ZW CABLE took the initiative of collecting essential information on wires and cables. The information is summarized in the form of 80 questions listed below. Please feel free to go through them because they are a good source of critical knowledge concerning wires and cables. The information provided relates to various aspects, including;
- Basic knowledge questions
- Cable laying problems
- Cable protection regulations
- Cable classification problems
- Cable laying methods
- Cable laying specific requirements
- General cable-related issues.
Basic Knowledge Questions:
1. What kinds of wires and cables are commonly used according to their purpose?
Answer: According to their use, wires and cables can be divided into bare wire, insulated wire, heat-resistant wire, shielded wire, power cables, control cables, communication cables, radio frequency cables, among others.
2. What types of insulated wires are there?
Answer: The most common types of insulated wires include; flexible wire, PVC flexible cable, rubber insulated wire, agricultural, underground direct buried aluminum cable, rubber insulated cable, PVC insulated wire, and PVC insulated cable.
3. What kind of occasions is the cable tray suitable for?
Answer: Cable bridges are suitable for general industrial and mining enterprises indoor and outdoor overhead laying of power cables, control cables. They are also ideal for telecommunications, radio, and television departments in indoor and outdoor erection.
4. What are cable accessories?
Answer: The most commonly used electrical wire and cable accessories include cable terminal junction boxes, cable intermediate junction boxes, connection tubes and terminals, steel plate junction channels, cable bridges, among others.
5. What are cable intermediate joints?
Answer: Cable intermediate joints are cable line connection devices that connect the cable and cable conductor and the insulation shield and protection layer.
6. What is mains electrical wiring?
Answer: Mains electrical wiring refers to a power plant electrical installation, substation in the leading electrical equipment, and busbar connection, including the primary cable and wire busbar and plant power system according to specific functional requirements of the connection.
7. What principles should one follow when choosing an ideal power cable cross-section?
Answer: You should follow the following principles when selecting an ideal power cable cross-section;
- 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
- The continuous allowable current of the cable should be equal to or greater than the maximum constant current of the supply load
- The core cross-section should meet the requirements of the stability of the power supply system in the event of a short circuit
- Verify that the voltage drop meets the requirements according to the length of the cable.
- The minimum short-circuit current at the end of the line should enable the protective device to act reliably
8. What are the advantages of XLPE cables compared with oil-paper cables?
- It is easy to install because it allows a small minimum bending radius and lightweight.
- It is not limited by line drop.
- It has good thermal performance thus, allowing high operating temperatures and high transmission capacity.
- Simple wire and cable accessories, all of which are of dry construction
- Simple operation and maintenance, no oil leakage problems
- It is cheap
- High reliability and low failure rate
- The manufacturing process is less complex, and it has significant economic benefits
9. What are the requirements for clamps for fixing AC single-core cables? Why are such requirements necessary?
Answer: The main requirement is that iron should not be used as a fixture for single-core AC cables. When current flows through the single-core AC cable, magnetic forces are formed around it.
Generally, the magnetic force produced is proportional to the amount of current transmitted through the single-core AC cable. If you were to use iron or any other magnetic materials, the electromagnetic induction would produce eddy currents. These eddy currents will make the cable hot or even burn the cable.
10. What is the most critical feature of accessories for heat shrinkable cable heads? What are the advantages of this feature?
Answer: The most crucial feature of the heat-shrinkable accessories is that the traditional stress cone is replaced by a stress tube. It not only simplifies the construction process but also reduces the size of the terminal of the joint.
Its advantages include the fact that it makes installation easy, saves time and labor, has superior performance, and saves metal. Essentially, the heat-shrinkable wire and cable attachment combine the advantages of both the potted and dry pack types.
Cable Laying Problems:
11. What inspection work should be carried out before the cable is laid?
- The bracket should be complete, and the paint should be intact
- The cable type, voltage, and specification are per the design
- You should carry out an inspection to ensure that cable insulation is good. The cable should also be inspected to eliminate any doubt about the sealing of the oil paper cable. It is also essential to test the moisture that the wire will be exposed to
- For direct burial wire and cable and small bottom cable, a DC voltage test should be done to ascertain whether the cable can withstand the current
- Oil-filled cable oil samples should be tested and qualified
- Oil-filled cable oil pressure should not be less than 1.47MPa
12. How to use a more straightforward way to check the cable insulation paper moisture when you have doubts about oil paper insulated power cable seals?
