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Detailed explanation of 80 common questions about wires and cables, recommended for collection

2023-05-09 超级管理员 Read 57

1: Basic knowledge issues


1. What are the types of commonly used wires and cables according to their purpose?


Answer: According to the purpose, it can be divided into bare wires, insulated wires, heat-resistant wires, shielded wires, power cables, control cables, communication cables, RF cables, etc.


2. What are the types of insulated wires?


Answer: There are several common types of insulated wires: polyvinyl chloride insulated wires, polyvinyl chloride insulated flexible wires, nitrile polyvinyl chloride mixture insulated flexible wires, rubber insulated wires, agricultural underground buried aluminum core plastic insulated wires, rubber insulated cotton yarn textile flexible wires, polyvinyl chloride insulated nylon sheathed wires, polyvinyl chloride insulated flexible wires for power and lighting, etc.


3. Where is cable tray suitable?


Answer: cable tray is suitable for indoor and outdoor overhead laying of power cables and control cables in general industrial and mining enterprises, and can also be used for indoor and outdoor erection of telecommunications, radio and television departments.


4. What are the cable accessories?


Answer: Common electrical accessories include cable terminal junction box, cable intermediate junction box, connecting pipe and terminal, steel plate junction slot, cable tray, etc.


5. What is a cable intermediate joint?


Answer: The device that connects the cable to the conductor, insulation shield, and protective layer of the cable to connect the cable line is called the cable intermediate joint.


6. What is the main electrical wiring?


Answer: The main electrical wiring is the connection method between the main electrical equipment and busbar in power plants and substations, including the connection method between the main busbar and the auxiliary power system according to certain functional requirements.


7. What regulations should be followed when selecting the cross-section of power cables?


Answer: The selection of power cables should follow the following principles:


(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 point;


(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-section of the wire core should meet the stability requirements of the power supply system during short circuits;


(4) Check whether the voltage drop meets the requirements based on the length of the cable;


(5) The minimum short-circuit current at the end of the line should enable reliable operation of the protection device.


8. What are the advantages of cross-linked polyethylene cables compared to oil-paper cables?


Answer: (1) Easy to install because it allows for a small minimum bending radius and lightweight;


(2) Not limited by line drop;


(3) Good thermal performance, allowing for high working temperature and large transmission capacity;


(4) The cable accessories are simple and all of them are of dry structure;


(5) Simple operation and maintenance, no oil leakage issues;


(6) The price is relatively low;


(7) High reliability and low failure rate;


(8) The manufacturing process is few, the process is simple, and the economic benefits are significant.


9. What are the requirements for fixing fixtures for AC single core cables? Why?


Answer: The fixture should have no iron components to form a closed magnetic circuit. This is because when the cable core passes through current, magnetic lines of force are generated around it, which are proportional to the current passing through the core. If magnetic conductive materials such as iron are used, according to electromagnetic induction, eddy currents will be generated in the iron components, causing the cable to heat up or even burn out. Therefore, iron parts cannot be used as fixing fixtures for single core AC cables.


10. What are the biggest features and advantages of the following accessories for heat shrink cable heads?


Answer: The biggest feature of heat shrink accessories is the use of stress tubes instead of traditional stress cones. It not only simplifies the construction process, but also reduces the size of the joint terminals, making installation convenient, time-saving, labor-saving, superior performance, and metal saving. The heat shrinkable cable accessory integrates the advantages of both perfusion type and dry pack type accessories.


Detailed explanation of 80 common questions about wires and cables, recommended for collection


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2: Cable laying issues


11. What inspections should be carried out before cable laying?


Answer: (1) The bracket should be complete and the paint should be intact.


(2) The cable model, voltage, and specifications comply with the design.


(3) The cable insulation is good, and when there is doubt about the sealing of oil-paper cables, it should be judged that they are affected by moisture; Directly buried cables and small bottom cables should pass the DC withstand voltage test; The oil sample of the oil filled cable should pass the test.


(4) The oil pressure of oil filled cables should not be lower than 1.47MPa.


12. When there is doubt about the sealing of oil-paper insulated power cables, how can a simpler method be used to check whether the cable insulation paper is damp?


Answer: Ignite the cable insulating paper or put it into the cable oil at about 150 ℃ for inspection. There is no "hissing" sound or white foam, indicating that it is not affected with moisture.


13. What should be indicated on the cable signboard? What are the requirements for writing?


Answer: The signboard should indicate the cable route design number, cable model, specification, and starting point. Cables used in parallel should have a sequence number. The handwriting should be clear and not easy to fall off.


14. Where should the directional signs of directly buried cables be set?


Answer: At both ends of the cable, at the straight section of the cable at a distance of 50-100m, the cable joint is located at the bend where the cable changes direction.


15. How to carry out moisture removal treatment for yellow wax silk tape, black glass paint tape, alkali free glass ribbon, etc. used for making cable joints before construction?


