voltage is about twice that of ordinary cables, and it is suitable for ultra-high voltage underground transmission cables above 44KV. Synthetic resin cable: the insulation layer is made of polyethylene, and the outermost layer is made of polyvinyl chloride, also known as EV cable. If cross-linked polyethylene replaces polyethylene, it is called CV cable. This kind of cable has low insulation failure strength and is not suitable for high voltage applications. Because of its light weight, it gradually replaces the application of solid cable in low voltage occasions. Telecommunication cable, a communication cable, is mainly used to transmit digital or analog signals with low voltage and current, so the main requirements are to avoid noise interference and signal attenuation with high frequency. For example, the electrostatic capacity between cores should be small, and the core configuration should avoid electrostatic induction in the loop. According to its use, telecommunication cable can be divided into: local cables: insulated soft copper wire is used as the core wire, and two round-trip cores are twisted, which is called twisted pair. Or two groups of two-core wires are twisted in a square, which is called a star-shaped square strand. Because the round-trip current is opposite, electrostatic induction can be prevented. The plastic insulation used for the core wire is also identified by color. A plurality of pairs of twisted wires or star-shaped square strands are twisted in layers to form a concentric arrangement, and an insulating paper tape is wound, and finally a lead layer is used for protection. This kind of cable is mainly used in the local telephone joining line and the trunk line of the telephone branch office. Cable outside the city: use copper wire thicker than cable inside the city as the core wire, intertwine with thin paper, and then use insulated paper roll to form a wire group. A plurality of wire groups are twisted in layers by concentric cylinders. External insulating paper tape is wound and finally protected by lead layer. According to the different twisting modes, it can be divided into three types: heavy signal cable twisted with DM square strand, outdoor star cable with the same structure as the local star cable, and transmission cable twisted with four cores in star square strand. This kind of cable is mainly used in telephone lines between cities and long-distance communication lines. Local cable: winding wire and colored stranded wire on enameled wire to form core wire. Now twist 2 or 3 stranded wires into a unit cable. The core wire can also be made of vinyl resin, and the outer layer is covered with vinyl resin. It is called PVC local cable. Simple process, moisture resistance and incombustibility. Generally, it is used to connect the machines in the telecommunication bureau. High-frequency cable: mainly transmitting high-frequency signals. As far as the structure is concerned, it can be divided into the following two kinds of coaxial cables: hollow cylindrical conductor inside, outer conductor, polyethylene as the insulator between the inner and outer conductors, and PVC as the outer insulator. Balanced cable: video cable generally used for TV signal transmission. In order to avoid high-frequency attenuation, a pair of conductors are twisted and insulated with foamed polyethylene, and the outside is still shielded by the same coaxial cable. The cable impedance is 1252.5, which can be used to 6MHz, and the attenuation constant is 11~13dB/km. Submarine cable: In addition to the characteristics of general communication cables, this kind of cable must also have the characteristics of waterproof and pressure resistance (water pressure). According to its use, it can be divided into two types: dry lead paper cable and lead-free cable: dry lead paper cable: soft copper wire as the core, and aluminum foil inserted in the center of the cable for electrostatic shielding of the line to prevent leakage. Both ends of the cable are wrapped with copper and iron belts to shield the electromagnetic field. There are two layers of aluminum coating on the surface to make it waterproof. The outermost lead coating is covered with rubber. This kind of cable is not suitable for deep sea, but it can accommodate more lines. Lead-free cable: soft copper wire or stranded wire is used as the core, and gutta percha is used as the insulation material. High permeability alloy is used to cut into strips and wound on conductors, which can maintain the transmission capacity. Although it can accommodate fewer circuits, it is suitable for deep sea. Optical fiber cable: The transmission of signals by light has many advantages, and it has gradually replaced the traditional telecommunication cable.