How to prevent cable from fire due to conductor overload

During the operation of wires and AAC Cables, they will heat due to the existence of resistance. The resistance of the conductor is generally very small, and its heating power can be expressed by the formula q = I ^ 2R. Q = I ^ 2R shows that for a section of conductor in actual use (r has been basically constant), the greater the current passing through the conductor, the greater the heating power; If the electric flow is constant, the heating power of the conductor is also constant. The heat released during operation will be absorbed by the conductor itself, resulting in the increase of conductor temperature. Although the conductor keeps absorbing the heat released by current work during operation, its temperature will not rise indefinitely. Because the conductor is absorbing heat and constantly releasing heat to the outside world, the facts show that the temperature rises gradually after the conductor is powered on, and finally the temperature is constant at a certain point. At this constant point, the heat absorption and release power of the conductor are the same, and the conductor is in thermal equilibrium. The ability of conductor to withstand high temperature operation is limited, and operation beyond a certain maximum temperature will be dangerous. This maximum temperature naturally corresponds to a maximum current. If the conductor exceeds this maximum current, it is overload. The overload of the conductor directly leads to the temperature rise of the conductor itself and its nearby articles. Temperature rise is the most direct cause of this kind of fire.

Overload destroys the insulation layer between double stranded conductors, causing short circuit, burning equipment and fire. The double strand conductors are separated by the insulating layer between them. Overload softens and destroys the insulating layer, resulting in direct contact between the two strands of conductors, causing short circuit and burning the equipment. At the same time, the high temperature generated by large current at the moment of short circuit causes the line to catch fire and fuse, and the generated molten beads fall to combustibles to cause fire. Overload temperature rise can also directly ignite nearby combustibles. The heat transfer of overload conductor increases the temperature of nearby combustibles. For combustibles with low ignition point nearby, it is possible to ignite them and cause a fire. This danger is particularly prominent in warehouses for storing inflammables and buildings with easy and combustible decoration.
Overload also puts the connection in the line under overheated conditions, which accelerates its oxidation process. Oxidation causes a thin layer of oxide film that is not easy to conduct electricity at the connection point. The oxide film increases the resistance between the contact points, resulting in ignition and other phenomena, resulting in fire.
So, how to prevent fire caused by overload of wires and ACAR Cables?
1. In the process of line design, the capacity of the site shall be accurately verified, the possibility of new capacity in the future shall be fully considered, and the appropriate type of conductor shall be selected. For large capacity, thicker conductor shall be selected. Line design and reasonable type selection are the key steps to prevent overload. If the design and selection are improper, it will leave congenital hidden dangers that are difficult to rectify. Some small projects and places are not carefully designed and selected. It is very dangerous to choose and lay lines at will. The bearing capacity of the original line shall be fully considered for new electrical appliances and electrical equipment. If the original line does not meet the requirements, it should be redesigned and reconstructed.

2. The line shall be laid by qualified electricians in accordance with relevant specifications. The laying conditions of the line directly affect the heat dissipation of the conductor. Generally speaking, the line laying shall not pass through inflammable and combustible materials and stacking, which will lead to poor heat dissipation of the conductor, heat accumulation, the possibility of igniting the surrounding combustible materials, and increase the risk of fire under overload; The lines laid in the decoration ceiling of public entertainment places shall be protected by steel pipes to separate the ceiling from the line. Even if there are molten beads under overload, short circuit and other conditions, they will not fall, so as to avoid fire.
3. Strengthen power management, avoid random wiring and grounding, and use mobile sockets carefully. Random ACSR Cables, random grounding and the use of mobile sockets are actually adding electrical equipment to a certain section of the line, increasing the amount of current, which may cause overload. The jack of mobile socket is obviously more than the fixed socket on the wall. If too many electrical equipment are used on the mobile socket, the original line must be difficult to bear. For high-power equipment and electrical appliances, separate lines should be set, and mobile sockets should not be used as wiring sources.

4. Speed up the renewal and transformation of old lines and eliminate potential fire hazards. Due to the long service life of old enterprises, old residential areas and other units, many lines have aged and exceeded the service life. Even if the current carrying capacity of some lines is small, it is difficult for aging lines to bear such current carrying capacity, and it also has the risk of overload. Especially in the old residential areas, the lines have long been aging, but with the improvement of people’s living standards and the increase of household appliances, their power consumption is still increasing year by year, which is really worse. For old lines, timely supervision and coordination shall be carried out to promote their rectification as soon as possible, so as to eliminate potential fire hazards and ensure safety.