Answer: The cable insulation paper is lit or put into the cable oil at about 150 ℃ to check, no “hissing” sound or white foam appears, indicating that no moisture
13. What should be displayed on the cable signage? What are the requirements for preparation?
Answer: The signage should indicate the cable line design number, cable type, cable specification, start point, and the parallel use of the cable should have the sequence number. Requirements for preparing a wire and cable include clear handwriting and ink that is not easy to fall off.
14. In which positions should the orientation mark of the direct buried cable be set?
Answer: At both ends of the wire and cable, at cable joints, specifically at 50~100m cable joints, and at the corner where the cable changes direction
15. How to drain the moisture treatment before construction during the production of cable joints with yellow wax silk tape, black glass paint tape, and an alkali-free glass ribbon?
- Thermostatic drying method: insulating tape is rolled into small rolls with a diameter of 25~30mm and put into 110~120℃ thermostatic drying oven for 4~5h. When it is cool dry, it is taken out and put into a dry sealed cylinder.
- Oil immersion and drainage method: insulating tape rolls into a constant temperature of 120 ~ 130 ℃ cable oil from the bottom of the pot to maintain a distance of 30mm. After some time, the oil surface will no longer produce foam after removal. The cable and wire are then loaded into a barrel with cable oil. The oil level should be more than all the objects loaded and should be sealed.
16. What is the role of the outer sheath of the cable?
Answer: To protect the inner sheath from mechanical damage and chemical corrosion and enhance mechanical strength.
17. What requirements should an outdoor cable trench meet?
Answer: The upper part of the cable trench should be slightly higher than the ground and should have a cover made of concrete. The cable should also be laid flat on the support, and good drainage must be provided in the trench.
18. What is the role of the inner cable sheath?
Answer: It ensures that the wire and cable insulation layer does not contact water, air, or other objects. Essentially, it protects the cable’s insulation from moisture and mechanical damage.
19. What should I pay attention to when using the wire clamp?
Answer: Before use, be sure to check whether the insulation of the wire clamp handle is intact. Also, you cannot use wire pliers to cut more than the specifications allowed by the metal wire. The use of wire pliers instead of hammer knocking tools is encouraged to avoid damage.
20. What do you mean by insulating strength?
Answer: When the electric field strength increases to a specific limit, the insulating material will eventually break down. Insulating strength is the measure of an insulating material’s ability to withstand electric tension without breakdown.
Cable Protection Provisions:
21. What are the advantages of cross-linked heat-shrinkable cable accessories?
Answer: This is a new type of material, which has the advantages of superior electrical performance, small volume, small mass, easy installation, and matching materials compared with other kinds of accessories nowadays. In addition, cross-linked heat-shrinkable cable accessories are weather-resistant, dirt-resistant, and have flame retardant self-extinguishing capability.
22. What are the requirements for the mechanical strength of the 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 of power cables, the tensile strength of the connection point requires no less than 60% of the tensile strength of the conductor itself.
23. What are the main properties of the insulation layer materials of power cables?
Answer: Insulation layer materials of a wire and cable should have the following main properties;
- High breakdown strength
- Low dielectric loss
- A reasonably high insulation resistance
- Excellent discharge resistance
- A certain degree of flexibility and mechanical strength
- Stable insulation performance over time
24. When laying cables mechanically, what are the provisions of traction strength?
When dealing with copper core cable traction head, the allowable traction strength is 70N/cm. For aluminum-core cables traction head, the acceptable traction strength is 40N/cm. If you are using a wire mesh set traction, the acceptable power for a lead sheathed cable is 10N/cm, and for a pure lead sheathed cable is 40N/cm.
25. What are the provisions for a cable protection tube?
- The cable needs to be inside the protection tube when laying. The inner diameter of the tube should also not be less than 1.5 times the outer diameter of the cable. Elsewhere, the concrete pipe, terra cotta pipe, asbestos, cement pipe should not be less than 100mm inner diameter.
- The bending radius of the cable pipe shall be per the provisions of the bending radius of the cable penetrated.
- Each tube should not exceed a maximum of three elbows, and right-angle bends should not be more than two.
