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Knowledge that needs to be known in weak current engineering construction
01. What types of commonly used wires and cables are divided into based on their purpose?
Answer: Based on their purpose, wires and cables can be categorized into bare wires, insulated wires, heat-resistant wires, shielded wires, power cables, control cables, communication cables, radio frequency cables, and more.
02. What types of insulated wires exist?
Answer: Insulated wires include polyvinyl chloride insulated wires, rubber insulated wires, nitrile polyvinyl chloride mixture insulated wires, agricultural underground direct-buried aluminum core plastic insulated wires, rubber insulated cotton yarn textile soft wires, polyvinyl chloride insulated nylon sheathed wires, and flexible polyvinyl chloride insulated wires for power and lighting, among others.
03. In what situations are cable trays suitable?
Answer: Cable trays are ideal for laying power and control cables indoors and outdoors in general industrial and mining enterprises. They can also be used for installations in telecommunications, radio, and television departments like telecommunications, radio, and television.
04. What are the common cable accessories?
Answer: Common electrical accessories include cable terminal junction boxes, cable intermediate junction boxes, connecting pipes and terminals, steel plate wiring troughs, cable trays, etc.
05. What is a cable joint?
Answer: A cable joint is a device that connects the conductors, insulating shielding, and protective layers of a cable to another cable, thereby connecting cable lines.
35 issues that must be known about wires and cables during the construction of Yatel weak current projects:
06. What is the main electrical wiring?
Answer: Main electrical wiring refers to the connection method of major electrical equipment and busbars in power plants and substations, including the connection method of the main busbar and the factory power system according to specific functional requirements.
07. What are the principles for selecting the cross-section of power cables?
Answer: When selecting power cables, the following principles should be followed: 1) The rated voltage of the cable should be greater than or equal to the rated voltage of the power supply system at the installation location; 2) The continuous allowable current of the cable should be equal to or greater than the maximum continuous current of the power supply load; 3) The cross-sectional area of the wire core must meet the stability requirements when the power supply system is short-circuited; 4) Check whether the voltage drop meets the requirements based on the cable length; 5) The minimum short-circuit current at the end of the line should enable the protection device to operate reliably.
08. What are the advantages of cross-linked polyethylene cables compared to oil-paper cables?
Answer: 1) Easy to install due to small bending radius and lightweight; 2) Not affected by line drop; 3) Excellent thermal performance, allowing for high working temperatures and large transmission capacities; 4) Simple cable accessories, all of which are dry structures; 5) Simple operation and maintenance, no oil leakage problems; 6) Lower cost; 7) High reliability and low failure rate; 8) Fewer manufacturing processes, simpler processes, and significant economic benefits.
09. What are the requirements for fixing AC single-core cable clamps? Why?
Answer: The clamp should not contain any iron parts to form a closed magnetic circuit. This is because when the cable core carries current, a magnetic field is generated around it, and the magnetic flux is proportional to the magnitude of the current passing through the wire core. If magnetic permeable materials such as iron parts are used, according to electromagnetic induction, eddy currents will be generated in the iron parts, heating the cable and potentially burning it. Therefore, iron parts cannot be used as fixing clamps for single-core AC cables.
10. What are the most notable features and advantages of heat-shrink cable head accessories?
Answer: The most prominent feature of heat-shrink accessories is the use of stress tubes instead of traditional stress cones. This not only simplifies the construction process but also reduces the size of the terminal of the joint, making it easier to install, saving time and labor, offering superior performance, and conserving metal. Heat-shrink cable accessories combine the advantages of the filling type and the dry-pack type.
35 issues that must be known about wires and cables during the construction of Yatel weak current projects:
11. What inspections should be carried out before cable laying?
Answer: 1) Ensure that the brackets are complete and painted properly; 2) Verify that the cable model, voltage, and specifications comply with the design; 3) Check that the cable is well insulated, and if there is doubt about the sealing of the oil-paper cable, judge for moisture. Direct-buried cables and small-bottom cables should pass the DC voltage resistance test. The oil sample of the oil-filled cable should pass the test; 4) The oil pressure of the oil-filled cable should not be less than 1.47MPa.
