Types of medium and low voltage cable accessories

The main types of products that are currently used for medium and low voltage cable accessories are heat shrinkable accessories, prefabricated accessories, and cold shrinkable accessories. They have the following characteristics:

1 Heat shrinkable accessories

The material used is generally a blend of polyethylene, ethylene-vinyl acetate (EVA), and ethylene-propylene rubber. This kind of product mainly uses stress tube to deal with the problem of electric stress concentration. That is, the parameter control method is used to relieve the electric field stress concentration. The main advantages are light weight, easy installation, good performance and low price.

The stress tube is a kind of heat-shrinkable tube with special electrical parameters with moderate volume resistivity (1010-1012Ωcm) and large dielectric constant (20-25). The electrical parameters are used to force the stress at the power cable insulation and shielding fracture to evacuate. The stress tubes are more evenly distributed. This technology is generally used in cable accessories of 35kV and below. Because the stress tube will heat up when the voltage level is high and cannot work reliably.

The key technical issues in its use are:

To ensure that the electrical parameters of the stress tube must reach the values ​​specified in the above-mentioned standards, it can work reliably. In addition, attention should be paid to filling the air gap at the fracture of the cable insulation semi-conductive layer with silicone grease to eliminate gas and achieve the purpose of reducing partial discharge. Cross-linked cables will shrink greatly during operation due to poor internal stress handling, so when installing accessories, pay attention to the stress tube and the insulation shield to cover not less than 20mm to prevent the stress tube from separating from the insulation shield during shrinkage. Due to the small elasticity of heat-shrinkable accessories, air gaps may occur at the interface during thermal expansion and contraction during operation. Therefore, the sealing technology is very important to prevent moisture intrusion.

2 prefabricated accessories

The material used is generally silicone rubber or ethylene propylene rubber. The geometric structure method is mainly used to deal with the stress concentration problem. Its main advantages are excellent material performance, easier and faster installation, installation without heating, good flexibility, and greatly improved interface performance. It is the main form used in low-voltage and high-voltage cables in recent years. The disadvantage lies in the high requirements for the outer diameter of the cable insulation layer. The usual interference is 2-5mm (that is, the outer diameter of the cable insulation is larger than the inner hole diameter of the cable accessory by 2-5mm). The interference is too small, and the cable The accessories will malfunction; the interference is too large, and the installation of the cable accessories is very difficult (high process requirements). Especially in the middle joint, the problem is prominent, the installation is not convenient, and it often becomes the point of failure. In addition, the price is more expensive.

The key technical issues in its use are:

The size of the accessory and the size of the cable to be installed must meet the specified requirements. In addition, it is necessary to use silicone grease to lubricate the interface for easy installation.

Countermeasures for cable failure

Strengthen cable construction management.

The following points should be done during cable construction: a. Strengthen the handover and acceptance. Before laying the cable, verify whether the type, specification, and quantity of the cable are consistent with the design drawings, and conduct insulation tests, and do not use unqualified cables; b. Strengthen the management of cable laying facilities. In order to facilitate the installation and reduce errors, hang up the cable laying cross-section diagrams at appropriate places such as cable supports, pipe channels, shafts, and turns. Cable laying is strictly forbidden to twist, flatten the armor, break the protective layer and severely scratch the surface; for direct buried laying, parallel laying on and below the pipeline is strictly prohibited. C. Strengthen the management of cable head production and construction. When installing the cable head, avoid installing it in windy, rainy days or in a humid environment, and take measures to prevent dust when installing outdoors. The ambient temperature for installation must be above 0°C. The relative humidity is below 70%. For long-running cables, moisture and small impurities are very harmful, and are likely to cause water treeing and partial discharge. Therefore, attention must be paid to environmental humidity and dust during joint construction. Pay attention to cleaning the environment before construction. In summer, people who construct joints should wear gloves. If the humidity in the environment is too high, they should be dehumidified (increasing the ambient temperature or using a dehumidifier).Use a blower to dry the insulating surface before inserting the stress cone. When installing the cable accessories, insulation treatment is carried out on the spot. Except for the influence of climate on the installation quality, dust and debris in the environment will have adverse effects. Special attention should be paid to cleaning when installing cross-linked cables. When using a heat-shrinkable tube with a lamp, soot particles that are not fully burned in the flame will be attached to the surface of the tube, causing poor contact between the insulating layers, lowering the insulation level, and increasing the leakage current. Therefore, it is required to scrub the surface with solvent for every shrinkage of a pipe. The construction of the cable head should be continuous and the time should be shortened as much as possible. After production, the cable head is tightly closed, the filler is filled with full, no bubbles, and no oil leakage; the core wires are tightly connected, the insulating tape is tightly wrapped, and the moisture-proof paint is evenly brushed; the lead sealing surface is smooth, free of blisters and cracks, and ensure the correct phase sequence .

