It is composed of core, insulating layer and protective layer. The wire core is used for conducting current and is generally made of stranded copper wires or aluminum wires. Low voltage cables include single core, double core, three core and four core cables. Double core cables are used for single-phase lines. Three core and four core cables are used for three-phase three wire lines and three-phase four wire lines respectively. Single core cables can be used for single-phase lines or three-phase lines as required. The sectional area of commonly used low-voltage cable cores is 10, 16, 25, 35, 50, 70, 95120150185240 mm, etc.
Cable selection
Model Selection
The selection of power cable model shall be determined according to environmental conditions, laying methods, special requirements of electrical equipment and other factors, and generally considered according to the following principles:
(1) Aluminum core cables should be used in general environment and places, but copper core cables should be used in places with severe vibration, special buildings and special requirements.
(2) Buried cables generally adopt armored cables with outer sheath. In places where there is no possibility of mechanical damage, plastic sheathed cables and lead (aluminum) clad cables with outer sheath can also be used.
(3) In the soil with chemical corrosion or stray current corrosion, buried cables shall not be used as far as possible. If buried laying is necessary, anti-corrosion cable shall be used.
(4) Cables laid in pipes or rows of pipes are generally plastic sheathed cables or bare armored cables.
(5) Cables laid in cable trenches or cable tunnels shall not be covered with flammable or flame retardant outer coverings. Generally, bare armored cable, plastic sheathed cable and bare lead (aluminum) wrapped cable are used.
(6) When cables are laid in places with large height difference, plastic insulated cables, non drip cables and dry insulated cables should be used.
The cross-sectional area of the cable shall be selected according to the following four conditions: ① Select according to the heating conditions; ② Select according to the allowable voltage loss; ③ Select according to mechanical strength conditions; ④ Select according to the coordination with line protection (device) equipment. (See low-voltage line)
Low voltage cable
Low voltage using cables. The cable is composed of wire core, insulating layer and protective layer. There are many kinds of cables, and low-voltage power cables are mainly used for low-voltage cable lines.
Common laying methods of low-voltage cable lines include: ① directly buried; ② Laying in cable trench; ③ Laying along the wall; ④ Laying in cable tunnel, etc. The model of low-voltage power cable is mainly selected according to the environmental conditions and laying methods. Compared with low-voltage overhead lines and low-voltage overhead insulated lines, low-voltage cable lines are more expensive and difficult to lay and maintain, but they are widely used in low-voltage power distribution systems due to their reliable operation, no pole erection, no occupation of the ground, unobtrusive, and less affected by the outside world. Low voltage cable lines are mainly used in places with high requirements for power supply reliability (see power supply quality), corrosive gases, inflammables, explosives and other places where low-voltage overhead lines or low-voltage overhead insulated lines are not suitable.
Cable laying
When laying cables, first select the cable laying path, and then consider the laying method and laying requirements. When cable trench or cable tunnel is used for laying, fire prevention shall also be considered.
Cable routing path
Laying method
The common laying methods of low-voltage cables include direct buried laying, cable trench laying, tunnel laying and wall laying.
(1) Directly buried. When the number of cables laid along the same path is not more than 8, direct buried laying shall be adopted as far as possible. The buried depth shall not be less than 700 mm, and it shall not be less than 1 m when crossing farmland, and 100 mm thick fine sand or soft soil shall be evenly laid on the upper and lower part of the cable, and then covered with concrete protection plate (see Figure 1). The clear distance between directly buried cables and parallel or cross facilities shall not be less than the value in Table 1. Cables passing through buildings, railways, roads and other sections shall be protected through pipes. Directly buried laying, convenient construction, cost saving and good heat dissipation; The disadvantage is that cable maintenance is inconvenient, external mechanical damage cannot be reliably prevented, and it is easy to be corroded. Direct buried laying is applicable to situations where the number of cables is small and the laying path is long.
(2) Cable trench laying. The cable trench is divided into indoor cable trench, cable trench in low-voltage power distribution device and general outdoor cable trench. See Figure 2 for cable trench structure. The cover plate of indoor cable trench shall be leveled with the ground. The cover plate of the cable trench for the internal line of the outdoor power distribution device shall be higher than the ground and serve as the operation walkway. Generally, the cover plate of outdoor cable trench shall be 0.3m lower than the ground in order not to interfere with drainage, and sand or crushed soil shall be paved on it. The investment in cable trench laying is less than that in cable tunnel and pipe laying. It is convenient to repair and replace cables, occupies a small area, and routes are easy and flexible. When there are not many cable trenches crossing the underground pipe network, and the number of cables in the same path is not more than 18, cable trench laying is generally recommended.
