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Changhui Instruments explains, through questions and answers, the appropriate use and precautions for the four cable installation methods: direct burial, conduit installation, tray installation, and cable trenches – something that is worth learning. Cable laying: yunrun.com.cn/tech/4070.html Direct burial method for cable installation. 1. What should be considered when using direct burial? Under what circumstances should it be used? For direct burial installation, it is necessary to consider whether the cable is susceptible to damage due to external forces. ①If it will not be subject to significant stress, direct laying is acceptable. ②If it may be subject to relatively large impacts, but the intensity can be kept within a certain range, armored direct burial can be considered. ③If the external force is greater, a protective sleeve is required, which can be installed in specific areas (such as those where roads pass through). 2. Advantages and disadvantages of direct burial installation of cables: Advantages: Easy to install, saves materials and labor. Disadvantages: It is inconvenient to maintain; if maintenance is required, the soil covering must be removed. It is only recommended for use in areas where maintenance is not a concern, or where such maintenance methods are acceptable. When cables are buried directly in the ground, it is generally necessary to use yellow sand as a padding layer. For directly buried cables that are grounded, how to locate the fault point? Answer: Use a cable inspection tester. Is a cable trench needed for directly buried cables? Additional question: For cable installation within the factory premises, armored cables are used for direct burial. Are cable trenches necessary at areas where the cables pass over obstacles or enter the workshop’s distribution room? Is it necessary to install a cable trench where the municipal 10kV cables enter the factory area? Answer: Cable trenches are not necessary when there are 6 cables or fewer; trenches should be installed when there are more cables, to facilitate their replacement and addition. At the point where the municipal 10kV cable enters the plant area, it is not necessary to install a cable trench; the cable can be led down from the terminal pole and buried directly up to the high-voltage distribution cabinet. How should temporary cables at construction sites be laid? Additional question: The company is building a new factory building. The construction transformers and high-voltage lines are located at a distance from the construction site. Since it is a steel-structure building, it is not possible to use overhead lines due to potential conflicts with the installation of the steel structure; therefore, low-voltage cables must be used to carry power from the transformers to the primary distribution boxes at the construction site, from where the power is then distributed to the secondary distribution boxes of various construction teams on site. How should these low-voltage cables be laid? Is it buried directly or placed directly on the ground? Answer: It is strictly prohibited to lay it directly on the ground. This low-voltage cable is laid directly buried. Cables are laid by being run through conduits. What precautions should be taken when laying cables in this manner? Under what circumstances should it be used? Cables are installed in conduits, which makes later maintenance and the addition of new lines easier compared to direct burial. For cables laid in conduits, it is advisable to consider using some spare conduits to prepare for future maintenance of the lines and capacity expansion. ①When laying cables in conduits, it is necessary to consider installing cable pits when the turning angle of the route is large or when the length of the straight section is considerable. ②When the number of cables is small and their diameter is low, a cable handhole can be used ； ③When there are many cables and their diameters are large, it is necessary to consider installing cable manholes. Cable shafts can be constructed according to the methods specified in the atlas. Apart from the atlas method, many small access shafts can also be built directly with bricks or concrete; in such cases, it is necessary to consider installing drainage holes at the bottom. ④Common pipe materials used for threading include cast iron pipes, steel pipes, polyethylene pipes, nylon pipes, carbon pipes, etc., which can be selected according to needs. When passing a single-core cable through a metal tube, attention should be paid to the effect of eddy currents. Cable laying using conduit methods – What should be considered when using conduits for laying? Under what circumstances should it be used? Pipe laying is mainly used in areas with a large number of lines and concentrated routes; relatively speaking, it is more convenient for laying pipes and also saves materials. When laying cables in outdoor conduit systems, should there be one conduit per cable, or can multiple cables share the same conduit? Answer: Only one cable should be run per tube. Except for important locations such as power plants and substations, no more than 3 power cables or multiple control cables may be run in each conduit for all circuits of a single motor or for all circuits of low-voltage motors in the same equipment. For details, see GB 50217-2007 P32 5.4.4. What to do if the pipe layout depth is insufficient? Additional question: How can cable trays be buried in soil with a thickness of less than 1 meter? Below is the basement; the ceiling of