Is the zero line current greater than the live line?

In low-voltage distribution system, the current of zero line is smaller than that of live line. This is because when the three-phase load is balanced, the three-phase current is completely offset, and the zero line current is zero; However, when the AAC Cables consumption of three-phase load is unbalanced, the three-phase current cannot be completely offset, then the unbalanced current will return to the transformer through the zero line.
In three-phase four wire distribution system, the current of zero line is generally smaller than that of live line, which is the consensus of all people in the industry. However, more and more facts make people subvert this concept.
One day in class, a student called to ask me a question.
Some time ago, he received the installation of wall advertising lines in a building. All the advertising light boxes around the building are illuminated with fluorescent lamps, nearly 1200.
The power supply of the advertising light box is led from the distribution box by four wires (three 25 square meters and one 16 square meters), and 1200 fluorescent lamps are evenly distributed in three-phase lines.
After connection, the power on test is normal and the acceptance is passed. But after a while, it was found that the zero line was burnt out. At that time, I thought the zero line was too small, and then I replaced it with a 25 square one. ACSR Cables was like this for some time. Before long, the zero line burned out again.
The customer is very dissatisfied. If this problem is not solved again, he may not get the balance payment.
I calmed his emotions while thinking. According to my many years of experience, this situation is most likely due to excessive zero line current due to harmonics.
I asked him to measure the current of each wire, and finally measured it with a clamp meter. He was stunned!
The three-phase live line current is almost 90a per phase, but the zero line current reaches an amazing 158a.
In fact, the phenomenon of excessive zero line current is becoming more and more common. Why is there still current on the zero line when the three-phase power load is balanced, and the current reaches more than 150% of the phase line current?
This is caused by harmonics.
We all know that the alternating current we use now is sinusoidal alternating current with frequency of 50Hz.
The waveform of sinusoidal alternating current is as follows:

sinusoidal alternating current
When sinusoidal alternating current is mixed with higher frequency clutter, this is called harmonic.
The figure mixed with harmonics is as follows:

The picture is a sinusoidal alternating current waveform mixed with harmonics
In the above figure, red is our normal sinusoidal AC waveform, in which green and blue are harmonic waveform.
AC non sinusoidal signals can be decomposed into linear combinations of sinusoidal components of different frequencies. When the frequency of the sine wave component is the same as that of the original AC signal, it is called the fundamental wave. Harmonic, in a narrow sense, refers to the electric quantity whose frequency is an integral multiple of the fundamental wave contained in the current. Generally, it refers to the Fourier series decomposition of periodic non sinusoidal electric quantity, and the electric quantity generated by other currents greater than the fundamental wave frequency. Broadly speaking, since the effective component of AC power grid is a single frequency of power frequency, any component different from power frequency can be called harmonic. When the frequency of the sine wave component is a non integer multiple of the frequency of the original AC signal, it is called fractional harmonic, also known as fractional harmonic or interharmonic. For any compound periodic vibration function y (T), it is expressed by Fourier series decomposition: the first term is called mean or DC component, the second term is fundamental wave or basic vibration, the third term is called second harmonic, and so on, or the following harmonics are collectively referred to as higher harmonic.
Harmonic generation
When the current waveform of three-phase alternating current is sine wave, they differ by 120 ° and have the same amplitude. The result of vector superposition on the zero line is that the sum is zero.
However, if the current on the phase line is pulsed and the difference is 120 °. Then, the pulses superimposed on the middle line are staggered, so they cannot be offset, and the current of the zero line is superimposed.
Since most modern electrical equipment contain rectifier circuits, even if the three-phase load is balanced, there will be a large current on the zero line.
The harm of excessive zero line current is very serious, mainly due to two reasons,
First, the cross-sectional area of the zero line is not larger than that of the phase line. The current exceeding the phase line will inevitably cause the zero line to overheat and burn out;
Second, when the zero line is disconnected, because the three phases are not completely balanced, the voltage of each phase will be inconsistent and the three-phase equipment can not work normally. The ABC Cable of one phase with less power will rise; The voltage of one phase with more electricity will be reduced. Single phase equipment on one phase of voltage rise will be damaged; Single equipment on one phase with reduced voltage cannot work normally.