26. How to measure the outer diameter of the cable sheath?
Answer: Carry out measurements at five points evenly distributed around the outer circumference of the wire and cable sheath. After getting the measurements of each of the five points of the sheath’s outer diameter, calculate the average value using the five measures obtained. The average value is the outer diameter of the cable sheath.
27.How to connect copper-core cables of different cross-sections?
Answer: A different copper-core cable and wire cross-section can open a weak back copper receiver and then be connected using the tin welding method. Alternatively, you can use a pure copper rod to connect the copper receiver and connect the copper-core cables using the crimping technique.
28. Briefly describe the process of 10KV cross-linked cable heat-shrinkable production of the indoor terminal head?
- Preparation stage: check whether the heat-shrinkable cable accessories are available and whether the models match. Also, check and confirm that the cable has no moisture.
- Excise the excess cable and decide on the cable length according to the site conditions.
- Strip the sheathing.
- Weld the earthing wire to the steel strip.
- Fill trig port and wrap around sealant.
- Install the three-core branch sheath, sheathing into the root and shrinking from the middle, first towards the root and then towards the fingers.
- First, strip copper tape and outer semi-conductive layer. Then, strip and cut copper tape over 20mm from the mouth of the three-core branch sheath. It would be best if you were careful to avoid damage to the primary insulation of the wire and cable and remove the clean semi-conductive layer.
- Install the stress pipe, the pipe mouth, and the end part of the branch sleeve butt after heat shrinkage.
- Install terminal blocks.
- Install the insulating tube.
- Install the sealing tube.
- Installation of phase colour pipe after verification
29. What are the installation steps for indoor and outdoor prefabricated terminals?
- Position the cable at the intended location, fix it, clean the surface, and strip the outer sheath in line with the instructions. If there is a particular need, the length of the outer sheath can be adjusted. Lead-out two groups of earth wires from the copper tape and outer sheath armour,
- Measure the required size of the wire and cable upwards from the outer sheath mouth and remove the excess wire. After that, put in the heat-shrinkable three-finger sleeve to the root of the three-fold mouth by the middle heating shrinkage. Measure the required size upwards from the finger sleeve upper mouth (350~380mm for 35kv). Remove the excess copper tape (retain 20mm), then keep the predetermined semiconductor location and crimp out the line terminal seal
30. The application of wire and cable is divided into three main categories. Which categories are these?
The power system
The power system uses wire and cable products. The wires used are mainly overhead cable, electrical cable, solar cable, teflon wire, battery cable, high-temperature wire, trailer wire, and electrical wire and cable for power equipment.
The information transmission system
Wires and cables used for information transmission systems are municipal telephone cables, television cables, electronic cables, radio frequency cables, fiber optic cables, data cables, electromagnetic lines, power communications cables, or other composite cables.
The machinery and equipment, instrumentation systems
In this part of the bare wire, almost all other products are used in addition to the overhead cable. However, the main cables used are power cables, electromagnetic wire, data cables, and instrumentation cables.
Cable Classification Issues:
31. Wire and cable products are mainly divided into five categories. Which categories are these?
- bare wire and bare conductor products
The main features of a bare wire and cable include pure conductor metal, no insulation, and a sheathing layer. Examples of such products include a steel-core aluminum stranded wire, copper and aluminum busbar, electric locomotive wire, to name a few. The processing techniques for bare wire and bare conductor products are mainly pressure processing, such as melting, calendaring, drawing, and stranding/tight compression stranding.
- Power cables
The main feature of these products is extrusion (winding) of the insulation layer outside the conductor. Examples of power cables include overhead insulated cables, several cores stranded (corresponding to the phase, zero, and ground lines of the power system), such as two or more cores of overhead insulated cables.
Some power cables and wires contain additional sheathing layers, such as plastic/rubber wire and cable. The primary process technologies used in the manufacture of power cables are pulling, stranding, insulation extrusion (wrapping), armoring, and sheathing extrusion. The different process combinations of various products are somewhat different.
- Electrical cable and wire
The products have a wide range of specifications, a wide range of applications, and voltage in 1kV and below. In the face of special occasions, we have continued to derive new products. The most cutting-edge wires and cables include fire-resistant cables, flame-retardant cables, low smoke halogen-free cables, mineral insulated cable, oil cables, heat-resistant cables, mining cables, and thin wall cable
- Communication cable and optical fiber (brief introduction)
With the rapid development of the communications industry in the last two decades, the products have also had an excellent growth rate. From the past simple telephone and telegraph cable development to thousands of pairs of words cable, coaxial cable, optical cable, data cable, and even combined communications cable.