Strengthen the completion acceptance and data management. After the completion of the cable project, strict inspection and acceptance. Production management, operation and other departments shall carry out acceptance inspections in accordance with the “Code for Construction and Acceptance of Cable Lines of Electrical Installation Engineering” and other relevant standards.(For example:Aerial Bundled Cable ASTM B231 Standard)

With the further development of the construction and transformation of urban power grids, the utilization rate of power cables has increased greatly, construction quality has been improved, project completion acceptance and operation management of power cables during normal operation have been strengthened, and potential accidents have been reduced, which is of great significance to improving the reliability of power supply . The majority of installation and operation personnel have accumulated experience in practice and summarized them in time, which also greatly promotes the improvement of the level of installation and operation.

Application of XLPE cable in urban network

Actively develop the ABC cable distribution network to meet the needs of load growth and urban construction (improvement of the city appearance and reduction of land occupation). In recent years, with the extensive use of cables in urban network construction and transformation, cable failures have gradually increased.

It is an important task for electric power workers to discuss the problems that should be paid attention to in the use of cables in urban networks.

1. Analysis of common cable faults The most direct cause of cable faults is breakdown due to insulation degradation.

Analyze the causes of cable failures in recent years, mainly include:

(1) Overload operation. Long-term overload operation will cause the temperature of the power cable to rise and the insulation aging, which will lead to insulation breakdown.

(2) Construction quality. Electrical aspect: the construction process of the cable head fails to meet the requirements, the sealing of the cable head is poor, moisture invades the inside of the cable, and the insulation performance of the cable is reduced; protective measures are not taken when the cable is laid, the protective layer is damaged, and the insulation is reduced. In terms of civil engineering: poor drainage of pipe trenches in industrial wells, long-term soaking of cables in water, impairing the insulation strength; too small industrial wells, insufficient cable bending radius, and long-term extrusion.

(3) Damage by external force. Mainly, it was the brutal construction of machinery in municipal construction, digging wounds and cutting cables.

(4) Corrosion. The protective layer suffers from chemical corrosion or electric corrosion for a long time, which causes the protective layer to fail and the insulation decreases.

(5) The quality of the cable itself or the cable head accessories is poor, and the insulating glue dissolves and cracks, resulting in insulation degradation and accidents.

2. Response measures

(1) The design model should have a proper margin. On-site operating experience shows that for the important load cables that are continuously produced, a proper margin should be left in the design and selection. Although the investment is slightly larger, it can ultimately reduce cable failures and extend cable life.

(2) Laying methods should be adapted to local conditions. Different laying methods should be adopted for different areas. Commonly used laying methods are: direct buried laying, pipe laying, trench laying, laying in tunnels, etc. In urban areas and rainy and humid areas, direct burial should not be used; cable tunnels or cable wells should be used in areas where the number of cables is relatively concentrated; when trenches are not suitable, pipe laying methods can be used.

3) Choose good quality cables and cable head accessories. The quality of the cable is very important to prevent the deterioration of water branches. When choosing a cable, you should have a certain understanding of the production process and management of the cable, so that you can buy a good quality cable and lay the foundation for reducing failures. The quality problems of the cable head accessories will dissolve and crack the cable head insulation, causing the cable to be damp, the insulation drop, and cause accidents; in recent years, we have begun to adopt cold shrinkage on the cable heads and middle heads on both sides of the main cable (especially outdoors) Head craft. Because the processing and installation are more convenient, and it is easy to store and not easy to deform, the construction quality is less affected by human factors, so it is gradually promoted.

