What are the effects of temperature on cable selection?

Whether our colleagues in electrical design or factory electrical maintenance can not open the topic of cable selection, we often see that the AAC Cable selection size of the same equipment under the same power will be different in the work, some may be less than one or two levels, and some friends may be confused by which right? Why do you choose different? Today we will learn how to choose the cable and how to consider it from? What is the effect of temperature on the selection of cables?

1、 Cable selection
(1) Select cable according to temperature rise
We all know that when the conductor passes through the load current, the temperature of the conductor will rise, the conductor has insulation layer, and the conductor temperature shall not exceed the maximum temperature allowed for a long time by the conductor insulation, and the conductor will be damaged after exceeding.
(2) According to mechanical strength
When laying, the conductor has a minimum section allowed according to the laying mode and the distance between the support point and the distance.
(3) Select according to economic conditions
That is, choose the most economical way.
(4) Select by line voltage drop
We all know that when the current passes through the conductor, the voltage drop will be generated in the line due to the existence of line impedance. If the voltage drop is too large, the voltage at the end of the line, that is, the load end is too low, which will cause the equipment to fail to work or affect the equipment life. Therefore, it is required that the voltage drop cannot be too large, so the cable section should be selected reasonably.

(5) Cable selection shall meet the requirements of load protection
When the circuit is in fault, the protective device should act according to the specified time, whether it can operate reliably or not has a great relationship with the cable section. Therefore, we should consider the requirements of reliable action when choosing the ACSR Cable.
(6) Select cable according to the requirements of thermal stability
In case of failure of cable or electrical equipment, the cable shall meet the requirements of thermal stability, and the size of cable section shall be considered in order to meet this requirement. The maximum cross section is the final result by combining the above conditions.

What are the cable maintenance items?

1. Manhole and drainage pipe
(1) water samples were taken for chemical analysis.
(2) remove the water in the well and remove the sludge.
(3) paint the iron parts such as cable bracket and hook.

(4) check the well cover and ventilation, and check the well body for settlement and cracks.
(5) dredge the spare pipe hole.

(6) check the condition of ACSR Cables and joints in the manhole to see if there is any oil leakage, whether the insulation gasket on the bracket is in good condition and whether the grounding is good.
(7) check the circuit name plate.
(8) check whether there is electric erosion, and measure the potential and current distribution of cables in the manhole.
2. Cable trench and tunnel
(1) check whether the door lock is normal and whether the access is smooth.
(2) check whether there is water seepage and ponding in the tunnel, remove the ponding and repair the leakage.

(3) check whether the ABC Cable bracket falls.
(4) check whether the insulation gasket between the metal sheath of the cable and connector and the bracket is intact, and whether there is any damage on the bracket; Whether the bracket has fallen off.
(5) check whether the fireproof tape, coating, blocking material and fireproof box are in good condition, whether the fireproof equipment and ventilation equipment are perfect and normal, and record the room temperature.

(6) check whether the grounding condition is good, and measure the grounding resistance if necessary.

(7) clean cable trench and tunnel.
(8) check the cable and cable joint for oil leakage.
(9) check cable tunnel lighting.

How to measure the temperature of cable terminal tail contact?

The contact resistance of the tail wire contact of AAAC Cable terminal (especially outdoor, sometimes connected by copper aluminum two different metals) will increase under the influence of long-term load and fault current, which will lead to overheating. When the line fails, the fault current flows through the contact, which will burn the contact. Measuring the contact temperature under the condition of no power failure, that is, to conduct live temperature measurement (preferably when the load is large), is an effective measure to monitor and check the contact condition. There are three main methods to check the contact temperature.

(1) Paste the temperature wax sheet: the temperature wax sheet is divided into three types: 60 ℃, 70 ℃ and 80 ℃, and are respectively expressed in different colors, and the commonly used ones are yellow, green and red. The temperature range can only be roughly checked by using the temperature wax plate. The reaction time is slow and the paste is not convenient. It is less used at present.
(2) Discoloration thermometer: this kind of color change thermometer is a surface measuring tool. It was used to monitor the temperature of heating working parts during welding in shipbuilding industry. It is used to indicate temperature according to the color changing characteristics of pigment in the pen at a certain temperature. The temperature measurement is quick and easy to use. Generally, the one with lower temperature can be selected for ABC Cable line. For example, the color changing temperature pen with color change temperature of 70 ℃ is selected, and it is marked as the original color. If the temperature is more than 70 ℃, it will become lake blue. The price of this color change thermometer is also cheaper. The temperature measurement reaction is fast (only 1-2s), and it is convenient to use.