12. When you have doubts about oil-paper insulated power cable seals, how can you use a simpler method to check if the cable insulated paper is damp?
Answer: Light the cable insulation paper or place it in cable oil at around 150℃ for inspection. If there is no "hissing" sound or white foam appears, it indicates that it is not damp.
13. What information should be indicated on cable signs? What are the requirements for writing?
Answer: The cable line design number, cable model, specification, and starting point should be indicated on the signboard. Cables used in parallel should have sequence numbers. It is required to have clear handwriting and not easily fall off.
14. Where should the directional signs of directly buried cables be placed?
Answer: At both ends of the cable, at the cable joints within 50~100m along the straight-line segment of the cable, and at the corners where the cable changes direction.
15. How to remove moisture before construction for yellow wax tapes, black glass paint tapes, alkali-free glass tapes, etc.?
Answer: 1) Constant temperature drying method: Roll the insulating tape into a small roll with a diameter of 25~30mm, place it in a constant temperature drying box of 110~120℃ for 4~5 hours, cool it, take it out, and store it in a dry sealed cylinder. 2) Oil immersion moisture removal method: Place the small roll of insulating tape in cable oil with a constant temperature of 120~130℃, keeping a distance of 30mm from the bottom of the pot. After a certain period of time, when the oil surface no longer produces foam, remove it and place it in a barrel filled with cable oil. The oil level should exceed all loaded objects and seal it.
16. What is the function of the cable outer sheath?
Answer: To protect the inner protective layer from mechanical damage and chemical corrosion, enhancing mechanical strength.
17. What requirements should outdoor cable trenches meet?
Answer: The top of the cable trench should be slightly higher than the ground. Concrete covers should be used. The cable should be flat on the brackets and there should be a good drainage pipe.
18. What is the function of the inner protective layer of the cable?
Answer: To prevent the insulating layer from coming into contact with water, air, or other objects, preventing moisture from affecting the insulation and mechanical damage to the insulating layer.
19. What should you pay attention to when using wire cutters?
Answer: Before use, ensure that the insulation of the insulating handle is intact. Do not use wire cutters to cut metal wires exceeding the specified limits. It is prohibited to use wire cutters as hammers to avoid damage.
20. What is insulation strength?
Answer: Insulating substances in an electric field will break down when the electric field strength reaches a certain limit. The electric field strength that causes insulation breakdown is called insulation strength.
21. What are the advantages of crosslinked heat-shrink cable accessories?
Answer: This is a new type of material. Compared to other types of accessories today, it has the advantages of superior electrical performance, small size, light weight, simple installation, and material compatibility. Additionally, it has the ability to resist climate, anti-fouling, flame retardant, and self-extinguishing.
22. What are the requirements for the mechanical strength of cable conductor connection points?
Answer: The mechanical strength of the connection point is generally lower than the tensile strength of the cable conductor itself. For fixedly laid power cables, the tensile strength of the connection point should not be less than 60% of the tensile strength of the conductor itself.
23. What are the main properties of the insulation layer material of power cables?
Answer: It should have the following main properties: 1) high breakdown strength; 2) low dielectric loss; 3) very high insulation resistance; 4) excellent discharge resistance; 5) certain softness and mechanical strength; 6) long-term stability of insulation performance.
24. What are the regulations on traction strength when laying cables mechanically?
Answer: For copper-core cables, the allowable traction strength is 70N/cm when pulling the head; for aluminum-core cables, the allowable traction strength is 40N/cm when pulling the head; if traction is made with a wire mesh sleeve, the allowable strength is 10N/cm when pulling; and for lead-sheathed cables, it is 40N/cm.
