Q&A on Basic Cable Knowledge

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1. What are the types of commonly used wires and cables according to their uses? Answer: According to their uses, they can be divided into bare wires, insulated wires, heat-resistant wires, shielded wires, power cables, control cables, communication cables, radio frequency cables, etc. 2. What types of insulated wires are there? Answer: Commonly used insulated wires include the following: PVC insulated wires, PVC insulated flexible wires, nitrile-PVC mixture insulated flexible wires, rubber insulated wires, agricultural directly buried underground aluminum core plastic insulated wires, rubber insulated cotton gauze textile flexible wires, PVC insulated nylon sheathed wires, PVC insulated flexible wires for power and lighting, etc. 3. What kind of occasions are cable trays suitable for? Answer: Cable trays are suitable for overhead laying of power cables and control cables indoors and outdoors in general industrial and mining enterprises, and can also be used for indoor and outdoor erection in telecommunications, radio and television and other departments. 4. What are the cable accessories? Answer: Commonly used electrical accessories include cable terminal junction boxes, cable intermediate junction boxes, connecting pipes and terminal blocks, steel plate junction troughs, cable bridges, etc. 5. What is a cable intermediate joint? Answer: The device that connects the conductors, insulation shields and protective layers of cables to connect cable lines is called a cable intermediate joint. 6. What is electrical main wiring? Answer: Electrical main wiring is the connection method of main electrical equipment and busbars in power plants and substations, including the connection method of main busbars and plant power systems 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 must 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 core cross-section must meet the stability requirements of the power supply system during a short circuit; (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 the protective device to operate reliably. 8. What are the advantages of cross-linked polyethylene cables and oil-paper cables? Answer 1) Easy to install, because it allows a small minimum bending radius and is light in weight; (2) Not limited by line height; (3) Good thermal performance, allowing high operating temperature and large transmission capacity; (4) Simple cable accessories, all dry structures; (5) Simple operation and maintenance, no oil leakage problems; (6) Lower price; (7) High reliability, low failure rate; (8) Few manufacturing steps, simple process, and significant economic benefits. 9. What are the requirements for clamps for fixing AC single-core cables? Why? Answer: The clamp should have no iron parts to form a closed magnetic circuit. This is because when a current passes through the cable core, magnetic lines of force are generated around it. The magnetic lines of force are proportional to the size of the current passing through the core. If magnetically conductive materials such as iron parts are used, according to electromagnetic induction, eddy currents will be generated in the iron parts to heat the cable and even burn the cable. Therefore, iron parts cannot be used as fixed fixtures for single-core AC cables. 10. What are the biggest features and advantages of the following accessories for heat-shrinkable cable heads? Answer: The biggest feature of heat-shrinkable accessories is that it uses stress tubes instead of traditional stress cones. It not only simplifies the construction process, but also reduces the size of the terminals of the joints. It is easy to install, saves time and labor, has superior performance, and saves metal. Heat-shrinkable cable accessories integrate the pouring type and dry-packing type, integrating the advantages of these two types of accessories. 11. What inspections should be performed before cable laying? Answer 1) The brackets should be complete and the paint is complete. (2) The cable model, voltage, and specifications comply with the design. (3) The cable insulation is good. When there are doubts about the sealing of oil-paper cables, moisture judgment should be made; directly buried cables and small bottom cables should pass the DC withstand voltage test; the oil sample of oil-filled cables 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 to use a simpler method to check whether the cable insulation paper is damp? Answer: Light the cable insulation paper or put it into cable oil at about 150℃ for inspection. If there is no "hissing" sound or white foam, it means it is not affected by moisture. 