- Electromagnetic wire (winding wire)
This type of wire and cable are mainly used for motors, a wide variety of instruments, and meters, to name a few.
32. When Busbar device construction is completed, what checks should be carried out?
Answer: The following checks should be carried out;
- Metal components of the processing, preparation, welding screwing should align with the wire and cable
- Each bolt, washers, cotter pins, and other parts should be complete and reliable.
- Busbar configuration and installation frame should be in line with the provisions, phase, and ground electrical distance to meet the requirements.
- Porcelain parts, iron parts, and glue joints should be complete. The oil-filled casing should be free of oil seepage and average oil level.
- The paint is complete, the phase colour is correct, grounding is good.
33. In the 35 kV and below power cable joints, several methods are used to improve the electric field distribution of its sheath disconnection. Please list five and briefly describe their methods.
- Swell the horn: pry up the edge of the lead package and cut it off into a horn shape, the brink of which should be smooth, round, and symmetrical.
- Leave the tundish insulation: leave a section of tundish insulation paper between the lead pack cutout and the cable core separation point.
- Removal of semi-conducting paper: remove the semi-conducting paper below the flare.
- Wrapping stress cone: use insulating wrapping tape and conductive metal material wrapped into a cone to expand the shield to improve the electric field distribution artificially.
- Equipotential method: For dry-wrapped or cross-linked polyethylene cable heads, a section of the metal tape is wrapped around the insulating surface of each wire and cable core profile and joined together.
34. What requirements should the processing of cable brackets meet?
- Steel should be straight, with no noticeable distortion, and the undercutting error should be within 5mm. The cut should also be free of rolled edges and burr.
- The bracket shall be firmly welded without noticeable distortion, and the vertical net distance between the cross braces shall not deviate from the design by more than 5mm.
- Metal brackets must have anti-corrosion treatment, located in hot and humid, salt, fog, and chemical corrosion areas, according to the design of special anti-corrosion treatment.
35. List some cable racks you are familiar with
Answer: Spell-welded E-frame, assembled E-frame, bridge-type cable rack, cable bracket, hook-type bracket, and single wire and cable bracket.
36. What requirements should be met before laying cables?
Answer: The following requirements should be met;
- Safe operation, as far as possible to avoid all kinds of external damage and to improve the reliability of the cable line power supply
- Economic aspects must be considered from the point of view of the most economic investment concerns
- Construction should be done such that the path of the cable line must be easy to operate and maintain after commissioning
37. What are the insulating materials used to make cable terminations or intermediate joints?
Answer: The primary insulating materials for wire and cable terminations are insulating rubber, insulating tape, insulating tube, insulating gloves, and insulating resin, to name a few.
38. Briefly describe the general operating procedures of cable head production
- Preparation before fabrication: The first step is reading the wire and cable installation instructions, inspecting the site, preparing materials, and testing the cable for overall moisture testing before fabrication.
- The fabrication process of the joint: involves cutting off excess cables, stripping of the cable protection layer, carrying out conductor connection, wrapping insulation (or shrink tubing), installing the joint shell, and filling with the insulating agent, and performing a sealing treatment.
- Electrical testing after fabrication
39. What requirements should the processing of the cable protection tube meet?
- The wire and cable orifice should be free from harshness and sharp corners and should be made into a flared shape.
- After bending, there should be no cracks and a significant deflated phenomenon on the cable protection tube. Also, the bending flat degree should not be greater than 10% of the outer diameter of the tube. Ideally, the cable tube bending radius should not be less than the minimum allowable bending radius of the wire and cable.
- The metal pipe should be coated with anti-corrosion paint or a leach table on the exterior. The galvanized pipe zinc layer flaking should also be coated with anti-corrosion paint.
40. What requirements should be met during the arrangement of cables?
- Power cables and control cables should not be configured on the same level of support.
- Both high and low voltage cables and weak and robust power control cables should be configured in layers according to their sequence. Generally, the situation is appropriate from top to bottom. However, high-voltage lines containing more than 35kv should be arranged in the cabinet tray. High voltage cable can be configured from the bottom up to meet the bending radius.