Cable quality defects and troubleshooting methods

The degree of crosslinking (hot extension) is unqualified
If the degree of cross-linking does not meet the standard, the thermal-mechanical properties of the cable are unqualified and cannot meet the requirement of 90°C working temperature.
The reasons for the unqualified cross-linking degree are the unqualified formula and the improper proportion of the compounding agent. The formula should be adjusted to solve it; the second is the improper vulcanization process, such as low air pressure, fast line speed, high cooling water level and other factors.
The solution is to first find out what is the reason, which may be one or several reasons at the same time, and eliminate the reasons.
Unqualified structure and appearance
(1) The thinnest point of the insulating layer thickness is lower than the minimum value specified in the standard, or the average thickness is lower than the nominal value. The reasons for the unqualified insulation thickness are the fast line speed, the small amount of glue from the extruder, and the improper mold selection.
The solution is to reduce the linear speed or increase the speed of the extruder and adjust the size of the die.

(2) Eccentricity
The reason for the eccentricity is that the mold is not adjusted or the suspension control has changed after the recruitment is adjusted.
The solution is to control the drape as stable as possible during driving.
The reason for the appearance of the bamboo-shaped outside of the cable is electrical and mechanical systems that cause the traction speed to be unstable, and the second is that the core is too small, or the conductor outer diameter is uneven.
The troubleshooting method is to check the mechanical and electrical system and eliminate the fault. Properly adjust the size of the core, control the outer diameter of the suburbs and counties as uniform as possible, and the stranded wire that exceeds the process regulations can be used.
Surface scratches
(1) The cable touches the upper or lower wall or foreign matter in the vulcanizing tube. Therefore, it is required to adjust the degree of suspension, try to make the wire core move in the middle of the cross-linked tube, and clean up in time if foreign matter is found.
(2) There is burnt on the outer edge of the mold sleeve. The solution is to adjust the temperature of the mold sleeve when starting the car to prevent overheating. Once it is found to be burnt, it should be stopped immediately to remove it, or it will not get better by itself.

Most of the impurities in insulating materials and semi-insulating materials are brought in during mixing and during the feeding process of the extruder. Strict attention should be paid to the cleanliness of materials during operation to prevent the mixing of external impurities. Another kind of impurity is scorch, which affects the performance and service life of the cable. Therefore, the temperature is strictly controlled during mixing and extrusion to prevent the occurrence of scorching.
There may be two reasons for bubbles generated in the insulation, one is caused during extrusion. The solution is to select appropriate molds. There are bubbles in the shielding layer. The main reason is that there is water in the material. It should be dried before extrusion. The second is insufficient cooling. At this time, the following will appear. A circle of bubbles will appear on the circumference equidistant from the core.
The elimination method is to strengthen cooling, raise the water level and lower the temperature of the cooling water.
Unqualified ABC cable performance

(1) Unqualified free discharge and dielectric loss
The causes of dissociation discharge and dielectric loss unqualified are complex, and they are shielded from the outside. Whether the insulation contains bubbles and impurities is closely related to the performance of the raw materials. The elimination method is to keep the raw materials clean and strictly follow the production process.
(2) Cable insulation breakdown
The main cause of cable insulation breakdown is the mixing of insulating materials and external damage. Due to strict avoidance of external damage, the mixing of impurities should be avoided as much as possible. The semi-finished products are strictly managed to prevent stumbling.
(3) Cushion breakdown
The main reason for the cushion layer breakdown is the steel with burrs, curling, and puncture the cushion. The method of elimination is the use of different quality requirements for the unqualified steel belt. The cushion shall be made of plastic tape with higher hardness, and ensure The thickness of the cushion.If you want to buy cables, you can learn about our Huaxing cables

Overhead lines need “nine checks”

The inspection of overheadcable is one of the basic contents of the operation and maintenance of overhead lines. Defects can be found in time through inspections so that preventive measures can be taken to ensure the safe operation of the line. Usually, line inspectors should do “nine inspections” when inspecting overhead lines.
Check the pole tower. Check whether the tower is collapsed, tilted, deformed, decayed, damaged, whether the foundation is cracked, and whether the iron components are bent, loose, skewed or rusted. Check whether the wire length of the iron bolts or iron screw caps of the tower is insufficient, the screws are loose, the binding wires are broken and loose. Check whether there are bird nests and other objects on the tower.
Second, check the crossarm and fittings. Check whether the cross arm and fittings are displaced, whether they are firmly fixed, whether the weld seam is cracked, whether the nut is missing, etc.
Three check the situation along the line. Check whether flammable, explosive or strongly corrosive substances are piled on the ground along the line, whether there are illegal structures near the line, whether there are buildings and other facilities that may harm the line during thunderstorms or strong winds; check the poles and towers Whether to erect other power lines, communication lines, broadcast lines, and install broadcast speakers, etc.; check whether the lines are connected to electrical equipment without authorization.