13. What content should be indicated on the cable sign? What are the requirements for writing? Answer: The cable line design number, cable model, specification and starting point should be indicated on the sign. Cables used in parallel should have sequence numbers. The writing must be clear and not easy to fall off. 14. Where should the orientation signs of directly buried cables be set? Answer: At both ends of the cable, the cable joints at 50~100m of the straight section of the cable and the corners where the cable changes direction. 15. How should the yellow wax silk ribbons, black glass paint ribbons, alkali-free glass ribbons, etc. used to make cable joints be drained before construction? Answer 1) Constant temperature drying method: roll the insulation tape into a small roll with a diameter of 25~30mm, place it in a constant temperature drying oven at 110~120°C for 4~5 hours, cool and dry, take it out, and put it into a dry sealed cylinder. (2) Oil-immersed moisture drainage method: Place a small roll of insulating tape into cable oil with a constant temperature of 120~130°C, and keep a distance of 30mm from the bottom of the pot. After a certain period of time, the oil surface no longer produces foam, take it out, and put it into a barrel containing cable oil. The oil level should exceed all the objects put in and seal it. 16. What is the function of the cable outer sheath? Answer: Protect the inner sheath 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 and covered with a cover made of concrete. The cable should be laid flat on the bracket and have good drainage pipes. 18. What is the function of the inner sheath of the cable? Answer: To prevent the insulation layer from coming into contact with water, air or other objects, to prevent the insulation from getting damp and from mechanical damage. 19. What should you pay attention to when using wire cutters? Answer: Before use, be sure to check whether the insulation of the insulating handle is intact. Do not use wire cutters to cut metal wires that exceed the specifications allowed, and it is prohibited to use wire cutters instead of hammers to hit tools to avoid damage. 20. What is insulation strength? Answer: Insulating materials will be broken down in an electric field when the electric field strength increases to a certain limit. This electric field strength that leads to insulation breakdown is called insulation strength. 21. What are the advantages of cross-linked heat shrinkable cable accessories? Answer: This is a new type of material. Compared with other types of accessories today, it has the advantages of superior electrical performance, small size, low mass, easy installation, and matching materials. It also has weather resistance, pollution resistance, flame retardant and self-extinguishing capabilities. 22. What are the requirements for the mechanical strength of the cable conductor connection points? Answer: The mechanical strength of the connection points is generally lower than the tensile strength of the cable conductor itself. For fixedly laid power cables, the tensile strength of the connection points is required to be no less than 60% of the tensile strength of the conductor itself. 23. What main properties should the insulation layer material of power cables have? Answer: It should have the following main properties: (1) High breakdown strength; (2) Low dielectric loss; (3) Quite high insulation resistance; (4) Excellent discharge resistance; (5) Certain flexibility and mechanical strength; (6) Long-term stable insulation performance. 24. What are the requirements for traction strength when laying cables mechanically? Answer: For copper core cables, when the head is pulled, the allowable traction strength is 70N/cm; for aluminum core cables, when the head is pulled, the allowable traction strength is 40N/cm; if a steel mesh sheath is used for pulling, the allowable strength of lead sheathed cables is 10N/cm; lead sheathed cables are 40N/cm. 25. What are the regulations for cable protection tubes? 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. 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 tube should comply with the regulations on the bending radius of the cable being penetrated; (3) Each pipe should not have more than three elbows at most, and there should be no more than 2 right-angle bends. 26. How to measure the outer diameter of the cable sheath? Answer: Measure the outer diameter of the sheath and its average value at five points evenly distributed on the circumference of the sheath. The average outer diameter is the outer diameter of the sheath. 27. How to connect copper core cables with different cross-sections? Answer: To connect copper core cables with different cross-sections, open weak-backed copper pipes can be used to connect by soldering, or pure copper rods can be used to connect copper pipes according to different cross-section requirements and connected by crimping. 