Cable Laying Mode:
41. What is the method of laying cables in the cable trench?
Answer: The method of laying wires and cables in the cable trench is similar to laying direct burial cables. Generally, the pulley can be placed in the groove, the application is completed, the wire is placed in the bottom of the trench or on the support, and the cable is tied to the cable label.
42. What is the standard equipment used for cable laying?
- Air compressor-mainly used to destroy the road surface and to prepare for the laying of cables later.
- Electric winch or cable tractor- mainly used to drag the cable.
- Cable conveyor- used in conjunction with the traction machine to overcome the considerable friction and reduce damage to the cable
- Cable tray discharge
- Roller devices
- Anti-twisting- to reduce the twisting of the wire rope
- Cable tray braking device
- Tension meter
43. What are the requirements for the installation distance of the bracket in the cable trench?
- When the cable’s outside diameter is equal to or less than 50mm, support should be added every 1m
- When the outside diameter is greater than 50mm, cable support should be put in place every 0.6m.
- For a single cable in a triangle, every 1m should be tied firmly, and for a vertical device, every 1 ~ 1.5m has to have fixed support.
44. How to deal with 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. However, in a scenario where the control wire and cable and power cable are laid in the same frame, a separation plate should be applied.
45. How to install a grounding wire in the cable tunnel?
Answer: The entire length of the tunnel and trench should be installed with a continuous grounding wire. The grounding wire in question should be connected to all the brackets, the two ends, and the grounding electrodes. The specification of the earthing wire and cable should be as per the design requirements.
The cable lead pack and armouring should also be connected to each other and the earthing wire. In addition to the insulation requirements, the cable frame and the earthing wire should be painted with rust-proof paint or galvanized.
46. What are the requirements for fire blocking of cable holes?
For larger wire and cable penetration holes, such as cable through the floor, the use of fire plugging material should be based on the actual situation. First, the cable surface should be coated with four to six layers of fireproof paint. Secondly, the length of the hole should be about 1.5m below and should be blocked using fire-resistant materials with a specific strength of the board to fire plugging material. The cable blocking should be dense and without gaps to plug the smoke and fire effectively.
47. What checks should be noted before the torch ignition?
Answer: The following checks should be carried out;
- You should check whether the pump is leaking or seeping oil
- You should check whether the amount of oil in the barrel is more than 3/4 of the capacity of the barrel and whether the screw plug for refueling is tightened
48. What matters should be noted when using the torch?
- The maximum amount of oil injected into the torch is 3/4 of the volume of the oil drum.
- Do not start pumping too much pressure. Also, ignite the flame from yellow to blue can be used.
- There should be no flammable materials around the wire and cable, and there should be adequate air circulation.
- Close the regulating switch when stopped. After the fire is extinguished, slowly screw loose the oil hole cover to release the air. After the air is released, screw loose the regulating switch, completely cool, and then screw open the hole cover.
- Paraffin torch and gasoline torch should be used separately.
49. What is the use of manual oil pressure pliers? How to operate?
Answer: The connection of two wires and cables is usually the end of the two wires into the same material made of a pressure tube. Crimping pliers squeeze several pits so that the wires are connected. When the crimping handle is lifted, the plunger moves outward into the lower cavity of the oil valve to produce a vacuum. The oil in the oil tank is pushed into the plunger cavity.
When the handle is pressed down, the plunger moves inward, and the oil is pressurized so that the inlet valve is closed. The outlet valve opens so that the oil pressure goes into the hydraulic cylinder. When pushed, the piston and Yang die, Yang, Yin die are placed between the pressure tubes. When the crimp is squeezed pit deep to a particular value, the return valve opens, the piston automatically returns. After pressing the recess, move the pressure clamp and then press the next one.
50. Commonly used low-voltage power cables are divided into several categories according to their different insulation and protection layers. Which categories are these?
- Oil-impregnated paper-insulated lead-clad (or aluminum-clad) power cables
- Non-drip oil-impregnated paper-insulated power cables and wires
- Polyvinyl chloride insulated polyvinyl chloride sheathed power cables
- Cross-linked polyvinyl chloride insulated polyethylene sheathed power cables
- Cross-linked polyvinyl chloride insulation polyvinyl chloride sheathed power cables
- Rubber insulated cables
51. What are the common types of insulation materials? Give examples?
- Inorganic insulating wire and cable materials: mica, asbestos, marble, porcelain, glass, to name a few.