(3) Infrared thermometer and thermal imager: the temperature wax plate and the color change thermometer shall be directly contacted with the live equipment, and the insulation rod shall be used for operation. Infrared thermometers and thermal imagers can measure temperature outside the measured point, so they are safe and reliable, and their measurement accuracy is much higher than the above two. The commonly used infrared thermometers or thermal imagers generally adopt portable type. See Fig. 9-4 and Fig. 9-5. The measuring distance is 5m, 10m, 100m, or even larger. The thermal imager can display the image of temperature distribution, and can store the image data in disk or computer, with good effect. But the price of this instrument is relatively expensive, and special personnel shall be responsible for the use and maintenance.

What is the content of periodic test for high voltage cable?

“Power cable operation regulation” stipulates that under the condition of not losing normal oil pressure, it is not necessary to conduct DC withstand voltage test regularly for high voltage oil filled AAAC Cable. As long as the oil pressure is normal and the dielectric loss and breakdown strength of oil are qualified, it can be considered that the cable insulation is reliable. Therefore, oil filled cable should be tested regularly to measure its dielectric loss and breakdown strength. The test period is generally sampled one year after putting into operation. If the results of two consecutive tests do not change, the test period can be gradually extended. In addition to the oil sample test, the following tests should also be carried out.

1. Measure the insulation resistance of outer sheath
the outer sheath of oil filled cable not only insulates the metal sheath from the ground, but also has anti-corrosion effect on the lead sheath and reinforcing tape. The insulation resistance of the sheath of oil filled cable shall be measured once a year by insulation resistance meter or other methods to check the integrity of the sheath insulation and the degree of water seepage. The high pressure oil filled cable is under the positive oil pressure, and the moisture in the air cannot invade people. Periodic measurement of outer sheath insulation can find out the early mild damage of sheath. When measuring the insulation of protective layer, the transposition bar and protector on transposition board in cross interconnection box shall be disassembled, and the insulation resistance of protective layer shall be measured by sections. Long lines shall be measured with electric insulation resistance meter.

2. Measure the grounding resistance
in the sheath grounding system with two ends grounded, one end grounded or cross interconnection connection, the value of grounding resistance will affect the operation of the protector. GEOX type grounding resistance measuring instrument can be used to measure the grounding resistance of the cable sheath connected with the grounding electrode or the grounding connecting line of the protector.
3. Measure the DC resistance of conductor
by measuring the DC resistance of the conductor and comparing it with the test values in previous years, we can check the change of the contact resistance between the outgoing stem or connecting pipe and the cable core, and judge whether the conductor connection is good.

Why measure the surface temperature of the cable in operation?

The temperature of the cable is closely related to the load, but only checking the load can not ensure that the AAAC Cable is not overheated
(1) the thermal resistance coefficient and side-by-side correction coefficient used in calculating the allowable ampacity of cables may be different from the actual situation.
(2) when choosing cables to determine conductor cross-sectional area, designers may lack sufficient information about the laying conditions and surrounding environment of the whole line.
(3) newly built and rebuilt power lines or thermal pipelines have an impact on the original surrounding environment and heat dissipation conditions.

(4) the excessive density of cables greatly exceeds the regulations of design and operation departments. In addition to measuring the load frequently, the operation Department must also check the actual temperature of the cable surface to determine whether the AAC Cable is overheated. The inspection shall be carried out at the maximum load and the section with the worst heat dissipation condition (generally not less than 10m). Thermocouple or pressure thermometer can be used for measuring instrument. The pressure thermometer is easy to install. Two temperature measuring points shall be installed at each measuring point. When measuring the temperature of the cable, the temperature of the surrounding environment should be measured at the same time, but it must be noted that the temperature measuring point of the surrounding environment should be kept a certain distance from the cable (generally 3M away), and there is no influence of external heat source.
On site detection method
During infrared detection, the cable should be live running for more than 24 hours, and try to remove or avoid the shelter between the cable and the thermometer, such as glass window, door or cover plate, etc; It is necessary to measure all parts of the cable line to avoid missing the measuring parts; It is best to carry out the test in the peak load state of the equipment, generally no less than 30% of the rated load. The infrared detection of the arrester connected with the cable terminal can refer to the requirements of DL / t664.
a) The emissivity of the tested equipment should be selected correctly, especially considering the influence of metal material oxidation on the selection of emissivity. For the selection of emissivity, please refer to Appendix B; Generally, 0.9 for metal conductor and 0.92 for insulator;