28. Briefly describe the process of making indoor terminal heads with heat shrinkable type for 10KV cross-linked cables? Answer; (1) Preparation stage: Check whether the heat shrinkable cable accessories are complete and whether the models match. After checking and confirming whether there is moisture in the cable, check the cable. (2) Cut off the excess cable and determine the cable length according to the site conditions. (3) Strip off the sheath. (4) Weld the ground wire and weld the ground wire to the steel belt. (5) Fill the three-pronged opening and wrapping sealant. (6) Install the three-core branch sheath, put the sheath into the root, start shrinking from the middle, first to the root, and then to the finger. (7) Peel off the copper strip and outer semi-conductive layer, peel and cut the copper strip above 20mm of the three-core branch sleeve. It is strictly forbidden to damage the main insulation, and clean the semi-conductive layer. (8) Install the stress tube, and heat shrink the branch sleeve at the end of the pipe after docking. (9) Install the terminal block. (10) Install the insulating tube. (11) Install the sealing tube. (12) Install the phase color pipe 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, clean the surface, and peel off the outer sheath according to the instructions. If there are special needs, the peeling length of the outer sheath can be adjusted, and 2 sets of ground wires are led out from the copper tape and the outer sheath armor. (2) Measure the required size from the outer sheath opening upward, cut off the excess cable, put it into the heat shrinkable three-finger sleeve to the root of the three-pronged opening, heat and shrink it from the middle, measure the specified size from the upper opening of the finger sleeve upward (35kv is 350~380mm). Remove the excess copper strip (retain 20mm), retain the predetermined position of the semiconductor, and crimp the outlet terminal to seal. 30. Under what circumstances should the motor installed on site be core-pulled and inspected? Answer: When the motor has any of the following conditions, core pulling inspection should be carried out: (1) The date of manufacture exceeds the manufacturer's warranty period; (2) When the quality is suspicious after visual inspection or electrical test; (3) When the open motor is suspicious after end inspection; (4) There are abnormal conditions during trial operation. 31. What conditions should be met for manual synchronization of generators? Answer: The three conditions for generator paralleling are that the difference between the voltage, frequency and phase of the generator to be paralleled and the voltage, frequency and phase of the operating system is less than the specified value. 32. What inspections should be carried out after the construction of the busbar device is completed? Answer: The following inspections should be carried out: (1) The processing, preparation and welding of metal components should comply with regulations; (2) All bolts, washers, cotter pins and other parts should be complete and reliable; (3) The busbar preparation and installation frame should comply with the regulations, and the electrical distance between phases and to the ground should meet the requirements; (4) The porcelain parts, iron parts and gluing parts should be complete, the oil-filled casing should have no oil leakage, and the oil level is normal; (5) The paint is complete, the color is correct, and the grounding is good. 33. In power cable joints of 35kv and below, there are several ways to improve the electric field distribution at the sheath disconnection point (please list five), and briefly describe the methods. Answer 1) Expanded bell mouth: Pry up the edge of the lead package at the cut point of the lead package to form a horn shape. The edge should be smooth, round and symmetrical. (2) Reserve overall insulation: leave a section of turnkey insulation paper between the lead package cut and the cable core separation point. (3) Cut off semi-conductive paper: Cut off the semi-conductive paper below the bell mouth. (4) Wrapping stress cone: Wrap insulating tape and conductive metal materials into a cone shape, and artificially expand the shielding layer to improve the electric field distribution. (5) Equipotential method: For dry-packed or cross-linked polyethylene cable heads, wrap a section of metal tape on the surface insulation surface of each wire core and connect them together. (6) Installation of stress control tube: For 35kv and lower heat shrinkable tube cable heads, first wrap 2 layers of semiconductor tape from the end direction of the copper shielding layer of the wire core through the semiconductor tape to the surface of the wire core insulation. Then, fold the stress tube of the corresponding specifications, put it at the end of the copper shield, and heat shrink it. 34. What requirements should the processing of cable brackets meet? Answer 1) The steel should be straight without obvious distortion, the blanking error should be within 5mm, and the cuts should have no curling or burrs; (2) The bracket should be welded firmly without obvious deformation, and the vertical net distance between each cross brace should not exceed the design deviation of more than 5mm; (3) The metal bracket must be treated with anti-corrosion treatment. When it is located in areas with humid heat, salt, fog, or chemical corrosion, special anti-corrosion treatment should be carried out according to the design. 