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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Discussion of the cable production process: conductor stranding

Discussion on the cable production process: conductor twisting and twisting
: the process of twisting many small diameter monofilaments into a large cross-section conductive core according to certain rules.
1. There are two types of twisted wires: normal twisted wires and irregular twisted AAC Cables.
Ordinary strands can be divided into ordinary concentric single stranded strands and ordinary concentric single stranded strands
(1) Ordinary stranded wire: single wires of the same diameter are twisted layer by layer according to concentric circles, and the direction of each layer is opposite.
(2) Combined stranded wire: made of a single wire of the same diameter, different materials or different diameters and different materials (representative products (such as overhead conductors))
Ordinary concentric stranded wire: a multi-stranded ordinary stranded wire or bundle Stranded wire concentrically stranded.
Irregular stranding (strand): strands formed by multiple single strands in the same direction, which does not comply with the law of twisting. The positions of the single strands are not fixed to each other, and the shape of the strands is difficult to maintain Round.

2. The biggest difference between the bundle wire and the ordinary stranded wire is that each single wire of the ordinary stranded wire has a fixed position and is regularly twisted layer by layer. There is no fixed position between the single wires of the bundle According to the law of twisting, the position will not be twisted together.
3. The characteristics of irregular twisting (bundling) : Since each single wire in the bundle is twisted in one direction, there is a residual amount of sliding between each single wire during bending. The amount is large and the bending resistance is small, so the bending performance of the bundle is particularly good. For wire and cable products that need to be flexible and frequently moved, the wire bundle is used as the conductor core.
4. The characteristics of the stranded core:
(1) Flexibility Good; by using a core composed of several single wires with smaller diameters, the bending resistance of the cable can be improved, and the processing, manufacturing, installation and laying of the wires and cables are convenient.
(2) Good stability; the core is It is composed of multiple single wires twisted according to a certain direction and twisting rules, because each single stranded wire is located in the stretched area above the twisted wire in the twisted wire, and is located in the lower compressed area When the stranded wire is bent in sequence, the stranded wire will not be deformed.
(3) Good reliability; due to material inhomogeneity or defects in twisting, using a single wire as the cable and the conductor of the cable easily affects the reliability of the conductor core. The defects of the conductor core formed by multiple single wires are scattered and will not be concentrated on a certain point of the conductor, so the reliability of the conductor core is much stronger.
(4) High strength; the strength of the single-stranded core is higher than that of the single-stranded core.

5. Explanation of terms:
(1) Pitch: the distance that a single filament advances one circle along the axis.
(2) Pitch diameter ratio: the ratio of the pitch length to the strand diameter.
(3) The relationship between the pitch and the flexibility of the strands: the smaller the pitch, the better the flexibility of the strands; on the contrary, the larger the spacing, the worse the flexibility of the strands.
(4) Stranding factor: the ratio of the actual length of a single wire to the pitch length in the pitch of the twisted wire.
(5) Twisting direction of the stranded wire: right direction (z direction) left direction (s direction)
(6) Compact conductor: common compact conductors are compact round, sector and compact tile-shaped (five-core cables) Semi-circular (two-core cable)
6. Compression purpose:
(1) Compact sector conductor: reduce the outer diameter of the cable, save product costs, and reduce the weight of the cable.
(2) Compact circular conductor: Improve the surface quality of the stranded conductor, reduce the diameter of the conductor, and increase the filling factor of the conductor. The compacted conductor surface is smooth without burrs, and the electric field on the conductor surface is uniform. Save materials and reduce costs (to learn more about cable technology, please click here. A lot of dry goods are waiting for your visit.)
7. Conductor classification:
According to GB/t3956 “Cable Conductor”, there are four types of conductors, namely the first One, the second, the fifth and the sixth. The first is a solid conductor, and the second is a stranded conductor, both of which are suitable for fixed laying of cables. The fifth and sixth types are stranded conductors, which are used for flexible cables and cords. The sixth is softer than the fifth.
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From January to March, China’s optical cable output increased by 25.2% year on year

The National Bureau of statistics released the output and growth rate of China’s major industrial products from January to March. Among the 100 major industrial products listed, except for fax machines, only 4 products showed a year-on-year decrease in output from January to March 2020. Among them, the cumulative output of abc cables was 63.996 million core kilometers, a year-on-year increase of 25.2%.