Four check the route. Check the wires and lightning protection wires for broken strands, back flowers, corrosion, damage from external forces, etc.; check whether the distance between the wires, the ground and adjacent buildings or adjacent trees, sag, etc. meet the requirements, and whether the sag of the three-phase wire is unbalanced Phenomenon: Check whether the wire connector is in good condition, whether there are signs of overheating, severe oxidation, and corrosion.
Five check insulators. Check the insulator for cracks, dirt, burns and flashover marks; check the deflection of the insulator string and the damage to the iron parts of the insulator.
Six check lightning protection devices. Check whether the size of the protection gap is qualified and whether the auxiliary gap is intact. Check whether the external gap of the tubular arrester changes and whether the grounding wire is intact. Check whether the porcelain sleeve of the valve-type arrester is cracked, dirty, burned, or flashover marks, and the sealing is good. Check whether the down conductor of the arrester is intact, whether the grounding body is exposed by water washing, and whether the connection between the grounding down conductor and the grounding body is firm.

Seven check pull lines. Check the power cable for rust, slack, broken strands and uneven force on each strand. Whether there is any decay or damage to the cable pile and protection pile Whether the cable anchors are loose, lack of soil and sinking of soil irrigation. Whether the wire rod, wedge-shaped wire clamp, UT-shaped wire clamp, and wire-holding hoop are corroded, whether the nut of the UT-shaped wire clamp is missing, and whether the stop device of the turnbuckle is in good condition. Whether the pull cord is pulled into the wood pole at the binding place.
Switch equipment on eight check poles. Check whether the switchgear is installed firmly, whether there is any deformation, damage or discharge traces, whether the operating mechanism is intact, and whether the distance between the leads and the ground meets the regulations.
Nine check crossing points. Check whether there are new crossing points, whether the crossing distance meets safety requirements, and whether the original crossing points endanger the safe operation of the line. Whether the protective measures are perfect.

Wire and cable commonly used plastic

Commonly used plastics for wires and cables include polyethylene, cross-linked polyethylene, polyvinyl chloride, polypropylene, polyolefin, fluoroplastics, nylon, etc.

Polyethylene is currently the most widely used and most used plastic. From the data in the table, it can be seen that polyethylene has low meson loss, high electrical resistivity, high breakdown field strength, good weather resistance, and good manufacturability. It is currently the best electrical Insulation Materials. However, due to its low operating temperature, it is mainly used as insulation for communication cables. Medium-density and high-density polyethylene have high strength and hardness, and their water permeability is low, and they are mostly used as cable sheaths. However, polyethylene has the biggest disadvantage, that is, it is easy to burn and has strong black smoke, so its application has brought many hidden dangers to the environment.

Cross-linked polyethylene is an excellent thermosetting insulating material formed by adding a cross-linking agent to low-density polyethylene. On the basis of inheriting many excellent properties of polyethylene, it has improved mechanical properties, weather resistance and allowable working temperature, thus becoming the best insulating material for power cables.

Due to the different cross-linking agents added, different cross-linking processes are formed. At present, there are three kinds of chemical cross-linking, warm water cross-linking, and radiation cross-linking that are most used. Chemical crosslinking is mainly used for medium and high voltage cables (such as 10KV and above); warm water crosslinking and radiation crosslinking are mainly used for low voltage cables (1kV and below).

The insulation performance of cross-linked polyethylene is closely related to its purity. High-voltage and ultra-high voltage cables above 35KV must be insulated with ultra-clean cross-linked polyethylene, which not only requires high purity of raw materials, but also requires high cleanliness of cross-linking process equipment and environment, and the process is stable and reliable.