35. List the cable racks you are familiar with. Answer: Tailor-welded E-frame, assembled E-frame, bridge cable rack, cable bracket, hook-type bracket, single cable bracket, etc. 36. What requirements should be met when laying cables? Answer: The following requirements should be met: (1) In terms of safe operation, avoid all kinds of external damage as much as possible and improve the power supply reliability of the cable line; (2) Economically, consider the aspect of saving the most investment; (3) In terms of construction, the path of the cable line must be convenient for rotation and maintenance after operation. 37. What are the insulating materials for making cable terminals or intermediate joints? Answer: Insulating glue, insulating tape, insulating tube, insulating gloves, insulating resin, etc. 38. Briefly describe the general operating procedures for making cable heads. Answer 1) Preparation before production: including ○1 reading the installation instructions; ○2 inspecting the site; ○3 preparing materials; ○4 cable moisture test; ○5 testing before production, etc. (2) The manufacturing process of the joint: including ○1 cutting off excess cable; ○2 stripping the cable protective layer; ○3 conductor connection; ○4 wrapping insulation (or shrink pipe); ○5 installing the joint shell; ○6 pouring insulating agent; ○7 sealing treatment, etc. (3) Electrical testing after production. 39. What requirements should the processing of cable protection tubes meet? Answer 1) The tube opening should be free of burrs and sharp corners, and the tube opening should be made into a trumpet shape. (2) After the cable tube is bent, there should be no cracks and obvious dents. The bending degree should not be greater than 10% of the outer diameter of the tube; the bending radius of the cable tube should not be less than the minimum allowable bending radius of the inserted cable. (3) The metal tube should be coated with anti-corrosive paint or leach surface, and the peeling parts of the zinc layer of galvanized tubes should also be coated with anti-corrosive paint. 40. What requirements does the arrangement of cables meet? Answer 1) Power cables and control cables should not be arranged on the same layer of brackets. (2) High and low voltage cables, strong and weak current control cables should be arranged in layers in order, generally from top to bottom. However, when high voltage cables above 35kv are introduced into the cabinet, in order to meet the bending radius, they can be arranged from below to meet the bending radius. 41. What is the method for laying cables in cable trenches? Answer: The method of laying cables in the cable trench is similar to that of directly buried cables. Generally, the pulley can be placed in the trench. After the laying is completed, the cable is placed on the bottom of the trench or on the bracket, and a cable sign is tied to the cable. 42. What are the commonly used equipment for cable laying? Answer: (1) Air compressor, mainly used to destroy the road surface to prepare for future cable laying; (2) Electric winch or cable tractor, mainly used to tow the cable; (3) Cable conveyor, used in conjunction with the tractor to overcome the huge friction and reduce damage to the cable; (4) Cable reel pay-off bracket; (5) Roller device; (6) Anti-twist, reduce the distortion of the wire rope; (7) Cable reel braking device; (8) Tension meter. 43. What are the requirements for the installation distance of brackets in cable trenches? Answer: Cables are fixed on the walls of cable trenches and tunnels. When installed horizontally, when the outer diameter of the cable is equal to or less than 50mm, a support should be added every 1m; for cables with an outer diameter greater than 50mm, a support should be added every 0.6m; for single cables arranged in a triangle, straps should be tied every 1m. When installed vertically, they should be fixed every 1~1.5m. 44. How to handle the installation of power cables and control cables in the same bracket? Answer: Power cables and control cables should generally not be laid in the same bracket. When there are few cables and the control cables and power cables are laid in the same bracket, they should be separated by partitions. 