b) If the safety distance is allowed, the infrared instrument should be close to the equipment under test as far as possible, so that the equipment under test can fill the field of view of the whole instrument, so as to improve the resolution and temperature measurement accuracy of the instrument to the surface details of the equipment under test. If necessary, medium and long focal length lenses should be used; Generally, medium and long focal length lenses are used for outdoor terminal detection;
c) The compensation parameters such as atmospheric temperature, relative humidity and measurement distance are input and corrected, and the appropriate temperature range is selected;
d) Generally, the infrared thermal imager is used to scan all the tested parts, focusing on the cable terminal and intermediate connector, cross interconnect box, grounding box, metal sleeve grounding point and other parts. After finding the abnormal parts of the thermal image, the abnormal parts and the key tested equipment are measured in detail;
e) In order to measure temperature accurately or track conveniently, several different directions and angles should be set in advance to determine the best detection position and mark it for future retest, so as to improve the comparability and work efficiency;
f) Record the actual load current and voltage of the tested equipment, the temperature of the tested object and the temperature value of the environmental reference body according to the format of Appendix C.

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Factors affecting insulation resistance of wires and cables

Small insulation resistance is often encountered in AAC Cable production, which is affected by many factors. In fact, there are four main factors that have a great influence on the coefficient of insulation resistance.

1、 The influence of temperature
With the increase of temperature, the insulation resistance coefficient decreases. This is due to the increase of thermal motion and the increase of ion production and migration. Under the action of voltage, the conduction current formed by ion motion increases and the insulation resistance decreases.
The theory and practice show that the insulation resistance coefficient decreases exponentially with the increase of temperature, and the conductivity increases exponentially with the increase of temperature.
2、 Influence of electric field intensity
When the electric field strength is relatively low, the mobility of ions increases in proportion with the increase of electric field strength. The ionic current and electric field strength follow Ohm’s law. When the electric field intensity is relatively high, with the increase of electric field intensity, the mobility of ions gradually changes from linear relationship to exponential relationship. When the electric field is close to breakdown, a large number of electrons will migrate and the insulation resistance coefficient will be greatly reduced.
The withstand voltage test voltage of various wire and ACSR Cable products specified in the standard is in the stage of ion mobility increasing in proportion to the electric field strength, so the influence of electric field strength on the insulation resistance coefficient cannot be reflected. The effect of electric field on the insulation resistance coefficient is obviously reflected when the sample is subjected to breakdown test.

3、 Influence of humidity
Due to the large conductivity of water, the size of water molecules is much smaller than that of polymer molecules. Under the action of heat, the polymer macromolecules and the constituent chain move relatively, so that water molecules can easily penetrate into the polymer, increase the conductive ions in the polymer, and reduce the insulation resistance.
The standard specifies the immersion test of various wires and cables. For example, the rubber specimen is immersed in water for 24 hours before the insulation resistance is measured. The purpose is to meet the influence of moisture and water on electrical performance during use.
Insulation resistance is one of the main electrical properties of insulating materials, and it is also an important index of wire and cable products or materials. Generally, the insulation resistance should not be lower than a certain value. If the insulation resistance value is too low, the leakage current along the wire and cable line will inevitably increase, resulting in the waste of electric energy. At the same time, electric energy will be turned into heat energy to prepare for thermal breakdown and increase the possibility of thermal breakdown.
4、 Influence of material purity
The impurity mixed into the material increases the conductive particles in the material and reduces the insulation resistance. Therefore, the insulation resistance of a certain rubber plastic material will reflect the purity of the material and verify whether it meets the standard.
In the process of wire and cable production, the process does not strictly comply with the operating procedures, mixed impurities and materials Blister due to moisture, insulation core deviation or outer diameter size is less than the standard, insulation delamination or crack, insulation scratch, etc., It will reduce the insulation resistance of the product.
Therefore, in order to check the insulation resistance, it is necessary to check whether there are any problems in the process operation. During the use of wires and cables, measuring the change of insulation resistance can also check the insulation damage and prevent accidents.