It should be particularly pointed out that the insulation performance of polyethylene and cross-linked polyethylene has a “quirk”, that is, it is suitable for AC insulation, not DC insulation, especially DC high voltage will reduce its insulation life. Therefore, the DC cable insulation is mostly rubber insulation or oil-paper insulation. In addition, polyethylene and cross-linked polyethylene insulation have “hydrophobia”, and their breakdown is often related to the presence of water, that is, the formation of “water branches” under high voltage, leading to insulation damage. Therefore, when polyethylene and cross-linked polyethylene are used for the insulation of high-voltage and ultra-high-voltage cables, they are particularly “water-proof” during their processing, storage and transportation, and insulation extrusion, and there should be a water-blocking structure outside the cable insulation shield, such as metal jacket.

Polyvinyl chloride has good physical and mechanical properties and excellent process performance. It is the most used plastic in the 20th century. It is also the main insulation material and sheath material for low-voltage wires and cables. But entering the 21st century, PVC cable will gradually shrink or even fade out in the cable market. There are two reasons for this. On the one hand, people’s safety awareness has increased and they hope to adopt halogen-free materials, so many halogen-free materials have emerged. There is no doubt that it will become the new favorite of the 21st century cable industry and squeeze the market. On the other hand, PVC has five weaknesses: one is its high density, which is about 1.5 times that of cross-linked polyethylene, and its insulation cost is high; the other is its low operating temperature; and the third is its higher dielectric loss than cross-linked polyethylene. One hundred times higher; fourth, poor cold resistance (brittle at -15 degrees); fifth, toxic gas (HCL) is released during combustion. In recent years, the mechanical properties, electrical heating properties, and insulation resistance of cross-linked polyvinyl chloride developed in recent years have been greatly improved. Some small cross-section cables have been introduced into the market by irradiation technology, and they have been used in equipment and installation wires, high-voltage lead wires, automotive wires and building wiring. Application, but its shortcomings of halogen cannot be changed.

Several reasons for cable aging

The most direct cause of wire and cable aging failure is the breakdown of the insulation due to degradation. There are many factors that lead to the reduction of conductive insulation. According to actual operating experience, it can be summed up as the following situations.

1) Reasons for cable aging: external force damage. Judging from the operation analysis in recent years, especially in the high-speed economic development of Haipudong, a considerable number of cable failures are now caused by mechanical damage. For example, non-standard construction during cable laying and installation can easily cause mechanical damage; civil construction on directly buried cables can also easily damage cables in operation. l Sometimes if the damage is not serious, it will take months or even years to cause the damaged part to be completely broken down and cause a failure. Sometimes the damage is serious and a short-circuit fault may occur, which directly affects the safe production of the electric power and the power user.

2) Reasons for cable aging: insulation is damp. This situation is also very common, and generally occurs at the cable joints in direct burial or piping. For example: unqualified cable joints and joints made in humid weather conditions will cause the joints to enter water or water vapor. For a long time, water branches will form under the action of the electric field, which will gradually damage the insulation strength of the power cable and cause failure.

3) Reasons for cable aging: chemical corrosion. The cable is directly buried in the area with acid and alkali, which will often cause the cable armor, lead skin or outer protective layer to be corroded. The protective layer suffers from chemical corrosion or electrolytic corrosion for a long time, which causes the protective layer to fail and reduce the insulation. The cable is faulty. Chemical: the unit’s cable corrosion is quite serious

4) Reasons for cable aging: long-term overload operation. Overload operation, due to the thermal effect of the current, the conductor will inevitably heat up when the load current passes through the cable. At the same time, the skin effect of the charge, the eddy current loss of the steel armor, and the insulation loss will also generate additional heat, which will increase the cable temperature. During long-term overload operation, excessively high temperature will accelerate the aging of the insulation, and even breakdown of the insulation. Especially in the hot summer, the temperature rise of the cable often leads to the first breakdown of the weak insulation of the cable, so in the summer, there are more cable faults.


5) Reasons for cable aging: cable connector failure. Cable joints are the weakest link in the cable line. Cable joint failures caused by direct faults (poor construction) by personnel often occur. In the process of making cable joints, if there are original nets such as insufficient joint crimping, insufficient heating, etc., the insulation of the cable head will be reduced, which may cause an accident.

6) Reasons for cable aging: environment and temperature. The external environment and heat source of the cable can also cause the cable to overheat, insulation breakdown, and even explosion and fire.