45. How to install grounding wires in cable tunnels? Answer: A continuous grounding wire should be installed along the entire length of the tunnel and trench. The grounding wire should be connected to all brackets and both ends should be connected to the grounding electrode. The specifications of the grounding wire should meet the design requirements. In addition to the insulation requirements, the cable lead jacket and armor should all be connected to each other and to the grounding wire. The cable rack and grounding wire should be coated with anti-rust paint or galvanized. 46. What are the requirements for fireproof sealing of cable holes? Answer: For larger cable penetration holes, such as cables penetrating floors, etc., when using fireproof plugging materials to seal them, you should first apply four to six layers of fireproof paint on the cable surface according to the actual situation, with a length of about 1.5m below the hole, and then use refractory materials to process the plate support fireproof plugging material with a certain strength to ensure that the plugging is firm and easy to disassemble when replacing the cable. The sealing is tight and without pores to effectively block smoke and fire. 47. What inspections should be paid attention to before igniting the blowtorch? Answer: The following inspections should be carried out: (1) Whether the pump is leaking or leaking, and whether the oil drum and the nozzle are leaking or air leaking; (2) Whether the amount of oil in the oil drum exceeds 3/4 of the capacity of the oil drum, and whether the screw plug for refueling is tightened. 48. What should you pay attention to when using a blowtorch? Answer: (1) The maximum oil filling amount of the blowtorch is 3/4 of the volume of the oil drum; (2) The starting pressure of the pump should not be too high. After ignition, the flame changes from yellow to blue before use; (3) There should be no flammable objects around, and the air must be circulated; (4) When out of use, turn off the adjustment switch first, and after the fire is extinguished, slowly loosen the oil hole cover to release the air. After the air is released, loosen the adjustment switch, and then loosen the hole cover after complete cooling; (5) Kerosene blowtorches and gasoline blowtorches should be used separately. 49. What is the use of manual hydraulic pliers? How to operate? Answer: The connection between two wires is usually to put the two wire ends into a pressure tube made of the same material, and use a crimping plier to squeeze several pits to connect the wires together. When crimping, when the handle is lifted upward, the plunger moves outward, a vacuum is generated in the lower chamber of the oil inlet valve, and the oil in the tank enters the plunger chamber. When the handle is pressed down, the plunger moves Move inward, and after the oil is pressurized, the oil inlet valve is closed, the oil outlet valve is opened, and the oil pressure enters the hydraulic cylinder to push the piston and the male mold. There is a pressure pipe between the male and female molds. When the depth of the squeezed pit reaches a certain value, the oil return valve is opened and the piston returns automatically. After pressing one pit, move the clamp and press another one. 50. What are the main categories of commonly used low-voltage power cables according to their insulation and protective layers? Answer: (1) Oil-impregnated paper insulated lead-clad (or aluminum-clad) power cable; (2) Non-drip oil-impregnated paper insulated power cable; (3) PVC insulated PVC sheathed power cable; (4) XLPV insulated polyethylene sheathed power cable; (5) XLPV insulated PVC sheathed power cable; (6) Rubber insulated power cable. 51. What are the categories of commonly used insulation materials? Give examples? Answer: (1) Inorganic insulation materials: mica, asbestos, marble, porcelain, glass, etc.; (2) Organic insulation materials: resin, rubber, paper, linen, cotton yarn, etc.; (3) Mixed insulation materials: Various insulation materials formed by processing the above insulation materials. 52. Under what circumstances is pipe laying used? What are the advantages? Answer: Pipe laying is generally used in places that intersect with other buildings, roads or railways, and is sometimes used in densely built areas. The main advantages are that it occupies a small area, can withstand large loads, and cables do not affect each other, which is relatively safe. 53. What is the long-term allowable carrying capacity of a cable? Answer: The long-term allowable carrying capacity of a cable refers to the current value when the specified current passes through the cable and the cable conductor reaches the long-term allowable operating temperature after thermal stabilization. 54. What are the factors that determine the long-term allowable carrying capacity of the cable? Answer: It is determined by the following three factors: (1) The long-term allowable operating temperature of the cable; (2) The heat dissipation performance of the cable itself; (3) The cable installation condition and the heat dissipation conditions of the surrounding environment. 