cable insualtion

Cable threading, tips you don’t know!

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In the wiring of weak current projects, we usually bury the wire tube in the wall first. Such a hidden design will not affect the beautiful effect of decoration. However, after the AAAC Cables tube is buried, threading from the inside is a troublesome thing. Sometimes we can’t wear a wire after tossing for a long time. What should we do at this time?

In fact, the reason is that we don’t use the right way to thread wires. Let’s introduce a new way to thread wires. It’s so fast and professional for wires to go through pipes. Teachers and Fu all install it like this. It’s too smart!
01
What’s good for threading wires
The traditional wire threading method is generally to put a small steel wire in the wire pipe, then bend the steel wire to buckle the outer edge of the pipe, tie the wire to one end of the steel wire, pull the steel ACSR Cables on the other side, and then slowly pull the wire out of the pipe, but this construction method tests patience. Moreover, it will greatly increase the decoration time, and the operation is also more troublesome. If you meet a corner, sometimes it takes half a day to get the line out. So now more and more people are not using this method.
However, with the development of technology, some tools in decoration are constantly updated, so now smart masters no longer use steel wire rope, but use this, which can thread quickly without jamming. Now the outer skin of professional threading device is made of steel wire wrapped rubber, which has good hardness and smoothness. It is much stronger than the previous steel wire rope, and it is not easy to jam at the elbow. In particular, a pulley is installed at the thread feeding head, which is very popular with teachers.
This threading device is also very simple in operation. It can be used for the first time. Use the traction head of the wire to pass through the wire head, and then fix it with a professional tensioner. This kind of tightener not only has good fixing effect, but also is convenient for construction, which saves the threading time of the master. After the thread head passes through the tightener, wrap it into an 8-shaped shape, and then push it up with the fastening spring. It is not too convenient. This is why more and more masters like to use the tightener instead of the steel wire rope.

02
Let’s introduce several standard threading methods
1. The first is that some wires will be long, or when it is necessary to turn and wind in the actual environment, talc powder can be added to the pipe to facilitate the extraction of wires. Remove a little of the insulation material in the front section of the wire, and then slowly fix the wire core in the coil.
2. The more important point is: wires with different voltages and different circuits must not be threaded in the same conduit. In addition, if the circuits of multiple lamps will not interfere with each other, remember not to thread too much.
3. After the wires are strung, remember to cut off the wires. If you want to connect the blood to the switches, sockets and junction boxes, we remember to reserve more than 15 cm. In addition, more than half of the perimeter of the distribution box should be left in the distribution box. Oh, the wires outside the door should be left about one and a half meters.
03
Wire threading cannot be carried out casually, and the following points shall be ensured
1. Be sure to select wires with good quality and large wire diameter. The threading pipe shall be flame-retardant PVC pipe. When purchasing, you can pinch it with your fingers. If it can’t be broken, it means that the quality is very good. If the economy permits, special galvanized pipe can also be selected.
2. In the same conduit, the number of wires passing through shall not be too much, and it shall be easy to pull. The popular standard is that the total cross-sectional area of the wires in the pipe shall not be greater than% of the total cross-sectional area of the inner diameter of the pipe, and shall not exceed


3. The inner wire shall be free of joints and kinks. The pipe sleeve shall be used when the pipeline is butted, the bend shall be connected with elbow or tee, and then glued with glue. The presser foot cap shall be used when connecting to the wire box, and the lock interface must be used between the wire pipe and the wire box.
4. High power electrical appliances such as floor heating and air conditioning should be wired separately, and electrical appliances such as refrigerators that are not suitable for frequent power cuts should also be wired separately. These electrical appliances should have separate switches at the main switch (air switch). If you go far away, you can turn off other power supplies, leaving only the refrigerator power on.
5. After the wire passes through the pipe, it is put into the slot and fixed with cement or quick drying powder, that is, several points are selected on a slot for sealing and fixing. When installing the cassette, the wall hole where the cassette is placed should be larger. Cement can be embedded in the gap between the cassette and the wall hole to fix the cassette firmly to prevent loosening in the future.
6. After the wiring is completed, the decoration company must leave a detailed “pipeline diagram”, number the wall, draw the direction and specific position of the wire with a pen, indicate the size from the upper floor to the lower floor and adjacent wall, especially the joint position of the pipeline. With such a detailed, it will be much more convenient to overhaul the ABC Cables in the future.
From the above points, we can know that it is necessary for wires to go through pipes in the process of decorating circuits. Now experienced masters will choose new types of threading appliances for wire threading. It is fast and professional to wear wires with it. Friends who like it might as well try it.

Completion of laying of high temperature AC superconducting cable

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The transmission capacity of superconducting AAC Cable is much larger than that of conventional cable. The power transmission capacity of a 10KV three-phase coaxial high temperature AC superconducting cable is greater than that of a conventional 110kV cable, but the transmission loss is only one fourth to one fifth of that of a conventional cable, which is very suitable for power supply in high load density areas. However, due to the great difficulty of project implementation, at present, the above application scenarios are still blank in China and even in the world.
The superconducting cable laid this time passes through the busy roads, subway areas and core business districts in the central urban area of Shenzhen. The laying environment space is narrow. There are special complex working conditions such as 24.2m vertical drop, more than 10 turns less than 90 degrees, short-span S-shaped corners and staggered height of transmission pipelines. There is almost no precedent experience to follow. It is a typical “small corridor” of complex transmission in large cities, Even laying conventional cables is challenging.