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High voltage cable intelligent anti breaking system

Project background

If the city grid is given life, the AAAC Cable is his blood vessel, which injects the energy of the power grid.
However, with the development, the construction frequency is high, which leads to the explosion of the risk of cable external force damage. Please see, this is the scene after the cable has been destroyed. Then, once the underground cable is damaged by the external force of construction machinery, it may cause chain reaction and lead to large-scale power failure.
If the power outage occurs in hospitals, transportation hub, government agencies and other important places, how much loss will be suffered to the people’s lives and property, and the indirect economic losses caused by the power failure are immeasurable; On the other hand, cable damage may also lead to the construction of personnel casualties. In view of this, it is urgent to prevent external damage!

Traditional cable inspection mainly relies on manual field inspection, which makes it difficult to find out the cable external breakage due to various mechanical construction on the ACSR Cable path in time and accurately. There are few operation and maintenance personnel, and the inspection mode mainly depends on the drawing and oral communication to confirm the cable location. Inspection efficiency is low and maintenance cost is high.

Function and characteristics of the scheme

With the national policy support for the digital transformation of state-owned enterprises and the promotion and application of 5g technology, our team seized the opportunity to break through the bottleneck, combined smart IOT technology with cable anti-external breaking work organically, and independently developed “intelligent brain – high voltage cable intelligent external breaking system based on artificial intelligence”.

Function 1
The full time visual monitoring device is applied to improve the real-time visibility of the equipment channel. The visual monitoring device is installed on the smart light pole in key cable channels and construction intensive areas, which can automatically identify the hidden dangers outside the cable channels such as large construction machinery.
The real-time data collected by camera is compared with the hidden danger image database of cable channel, and all kinds of external damage hazards on cable channel are automatically studied and identified.
At the same time, the system has the ability of autonomous learning, and with the deep use of the system, the recognition efficiency is improved.
Function 2
The fiber vibration technology is applied to improve the state perception of the equipment. The abnormal vibration caused by external force is automatically captured, and the warning is given before the external force endangers the power cable. According to the external vibration of optical fiber, the external force failure signal is automatically identified and the location of the hidden danger of external failure is accurately located.

Function 3
The application of “four-dimensional” visualization technology can enhance the visual operation and maintenance capability of underground cables. Realize the operation and maintenance personnel to stereoscopic view the cable lines and auxiliary facilities in VR form on site through PDA terminal, and guide the accurate operation.

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How to distinguish the rigidity or flexibility of mineral insulated cable

The structure of mineral insulated ABC Cables with different names is convenient for us to design and select better, and there is no confusion about many naming rules.

What are (rigidity) and (flexibility)? For a long time, the domestic mineral insulated cables are divided into two types: rigidity and flexibility.
The internationally used magnesium oxide mineral insulated cable (IEC 60702-1:2002) uses copper solid conductor instead of stranded conductor or soft conductor which is used in our common common cable.
Later, the following MI cables were developed by domestic production enterprises:
(1) It meets the industry standard: yttw series of jg/t 313 metal sheathed inorganic mineral insulated cable and terminal with rated voltage of 0.6/1kv and below (the release and update time of the standard is 2011 and 2014);
(2) It meets national standards: RTT series of gb/t 34926 mica tape mineral insulated corrugated copper sheath cable and terminal with rated voltage of 0.6/1kv and below (the standard release time is 2017);
(3) There are also some mica tape mineral insulated cables that meet the enterprise standards: non solid conductor.
All the above cables are generally classified into flexible mineral insulated cables in the industry, only to distinguish them from rigid MgO mineral insulated AAC Cables.
1. What form of (MI) cable is identified by the specification
The code for fire protection design of buildings GB 50016-2014, article 10.1.10 of the article description states that:
Mineral insulation non combustible cable is composed of copper core, mineral insulation material, copper and other metal sheath. Besides good conductivity, mechanical and physical properties and fire resistance, it also has good non flammability. Under fire conditions, this cable can ensure the fire power consumption within the fire duration.
That is to say, mineral insulation cable recognized by the specification must be made up of copper core, mineral insulation material, copper and other metal sheath as the basic composition.
BTT series rigid mineral insulated cable structure form
According to this requirement, we will see whether the structural forms of various rigid cables and flexible cables meet the requirements.
2. Rigid cable with unique naming rule (BTT)
BTT series mineral insulated cable is the only mineral insulated cable recognized internationally. It is a kind of cable with seamless copper tube sheath and dense compacted magnesium oxide crystal powder in the middle as the insulation material. The conductor is a single copper rod cable.
The reason for being recognized internationally depends on the material used is the real high temperature resistant material, among which the melting point of copper conductor and copper sheath is 1083 ℃, and the melting point of MgO mineral insulation layer reaches 2800 ℃, so that the line can still operate safely under fire temperature of more than 1000 ℃.
Because all the materials used are inorganic materials, and the structure is dense and hard, the product has the electrical, mechanical and environmental withstand properties that can not be compared with the conventional VV, YJV, yjy and other (organic cables).
It has high temperature resistance, fire prevention, explosion-proof and non combustion. It can run continuously for a long time at 250 ℃ and can also maintain 180min short-time operation under the limit state of 1000 ℃.
For BTT series Mi cables, the domestic implementation standard is gb/t 13033-2007 mineral insulated cables and terminals with rated voltage of 750V and below, and has not been updated so far. It can be seen from the release time of the specification that the stable state of product standards can be said to be the most mature Mi cable in technology.
BTT series rigid cables have good technical performance advantages, but also have problems such as difficult construction, short manufacturing length and complex intermediate joint processing technology. In order to solve the above problems, a variety of flexible cables have been developed in China.
This also brings us a lot of difficulties in the selection of engineering design.
3. Flexible cable named (bbtrz)
When the mineral insulated cable was introduced to China in the early stage, the copper sheath was often cracked because of the immature technology. Therefore, magnesium hydroxide was used as the sheath instead of copper sheath.
So (T) is changed to (b), and B is mineral.
The product structure consists of copper strand, mica tape insulation, irradiation crosslinked polyethylene water barrier, magnesium hydroxide fire protection sheath and low smoke halogen-free sheath.