Answer: (1) Constant temperature drying method: Roll the insulation tape into small rolls with a diameter of 25-30mm, place it in a constant temperature drying oven at 110-120 ℃ for 4-5 hours, cool it dry, and take it out. Place it in a dry sealed cylinder.


(2) Oil immersed moisture removal method: put a small roll of insulating tape into the cable oil with a constant temperature of 120~130 ℃, keep a distance of 30mm from the bottom of the pot, take it out after a certain period of time when the oil surface no longer produces foam, and put it into the barrel where the cable oil is stored. The oil level should exceed all the loaded objects and seal them.


16. What is the function of the cable outer protective layer?


Answer: Protect the inner protective layer from mechanical damage and chemical corrosion, and enhance mechanical strength.


17. What requirements should outdoor cable trenches meet?


Answer: The upper part of the cable trench should be slightly higher than the ground, covered with a cover made of concrete, and the cable should be laid flat on the bracket with good drainage pipes.


18. What is the function of the cable inner protective layer?


Answer:; Ensure that the insulation layer does not come into contact with water, air, or other objects to prevent moisture and mechanical damage to the insulation layer.


19. What should be noted when using wire cutters?


Answer: Before use, it is necessary to check whether the insulation of the insulating handle is intact. During use, it is not allowed to cut metal wires that exceed the specifications with wire pliers, and it is prohibited to use wire pliers instead of hammers to strike tools to avoid damage.


20. What is insulation strength?


Answer: Insulating materials in an electric field will be broken down when the electric field strength increases to a certain limit, and the electric field strength that causes insulation breakdown is called insulation strength.


Detailed explanation of 80 common questions about wires and cables, recommended for collection


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3: Cable protection regulations


21. What are the advantages of cross-linked heat shrink cable accessories?


Answer: This is a new type of material, which has advantages such as superior electrical performance, small size, small quality, easy installation, and material matching compared to other types of accessories nowadays. In addition, it also has the ability to withstand weather, pollution, 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 fixed laying power cables, the tensile strength of the connection point is required to be no less than 60% of the tensile strength of the conductor itself.


23. What are the main properties that the insulation layer material of power cables should possess?


Answer:; It should have the following main performance:


(1) High breakdown strength;


(2) Low dielectric loss;


(3) Relatively high insulation resistance;


(4) Excellent discharge resistance;


(5) Having a certain degree of softness and mechanical strength;


(6) The insulation performance is stable for a long time.


What are the regulations for 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 a wire mesh sleeve is used for traction, the allowable strength of the lead sheathed cable is 10N/cm;


The lead sheathed cable is 40N/cm.


25. What are the regulations for cable protection pipes?


Answer: (1) When cables need to be laid through protective tubes, the inner diameter of the tube 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 duct should comply with the regulations for the bending radius of the penetrated cable;


(3) Each pipe should have no more than three bends and no more than two right angle bends.


26. How to measure the outer diameter of the cable sheath?


Answer: At five evenly distributed points on the circumference of the sheath, measure the outer diameter of the sheath and its average value. The average outer diameter is the outer diameter of the sheath.


27. How to connect copper core cables with different cross-sections?


Answer: Copper core cables with different cross sections can be connected using an open weak back copper connection pipe, which can be connected by soldering. Pure copper rods can also be connected into copper connections according to different cross section requirements, which can be connected by crimping method.


28. Briefly describe the process of producing indoor terminal heads using heat shrink technology for 10KV cross-linked cables?


Answer:; (1) Preparation stage: Check if the heat shrink cable accessories are complete and if the models match. After checking and confirming that the cables are free of moisture, check the cables.


(2) Cut off excess cables and determine the cable length based on on-site conditions.


(3) Peel off the protective layer.


(4) Weld the grounding wire onto the steel strip.


(5) Fill the three prongs and wrap the sealant.


(6) Install the three core branch sheath, insert the sheath into the root, shrink from the middle, first towards the root, and then towards the finger.


(7) Peel off the copper strip and the outer semi conductive layer, and cut the copper strip with a diameter of 20mm or more from the three-core branch sleeve. It is strictly prohibited to damage the main insulation and clean the semi conductive layer.


(8) Install the stress tube and heat shrink the support sleeve at the end of the pipe after docking.


(9) Install wiring terminals.


(10) Install the insulation pipe.


(11) Install the sealing tube.


(12) Install a phase color tube after phase verification.


29. What are the installation steps for indoor and outdoor prefabricated terminals?


Answer: (1) Place the cable in the predetermined position, fix it, and clean the surface. Refer to the manual to remove the outer sheath. If there are special needs, the length of the outer sheath can be adjusted, and two sets of ground wires are led out by copper tape and outer sheath armor.


(2) Measure the required size upwards from the outer sheath opening, cut off excess cables, insert a heat shrink three finger sleeve to the root of the three fork opening, heat and shrink from the middle, and measure the specified size upwards from the upper opening of the finger sleeve (350-380mm for 35kV). Remove excess copper tape (retain 20mm), then reserve the predetermined position of the semiconductor, and crimp the outgoing terminal for sealing.