The cumulative output of pig iron was 220.971 million tons, up 8% year on year; The cumulative output of crude steel was 271042000 tons, with a year-on-year increase of 15.6%; The cumulative output of steel products was 329.401 million tons, a year-on-year increase of 22.5%; The cumulative output of ferroalloy was 9.021 million tons, a year-on-year increase of 4.7%; The cumulative output of alumina was 19.23 million tons, a year-on-year increase of 13.7%; The cumulative output of refined copper (electrolytic copper) was 2.487 million tons, with a year-on-year increase of 15.2%; The cumulative output of primary aluminum (electrolytic aluminum) was 9.758 million tons, up 8.8% year on year; The cumulative output of aluminum alloy was 2.266 million tons, with a year-on-year increase of 38.6%; The cumulative output of copper products was 4.516 million tons, a year-on-year increase of 14.8%; The cumulative output of aluminum products was 13.887 million tons, a year-on-year increase of 31.1%.
The cumulative output of generating units (generating equipment) was 31.319 million KW, up 55.5% year on year; The cumulative output of optical fiber cables was 63.996 million core kilometers, with a year-on-year increase of 25.2%; The cumulative output of lithium-ion batteries was 4790.314 million, with a year-on-year increase of 83.4%; The cumulative output of solar cells (photovoltaic cells) was 46.602 million kilowatts, up 78.2% year on year.
In March, among the 100 major industrial products listed, except for fax machines, only 8 products showed a year-on-year decrease in output compared with March 2020. Among them, the output of optical cable was 26.213 million core kilometers, up 14.2% year on year.
Pig iron output was 74.745 million tons, up 8.9% year on year; Crude steel output was 94.021 million tons, up 19.1% year on year; The steel output was 119.872 million tons, up 20.9% year on year; Ferroalloy production was 3.095 million tons, up 1% year on year; The output of alumina was 6.537 million tons, up 11.3% year on year; The output of refined copper (electrolytic copper) was 870000 tons, a year-on-year increase of 18.2%; The output of primary aluminum (electrolytic aluminum) was 3.276 million tons, up 8.5% year on year; The output of aluminum alloy was 870000 tons, a year-on-year increase of 23.9%; The output of copper products was 1.815 million tons, a year-on-year decrease of 2.9%; Aluminum production was 5.367 million tons, up 17.6% year on year.
The output of generating units (generating equipment) was 12.429 million KW, up 17.6% year on year; The output of optical cable was 26.213 million core kilometers, up 14.2% year on year; The output of lithium-ion batteries was 1929.547 million, up 52.7% year on year; The output of solar cells (photovoltaic cells) was 18.336 million KW, up 52.4% year on year.

Single-core Parallel Type and Multi-core Twisted Type Cable

The advent of power cables has greatly improved the safety of power generation, transmission, transformation, supply, distribution, and use of electricity. Single-core power cables appeared first. As there are more and more occasions for three-phase four-wire power supply, three-phase five-wire power supply and multi-loop power supply in actual use, the requirements for occupied space and laying occasions are also getting higher and higher. When multiple and multi-layer laying are required, and the space occupied and laying conditions are limited, single-core power cables cannot be used conveniently. Therefore, multi-core power cables have been developed and quickly entered the field of power applications, and are accepted and used by the majority of users.

With the rapid growth of power cable usage, even if single-core power cables are used in actual use, the joints and branches shall be stripped and insulated on site, and the branches or joints shall be crimped before using epoxy resin insulation The method of material encapsulation treatment still has disadvantages such as large site occupation, long construction time, high cost, multiple equipment, high technical requirements, and high difficulty, especially the joints or branches after the completion of the on-site construction, and their insulation Strength, reliability, and consistency are difficult to guarantee. Therefore, the busway was developed, and after the development was put into the market, it was quickly accepted and used by a large number of users.
With the increase in user usage, it is found that the bus duct also has some defects, such as too many parts connected by * screws, complicated installation and construction, and large maintenance and high maintenance costs. In the process of operation, it often encounters the influence of electromagnetic vibration, thermal expansion and contraction, expansion coefficient, external force and other factors, which will cause the loosening of the screw. If a screw is loose, there will be heat and high temperature at the fault point, which will affect the stability of the entire bus duct. In particular, the improper use of the five-wire bus duct will also cause the contact resistance of the PE wire to increase. It violates the basic requirements for the continuity of the PE wire that is clearly stipulated in the building electrical design code and construction code. However, bus ducts still have their own advantages in the case of large capacity. Because when the current reaches thousands of amperes, if a cable is used, even a single-core cable must be laid in multiple, otherwise the corresponding large current capacity will not be reached, and the busway will show its own advantages at this time.