- Organic insulating materials: these include resin, rubber, paper, hemp, and cotton yarn, among others.
- Mixed insulating materials: the above insulating materials are used to process and shape the name insulating materials
52. In which case is row pipe laying used? What are the advantages?
Answer: The laying of rows of pipes is generally used in places where they intersect. This is often done in buildings, roads, or railways, and sometimes also in dense structures. The main advantage of row pipe laying is that it occupies less space and can withstand large loads. Also, the wires and cables do not affect each other. Thus, it is safer.
53. What is the long-term permissible load capacity of a cable?
Answer: The long-term permissible load capacity of a wire and cable is its specified current after thermal stabilization. After current stabilization, the cable conductor reaches the long-term allowable working temperature.
54. What are the factors that determine the long-term allowable load capacity of a cable?
Answer: Three main factors determine the long-term allowable load capacity of a cable. The factors include;
- The long-term permissible operating temperature of the wire and cable
- The thermal performance of the wire itself
- Cable installation and the surrounding environment of the heat dissipation conditions
55. How many ways of laying a cable are there?
Answer: There are several ways of laying a cable. The most common methods are listed as follows;
- Directly buried in the ground
- Installed in the cable trench
- Installed in underground tunnels
- Installed on the internal wall or canopy of a building
- Installed on a bridge
- Laying in a drainage pipe
- Laying underwater
56. How to connect a cable pipe?
Answer: A wire and cable pipe must be connected with a buckle and tube joint. If you intend to use welding, you must avoid direct butt welding. The connection should also be set with a section of thick pipe and then welded to avoid welding slag falling into the tube.
57. What are the requirements for up and down bending cable core during the production of a cable head?
Answer: When bending the core, do not damage the paper insulation. The bending radius of the cable core should not be less than ten times the core of the cable. Production should be meticulous in making the core bending part with a uniform force. Otherwise, it will be easy to damage the insulation paper.
58. Give a brief description of the plastic cable heat shrinkage sealing method
Answer: The heat shrinkage method applies to medium and low voltage rubber and plastic wire and cable joints, and terminal seals. It is also applicable for non-drip and thick impregnated insulated cables.
Two main types of heat-shrinkable tubing are used: cross-linked polyethylene type and silicone rubber type, which can shrink evenly when exposed to heat. When heated to a specific temperature, the heat shrinkage tube will shrink, while the hot melted adhesive will naturally cool to form a good seal layer.
59. What should you pay attention to when dealing with an electric bending machine cable protection tube?
- The bender should understand its performance and familiar with the operating knowledge of the personnel to operate
- Proper checking before use to ensure that buttons, operating handle, travel switch should be intact, and the bender must also be reliably grounded
- The choice of molds should meet the requirements of the required bending radius
- There should be sufficient scope of activities around the construction site
- Hands and feet are strictly prohibited from coming in contact with its rotating
- The work should be completed in time to stop the power and release the oil pressure
60. What is the content of the cable inventory and the meaning of the cable number?
Answer: A wire and cable inventory form the foundation for placing a cable order. It also guides the construction, operation, and maintenance of the cable’s archival data. Cable inventory should contain data on cable number, starting point, type, specification, length, and cable classification statistics. The control cable should also be listed in each spare cable core.
The wire and cable number acts as a critical cable identifier. Thus, it is essential not to repeat a particular cable number throughout the plant. Ideally, a specific cable number has a particular meaning and regularity that can express the characteristics of the cable.
Cable Laying Specific Requirements:
61. Cable laying methods currently used can be divided into several categories. Which categories are these?
- Manual laying– involves the use of manual tactics, under the coordinated command of one or more people, following the wire and cable laying provisions
- Mechanisedlaying – involves the use of rollers, tractors, and conveyors, among others. Such machines are controlled by a synchronized power supply, which makes the entire process safer.
- Manual and mechanical combination- In certain instances, there is a need to combine manual and mechanized wire and cable laying methods. This often occurs when dealing with lots of cable turns and construction difficulties.
62. What requirements should secondary control circuit wiring meet?
- Correct wiring according to the construction diagram.