25. What are the regulations on cable protection pipes?
Answer: 1) When a cable needs to be laid through a protective pipe, the inner diameter of the pipe should not be less than 1.5 times the outer diameter of the cable, and the inner diameter of concrete pipes, clay pipes, asbestos, and cement pipes should not be less than 100mm; 2) The bending radius of the cable pipe should comply with the bending radius of the penetrated cables; 3) Each pipe should not exceed three elbows, and there should not be more than 2 right-angle bends.
26. How to measure the outer diameter of the cable sheath?
Answer: At five points evenly distributed on the circumference of the sheath, measure the outer diameter of the sheath and calculate its average value, which is the outer diameter of the sheath.
27. How to connect copper core cables with different cross-sections?
Answer: When connecting copper core cables with different cross-sections, open weak back copper connectors can be used, soldered, or pure copper rods can be connected to copper connectors according to different cross-section requirements, and connected by crimping method.
28. Briefly describe the process of heat-shrinking production of indoor terminal heads in 10KV crosslinked cables.
Answer: 1) Preparation stage: Check whether the heat-shrinkable cable accessories are complete and whether the models match. After checking and confirming whether the cable is damp, check the cable; 2) Cut off the excess cable and decide the cable length according to the on-site situation; 3) Peel the protective layer; 4) Weld the grounding wire and weld the grounding wire on the steel strip; 5) Fill the three-fork and wrap sealant; 6) Install the three-core branch sheath, put the sheath into the root, start to shrink from the middle, first to the root, and then to the finger; 7) Peel the copper belt and the outer semiconductor layer, peel and cut the copper belt with a mouth of more than 20mm of the three-core branch sleeve. It is strictly forbidden to damage the main insulation and remove the semiconductor layer; 8) Install the stress tube, and the support sleeve at the end of the pipe is docked and heat shrink; 9) Install the wiring terminals; 10) Install the insulating tube; 11) Install the sealing tube; 12) Install the phase color tube after the core phase.
29. What are the steps to install prefabricated terminals in indoor and outdoor settings?
Answer: 1) When a cable needs to be laid through a protective pipe, the inner diameter of the pipe should not be less than 1.5 times the outer diameter of the cable, and the inner diameter of concrete pipes, clay pipes, asbestos, and cement pipes should not be less than 100mm; 2) The bending radius of the cable pipe should comply with the bending radius of the penetrated cables; 3) Each pipe should not exceed three elbows, and there should not be more than 2 right-angle bends.
30. What are the regulations on cable protection pipes?
Answer: 1) Power System: The wire and cable products used in the power system mainly include overhead bare wires, busbars (busbars), power cables (plastic cables, oil-paper cables (basically replaced by plastic power cables), rubber-covered cables, overhead insulated cables), branch cables (replaced with some busbars), electromagnetic wires, and electrical equipment wires and cables for power equipment.
31. Five categories of wire and cable products:
(1) Bare wires and bare conductor products: The main features of this type of product are: pure conductor metal, without insulation and sheathing layer, such as steel core aluminum stranded wire, copper-aluminum busbar, electric locomotive wire, etc.; the processing technology is mainly pressure processing, such as smelting, calendering, drawing, twisting/pressure twisting, etc.; the products are mainly used in suburban areas, rural areas, user main lines, switch cabinets, etc.
(2) Power cables: The main features of this type of product are: extruding (wrapping) the insulating layer outside the conductor, such as overhead insulated cable, or twisting several cores (corresponding to the phase, neutral and ground wires of the power system), such as overhead insulated cables above two cores, or adding a sheath layer, such as plastic/rubber-sleeved wire and cables. The main process technologies include drawing, twisting, insulating extrusion (wrapping), cable formation, armor, protective layer extrusion, etc. The different process combinations of various products are different. The products are mainly used for the transmission of strong electric power in the transmission, distribution, transmission, conversion and power supply lines. The current through the flow is large (ten to thousands of amperes) and the voltage is high (220V to 500kV and above).