the basement is only 70 or 80 centimeters thick, and the depth of the shaft reaches 1.3 meters? Answer: In areas where there is no vehicle traffic, the burial depth should be at least 0.5 m (the cable standards require 0.5 m, while civil engineering standards require 0.7 m; the cable standards shall prevail here). If there is heavy traffic of vehicles, protective plates can be added to the exhaust pipes. Alternatively, thick-walled steel pipes can be used for installation, or cable trenches can be employed. If the cover soil over the basement ceiling is only 70–80 centimeters thick, a shaft depth of 1.3 meters would require the ceiling to sink, but this has little significance, as the drainage problems in cable shafts are difficult to resolve and are rarely given attention. Issues with the installation of strong-current conduit and weak-current conduits? Additional question: There is an external cable run in one factory facility, comprising a dozen or so low-voltage high-current cables, as well as cables for low-voltage systems (television, telephone, network). Since there are more than 8 high-current cables, it’s not possible to bury them directly; therefore, I used conduit for their installation. Now there is a problem: since low-voltage cables are also installed in pipes, can high-voltage pipes and low-voltage pipes be placed side by side? Answer: Try not to arrange them side by side; when working on minor municipal projects, place the electrical conduits for strong and weak currents on both sides of the road. If there are occasional intersections or the like, it is recommended to follow the specified spacing requirements. You can refer to the atlas, such as 05X101-2 for the installation of underground communication cables. Laying outdoor cable concrete conduit? Additional question: When drawing the general layout, at the client’s request, all outdoor cables are to be laid using concrete conduit. After reviewing the atlas (94D164), the following issues were identified: ① Due to the influence of other pipelines on site, except for the straight-shaped manholes, the manholes used for cable conduits cannot be constructed to the dimensions specified in the atlas. What are some reasonable solutions? ②Regarding the drainage issue in manhole wells. According to civil code requirements, a sump should be installed inside the manhole, but it is not clear what method should be used for draining water. ③The spacing between manhole wells is required to be 150 m according to the specifications, whereas the catalog specifies 50 m. I’m not sure which one should be taken as the standard, or if there are other regulations. ④Are there any other atlases that can be used as references? Besides standard wells, is it possible to design the shape and size of wells oneself? ⑤What considerations are needed for this type of laying method during design and construction? Answer: ① Non-standard wells can be designed as needed ； ②For drainage within the well bore, if it is possible to install a drainage pump, such a pump should be used. If it is not possible to install a pump, then overflow holes should be created at the bottom of the collection pit; these holes should not be sealed with concrete – it is fine to allow water to seep through naturally. However, this depends on the local groundwater level ； ③I usually consider the spacing between manhole wells to be between 50 and 70 ； ④In my opinion, it is not necessary for the power distribution in small municipal systems to follow the requirements outlined in the design manuals; non-standard solutions are sufficient ； ⑤Concrete ducts have not been used before, but please refer to clause 5.4.6 on page P32 of GB50217-2007 for the requirements regarding precautions when using ducts. Cables are installed in cable trenches. What should be taken into account when installing cables in trenches? Under what circumstances should it be used? When laying cables in cable trenches, it is important to use separate supports for cables of different voltage levels; moreover, proper drainage measures must be in place within the cable trenches themselves. And consider measures to prevent small animals such as rats from entering. How many cables are required to be laid in a cable trench? Answer: According to Section 6.10 of Measures 2009, it is recommended to use direct burial for 8 or fewer cables, conduit installation for 8 to 12 cables, cable trenches for 12 to 18 cables, and cable tunnels for 18 or more cables. ”These are all suggestions; they can be used as a reference for actual engineering projects, and they are not set in stone. As for cable trenches and cable tunnels, it’s not a matter of the number of cables, but rather a difference in terms of maintenance and importance. The selection is mainly based on factors such as engineering conditions, environmental characteristics, and the type and quantity of cables, as well as the requirements for reliable operation, easy maintenance, and technical and economic feasibility. It’s unclear whether there are any mandatory indicator requirements. For the determination of specific plans, reference can be made to the relevant content in Section 5.2, P28 of GB50217-2007. How is the current-carrying capacity of cables laid in cable trenches calculated? Answer: Netizen 1: (Considering the installation in cable trays, and taking into account that the cable trays are exposed to the air while the cable trenches have covers, a derating factor of 0.96 is applied.) ) Netizen 2: (The correlation coefficient related to current-carrying capacity can be referred to in GB T16895.15-2002 \"Electrical installations in buildings – Part 5: Section 523 Current-carrying capacity of wiring systems.