In the face of such special and complex working conditions, in order to ensure the smooth completion of superconducting cable laying, the superconducting cable project team of Shenzhen Power Supply Bureau has conducted dozens of field surveys and discussions in the past three months, and formulated a special research scheme of “remote simulation laying + on-site pre laying”. The project team built a simulation site at the location of the manufacturer, restored various special and complex working conditions, carried out many simulation laying drills, obtained a large amount of valuable data, accumulated on-site experience and laid a solid foundation for on-site laying. In the project site, analog cables of the same size and weight are pre laid twice, which provides the most direct reference for formal laying.
During the laying process, in addition to the conveyor and winch required for conventional AAAC Cable laying, the project team has developed special auxiliary guidance, turning radius limit protection, nozzle center positioning, well shaped conveying wheel set and other laying tools and instruments to ensure the safe and reliable laying of superconducting cable body.
Next, the project will enter the installation, test and commissioning stage of superconducting cable accessories, supporting low-temperature refrigeration system, measurement and control protection system, etc. When the project is put into operation, it is expected to “package” solve many problems and challenges such as the difficulty of power grid construction land in large cities, the continuous growth of power grid load demand and the saturation of urban transmission and distribution corridors, so as to provide a new scheme for solving the problem of regional power supply with high load density in mega cities around the world
Other media reports:
The first 10kV three-phase coaxial high temperature AC superconducting cable in China has been laid
Xinhua news agency, Shenzhen, July 30 (reporter Wang Feng) – the reporter learned from the Shenzhen Power Supply Bureau of China Southern Power Grid on the 30th that the first 10kV three-phase coaxial high-temperature AC superconducting cable in China was laid in Shenzhen on the 28th, laying the foundation for the first demonstration project of this type of superconducting cable to be put into operation within the year. It is expected to “package” solve the difficulty of power grid construction land and the continuous growth of power grid load demand in big cities There are many problems and challenges such as the saturation of urban transmission and distribution corridors.
It is reported that the project is a major science and technology project of China Southern Power Grid Corporation. It is developed by Shenzhen Power Supply Bureau of China Southern Power Grid and connects 220 kV Binhe substation and 110 kV Xinghe substation in Futian central area of Shenzhen. The cable length is 400 meters. It will supply power to important loads such as Shenzhen landmark Ping’an building.
High temperature superconducting power transmission refers to the use of the superconducting properties of superconducting materials to make the power transmission medium close to zero resistance and the power transmission loss close to zero in the liquid nitrogen environment close to minus 200 ℃ relative to absolute zero, so as to realize high-capacity power transmission at low voltage level. A 10KV three-phase coaxial high temperature AC superconducting cable has the power transmission capacity equivalent to a conventional 110kV cable.
It is understood that at present, most domestic superconducting ACSR Cable demonstration projects are experimental in nature, and the importance level of line load is low. The demonstration project plans to apply superconducting cable to the high load density power supply area in the city center to supply power to important loads, which will be a big test for the application of superconducting cable.
Next, the project will enter the installation, test and commissioning stage of superconducting cable accessories, supporting low-temperature refrigeration system, measurement and control protection system, etc. When the project is put into operation, it will provide a new scheme and model for solving the power supply problem in high load density areas of mega cities.

Analyze the development opportunities of cable accessories industry

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1. Market demand
China’s power grid construction investment will continue to grow steadily, including smart grid construction, new energy construction and distribution network transformation, which will bring a new round of development opportunities for the cable accessories industry.


The “long-distance, large-scale, high-capacity and high-voltage” pattern of power grid construction will further stimulate the demand for high-voltage and EHV cables and ABC Cable accessories. At the same time, with the development of new energy such as photovoltaic, wind power and nuclear power, the demand for high-voltage and ultra-high voltage DC cable accessories will also increase significantly. Among them, offshore wind power has become a research hotspot of global wind power development. All countries in the world regard offshore wind power as an important direction of renewable energy development, and China has also classified it as an important part of strategic emerging industries. China’s offshore wind power development has entered the stage of large-scale and commercial development. Therefore, it also brings opportunities for the development of submarine cable accessories (submarine cable terminals, intermediate joints and repair joints).
At present, China’s cable accessories industry is still dominated by the domestic market to meet domestic demand. Because Chinese products are competitive with imported products in cost and price. Along with one belt, one road construction will promote the rapid development of the regional economy along the line, and the demand for power construction will be sustained. One belt, one road, China’s home appliances network is relatively backward. In addition, China’s local equipment companies are backward in technology, limited in supply capacity and high in import dependence. This brings considerable business opportunities to China’s cable accessories enterprises with superior price performance.
In the next five years, the average growth of medium and low voltage power cables for power grid construction will still be 7% – 10%. High voltage ACAR Cable accessories of 66kV and above will also become the growth highlight of the industry.


2. Technological innovation and existing problems
The development of cable accessories is closely related to the development of cables. It has also experienced the process from oil filled insulation to dry insulation, from medium voltage to high voltage and ultra-high voltage. The development and production of cable accessories is not a single professional technology, but requires the use of multi-disciplinary comprehensive technology. The fields involved mainly include electrical, mechanical, chemical, polymer materials, etc.
Chinese enterprises lack the core technology of power cable accessories, especially in the research and development of EHV power cable accessories, which has been restricted by technology. In recent years, the cable accessories independently developed by a few domestic technology leading enterprises have gradually become mature, which not only fill the domestic gap and break the foreign monopoly, but also some products have been exported abroad, and the voltage level has reached 500kV. The quality and reliability of domestic cable accessories have also been greatly improved.
Except for a few leading enterprises, the main products of most domestic cable accessories manufacturers are medium and low voltage cable accessories or fittings. Most of them have small production scale, relatively little R & D investment, weak R & D ability, forward-looking and basic common technology research lags behind foreign competitors, and the development of basic materials, basic processes and key components lags behind. Especially in terms of high-voltage, ultra-high voltage AAAC Cable accessories and special cable accessories, there is still a certain gap with the foreign advanced level. There is a long-term shortage of high-quality talents in the industry, the R & D and test ability is weak, and there are few production enterprises with sustainable innovation ability in the industry.
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18 key problems in cable stranding process