(3) Electrical equipment wires and cables: The main characteristics of this type of product include a wide variety of specifications, a wide range of applications, and a large number of voltages of 1kV or below. New products are constantly derived in special occasions, such as fire-resistant cables, flame-retardant cables, low-smoke, halogen-free/low smoke and low-halogen-low-halogen-low-termite-proof cables, mouse-resistant cables, oil-resistant/cold-resistant/temperature-resistant/wear-resistant cables, medical/agricultural/mining cables, thin-walled wires, etc.
(4) Communication cables and fiber optics: With the rapid development of the communications industry in the past twenty years, products have also developed at an astonishing speed. From the simple telephone and telegram cables of the past to thousands of pairs of speech cables, coaxial cables, optical cables, data cables, and even combined communication cables. This type of product is usually small and uniform in structural dimensions, and has high manufacturing accuracy requirements.
(5) Electromagnetic wire (winding wire): mainly used in various motors, instruments, etc. The derivatives/new products of wires and cables are mainly generated due to the requirements of different application occasions, different application requirements, convenience of equipment and reduced equipment costs, etc., and the use of new materials, special materials, or changing the product structure, or improving process requirements, or combining different varieties of products; different materials such as flame retardant cables, low-smoke, halogen-free/low-smoke, low-halogen-low-halogen-low-termite-proof cables, mouse-proof cables, oil-resistant/cold-resistant/temperature-resistant cables, etc.; changing the product structure such as fire-resistant cables, etc.; improving process requirements such as medical cables, etc.; combining products such as OPGW; facilitating installation and reducing equipment costs such as prefabricated branch cables, etc.
32. What inspections should be carried out after the busbar device is completed?
Answer: The following inspections should be carried out: 1) The processing, preparation, and welding screw connections of metal components should comply with the regulations; 2) Bolts, washers, opening pins, and other parts should be complete and reliable; 3) The busbar preparation and mounting frame should comply with the regulations, and the electrical distance between phases and ground meets the requirements; 4) Porcelain parts, iron parts, and glued parts should be complete, the oil-filled sleeve should be free of oil leakage, and the oil level should be normal; 5) The paint should be complete, the color should be correct, and the grounding should be good.
33. Among the power cable connectors below 35kV and below, there are several methods to improve the electric field distribution at the sheath disconnection (please list five), and briefly describe their methods.
Answer: 1) Flaring the horn: Pry the edge of the lead bag at the cutting point of the lead bag, making it a flared edge, and the edge should be smooth, round, and symmetrical; 2) Reserving the entire insulation package: Leave a piece of round-pack insulating paper between the lead bag cut and the separation point of the cable core wire; 3) Cutting off the semi-conducting paper: Cut the semi-conducting paper below the flared mouth; 4) Wrapping the stress cone: Wrap it into a cone shape with an insulating tape and conductive metal material, artificially expanding the shielding layer to improve the electric field distribution; 5) Equi-potential method: For dry-pack or cross-linked polyethylene cable heads, a metal strip is wrapped on the insulating surface of each wire core profile and connected them together; 6) Installing stress control tubes: For 35kV and lower heat shrink tube cable heads, first wrap the 2-layer semi-conductor strip from the end direction of the copper shield layer of the wire core through the semi-conductor tape to the insulating surface of the wire core, and then fold the stress tubes of the corresponding specifications at the end of the copper shield and heat shrink forming.
34. What requirements should the processing of cable brackets meet?
Answer: 1) The steel should be straight without obvious distortion, the cutting error should be within 5mm, the cut should be without curled edges and thorns; 2) The bracket should be firmly welded without deformation, and the vertical clear distance between the horizontal braces and the design deviation should not be greater than 5mm; 3) The metal bracket must be subjected to anti-corrosion treatment. When located in damp heat, salt, fog, and chemically corrosive areas, special anti-corrosion treatment should be carried out according to the design.
35. What requirements should be met when laying cables?
Answer: The following requirements should be met: 1) In terms of safe operation, avoid various external damages as much as possible, and improve the power supply reliability of cable lines; 2) In terms of economics, consider from the perspective of investment savings; 3) In terms of construction, the path of cable lines must be convenient for rotary work and maintenance after operation.