55. How many methods are there for laying cables? Answer: There are the following types: (1) Directly buried underground; (2) Installed in cable trenches; (3) Installed in underground tunnels; (4) Installed on walls or ceilings inside buildings; (5) Installed on bridges; (6) Laying in the pipe; (7) Laying under the water. 56. How to connect cable tubes? Answer: When connecting cable tubes, they must be connected with buckles and pipe joints. If welding is used, direct butt welding is not allowed. A thick tube must be put on the connection before welding to prevent welding slag from falling into the tube. 57. When making a cable head, what are the requirements for bending the cable core wire up and down? Answer: When bending the wire core, the paper insulation must not be damaged, and the bending radius of the core wire must not be less than 10 times the cable core. Special care must be taken during production to ensure that the bent part of the wire core is evenly stressed, otherwise the insulating paper will be easily damaged. 58. Briefly describe the heat shrink sealing method of plastic cables. Answer: The heat shrink method is suitable for the sealing of medium and low voltage rubber and plastic cable joints and terminal heads. It can also be used for non-drip and viscous impregnated insulated cables. Two types of heat shrinkable tubes, cross-linked polyethylene type and silicone rubber type, can shrink evenly when exposed to heat. This kind of tube is placed in the predetermined bonding and sealing part, and hot melt glue is applied to the bonding part. When heated to a certain temperature, the heat shrinking tube will shrink and the hot melt adhesive will melt at the same time. After natural cooling, a good sealing layer will be formed. 59. What should you pay attention to when bending cable protection pipes with an electric pipe bender? Answer: (1) The pipe bender should be operated by personnel who understand its performance and are familiar with the operation knowledge; (2) It must be inspected before use, the buttons, operating handles, and travel switches should be intact, and the pipe bender must be reliably grounded; (3) Select a mold that meets the requirements and determine the required bending radius; (4) There should be sufficient scope of movement around the construction site; (5) When in use, wait until the idling is normal before working with load. During operation, it is strictly forbidden to touch the rotating parts with hands and feet; (6) After the work is completed, the power should be cut off in time and the oil pressure should be released. 60. What is the content of the cable inventory and the meaning of the cable number? Answer: The cable inventory is the basis for laying cables and guiding construction, and is an archive of operation and maintenance. The number, starting point, model, specification, and length of each cable should be included, and the total length should be classified and counted. The spare core of each cable should also be listed for the control cable. The cable number is a sign to identify the cable, so the factory-wide number is required not to be repeated, and has certain meanings and rules to express the characteristics of the cable. 61. How many categories are currently used for laying cables? Answer: (1) Manual laying, that is, using the human sea tactic, under the coordinated command of one or more people, laying according to regulations; (2) Mechanized laying, that is, using rollers, tractors, and conveyors, controlled by a synchronous power supply, which is safer; (3) Combination of manual and mechanical, some sites have many turns and difficulty in construction, and it is difficult to use all machinery, so this method is adopted. 62. What requirements should be met to control the secondary circuit wiring? Answer: (1) The construction wiring is correct according to the drawing; (2) The electrical connection of the wires should be firm and reliable; (3) There should be no joints in the wires in the panel, and the core wires should be undamaged; (4) The ends of the cable core wires should be marked with their circuit number, which should be correct, clear and not easy to discolor; (5) The wiring should be neat, clear and beautiful, and the wire insulation should be good and undamaged; (6) There should be one wire on each side of each terminal block, and no more than two wires at most. 63. What are the requirements for cable ducting? Answer: (1) The distance from the top of the drain pipe to the ground is 0.2m in the factory building, 0.5m under the sidewalk, and 0.7m in general areas; (2) Pipe drain pits should be installed at changes of direction and branches, and additional pits should be added when the length exceeds 30mm; (3) The depth of the drain pipe should be no less than 0.8m, and the diameter of the manhole should be no less than 0.7mm; (4) The drain pipe should have a drainage slope of 0.5% to 1% toward the well pit. 64. What are the requirements for the resistance of the cable conductor connection point? Answer: The resistance of the connection point is required to be small and stable. The ratio of the resistance of the connection point to a conductor of the same length and cross-section should not be greater than 1 for newly installed terminal heads and intermediate heads; for running terminal heads and intermediate heads, the ratio should not be greater than 1.2. 