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In order to improve the flexibility and integrity of wires and cables, two or more single wires are intertwined in the specified direction. This process is called stranding. The whole stranding process is divided into conductor stranding, cable forming, braiding, steel wire armor and winding. Any problem in any process will directly affect the cable quality.
What should we pay attention to in the process of stranding? Which links are prone to problems? Do you know all this? Next, let’s introduce the matters needing attention and easy problems in the middle process of wire and cable twisting.
18 questions and answers about AAAC Cable stranding process


1. What are the quality requirements for stranded conductor cores?
Answer: the stranded wire core shall be tight, round, straight, without serious bending, strand skipping, scratch and flattening, the copper wire surface shall be free of oxidation, blackening, tin coating falling off or tin particles, uniform pitch, flat and firm joints, neat wire arrangement, wire diameter shall meet the tolerance requirements, and length shall meet the specified requirements.
2. What preparations should be made before strand production?
Answer: (1) check whether all parts of the stranding machine are normal and whether there are obstacles in the operating parts（ 2) Check whether the electromechanical equipment, electrical switch and fence insurance are normal（ 3) Check whether the butt welding machine is normal（ 4) Check whether the tools and measuring tools are complete（ 5) Select and match the die according to the requirements of the dispatch list and process card, replace the pitch lap gear, replace the twist direction, measure the wire diameter, etc.
3. What are the main reasons for the scratch of stranded wire core and jumper?
Answer: (1) the main causes of abrasion are: the guide wheel is damaged or the conductor jumps out of the guide wheel, the threading pipe is damaged, the die is damaged or too small, and the conductor is crushed or abraded when the disc is rolling. Prevention method: replace the guide wheel to prevent the conductor from jumping out of the guide wheel, replace the threading pipe, select the die and replace the reel（ 2) The main reasons for jumper are: the die is too far from the distribution board, the setting out tension is inconsistent, the die is too large, etc. Prevention method: reasonably adjust the distance between the die and the branching board, adjust the base number of setting out tension, and reasonably select the die.
4. What is untwisting? Under what circumstances is the effect significant?
Answer: untwist stranding is to keep each reel equipped with a single line in a horizontal position through a special device (untwist mechanism), so as to ensure that each single line does not produce self twisting deformation in the stranding process. Only when the diameter of the single line is large, the pitch is large and the single line is hard, the untwisting will have a good effect.


5. Which two requirements must the structure of sector core meet?
Answer: (1) the single lines on the center warp of the central layer shall also be arranged on the center line of the sector. If this requirement cannot be guaranteed, when the core is bent, these single lines will be stretched or compressed, resulting in the uplift of the single line, resulting in the change of the shape of the sector core（ 2) Slip requirement: that is, the single line around the fan-shaped outer circumference shall be able to slide on the central layer. This is because the insulated fan-shaped line is not twisted back when it is twisted into the ABC Cable core, and each single line around the outer circumference tries to slide around the central plane. Therefore, if this requirement is not met, the single line will jump up and damage the shape of the fan-shaped line.
6. For stranded wires in concentric layers, the direction of adjacent layers is specified to be opposite. Why?
Answer: (1) the stranded wire is round. If the adjacent layers are twisted in the same direction, it is easy to embed the single wire of the outer layer into the inner layer, thus damaging the complete circular structure（ 2) When the stranded wire is under tension, the rotating torque generated by each layer is opposite, which can offset each other to prevent the loose strand caused by the single wire of each layer rotating in the same direction. At the same time, it can also avoid the looping phenomenon of the stranded wire when it is not tensioned（ 3) In steel cored aluminum strand, AC impedance can be reduced, corona phenomenon and power loss can be reduced.
7. What are the two methods of twisting insulated cores together? What kind of communication cable is currently used in domestic cities?