- The electrical connection of the wire should be solid and reliable.
- The wire in the tray cabinet should not have joints, and the wire core should be free of damage.
- The end of the cable core should be marked with its circuit number, the number should be correct, and the handwriting must be clear and not easy to lose colour.
- The wiring should be neat, clean, and beautiful, and the wire and cable insulation should be excellent and undamaged.
- Each terminal block should be wired on each side for one and not more than two at most.
63. What are the requirements for cable ducting?
- The ducting pipe should be about 0.2m from the ground, under the pavement for 0.5m, the general area for 0.7m.
- The pit should be installed at the change direction and branches of the drainage pipe, and a recess should be added when the length exceeds 30mm.
- Pit depth should not be less than 0.8m with a utility hole diameter of not less than 0.7mm.
- The drainage pipe should tend to pit 0.5% ~ 1% of the drainage slope.
64. What are the requirements for the resistance of the cable conductor connection point?
Answer: The resistance of the connection point has to be small and stable and should be of the same length. Also, for a newly installed terminal and intermediate head, the cross-section conductor ratio should not exceed 1. For an operating terminal and intermediate head, the balance should not exceed 1.2.
65. What requirements should the design of cable joints and intermediate heads meet?
Answer: The design of wire and cable joints and intermediate heads should meet the following requirements;
- High-pressure strength and good conductor connection
- High mechanical strength and low media loss
- Simple structure, strong sealing
66. What is a cable fault? How many common types are there?
Answer: A Cable fault is an error that occurs when the insulation of the cable breaks down during preventive testing or in operation, forcing the cable line to blackout due to insulation breakdown, wires burnout, to name a few. Standard wire and cable faults include ground fault, short-circuit fault, fragmented wire fault, flashover fault, and hybrid fault.
67. How to deal with single-phase ground fault of the cable line?
Answer: Generally, a single-phase cable line ground fault involves local damage of the cable conductor. If mechanical damage occurs on the cable line and the error is near dry soil, all you’ll need is some local repair. Essentially, you have to add some insulation to the wire and cable fault point to strengthen the seal.
68. What tests and checks should be carried out before laying power cables?
Answer: Before laying the power cable, you should check whether the cable type, specification, and length meet all the requirements. Secondly, you should check whether there is any external damage to the cable. It would be best if you also considered the resistance of the wire and cable in relation to its intended use. Low-voltage cables with 1000V have a resistance value that is generally not less than 10MΩ. High-voltage wires with 2500V have a resistance value that is usually not less than 400MΩ.
69. What should one pay attention to when laying cables in the main plant?
Answer: Generally, one should pay attention to the following details when laying cables in the main plant;
- The control cable leading to the control room should be laid overhead
- Medium voltage cable should be applied in tunnels or rows of pipes or overhead at a high water table
- A 380V should be tunneled, placed in a secure ditch or a row of tubes, with one end of the equipment on the top and the other end at the bottom. Also, the wire and cable can be partially laid overhead when the water table is high
70. What materials are used in the inner and outer shielding layers of a power cable? What role do they play?
Answer: Generally, inner and outer shielding layers of power cables are either made of braided strands of copper or, in certain instances, a non-braided spiral winding of copper tape or a layer of conducting polymer. Wire and cable shielding play a role in mitigating electrical noise and minimizes electromagnetic radiation.
Issues Related To Cables:
71. Briefly describe the composition and properties of the epoxy resin compound
Answer: The epoxy resin compound consists of epoxy resin with a hardener, filler, toughener, and thinner. It has the following properties;
- Sufficient mechanical strength
- Excellent electrical properties
- Stable electrical properties
- Sufficient adhesion to non-ferrous metals
- Good corrosion resistance
- When used outdoors, it is resistant to rain, light and heat, and humidity
72. When using a single-core cable sheath end grounding method, why must you install the cable parallel to the return line?
Answer: A wire and cable conductor must be installed along the cable line laid in parallel in the metal sheath end of the grounded cable line to ensure that the induced voltage in the sheath does not exceed the acceptable standard. The two ends of the grounded conductor are referred to as the return line.
When a single-phase ground fault occurs, the grounded short-circuit current can flow back through the return line to the system’s center. Due to the magnetic flux generated by the grounding current transmitted through the return line, a magnetic flux is caused by the grounding current of the cable conductor. This reduces the induced voltage in the wire and cable sheath during a short-circuit fault.