65. What requirements should be met for the design of cable joints and intermediate heads? Answer: The requirements that should be met are: (1) High voltage resistance and good conductor connections; (2) High mechanical strength and low dielectric loss; (3) Simple structure and strong sealing. 66. What is a cable fault? How many common types are there? Answer: Cable fault refers to a fault in which insulation breakdown occurs during a preventive test or during operation, due to insulation breakdown, wire burnout, etc., which forces a power outage in the cable line. Common faults include ground faults, short circuit faults, disconnection faults, flashover faults and mixed faults, etc. 67. How to deal with single-phase ground faults in cable lines? Answer: Generally speaking, for single-phase grounding faults in cable lines, the damage to the cable conductor is only local. If it is mechanical damage and the soil near the fault point is relatively dry, local repairs can generally be carried out and a false joint can be added. That is, the cable core is not cut off, but the insulation of the fault point is reinforced and sealed. 68. What tests and inspections should be carried out before laying power cables? Answer: Before laying, check whether the model, specification and length of the cable meet the requirements and whether there is any external damage. Use a 1000V megohmmeter to remotely measure the insulation resistance of low-voltage cables. The resistance is generally not less than 10MΩ. Use a 2500V megger to measure the resistance of high-voltage cables. The resistance is generally not less than 400MΩ. 69. What should you generally pay attention to when laying cables in the main factory building? Answer: When laying cables in the main factory building, you should generally pay attention to: (1) All control cables leading to the centralized control room should be laid overhead; (2) 6KV cables should be laid in tunnels or pipes, and high groundwater levels can also be laid overhead or in pipes; (3) 380V cables should be laid in tunnels, trenches or pipes when the two ends of the cables are at zero meters. When the equipment at one end is up and the other end is down, it can be laid partially overhead. When the groundwater level is high, it should be laid overhead. 70. Where are the inner shielding layer and the outer shielding layer of the power cable located? What materials are used? What are their functions? Answer: In order to have a good contact between the insulation layer and the cable conductor and eliminate the increase in electric field intensity on the conductor surface caused by the roughness of the conductor surface, the conductor surface is usually covered with an inner shielding layer of metallized paper or semiconductor paper tape. In order to have a good contact between the insulation layer and the metal sheath, an outer shielding layer is generally wrapped around the insulation layer. The material of the outer shield layer is the same as that of the inner shield layer, and sometimes copper tape or braided copper wire tape is also used. 71. Briefly describe the composition and properties of epoxy resin composites. Answer: The epoxy resin compound is composed of epoxy resin added with hardener, filler, toughener and diluent. It has the following properties: (1) Sufficient mechanical strength; (2) Excellent electrical properties; (3) Stable electrical properties; (4) Sufficient adhesion to non-ferrous metals; (5) Good corrosion resistance; (6) Resistant to rain, light, humidity and heat when used outdoors. 72. Why is it necessary to install a return line laid parallel to the cable when one end of the single-core cable sheath is grounded? Answer: In a cable line with a metal sheath that is grounded at one end, in order to ensure that the induced voltage in the sheath does not exceed the allowable standard, a conductor must be installed parallel to the cable line and both ends of the conductor are grounded. This conductor is called a return line. When a single-phase connection occurs During a ground fault, the ground short-circuit current can flow back to the center point of the system through the return line. Since the magnetic flux generated by the ground current passing through the return line offsets part of the magnetic flux generated by the ground current of the cable conductor, the induced voltage of the sheath during a short-circuit fault can be reduced.