Answer: twisted pair and star twisted pair. At present, twisted pair communication cables are used in domestic cities.
8. What’s the reason why the inlet of the die hole of the die used in twisting the ACSR Cable is in the shape of a horn?
Answer: the horn shape at the inlet of the die hole has two advantages: (1) to gradually compress the twisted cable core（ 2) It will not scratch the cable core.
9. What are the main differences between a winch and a cage winch?
Answer: the stranding part of the cage winch is connected with the paying off device, while the stranding part of the winch is not connected with the paying off device, but with the traction device and take-up device.
10. In terms of structure, what are the wrapping forms of the package for cabling?
A: there are three types: simple type, plane type, tangent or semi tangent type. The simple wrapping head is a wrapping in which the wrapping reel is at an angle with the group or the cable core axis. The flat winding head is the winding in which the tape reel is perpendicular to the wire group or cable mandrel, and the tangent (half tangent) winding head is the winding in which the tape reel is parallel to the wire group or cable mandrel.
11. The trial fraction shows the relationship between the package noise pitch H (mm) and the forward speed V (M / min) of the cable core and the number of revolutions n (R / min) of the package head rotating around the cable core.
Answer: H = V / N = 1000 (mm)
12. What is the difference between unit stranding and concentric stranding?
Answer: the stranding direction of each layer of unit stranding cable core is the same, and the multiple of stranding distance is large, which is about 1.5 times that of concentric stranding. The unit quantity in each layer of cable core does not follow the rule of N + 6. The reason is that it is necessary to completely fill the cable core to cause deformation, so that each unit no longer maintains the original circle. The unit quantity of each floor basically follows the rule of N + 4.
13. What is a cable? How many deformations are there when the insulated core is cabled?
Answer: the process of twisting multiple insulated cores into cables according to certain rules is called cable. When the insulated core is cabled, there are two kinds of deformation: one is the bending deformation caused by the core around the cylinder, and the other is the torsional deformation caused by the direction of the spiral wire.
14. What is reverse twist? Why do we use untwist to form cables with circular cores?
Answer: when stranding, the method of reverse twisting of the wire is called untwisting. For the circular core cable, the untwisted cable is mostly used to reduce the core deformation, avoid damaging the insulation due to deformation, and make the cable have good flexibility.
15. What are the main qualities and requirements of auxiliary materials in the cable forming process of power cables?
Answer: the auxiliary materials in the cable forming process of power cable include: filler rope liner, tie belt and shielding belt, which shall have heat resistance, non water absorption, consistent cable working conditions and no corrosive insulating materials. The tie belt for filling and lining shall have considerable mechanical properties and meet the process requirements.
16. General selection principle of cable forming die for sector core? What should we pay attention to in the specific operation process?
Answer: the first mold is 1.0 ~ 3.0mm larger than the outer diameter of the cable, and the second mold is 0 ~ 0.6mm smaller than the outer diameter of the cable. The third die is 0 ~ 0.2mm smaller than the outer diameter of the finished cable. Pay attention to the actual use of the die. The cable cannot swing in the die, the cable core does not feel loose when rotating by hand, and the heat generated by the friction between the die and the insulated wire core should not be hot. The surface of the pressing die of the insulated wire core shall be smooth without damage and other marks.
17. After the completion of a certain specification of control cable, it is found that the conductor is thin or even broken, and the main causes are analyzed?
A: the following reasons lead to fine drawing and fracture: (1) excessive setting out tension（ 2) Wire clamp at wire nozzle（ 3) The insulation of the wire core is wrapped, and the part is thick（ 4) The wire joint is not firm（ 5) The take-up force is too large.
18. Causes and preventive measures of fan-shaped insulated wire core turning over?
Answer: the causes of fan-shaped insulated wire core turning over are: (1) insufficient pre torsion angle（ 2) When the pay off reel is on, the core row turns over, and the core is thick（ 3) The wire core is twisted back during the split footwall, resulting in turning over. The prevention methods are: (1) adjust the distance between the pre angle spring die and the core guide wheel（ 2) Adjust the pre twist angle, arrange the line on the pay off reel once, turn the reel if necessary, and the line shall be neat and not loose.

On the service life of control cable

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The service life of control cable is determined by the air oxidation induction period of sheath raw materials. Generally, the design scheme of control cable is applied for 20 years, which is the most applied life of control cable. Although the specific daily life is likely to be longer, from the perspective of safety, the design scheme period is the most life of control cable.

Life of control cable
Elements endangering the service life of control cables and ways to increase the service life of control cables:
1. The control cable does not have to return to moisture to avoid heat and corrosion. Because the sheath of the cable has a certain maintenance effect on the cable, the corrosion is more harmful to the cable in case of heat.
2. Control cables do not have to be overloaded. Overload application of the cable will cause immediate damage to the insulation and sheath of the cable, endanger the service life of the cable or destroy the cable immediately.
3. For the route of old engineering buildings, if it is found to be swallowed or wet by water, especially if the route is worn and brittle, electric welders should be asked to repair it immediately.
4. The control cable shall prevent common faults of cable connector, which will lead to ABC Cable short-circuit fault, which will not only damage the cable, but also destroy household appliances.


5. Control cable commodity itself: select national and industrial standard cables as much as possible. Cost-effective non-standard cables are very easy to cause short-circuit fault, and the service life of non-standard AAC Cables is short.
6. Control the natural environment and temperature of the cable. The external natural environment and heat source where the control cable is located will also lead to excessive temperature of the control cable, penetration of the insulation layer, and even explosion and fire.

Don’t say how cheap the cable is. Let’s see if these tests can be passed

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A quality inspection organization has conducted a sampling survey on domestic wires and cables. The results show that the qualified rate of wires and cables in production enterprises that have passed ISO 9000 certification is less than 90%, while the qualified rate of products in small-scale wire and cable production enterprises that have not passed certification is even less than 30%. The quality of wires and AAC Cables in China is worrying.


So, don’t be too busy talking about how cheap the AAAC Cable is. Let’s take a look at the following common testing items?
1. Electrical performance test
It mainly includes conductor DC resistance, insulation resistance, finished product voltage test and insulated wire core voltage test, each of which is very important.
The conductor resistance directly reflects the electrical transmission performance of the cable and directly affects the temperature, service life, voltage drop and operation safety of the cable during power on operation. It mainly examines the material and cross-sectional area of the conductor. If the material of the conductor is poor or the cross-sectional area is seriously insufficient, the DC resistance of the conductor will seriously exceed the standard, This kind of cable laying in the line will increase the loss of current passing through the line, cause the heating of the cable conductor itself, cause the insulation aging and cracking of the coated conductor, cause electric leakage and short circuit of the power supply line, and even cause fire, endangering the safety of personal and property. The standard has strict provisions on the DC resistance of conductors of cables of different specifications, which shall not be greater than the value specified in the standard.
Insulation resistance, finished product voltage test and voltage test between insulated cores all examine the electrical insulation performance of cable insulation layer and sheath layer. Insulation resistance is to detect the resistance of insulating material between two conductors, which shall be large enough to play the role of insulation protection. The voltage test of finished products and the voltage test between insulated cores not only require that the ABC Cable has sufficient insulation capacity, but also that the insulation or sheath material is uniform without impurities, the thickness is uniform enough, and there shall be no invisible trachoma, pinhole, etc. on the surface, otherwise it will cause local breakdown during voltage withstand test.