Bbtrz, as a series of flexible mineral insulated cable models, has been highly recognized by many design institutes, owners and users, and has been widely used in various large-scale building fire lines in China.
Although the fire resistance tests of C, W and Z of the circuit integrity of bs-6387 can be passed, the mineral insulated cable does not use metal sheath such as () copper).
With the development of national comprehensive level, the structure of non combustible cable of mineral insulation is officially defined as “composed of copper core, mineral insulation material, copper and other metal sheath” in the code for fire protection design of buildings (GB 50016-2014) promulgated and implemented in 2015.
Because the series does not contain metal sheath, it has gradually withdrawn from the stage of fire protection application, and is no longer classified as “mineral insulation non combustible cable”.
4. Flexible cable named after [bttrz, yttw, rttz]
In fact, the three named Mi flexible cables have the same structure, all of which are called flexible rolled copper sheathed mica tape mineral insulated cables.
The product structure consists of copper pipe sheath composed of multi strand copper strand, mica tape mineral insulation wrapping, alkali free glass fiber dense filling and copper strip longitudinal welding.
Before the relevant domestic standards were not issued, the bttrz flexible cable, which was developed and named by the manufacturer, was mainly different from bttz rigid cable, which highlighted (soft), so it was added (R) on the basis of bttz naming.
Later, the domestic construction industry standard: jg/t 313 was promulgated and implemented in 2011, and bttrz named by the manufacturer was changed to yttw. However, this naming rule is not in line with the domestic cable naming method, and many people can not accept it.
Later, the national standard was promulgated and implemented by the National Standardization Commission in 2017: gb/t 34926, which named the former flexible cable rttz.
So bttrz = yttw = rttz. In addition, before the relevant standards were issued, the flexible cables were named by different manufacturers: bttw, Btte and btwtz, which can be described as various.
It is obvious that the flexible cables meet the requirements of gb50016-2014, and the fire resistance type test meets the most stringent requirements of C, W and Z of bs-6387.
5. Flexible cable named (ng-a, btly)
Ng-a and btly series cables belong to the same category. They use aluminum as the main material metal pipe extrusion instead of copper tube pull sheath, which not only simplifies the production process and improves the efficiency, but also reduces the product cost (aluminum is only 1/10 of the comprehensive cost of copper).
The product structure consists of copper strand, mica tape insulation, seamless aluminum tube sheath, oxygen insulation layer filling, isolation refractory layer and low smoke and halogen-free sheath.
The reason why aluminum tube can replace copper pipe is not melted under high temperature flame, which is due to the expanded refractory layer extruded by the outer part of the aluminum pipe.
It is foamed and solidified under the fire, forming a thick barrier to block the direct effect of flame on aluminum pipe. The insulation stability of mica tape can be improved (the insulation resistance will decrease with the increase of temperature).
According to the structure form and type test, it also meets the requirements of grade C, W and Z of gb50016-2014 and bs-6387. The insulation performance of mica tape inside the insulation layer (Mg (OH) 2 or Al (OH) 2 mineral material) on the outer layer of aluminum sheath has been improved.
However, ng-a (btly) series cables are not flexible cables specified in jg/t 313 and gb/t 34926 standards, and only enterprise standards are supported at present.
6. Summary
The above Mi cables are simply combed as follows:
(1) Rigid cable: BTT, structure form and product performance are in accordance with international and domestic specifications.
(2) Flexible cable: RTT and yttw, the fire resistance performance of the product meets the most stringent level tests of C, W and Z of bs6387, and the structure form meets the national specifications.
(3) Flexible cable: ng-a and btly, the fire resistance performance of the product meets the most stringent level tests of C, W and Z of bs6387, and the structure form meets the national standards, but only [enterprise standard] is supported.
From the above situation, the performance of flexible cable meets the requirements of the specification, and the length of a single cable is long and the construction is relatively easy. We will surely think, “can flexible cable replace rigid cable?”

