A.Direct laying
Direct laying has the significant advantage of saving investment and is a widely used laying method.
Before laying the cable, check whether there is any mechanical damage on the surface of the cable; and use a lkV megger to to telemeter the insulation, and the insulation resistance is generally not less than 10MΩ.
①The depth of the cable trench should be determined according to the elevation provided by the relevant department, and the buried depth of the cable must be guaranteed. The depth of direct burial cable should not be less than 0.7m, and should not be less than 1m when crossing farmland. The bottom of the trench for direct burial cable should be no hard debris, and covered with a 100mm thick layer of fine soil or golden sand. When laying the cable, a margin of 0.5%-1% of the total length should be left. After laying, covering 100mm of fine soil or golden sand, and then protect it with a cement cover or bricks. The coverage width should exceed 50mm on both sides of the cable. When backfilling to half the depth of the trench, it is recommended to lay a layer of color striped cloth with warning signs. After the backfill is completed, obvious orientation signs and stakes should be placed at special positions such as cable turns, intermediate joints, and intersections with other pipelines to enhance the ability to prevent external force damage.
②Cables that pass through roads and buildings or lead out parts within 2m on the ground should be protected by steel pipes. If the length of the protection pipe is less than 30m, the inner diameter should not be less than 1.5 times the outer diameter of the cable, and if it exceeds 30m, it should not be less than 2.5 times.
Both ends of the pipe should be trumpet-shaped, the inner wall of the pipe should be smooth without burrs, and the outside of the steel pipe should be coated with anti-corrosion paint. When the cable is introduced into and pulled out of the cable trench, building and penetrated into the protection pipe, the outlet port and the nozzle should be closed.
③When the AC 4 core cable is passed through the steel pipe or rigid plastic pipe, each cable is passed through one pipe. The single-core cable is not allowed to be penetrate into the steel pipe alone (except for those who take measures), and the clamp for fixing the cable should not have a closed magnetic circuit composed of iron pieces.
④For cables laid in parallel underground, the intermediate joints should be staggered to prevent other joints from being damaged in the event of a joint accident. When the cable is parallel or crossed with other pipelines, buildings, etc., it should be implemented in accordance with the specifications and must not be changed.
⑤Rural low-voltage power cables generally use PVC insulated cables or XLPE insulated cables. In places that may be damaged, armored cables with outer sheaths should be used; when laying cables in soils where displacement may occur (swamps, quicksand, backfill, etc.), steel wire armored cables should be used.
B．Pipe-laying method
As the most common laying method in cities, the cable channel is narrow and urban construction is frequent. In order to make better use of various terrains and protect the safe operation of cables, this is the most reasonable method. Its shortcomings are the cooling conditions of the cables are lowered, the ampacity is reduced and the construction cost is relatively high.
①If it is difficult to direct laying, not easy to repair the trench, the pipe-laying method can be used. The inner diameter of the pipe should not be less than 1.5 times the outer diameter of the cable, and the buried depth should be below 0.5m. When it is parallel and intersecting with other pipelines, buildings, etc., it should be carried out according to the specifications. There should be a 20mm gap between each tube to ensure heat dissipation.
②When laying cables, the nozzles of the pipes should be polished and smooth, and the dirt in the pipes must be removed to prevent the cables from being scratched. In order to facilitate inspection and maintenance, working wells should be set up every 150-200m or at turns. Cable joints should be located in the well.
③Plastic, asbestos or cement pipes can be used as the pipes. The most commonly used is plastic pipe.
However, when choosing plastic pipes, you should consider the material's impact resistance, pressure bearing capacity. It is not suitable to use pipes with a large thermal resistance coefficient. At present, the corrugated PVC pipes produced by many manufacturers have good performance and are suitable for selection.
C. Pipes used for pipe penetration
Generally, cement pipe or PVC pipe is used. Cement pipe is generally used for medium and low voltage cable pipe. The PVC pipe wall is smooth, the installation is simple, the friction force is small when the cable is laid, and the damage to the outer sheath is light. Brick or prefabricated trench laying is also a commonly used cable laying method. The advantage is that it can accommodate many types and quantities of cables at the same time, which can be distinguished and isolated by cable brackets. For high voltage cables, the cable laying in the open trench is safer and more intuitive. The disadvantage is that the ditch covers a wide area, which is not suitable for the layout of urban underground pipelines.
D．Tunnel or underground pipe gallery laying method
For some parts of the city, the underground pipelines are concentrated and it is difficult to arrange them. At this time, an underground corridor with a large space must be built. According to different pipelines, consider safety and reasonable factors to arrange. The issues that must be considered when laying cables in tunnels are fire and moisture resistance.
The direct laying method has the least investment, but due to poor safety, it is easy to be damaged by external forces, so it is not used as a permanent cable laying method, but only for temporary transition considerations.
E.Underwater laying method
The cable is required to have high mechanical strength (with reinforced armor), good waterproof of outer sheath (such as PE sheath). Before the cable is laid, choose a path environment with low water flow speed and less external interference. After laying, take strict protection measures , such as setting up signs, etc., to ensure safe operation. Cable laying is a key link between manufacturing and operation, and the quality of cable laying has a crucial impact on the safe and reliable operation of cables in the future. The construction personnel should pay great attention to the following work before and after cable laying:
①Make preparations before laying. First, check whether the cable laying path, civil construction facilities (cable trench, cable tunnel, protection pipe, etc.), and laying depth and width meet the requirements of the regulations. Prepare tools and equipment, remove various obstacles, and create conditions for laying.
②The cables must be hoisted as special material. Scraping, touching, squeezing and grinding are strictly prohibited. Arrange the position and direction of the cable tray according to the laying requirements, and do a good job of visual inspection. Before laying, the cable should be subjected to withstand voltage identification, and can be laid only after passing the test.