2. Mechanical property test
It mainly examines the tensile strength and elongation at break of insulation and sheath plastic materials, including before and after aging, as well as the bending test, bending test, load core breaking test, insulation core tear test, static bending test, etc. for the finished flexible cable.
Tensile strength before and after aging and elongation at break before and after aging are the most important and basic indicators of cable insulation and sheath materials. It is required that the materials used as cable insulation and sheath should not only have sufficient tensile strength, not easy to break, but also have certain flexibility. Aging refers to the ability of insulation and sheath materials to maintain their original properties under high temperature conditions. Aging shall not seriously affect the tensile strength and elongation of materials, which will directly affect the service life of cables. If the tensile strength and elongation at break are unqualified, it is very easy to break sheath or insulator during cable construction and installation, or in light The sheath and insulation of cables used in thermal environment are easy to become brittle and break, resulting in exposed live conductors and electric shock.
In addition, because the flexible cable is not fixed laying, there are repeated dragging and bending in use. Therefore, the flexible cable standard also stipulates that dynamic bending test, bending test, load core breaking test, insulation core tear test, static bending test, etc. are added to the finished cable, so as to ensure that the cable meets the requirements in practical use. For example, the dynamic bending test mainly examines whether the stranded wire of the flexible conductor is broken to reduce the electrical transmission performance, or pierce the insulation to reduce the electrical performance of the insulation when the flexible cable is subjected to external mechanical tensile and bending stresses; It is a test method for whether the insulation is deformed or cracked under stress to affect the electrical insulation performance of cables.


3. Performance test of insulation and sheath materials
Including thermal weight loss, thermal shock, high-temperature pressure, low-temperature bending, low-temperature tensile, low-temperature impact, flame retardant performance, etc. These are to examine the performance of plastic materials for insulation and sheath.
For example, the thermogravimetric test is to detect the degree of degradation and volatilization of the material after 7 days of high-temperature aging at 80 ℃; Thermal shock test whether the specially wound insulation surface is cracked after high temperature of 150 ℃ for 1H; High temperature pressure test the elastic retention of insulating materials after high temperature re cooling; All low temperature tests generally refer to the change of mechanical properties under the condition of – 15 ℃, which is to detect whether the cable material becomes brittle, easy to crack or easy to break under the low temperature environment.
In addition, the flame retardant performance of the cable is very important. The test to check this performance is the non delayed combustion test, that is, the finished cable installed according to the standard is ignited with a special flame for a certain time, and the cable is burned after the flame goes out. Of course, the less the burned part is, the better, indicating that its flammability is poor, its flame retardant is good, and it is safer.
4. Sign check
The standard requires that the cable package shall be attached with labels or marks indicating product model, specification, standard number, factory name and place of origin.
Specifications include rated voltage, number of cores, nominal section of conductor, etc; The cable surface shall be printed with continuous signs of manufacturer’s name, product model and rated voltage. The spacing of signs shall be ≤ 200mm (insulating surface) or ≤ 500mm (sheath surface). The contents of signs shall be complete, clear and scratch resistant. This requirement is convenient for users to understand the model, specification and voltage grade of cables to prevent wrong laying.
In addition, the color recommended by the standard shall be preferred for the insulated wire core of the wire, especially the Yellow / Green two-color wire core. This kind of wire is generally used in the power line of electrical products. This special two-color wire is specially used for grounding. The Yellow / Green matching standard also has the following provisions: for each 15mm long insulated wire core, one color shall cover at least 30% of the surface of the insulated wire core, And not more than 70%, while the other color covers the rest of the insulated core, that is, the Yellow / green two colors shall be basically balanced.
5. Structural dimension detection
Including the thickness, thinnest thickness and overall dimension of insulation and sheath.
The thickness of insulation and sheath plays an important role in how much voltage the cable can withstand and its mechanical performance. Therefore, for cables of different specifications, the standard has strict provisions on the thickness, which shall not be lower than the specified value of the national standard. The cable insulation thickness is too thin, which will seriously affect the use safety of the cable, and will bring potential safety hazards such as cable breakdown and electric leakage caused by bare conductor. Of course, the thicker the better, which should not affect the installation. Therefore, the standard also sets an overall dimension requirement to limit it.
Appendix: common certification
1. CCC certification
Compulsory certification is a pass to enter the domestic market.
2. CB certification
Electrical products directly related to personal safety that are conducive to the export of products for families, offices, workshops and similar places. Such products are subject to compulsory certification in some countries, that is, they are allowed to be exported to the country and sold in the market of the country after obtaining the certification certificate of the country. Even in countries without compulsory certification, consumers are willing to buy certified products with certification marks for their own safety.
3. CE certification
It is a pass for products to enter the markets of EU and European Free Trade Area countries. Products certified with CE mark will reduce the risk of sales in the European market:
1) The risk of being detained and investigated by the customs;
2) Risks investigated and dealt with by market supervision institutions;
3) Risk of peer allegations for competitive purposes.
4. UL certification
In the U.S. market, consumers and buyers are more willing to buy products with UL certification marks.

Seven reasons why transformers burn out

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(1) Being struck by lightning. Most of the high and low voltage lines of power transformers are introduced by overhead cables. Because they are located in mountainous forests, the probability of lightning strikes is high. Therefore, during the thunderstorm season each year, the proportion of distribution transformers damaged by lightning strikes accounts for more than 30% of the overhaul.
(2) Ferroresonance occurs in the system. 10kV distribution lines in rural areas have conditions for overvoltage. When the system resonant overvoltage occurs, the primary current of the transformer surges. At this time, in addition to the fuse on the primary side of the transformer, the transformer windings will also be damaged. In some cases, it can also cause flashover or explosion in the bushing of the transformer.