\" In my opinion, it is more appropriate to use the coefficients specified for cable trays. ) Can the bus duct be laid along the cable trench? Answer: There are no specific requirements. The depth of the cable trench is determined by the number of cables and the types of cables used. For example, if there are control cables, low-voltage power cables, and high-voltage power cables, three layers are usually established, with a spacing of 250–300 between each layer; this determines the overall height of the trench. Is a cable trench needed for directly buried cables? Additional details: It’s a construction project involving a high-rise hospital. To cut costs, an oil-immersed transformer has been chosen and will be installed outdoors, 15 meters away from the building (including a 6-meter-wide road in between). The low-voltage switchroom is located underground within the hospital. To save on investment, I plan to use busbars (with a current-carrying capacity of 3000A) laid along the cable trench to reach the underground switchroom. Is this feasible? Answer: It should be possible to use dense busbars that meet the corresponding IP rating requirements; for example, Siemens offers busbars designed for use underwater. However, the cost of using dense busbars is relatively high; it is necessary to consider carefully whether it is better to use a dry-type transformer or busbars. Personally, I recommend using a dry-type transformer placed indoors. The amount of maintenance work will be less, and the failure rate will also be lower. How many cables can be laid in a cable trench without the need for supports? Answer: Generally, a support is required; without one, it will be quite troublesome to maintain the cables in the cable trench. If the project is small and the number of cables is limited, it’s acceptable not to install one. However, the heat dissipation conditions will be poorer, and if there’s water in the trench, it can also have a negative effect on the cables. Can high- and low-voltage cables be laid in the same cable trench? Additional question: It is an outdoor cable trench with a width of 1200 mm, equipped with supports on both sides. 10 KV high-voltage cables run on one side, while 1 KV low-voltage cables and control cables are laid on the other side. Is this in compliance with the standards? What precautions should be taken? Answer: They can be laid in the same cable trench. For specific requirements, please refer to clauses 5.1.3 and 5.1.4 on page 24 of GB 50217-2007. The cable trench runs parallel to the road. What is the spacing between them? Answer: This is mainly to consider the stress on the cable trench cover plates; if there are no issues with the stress, then there’s no problem. It is generally installed on sidewalks or in areas where motor vehicles do not pass. Should armored cables be used when laying cables in cable trenches? Additional question: In terms of technical measures, when cables are installed openly in cable trenches or cable tunnels, it is advisable to use cables with a steel tape armor and plastic outer sheath that provides protection against ants and rodents. But it seems that the specifications don’t clearly state this point! Is it possible to not use armor? Answer: There is no mandatory requirement for armored cables when they are buried directly, let alone in cable trenches. Depending on the different environmental requirements (if rats are rampant in the area, armored versions are recommended), there is generally no mandatory requirement. In a cable trench, when high-voltage cables and low-voltage cables are laid in layers on the same side of a support, what is the minimum distance required? Answer: Separate the low-voltage cables with metal plates, and ground those metal plates as well! It is best to place iron plates (or metal troughs) on the topmost support; the iron plates should be grounded, with the low-voltage cables placed on these plates (inside the troughs). According to Article 5.1.3 of GB 50217-2007, power cables with higher voltage levels should be installed first, followed by control and signal cables that carry strong electrical signals versus those carrying weak signals, and then communication cables – in this order from top to bottom. It can then be determined according to Table 7.6.23 of GB50054-2011. Further reading: The installation and layout of cables are part of the daily work of electrical designers. Related questions also appeared in the 2021 survey and design examination. It can be said that exams are becoming more and more aligned with actual project designs. As part of a certain technical upgrade plan, the company plans to add 6 three-core cables that will be buried directly beneath the lawn; the details of these cables are shown in the table below. To the left, parallel to the cable duct, are the underground gas pipes, while to the right are the underground drainage pipes. What is the minimum distance required between these gas pipes and drainage pipes in order to meet the requirement of laying these 6 cables in a single layer directly underground? http://yunrun.com.cn/upload/202112/06/202112061622370674.png 【A】1.90m/【B】2.40m/【C】2.80m/【D】2.85m Registered electrical answer: 【D】 According to GB50127-2018, 5.5.3, the outer diameter of the cable is: 0.1+0.1+0.05+0.05+0.05+0.05=0.4m; for voltages below 10kV, it is 0.1m. 0.25m for 10kV and above, 1m from gas pipelines. Drain pipe: 0.5m; total spacing: 0.4 + 0.25×3 + 0.1×2 + 0.5 = 2.85m