Cable drawing principle and die matching

In the field of wire drawing, the sliding water tank wire drawing machine is widely used, that is, there is a gap between the speed of the drum and the AAAC Cables, so that the wire can slip on the contact surface with the drum, resulting in sliding friction, which drives the wire to realize drawing before and after each die.

The first is the efficiency of wire drawing production. Referring to the calculation of steel wire production efficiency, the key is the utilization rate of the machine, the size of the outgoing line, and the fastest take-up speed. If the production efficiency is calculated according to the number of kilograms per hour, then the production efficiency = take-up speed * cross-sectional area of copper clad steel * density of copper clad steel * machine utilization rate. Machine utilization refers to the actual full speed running time of the machine within 24 hours. If we get the maximum and minimum utilization error under the assumption of 100% utilization through statistics, or do classified statistics, then we can get the average error, so as to determine the efficiency evaluation of wire drawing production.
The second is the mechanism of ABC Cables drawing. Referring to the sliding drawing process of composite wire, we know that the metal plastic deformation is generally realized by the movement of dislocation on the sliding surface, and the polycrystalline deformation is also carried out by the coordination of various grains. Because of the complexity and inhomogeneity of grain boundary and the inhomogeneity of original crystal particles, the plastic deformation will not be absolutely uniform in the metal, which will affect the subsequent deformation of copper-clad steel wire.
During cold deformation, the metal will produce strain hardening effect. Because the strain hardening index of the copper layer is larger than that of the steel core, the strain hardening of the copper layer is more obvious in the drawing process (as the saying goes, hardening becomes faster), that is, the increased stress required for continuous deformation is higher. Therefore, in the drawing process of copper-clad steel, the copper layer will not be damaged under greater stress, At the same time, due to the existence of strain strengthening, the deformation tends to be uniform with the increase of deformation. Through research, Korean scientific and technological workers found that the working area angle and total deformation will lead to different changes in the proportion of copper layer, which is directly related to strain strengthening. In our company’s conventional production, through analysis and statistics, it is found that the change of copper layer can be almost ignored.