The application of wires and cables is mainly divided into three categories:


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 power cables (mostly replaced by plastic power cables), rubber sheathed cables, overhead insulated cables), branch cables (replacing some busbars), electromagnetic wires, and electrical equipment wires and cables for power equipment.


2. Information transmission system


The wires and cables used for information transmission systems mainly include local telephone cables, television cables, electronic cables, radio frequency cables, fiber optic cables, data cables, electromagnetic cables, power communication or other composite cables.


3. Mechanical equipment and instrument system


Except for overhead bare wires, almost all other products in this section are applied, but mainly power cables, electromagnetic wires, data cables, instrument cables, etc.


Detailed explanation of 80 common questions about wires and cables, recommended for collection


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4: Cable classification issues


31. Wire and cable products are mainly divided into five categories:


1. Bare wires and bare conductor products


The main features of this type of products are: pure conductor metal, without insulation and sheath layer, such as steel cored aluminum strand, copper aluminum bus bar, electric locomotive line, etc; The processing technology mainly involves pressure processing, such as melting, rolling, drawing, stranding/compression stranding, etc;


2. Power cable


The main characteristics of this type of product are: extruding (winding) an insulation layer outside the conductor, such as overhead insulated cables, or twisting several cores (corresponding to the phase, zero, and ground wires of the power system), such as overhead insulated cables with two or more cores, or adding a protective layer, such as plastic/rubber sheathed wires and cables. The main process technologies include drawing, stranding, insulation extrusion (wrapping), cable forming, armor, sheath extrusion, etc. The different process combinations of various products have certain differences.


3. Wire and cable for electrical equipment


The main characteristics of this type of product are: a wide variety of specifications, a wide range of applications, and a wide range of voltages below 1kV. In the face of special occasions, new products are constantly derived, such as fire-resistant cables, flame-retardant cables, low smoke zero halogen/low smoke low halogen cables, termite resistant, mouse resistant cables, oil resistant/cold resistant/temperature resistant/wear-resistant cables, medical/agricultural/mining cables, thin-walled wires, etc.


4. Communication cables and optical fibers (brief introduction)


With the rapid development of the communication industry in the past twenty years, products have also experienced an astonishing pace of development. From simple telephone and telegraph cables in the past to thousands of pairs of telephone cables, coaxial cables, optical cables, data cables, and even combined communication cables.


5. Electromagnetic wire (winding wire)


Mainly used for various motors, instruments, etc.


What inspections should be carried out after the construction of the busbar device is completed?


Answer: The following inspections should be conducted:


(1) The processing, preparation, and welding of metal components should comply with regulations;


(2) All bolts, washers, split pins and other components should be complete and reliable;


(3) The busbar configuration and installation frame should comply with regulations, and the electrical distance between phases and ground should meet the requirements;


(4) Porcelain parts, iron parts, and bonding points should be intact, and the oil filled sleeve should be free of oil leakage, and the oil level should be normal;


(5) The paint is complete, the phase color is correct, and the grounding is good.


33. There are several methods to improve the electric field distribution at the disconnection point of the sheath in power cable joints of 35kV and below (please list five), and briefly describe their methods.


Answer: (1) Expansion bell mouth: Pry up the edge of the lead package at the cutting point, forming a trumpet shape, and the edge should be smooth, round, and symmetrical.


(2) Reserved blanket insulation: Leave a section of blanket insulation paper between the lead wrap cut and the separation point of the cable core.


(3) Cut off semi conductive paper: Cut the semi conductive paper below the bell mouth.


(4) Wrapping stress cone: Using insulating tape and conductive metal material to wrap it into a cone, artificially expanding the shielding layer to improve the electric field distribution.


(5) Equipotential method: For dry wrapped or cross-linked polyethylene cable heads, wrap a section of metal tape on the insulation surface of each core profile and connect them together.


(6) Install stress control tube: For 35kV and lower heat shrink tube cable heads, first wrap two layers of semiconductor tape from the direction of the end of the copper shielding layer of the wire core to the insulation profile of the wire core, and then wrap the corresponding specifications of the stress tube at the end of the copper shielding layer for heat shrink forming.


34. What requirements should the processing of cable supports meet?


Answer: (1) The steel should be straight without obvious distortion, the cutting error should be within 5mm, and the incision should be free of curls and burrs;


(2) The bracket should be welded firmly without obvious deformation, and the vertical net distance between each transverse brace should not exceed 5mm from the design deviation;


(3) The metal bracket must undergo anti-corrosion treatment. When located in areas with damp heat, salt, fog, and chemical corrosion, special anti-corrosion treatment should be carried out according to the design.


35. List the cable racks that you are familiar with.


Answer: Splice welded E-frame, prefabricated E-frame, bridge cable frame, cable tray, hook bracket, single cable support, etc.


36. What requirements should cable laying meet?


Answer: The following requirements should be met:


(1) In terms of safe operation, try to avoid various external damages as much as possible and improve the power supply reliability of cable lines;


(2) In terms of economy, from the perspective of investment


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