2. Insulation damage
(1) The short-circuit fault of the low-voltage cable and the sharp increase of the load make the current of the transformer more than tens of times the rated current. At this time, the winding is affected by a large electromagnetic torque and shifts and deforms. Due to the sharp increase in current, the temperature rises rapidly, leading to accelerated aging of the insulation.
(2) The winding insulation is damp. This is caused by poor insulating oil or lowered oil level. One is that in the storage, transportation, or operation and maintenance of transformer insulating oil, moisture, impurities or other oils are accidentally mixed into the oil, which greatly reduces the insulation strength. The second is that the inner layer of the winding is impervious to impregnation, incomplete drying, poor welding of winding lead joints, and incomplete insulation, resulting in short circuits between turns and layers. Third, the lowering of the oil level increases the contact surface between the insulating oil and the air, and accelerating the entry of moisture in the air into the oil will also reduce its insulation strength. When the insulation is reduced to a certain value, a short circuit will occur.
3.tap switch
(1) Transformer oil leaks, so that the tap changer is exposed to the air, and the insulation performance decreases when the insulation is damp, resulting in a short circuit of discharge and damage to the transformer.
(2) The oil temperature is too high. The oil in the transformer is mainly used to insulate the windings, dissipate heat and prevent moisture. The oil temperature in the transformer is too high, which will directly affect the normal operation and service life of the transformer.
(3) The quality of the tap changer is poor, the structure is unreasonable, the pressure is not enough, the contact is unreliable, the position of the external character wheel is not completely consistent with the actual position of the internal, resulting in incomplete contact of the star moving contact position, dislocation of the dynamic and static contacts The tip makes the insulation distance between the two taps smaller, and short-circuits or discharges to the ground under the action of the potential between the two taps. The short-circuit current quickly burns the tapped turns and even damages the entire winding.

4. oil seepage
Oil seepage is the most common appearance abnormality of transformers. Since the transformer body is filled with oil, there are rubber beads and rubber pads at each connection part to prevent leakage. After a long time operation of the transformer, the rubber beads and rubber pads will age. Crack and cause oil seepage. Of course, if the screw is loose or the oil drain valve is not closed tightly, there will be blisters or poor welding quality during manufacturing, which will also cause leakage.
5. the iron core is grounded at multiple points
(1) It is not easy to find and test the multi-point grounding of the iron core of a 10kV distribution transformer. This is because the iron core grounding of the distribution transformer is internally sandwiched between the iron core (silicon steel sheet) with a thin copper sheet. The other end is pressed on the iron core splint and directly connected to the transformer shell.
(2) Short circuit between iron core silicon steel sheets. Although the silicon steel sheets are coated with insulating paint, their insulation resistance is very small, which can only block eddy currents but cannot prevent high-voltage induced currents. If the insulating paint on the surface of the silicon steel sheet is naturally aging, it will produce a large eddy current loss and increase the local overheating of the iron core.
6. overload
The uneven distribution of the three-phase load of the distribution transformer results in asymmetrical three-phase currents. The asymmetrical currents make the impedance drop of the transformer asymmetrical, so the low-voltage three-phase voltage is unbalanced, which is detrimental to the transformer and the user’s electrical equipment.
7.silica gel discoloration
Old-fashioned silicone is blue before being damp, and pink after being damp. However, this type of silica gel contains cobalt, which has an impact on human health, and its use has been banned in Europe. There is currently no cobalt-free silica gel in China, which is pink before being damp, and dark green after being damp.sformers burn out

Discussion on cable process: cable forming

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1、 What is cable formation
Definition of cable forming: the process of twisting multiple insulated cores into AAC Cables according to certain rules.
2、 The role of cable forming in cable production
Cable forming is one of the important processes in the production of multi-core cables. The three core, four core (one core is the ground wire) and five core (one core is the ground wire and the other is the neutral wire) of the three-phase power supply is commonly used for power cables. The number of control cable cores is more (more than 2 cores to 61 cores). During the cable forming process, several insulated cores are twisted together according to certain rules, The process of forming a multi-core cable. In addition to the stranding, the process of cable forming includes filling of the gap between the cores of the insulated wires, wrapping and shielding on the core after the cable forming.
When forming cables, the twisted form of insulation core adopts concentric normal stranding. If the diameter of the insulated core is identical, it is called symmetrical cable forming. If the diameter of the insulation core is different, the cable formation is called asymmetric cable formation. In order to avoid the influence of torsional stress on the core during the process of cable forming, the cable forming machine with the torsion device or bow shaped cable forming machine is used for the cable forming of circular insulated core to conduct the twisting and twisting.
3、 The way of cable forming
There are two ways to twist the strand and the cable, one is to twist back and the other is not to twist.
The rewinding and twisting is that the cable tray frame equipped with the setting out plate keeps the setting out plate at all times horizontal position when the machine rotates by means of the special device (the torsion device) on it. When the cable is formed, the insulation core is only subjected to the bending action, but does not twist. The back twisting is often used in the process of forming the circular insulated core. The core has no rebound stress after the cable forming, which can ensure the accuracy of the roundness and diameter of the cable.
The non twisted twisted cable is mostly used for the formation of fan-shaped lines. After the compression of the die, it becomes plastic deformation, thus eliminating the original torsional stress and ensuring the round after the cable is formed.