Thirdly, it is the working problem of the die. By studying the section diagram provided by the die supplier, we can know that the internal structure of the die is mainly divided into six areas: the entrance area, the lubrication area, the compression area, the sizing area, the safety angle and the exit area. The key is the yield of the compression area, the extrusion stress and the friction of the sizing area. The drawing stress is determined by the yield stress, compression ratio, working area angle, material friction coefficient and post tensile stress of copper clad steel. On the other hand, the yield stress of copper clad steel itself is also obtained by adding the yield stress of copper and steel in proportion.
Finally, through the tower wheel work on the equipment, the drawing is completed. As mentioned earlier, sliding wire drawing relies on sliding friction, that is to say, the movement speed of copper-clad steel on the tower wheel is less than the linear rotation speed of the tower wheel. In this way, it is always relaxed at the inlet end (the back tension is 0). Otherwise, if the inlet end is not tight, it will increase the back tension, thus increasing the front tension, which is easy to cause wire breakage. The final result is that the elongation coefficient of the wire through the drawing die should be greater than the gradient of the adjacent tower wheel, expressed as μ/ε> 1. In this way, during the drawing process, the wire is tightly wound on the tower wheel and moves forward synchronously, and sometimes it looses and slips. Of course, this will wear the surface of the tower wheel and increase the power loss.
The ratio of the linear speed of the tower wheel and the speed of the wire in drawing is called the sliding coefficient; The difference between the linear speed of the tower wheel and the speed of the wire during drawing is the absolute sliding amount; The ratio of the absolute sliding amount to the linear speed of the tower wheel rotation is called sliding rate; The cumulative slip coefficient is the multiplication of the slip coefficients of each pass, and the cumulative slip rate is 1-1 / cumulative slip coefficient.
The data show that the sliding coefficient is generally between 1.02-1.10. The copper-clad steel has good lubrication effect with the mold, and the relative wear with the tower wheel is also small. Therefore, some scholars suggest that the sliding coefficient should be within 1.01-1.04. We tend to 1.02.
In the actual drawing process, because each pass has preset sliding, the farther the pass is from the finished die, the greater the sliding between the tower wheel and the copper-clad steel wire, and the more serious the surface wear of the tower wheel. The uneven sliding will shorten the service life of the tower wheel. Therefore, a cumulative sliding effect should be considered, It spreads and accumulates continuously from the finished die to the incoming line. The higher the pass, the greater the slip and the more serious the wear. At the same time, the thicker the wire diameter is, the greater the drawing load is, the greater the power loss is, and the more serious the damage between the wire and the tower wheel is, which leads to the tower wheel grinding out of the groove, or the wire is thrown up to drive the mold shaking during the drawing, and the wire is not evenly stressed, Bamboo like or broken.
The tower wheel gradient (also known as tower pole ratio) of our ordinary wire drawing machine is about 10-12%. With the sliding rate, the ratio is generally set at 13-15%. According to the diameter of the outgoing line of the adjacent die, we can directly calculate the surface reduction rate or elongation, or vice versa, we know the size of a die and the required elongation, The size of the last die can be calculated. It is worth mentioning that when drawing the flexible cord, the local compression of the die must not be too large, otherwise the tension of the constant speed wheel will strain the flexible cord, resulting in the reduction of the wire diameter and the decline of the extension.

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What should be paid attention to when storing wire and cable reel?

Attention should be paid to the following points when storing and transporting wires and AAAC Cables on shelves:
1、 In storage, first of all, it is forbidden to contact with acidic, alkaline and corrosive substances and store them separately. Insulated wires and cables should be stored in a dry and ventilated warehouse. The suitable temperature in the warehouse is 5-30 degrees, and the relative humidity should not exceed 75%. The glass on the warehouse window should be painted with white paint to avoid direct sunlight and premature aging of insulation layer.

2、 There shall be no harmful gas that can damage the insulation layer or corrode the conductor metal around the wire and cable storage place.
3、 Avoid storage in open air to prevent accidental damage.
4、 Small quantities of wires can be stacked in the shelf, and the stack height should be 5-10 rolls. Large quantities of wires can be stacked into plum blossom style ventilated round stacks. Single core wire and cable can be appropriately high code; The stack height is 2 meters in spring and winter and 1.5 meters in summer and autumn. When stacking, the pallet should be padded with sleepers, and the sleepers should be paved with planks, and two layers of moisture-proof paper should be added, and good ventilation should be maintained. During the storage of wires and cables, they should be stacked once every three months, and the upper and lower layers should be replaced to prevent long-term compression and deformation of the lower layer. Always check whether the sealing package of wires and cables is intact during storage.
5、 The storage life of wires and ACSR Cables shall be subject to the date of production. Generally, the storage life shall not exceed one and a half years, and the longest shall not exceed two years. Pay attention to the change of temperature and humidity in the process of storage, and take cooling measures immediately if the temperature in the warehouse exceeds 30 ℃. If the relative humidity reaches 75% for 3 consecutive days, desiccant or other measures should be taken to absorb moisture.

6、 It is strictly forbidden to drop wires and cables from high places during transportation, so as to avoid cracking of insulation layer and sheath layer, damaging the electrical and mechanical properties of wires and cables.
7、 During hoisting operation, it is strictly forbidden to hoist multiple coils of wires and cables at the same time. On vehicles, ships and other means of transport, wires and cables must be laid flat and properly fixed to prevent collision or overturning and damage to the outer or internal structure of wires and cables.

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