73. Cable works can be divided into several projects. What projects are these?
- Site transport: involves loading and unloading engineering materials from the warehouse to the construction point, transportation, and empty backhaul.
- Local works: includes road excavation, tunnels, trench construction, among others.
- Laying works: involves laying cables, intermediate head fabrication, lifting of cover, the burial of pipes, tide proofing, and traction head fabrication.
- Both ends of the project: include the production and installation of support and suspension bridges and their foundations, the output of terminal heads, the structure of oil pressure and signal devices, the electrical performance test of the name species, to name a few.
- Plugging works: includes the production of oil-filled wire and cable sealing heads, the installation of oil supply tanks, automatic drainage, and signaling devices, among others.
- Grounding works: includes insulation joints, transposition boxes, protectors, grounding box installation, to name a few
74. What checks should be carried out for the acceptance of cable lines?
- Cable specifications should be in line with the requirements of the project in question. The wire and table arrangement should also be neat, without damage, complete, and have correct and clear labeling
- The fixed bending radius of the cable, the distance concerned, and the wiring of the metal sheath of the single-core power cable shall comply with the requirements
- Wire and cable terminals, intermediate heads do not leak oil, firmly installed, oil-filled cable oil pressure and meter calibration values should meet the requirements
- Good grounding
- The cable terminal is correctly coloured, and the metal parts of the brackets should be painted completely
- Cable trenches and tunnels, bridges should be free of debris, complete cover
75. What are the requirements of the cable line positive sequence impedance measurement process?
Answer: The AC resistance of the wire and cable conductor and the phase sum of the three equivalent inductances of the cable is called positive sequence impedance.
The cable treasure application technology research institute revealed that the cable line positive sequence impedance could generally be measured directly on the cable tray. Measurement typically uses lower voltage. Therefore, there is a need to use a step-down transformer for step-down measurement.
A step-down transformer uses star-shaped wiring, and its capacity is generally 10kVA or more. There is a wide range of voltage regulation. When carrying out measurement, the AC power supply should be more stable to ensure that the current achieves the specified requirements. The actual voltage meter reading value must be the voltage at the end of the cable. Ideally, the test current should be close to the cable’s long-term allowable load capacity and the measurement of the value of each meter close to the current after reading the value of the three meters at the same time.
76. According to the “electrical production safety regulations,” what conditions must electrical staff meet?
Answer: For one to qualify as an electrical staff member, he or she must meet the following conditions;
- Must visit the doctor for a check-up to ensure that they are in good physical health and not ailing from any diseases that might impair their working ability
- They must have the necessary electrical knowledge. He or she must also master the relevant regulations on the particular area of wire and cable specialization and the professional technology and safety operation techniques. They must also have passed their electrical exams
- Must be skilled in first aid methods of electric shock
77. What should be noted in the process of transporting and loading cables?
In the process of transport loading and unloading, the cable and cable tray should not be damaged. It is strictly prohibited to push the cable tray directly from the car, and the cable should not be transported flat and stored flat.
Before transporting or rolling the wire and cable tray, you must ensure that the cable tray is firm. The cable winding should be tight. For an oil-filled cable, the pressure tank between the oil pipes should be fixed to avoid damage. The pressure tank should be firm, and the pressure indication should meet the ideal universal requirements.
78. What measures are recommended for cable fire prevention?
- Use of fire-retardant cables
- Use of a fire-resistant wire and cable bracket
- Use of fireproof paint
- Set a fire partition wall and a fire baffle in the cable tunnel, mezzanine exit, and other similar high-risk places
- For overhead cables, you should avoid oil pipes and install explosion-proof doors
79. Which aspects should be considered when choosing a power cable cross-section?
Answer: You should consider the following aspects;
- The long-term allowable working current through the cable
- The thermal stability in the event of a short circuit
- The voltage drop on the line should not exceed the permissible operating range
80. What are the advantages of power cables over overhead lines?
- Reliable operation plus the fact that external damage is slight, the chance of failure is less, there is optimum power supply safety, and they will not cause personal harm because they are installed underground
- Less maintenance workload and no need for frequent wire and cable inspections
- No need to erect poles.
- It helps to improve the power factor