If you want to buy a cable, you can send us Huaxing Wire and Cable for consultation

Smart grid spawns smart cables, experts may have temperature sensing function

What should a “smart cable” look like? Some experts gave an example of whether the wire and cable can understand the conveying capacity through the color of the surface, can have a temperature sensing function, and the color display can be different at different temperatures. For example, if the wire is transported more than 1000 A, the color of the wire will change from white to other colors if the transmission capacity is exceeded. If this can be achieved, it will be of great help to the operation of the line. Another example is insulation, which has a self-recovery function in the case of discharge, and the defect can be recovered by adding other materials. It is hoped that enterprises can work hard to develop such products, which will also play a great role in the upgrading and innovation of enterprises.

Some experts believe that the purpose of my country’s construction of a strong smart grid is to ensure the safety and reliability of the grid, so that people can use it more conveniently and at ease. This requires more reliable grid-related equipment. “The measure to reduce the occurrence of cable failures is to use qualified cables, set up safe cable channels and lay them in accordance with regulations, check them regularly, and conduct state assessments.” said Chen Peiyun, chief engineer of Qingdao Hancable Co., Ltd., currently laying in ultra-high voltage cables Some fiber optic cables can detect its partial discharge and temperature through the fiber optic cable, which is equivalent to having a certain perception effect. These are smart cables.

The double-ring network power cable  supply in big cities, the increasingly narrow space corridors, and the increase in the rate of underground cables in the city center have brought great demand for cables. The maintenance-free requirements of cables and the attention to the life of insulation withstand voltage put forward higher requirements on the insulation medium, performance indicators, and brand reputation of distribution cables.

With the increasing application of cables, coupled with the improvement of the power supply reliability requirements of the smart grid, cable faults occur from time to time, and there is an urgent need to study new and smarter cable detection technologies. “Diagnosing and evaluating the status of cables is an important technical means for rationally arranging cable replacement and ensuring the safety and reliability of power supply. It is also an extremely important part of effective cable management in smart grids.” National Wire and Cable Quality Supervision and Inspection Center Director Wu Changshun said.

Some cable companies have seen the business opportunities brought by smart cables and want to dig money in the field of smart cables. Recently, the intelligent ultra-flexible fireproof cable successfully developed by Far East Smart Energy has passed the full performance type test of the “National Fireproof Building Material Quality Supervision and Inspection Center” and the “National Wire and Cable Quality Supervision and Inspection Center”. The comprehensive performance has been appraised by relevant experts. As an internationally advanced level, the products have obtained national invention patents and were selected into the list of “the first batch of Jiangsu high-tech products in 2014”.

Requirements of Power Cable Cross Section

Power cable section
The selection of power cable conductor cross-section should meet the following requirements:
1 The temperature of the cable bare conductor under the action of the maximum operating current shall not exceed the allowable value of the service life of the cable. Cable conductor work of continuous working loop
The temperature should meet the requirements of Appendix A of this code.
2 The temperature of the cable conductor under the action of the maximum short-circuit current and short-circuit time shall comply with the provisions of Appendix A of this code.
3 The voltage drop of the connected circuit under the maximum operating current shall not exceed the allowable value of the circuit.
4 In addition to meeting the requirements of paragraphs 1 to 3 above, the cross-section of power cables of 10kV and below should be based on the initial investment of the cable and the operation during its service life.
The principle selection of comprehensive cost economy. The selection method of economical current cross-section for power cables of 10kV and below should comply with the regulations in Appendix B of this code.
5 The minimum cross-section of multi-core power cable conductors, copper conductors should not be less than 2.5mm2, and all aluminum conductors should not be less than 4mm2.
6 For cables laid underwater, when the conductor is required to withstand the tensile force and is reasonable, the cross section can be selected according to the tensile requirements.

Commonly used cables of 10kV and below shall determine the allowable minimum cross-section of the cable conductor at 100% continuous working current, which should meet the requirements of Appendix C and Appendix D of this code. The current carrying capacity shall be greater than The working current of the loop.
1 Difference in ambient temperature.
2 The difference in soil thermal resistance coefficient when directly buried.
3 The influence of multiple parallel cables.
4 The influence of sunlight on outdoor overhead laying without shading.