4、 Cable forming direction and pitch diameter ratio

The cable forming direction is generally right. The confirmation of cable forming direction is in the direction of ACSR Cable core forward, if the cage turns left, it is right direction, otherwise, otherwise, it is opposite.
The ratio of cable pitch diameter is different according to different types of cables. The circular core insulated by cross-linked polyethylene is hard and the diameter of cable forming is larger, with a disk of 30-40; The pitch diameter ratio of PVC insulated power cable is 30-40, and that of sector line is 40-50; The diameter ratio of the cable core of plastic insulated control cable is specified in the national standard, and generally it shall not be more than 16-20.
5、 Stranding coefficient and twist rate
In a pitch of the cable forming, the ratio of the actual length of the insulation core to the length of the pitch of the cable is called the stranding coefficient (k=l/h);
The stranding rate is the ratio between the difference between the actual length of the insulating core and the length of the cable forming pitch and the length of the formed pitch within a cable forming pitch λ=( L-H)/H × 100%
It is convenient to adopt the coefficient K, and the K value is always greater than 1. Thus, the actual value of K is increased by one K value for the single insulated core after the cable forming. The resistance of the core is proportional to the core, that is, the resistance of the core is also increased by a k value. If the insulation resistance is inversely proportional to the length of the insulating core, the insulation resistance of each core will be reduced by a k value. From the angle of reducing the core resistance and increasing the insulation resistance value, it is hoped that the smaller the coefficient of cable winding is, the better.
The ratio of the cable formation to pitch ratio is inversely proportional to the square of the pitch ratio. Therefore, the smaller the pitch ratio, the greater the coefficient of the cable formation, the greater the amount of insulating core material is, otherwise, from the perspective of saving the material consumption, the smaller the coefficient of cable formation is, the better.
6、 Wrapping process
The production process of wrapping all kinds of metal or non-metallic materials on the core or core of the guide cable is covered with the specified pitch spiral in the form of ribbon or wire.
7、 Non armored cable wrapping
In order to prevent the cable core from deformation after the cable forming and to prevent the adhesion with sheath, the insulating core shall be wound with the wrapping layer while forming and filling on the cable forming machine. For the non armored plastic insulated cable, the 1-2-layer non-woven fabric belt is usually wrapped by covering (the first layer or two layers are used for the specific purpose, and the principle of cable forming and tightening is adopted), The covering size is 10% – 15% of the bandwidth, and the wrapping angle is 25% °~ forty ° Within the scope.


8、 Armouring of cables
Steel belt armored cable is mainly suitable for underground direct burial, and can bear certain mechanical pressure; Steel wire armored cable is mainly suitable for laying with drop or vertical, and can bear large mechanical tension. Armored cable is mainly divided into steel strip armor and steel wire armour, and their combination armouring mode.
If there is shielding material (including unified shielding) on the core of the steel belt armored cable, the cable core shall be replaced by the extruded insulating sleeve instead of the inner layer. If the insulation core has no metal shielding layer, the inner layer of the insulation wire can be extruded or wrapped. The inner layer of the wrapping is generally PVC or PE and other similar strip.
Generally, the steel wire armored cable adopts the extruded inner layer.
For the model of armored cable, such as yjv22, there will be two numbers, the first number is the armor Code: generally, there are 2, 3 and 4 numbers:
2 – indicates double layer steel belt armor
3 – indicates the thin steel wire armouring
4 – indicates the thick steel wire armouring
The steel strip thickness and copper clearance of armored cable, wire diameter and clearance of steel wire armored cable shall meet the relevant standards.

Factors Affecting XLPE Insulation Thermal Shrinkage

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The cross-linked polyethylene cable material (XLPE) can make the aggregate structure of the insulating material in a reasonable state through the cross-linking process, and can increase the long-term working temperature of the power cable to 90 ℃, and the instantaneous short-circuit temperature is 170 ℃ ~ 250 ℃. The performance remains unchanged while other performances are improved and enhanced. Therefore, the use of cross-linked polyethylene (XLPE) insulated cables is becoming wider and wider. However, the author found in the experiment that due to the relatively small contact area between the insulating material and the conductor of the small-area cable, especially when the surface of the single-core conductor is smooth and rounded and the adhesion is insufficient, the thermal shrinkage of the insulation is large, and it is difficult to reach the national standard GB/ T12706-2008 “Rated Voltage 1kv (Um=1.2kV) to 35kV) Extruded Insulated Power Cable,such as 1-35kV underground armoured power cable , and Accessories” stipulates the requirement of not more than 4%, and the cable with larger area and higher voltage level is due to insulation and conductor It is easier to pass the test if the contact area is larger and the insulation thickness is larger.
Cables that fail the insulation heat shrinkage test are prolonged during use. Because of the excessive shrinkage caused by the insulation, the conductors may be exposed, which may cause the danger of telephone calls. Therefore, we must try our best to solve the problems in the production and improve the product quality of the cable.
So what factors will affect the insulation thermal shrinkage, and what causes the insulation thermal shrinkage test to fail?

PE is a crystalline polymer that is subjected to shearing and traction stretching under a heated environment (melting temperature), which makes the crystal grains of PE molecules increase in size along the stretching direction (longitudinal) and decrease in lateral size. The ordering is improved, that is, PE molecules are oriented, which increases the number of crystal nuclei, shortens the crystallization time, and increases the crystallinity and strengthens the orientation. However, when the finished XLPE insulated cable is placed at room temperature, the internal stress (shrinkage stress) generated during the extrusion of the XLPE insulation increases, which makes the crystallized XLPE molecules easy to de-orientate (the trend of shrinking). These factors, and Mainly related to the melting temperature and time, cooling rate, and external force (traction and stretching) in these three aspects:

1) Melting temperature and time
At high melt temperature, the crystalline polymer is a melt containing crystal nuclei, and the longer the melting time, the fewer the number of crystal nuclei. Therefore, during the cable insulation extrusion process, the higher the heating and melting temperature of the XLPE insulation material, the longer the residence time (holding time) at the heating temperature, the less the number of crystal nuclei, and the lower the crystallization performance of PE Cable, which is beneficial to reduce The crystallinity of the insulation can make the insulation thermal shrinkage meet the standard requirements;