2. Prefabricated branch cable
With the development of technology and the increasing market demand, prefabricated branch cables have been developed and developed from single-core prefabricated branch cables to multi-core prefabricated branch cables, which also include flame-retardant, fire-resistant, and armored prefabricated branch cables. That is, in the factory, in accordance with the cable specifications, models, cross-sections and specific locations of the branches specified in the architectural design drawings, special production equipment and molds are used on the professional production line, which can be completed in one time. Its advantage lies in the high insulation strength, the encapsulated injection-molded branch joint connector and the outer insulating sheath of the cable are tightly bonded together, and it has excellent water tightness and air tightness. Because it is made of factory-specific equipment and molds, It has excellent reliability and consistency, and it is extremely convenient for users to install and use. When installing vertically, only the main cable can be evenly fastened to the corresponding bracket. In horizontal, pre-buried, overhead, tunnel, airport runway, port Construction and installation under environmental conditions such as docks, mines, and modern industrial plants are simpler and more convenient. The requirements for site, equipment, technical level of construction personnel, and costs during the installation and construction process are much lower than those for handling cable branches or cables at the construction site. Busway: It saves a lot of maintenance costs during operation and reduces the power outage time. In some cases, it can achieve the effect of maintenance-free. In the case of small cable shafts and cable channels, it can show its unique advantages. Therefore, branch cables are ideal products to replace bus ducts in medium and small capacity power supply occasions.

3. Stranded and twisted prefabricated branch cables
With the acceptance of prefabricated branch cables by the majority of users, a variety of prefabricated branch cables have entered the market under the situation of rapid increase in usage. This situation has brought a certain degree of selection to building electrical designers. difficult. The choice of stranded type, also called twisted type, branch cable is introduced below.

4. Multi-core prefabricated branch cable
The multi-core power cable is the core wire conductors are individually insulated, and are collectively and parallelly enclosed in the same outer sheath. Inside the outer sheath of the entire power cable, whether it is armored or unarmored, all power cable core wires that have been independently insulated and encapsulated are parallel and tightly encapsulated in the outer insulation. Set of interior. During the entire encapsulation process of the outer insulating sheath, no part or any power cable is allowed to “cross”, “displace”, or “twist” inside the sheath. Of course, “twisting” is not allowed. If any of the above phenomena occurs, the entire power cable will be judged as “unqualified” and not allowed to leave the factory. Therefore, the multi-core power cable itself is a qualified product that uses special production equipment in a professional production plant and strictly follows the relevant standards. Regardless of the distribution of inductance and capacitance and the basic requirement that the vector sum of any part is equal to zero, good consistency can be guaranteed. This cannot be achieved by any “stranded conductor type” or “twisted type”, and is a basic requirement that must be guaranteed on the power distribution circuit. Therefore, prefabricated multi-core branch cables made of qualified multi-core power cables can ensure that various technical parameters and basic requirements will not be destroyed, and can ensure the stability and reliability of operation. Of course, compared with single-core prefabricated branch cables, multi-core prefabricated branch cables are much more complicated and difficult regardless of the required technical level, the complexity of the manufacturing process, and the production equipment and investment.

In summary, the use of “twisted” or “twisted” prefabricated branch cables in the three-phase three-wire, three-phase four-wire, and three-phase five-wire power supply loops is used as the power supply loop to ensure the safety of its operation. And stability are extremely disadvantageous. The author also consulted related materials, regulations, specifications, standards such as IEC, NEC, etc., which clearly stipulated the distance between cables and cables when laying in parallel, and the correction coefficient of the current carrying capacity of cables at various distances. The author believes: This is also enough to explain the use of power cables. The series of processes from design to laying must fully consider the distribution of capacitance and inductance during operation, electromagnetic attraction, repulsion, vibration, etc.; the temperature rise during operation (capacitance And the distribution of inductance, electromagnetic attraction, repulsion, vibration, etc. are all part of the’temperature rise’) and heat dissipation environment and conditions. Therefore, only stipulating the different correction factors for the ampacity at different distances is sufficient to explain the nature and essence of the problem. What’s more, these are facts proved by theory and practice