Except for the conditions specified in Article 3.7.2 of this code, when the cable determines the minimum allowable cross-section of the cable conductor at 100% continuous working current, it shall be verified by calculation or test, and the calculation content or parameter selection shall meet the following requirements:
1 Non-coaxial cables used in power supply circuit cables with higher harmonic loads or intermediate frequency load circuits should be included in the skin effect and the increase in proximity effect
And other additional heating effects.
2 For single-core high-voltage cables that are cross-connected and grounded, when the three sections in the unit system are of unequal length, the effect of the additional loss and heating of the metal layer should be included.
3 The cables laid in the protective tube shall be included in the influence of thermal resistance; the cables with different holes in the pipe shall also be included in the influence of mutual heating factors.
4 Cables laid in closed, semi-enclosed or ventilated refractory trough boxes should be included in the heat resistance of the type of material and the thickness and size of the box.
The impact of increased resistance.
5 When the thickness of the fireproof coating, tape and other covering layer applied on the cable is greater than 1.5mm, the thermal resistance should be included.
6 When the cable in the trench is buried with sand and there is no regular water supplement, a thermal resistance coefficient greater than 2.0K·m/W should be selected according to the sand quality to include the influence on the increase of the thermal resistance of the cable.

When calculating the continuous allowable current-carrying capacity of the cable whose conductor working temperature is greater than 70℃, the following requirements should be met:
1 When a large number of such cables are laid in tunnels and shafts without mechanical ventilation, the impact on environmental temperature rise should be included.
2 Cables are directly buried in dry or moist soil. Except for the implementation of soil replacement treatment to avoid water migration, the soil thermal resistance coefficient is not
It should be less than 2.0K·m/W.

Control Cable and Its Metal Shield

1 The current and voltage of dual protection, as well as the two systems that need to enhance reliability, such as DC power supply and trip control loop, should use separate control cables.

2 In the following cases, the same control cable should not be used together:
1 Weak current signal, control circuit and strong current signal, control circuit.
2 Low-level signal and high-level signal loop.
3 Weak current control circuit of each phase of AC circuit breaker split-phase operation.
3 Each pair of round-trip wires of the weak current circuit should belong to the same control cable.
4 The phase wire and neutral wire of each group of secondary windings of current transformer and voltage transformer should be arranged in the same cable.
5 The control cables of strong current loops may not contain metal shields, except for those located in high-voltage power distribution devices or close to and parallel to high-voltage power cables and need to suppress interference.
6 The control cables of weak current signals and control circuits should have metal shields when they are located in an environment affected by interference and do not have effective anti-interference measures.7 The selection of the metal shielding type of the control cable should be included in the comprehensive interference suppression measures based on the possible electrical interference impact, and should meet the requirements for reducing interference or overvoltage, and should meet the following requirements:1 For the weak current control cables of power distribution devices above 110kV, general shielding or double-layer general shielding should be used.
2 The control cable used for the current, voltage and signal contacts of integrated circuit and microcomputer protection should be shielded.
3 The shield selection of the signal loop control cable of the computer monitoring system shall meet the following requirements:
1) Switch signal, general shield can be used.
2) For high-level analog signals, a pair of twisted cores should be used for general shielding, and a pair of twisted cores for separate shielding can also be used when necessary.
3) For low-level analog signals or pulse signals, it is advisable to use a pair of twisted cores and separate shields, and if necessary, a pair of twisted cores and a composite general shield can also be used.

4 In other cases, the appropriate shielding type should be selected according to factors such as electromagnetic induction, electrostatic induction and ground potential rise.
5 When the cable has steel armor or metal sheath,such as SWA cable , its shielding function should be fully utilized.
8 For control cables that need to reduce electrical interference, a grounded spare core can be added, and it should be grounded at one point on the side of the control room.
9 The grounding method of the metal shield of the control cable shall meet the following requirements:
1 The analog signal loop control cable shielding layer of the computer monitoring system shall not form two or more points of grounding, and shall be grounded at one point in a centralized manner.
2 The cable shielding layer of the current, voltage and signal of the integrated circuit and microcomputer protection shall be grounded at the same time as the switch placement place and the control room.

3 For the shielding layer of control cables other than the above, when the electromagnetic induction interference is large, two points should be grounded; when the electrostatic induction interference is large,
One point grounding can be used.
For double shielding or composite general shielding, it is advisable to use one or two grounding points for the inner and outer shields respectively.
4 In the choice of two-point grounding, the shielding layer should not be melted under the action of transient current.
10 The conductor cross-section of the strong current control loop should not be less than 1.5mm2, and the weak current control loop should not be less than 0.5mm2.