2) Cooling rate
The temperature decrease rate of the polymer melt from above the melt temperature to below the glass transition temperature is called the cooling rate, and the cooling rate is the key to the crystallization of the polymer. The cooling rate is not only related to the melt temperature and room temperature, but also related to the crystallization rate and thermal properties of the polymer itself. The crystallization rate of PE itself is very large, and PE insulation can also get a higher crystallinity under extremely fast cooling conditions. Therefore, this situation is especially obvious in winter, and special attention should be paid to the control of the cooling rate during the extrusion process of XLPE cable insulation. The specific heat capacity of PE is large and the thermal conductivity is small. If the PE melt has a slow cooling rate and sufficient cooling is obtained, the relaxation process of the PE molecules will be prolonged, the orientation can be easily de-orientated, the degree of orientation can be reduced, and the generation of PE nuclei can be controlled and the grains can be delayed Grow up;
In addition, the conductor temperature also affects the cooling rate of the XLPE insulation. The conductor temperature is too low. When the high-temperature PE melt at the die of the extruder is coated on the surface of the conductor, the LPE insulation will shrink and shrink due to contact with the low-temperature conductor to produce shrinkage stress, and reduce the adhesion between the XLPE insulation and the conductor. Focus on reducing the resistance to heat shrinkage, and ultimately affect the heat shrinkage performance of the insulation layer of the XLPE insulated cable.


3) External force (traction and stretching)
In the cable insulation production process, the molecules are oriented along the force side under the action of external force (traction and stretching), which will promote the formation of nuclei, increase the speed of crystal nucleus generation, increase the number of crystal nuclei, and shorten the knot time and crystal Degree increases. Extrusion tube extrusion is commonly used in current electric winding production enterprises. Compared with extrusion molds, the insulation heat recovery of Si-XLPE insulated electric windings produced by extrusion tube molds that must be stretched during the extrusion process It is much larger. The disadvantage of the poor compactness of the plastic layer of the extrusion tube extrusion is likely to cause the marginal heat shrinkage test to fail, but because the process inspection and final inspection do not do the type test item, the insulation heat shrinkage test is ignored, even if it is extrusion In order to improve the production speed and the smoothness of the extrusion surface, the compression mold is generally several millimeters larger than the insulation outer diameter of the cable. In this way, the insulation outer diameter is ensured during the insulation production process, and insulation is inevitable. It will be stretched, and the molecules are still affected by external forces during the stretching process, resulting in orientation, which makes the insulation thermal shrinkage of the produced cable also larger, even far exceeding the standard requirements.

Cable Insulation Resistance Decreases Due to Moisture

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The insulation resistance of directly buried power cables is reduced and the phenomenon of cable line failures often occurs, especially when the cable insulation is damp, it is easy to cause the insulation resistance of directly buried power cables to decrease and substandard

1. The cable raw materials are damp

The raw materials used for cable insulation and sheathing are mainly plastic and rubber materials, and many kinds of materials with special functions are derived from this modification. When manufacturing materials, material manufacturers go through the processes of compounding, mixing, granulation, cooling and drying, as well as during the transportation and storage of the materials, dampness of varying degrees often occurs, resulting in varying degrees of material content. Moisture. Therefore, the cable manufacturer must dry the material before squeezing the material on the cable conductor. The extrusion unit is equipped with a material drying device to prevent the extruded insulation layer and sheath from happening. No defects such as bubbles and blisters, and no bubbles on the surface. This is the rigid process regulation of the cable manufacturer, otherwise the finished cable will not pass the factory withstand voltage test.

2. the cable manufacturing process is damp

In the insulation extrusion process, the insulation layer is scratched, causing holes or degumming of the insulation layer, and the insulation core enters the cooling water tank, which causes the insulation resistance to drop. Or when the protective layer is squeezed, the protective layer is damaged and water enters, so that the insulating layer is damp and the insulation resistance decreases. When manufacturing a multi-core cable, even if the insulation layer is extruded intact, when the insulated core is twisted into a cable, and when the sheath is extruded, damage may occur and the water may enter and become damp, so the finished cable cannot pass the factory withstand voltage test .

3. Moisture during cable construction

In the process of direct buried cable construction, if the cable trench is excavated, the cable burying operation, the cable intermediate joint and the terminal joint are not made standardly, it is very likely to damage the cable sheath and insulation layer. If the soil is wet or the cable trench is filled with water, water in the cable will definitely occur. After the insulation is damp, the surface resistance of the cable insulation will decrease and the surface leakage current will increase, and the insulation resistance will decrease, which will also cause the electric field distortion between the conductor and the insulation layer. The uneven electric field distribution in the insulation will cause free discharge inside the insulation and even lead to cable breakdown. After-sales service practice has proved that more than 95% of the direct-buried cable insulation resistance drop accidents are caused by improper construction.

 

As we all know, water and damp in the process of cable manufacturing and laying operation are the main factors that endanger the electrical performance and service life of the cable. Both cable manufacturers and users attach great importance to this.
Practical experience has proved that the main reasons for the ingress of water and moisture in the cable are as follows.

1) Material purity
If the cable insulation material is mixed with impurities, especially metal impurities, even the pigments of different colors used by China Wire and Cable manufacturer  will directly affect the electrical performance of the insulation and reduce the insulation resistance. The reason is that the non-metallic impurities in the insulating layer will absorb moisture when the cable is damp, and form numerous conductive points; second, the m

etallic impurities in the insulating layer are directly conductive points. Under the combined action of the conductor operating temperature and the external ambient temperature, these conductive points form conductive channels in the insulating layer, resulting in a decrease in insulation resistance and an increase in leakage current, which in turn leads to insulation breakdown.

2) The material is damp
If the cable insulation material has been damp and is not dried before being squeezed on the conductor, there will be quality defects such as a large number of pores in the insulation layer, an unsmooth extrusion surface, reduced mechanical strength, and even cracking. Therefore, when extruding the cable insulation layer, the cable manufacturer must dry the material. When extruding low-smoke and halogen-free materials, more attention should be paid to drying. These are the basic technical knowledge of cable manufacturers.