Analysis of Operation Failure of Armored Cable

At present, there are various on-site treatment methods for the armor layer of low-voltage direct-buried cables, including single-ended grounding and two-end grounding. There are also those that are not grounded at both ends. According to the different treatment methods of the steel tape armor at both ends of the field cable, after the cable fails, the appearance of the fault point will be different.
The steel strips at both ends of the cable are all suspended and not grounded. After a short-circuit fault occurs in the cable, the breakdown point may only be a burn-through hole in a local location of the cable line, which will not cause long-distance and large-area burning and carbonization. Because when the cable is partially damaged by accidental mechanical damage and the sheath insulation is damaged, the system may not immediately trip and power off. The damaged point will cause intermittent flashover discharge on the earth due to the action of moisture and moisture in the soil, which will eventually develop into Permanent grounding and short-circuit between phases cause a trip and power failure. Since the discharge current of the live wire to the ground is limited to the location of the damaged point of the cable, the discharge current does not form a branch circuit to the ground through the steel belt, so there is generally only one point in the entire cable after the cable fails. malfunction. However, the surface of the armor layer will be electrified at this time. In consideration of safe use of electricity, the armor layer exposed at both ends of the cable must be insulated and sealed.


The steel belt of the cable line adopts single-ended grounding or double-ended grounding. After a short-circuit fault occurs in the cable, the fault may be a section of the cable, and the local area of ​​the cable may have long-distance surface burnt and carbonized adhesion. Because after the steel belt adopts this connection method, when a single-phase ground fault occurs in the cable, a relatively large ground short-circuit current will flow in the steel belt of the cable; at the same time, the three-phase load current of the cable will also appear unbalanced. Eddy current may also occur in the belt. After the two currents flow through the steel belt, the steel belt will be like a high-power electric furnace, heating the cable sheath and insulation, and the customer’s switch selection is not appropriate, and the soil is localized. Poor heat dissipation, excessive thermal resistance, partial accumulation of cable coils, poor heat dissipation and other unfavorable reasons may cause long-distance and large-area burning and carbonization of the cable insulation and sheath. The burned area is relatively random. It may be near the fault point, or in another section. It is often the section with the most difficult heat dissipation and the section with the largest thermal resistance that burns the most. The system may not trip until the single-phase grounding develops into a two-phase short circuit and cannot be reclosed to transmit power.


For low-voltage cable armored cables, it is necessary to strengthen the real-time online detection and monitoring of the three-phase current of the cable. At the same time, after the armor layer is grounded, an armor layer current transformer should be installed to monitor the current of the steel strip from time to time. The single-phase grounding short-circuit fault of the cable should be detected and dealt with in advance to avoid the long-distance burning of the cable and cause unnecessary power economic loss, and to ensure the economy, reliability, stability and safety of the grid operation.
According to normal analysis, after a short-circuit fault occurs in a directly buried low-voltage cable, there should generally be only one fault point. However, during the actual field excavation and processing of cable fault points, it is found that low-voltage cable faults may have two or more fault points, and may also be accompanied by long-distance insulation sheath heating and burning and carbonization adhesion. The author believes that the difference in the failure of low-voltage armored cables may be related to the grounding or non-grounding of the cable armor, and the opinions and opinions may not be correct. It is hoped that professionals who have sincere insights into such phenomena can provide more scientific and authoritative analysis and opinions. To uncover the underlying cause of this phenomenon.

What are the requirements for environmental protection cables

As the second largest manufacturing industry in China after automobile manufacturing, the wire and cable industry should also play a leading role in environmental protection. In recent years, environmentally friendly cables have become more and more popular in the market. In developed countries such as the European Union, non-environmental protection has been banned. Cables, China also attaches great importance to this aspect. China’s relevant laws and regulations explicitly require important buildings to prohibit the use of polyvinyl chloride wires and cables. Halogen-free and low-smoke cross-linked PVC wires and cables must be used to avoid a large amount of smoke and chlorine in the event of a fire. Casualties.
Environmental protection cables are the main direction of the future development of the cable industry, which can effectively enhance the competitiveness of cable companies. Because of the many excellent characteristics of environmental protection cables, they are also favored by more and more customers in the market.


Compared with ordinary cables, environmental protection cables have many excellent characteristics, such as no heavy metals, no halogens, no corrosive gas, no soil pollution, etc. If summed up, the requirements of environmental protection cables can be roughly divided into three aspects.
The first is that it does not contain heavy metals. Common PVC cables often contain heavy metals such as lead, cadmium and barium, which are harmful to the human body. Later, some countries stipulated that the content of eight types of heavy metals should not be higher than the specified value, and one of the main advantages of environmentally friendly cables It does not contain heavy metals, which is also its popular selling point in the market.
The second requirement of environmentally friendly cables is low-smoke and halogen-free. When ordinary cables are burnt, they will release a large amount of smoke and toxic gases, which will seriously pollute the air environment and cause harm to the human body. The low-smoke and halogen-free characteristics of environmentally friendly cables can not only protect the environment, but also It can help people evacuate and carry out fire rescue work when a fire occurs.


Secondly, it must be non-toxic. All component materials used in PVC formulations should be non-toxic. Cable material contains a lot of plasticizers, but non-toxic plasticizers must be used to make non-toxic materials. Therefore, this PVC material has higher requirements than lead-free and heavy metal-free materials, and the price is naturally more expensive.
Many countries have different requirements for environmental protection cables. For example, the European Union will restrict environmental protection cables containing asbestos. my country has not been as perfect as the EU in terms of related policy formulation and mandatory application. In this regard, China still has a long way to go. The way to go.

Difficulties in locating cable faults

Due to changes in the environment in which Power Cables are laid and the application of new materials for cables and their accessories in the cables, it is increasingly difficult to locate faults on the power cable site.
The difficulty of field cable fault location is mainly reflected in the fault location of directly buried power cable lines. At present, despite the relevant classic technical literature and advanced fault detectors, it is still available for high resistance faults in power cable lines. However, when using a professional cable fault locator to locate on site, sometimes you will encounter some special and difficult faults that cannot be located. For example, using a professional cable fault locator, part of the creepage flashover fault on the insulation surface of the medium voltage cross-linked power cable terminals and intermediate joints, and accurately determining the metal short-circuit fault, often appears powerless or powerless.

For common cable faults, you can use a cable fault locator purchased on the market to determine the location of the fault point within minutes or hours. However, when you encounter special difficult faults and the detection is not stable, you may need to call multiple cable fault detectors with different functions to repeat the test, and take turns to detect, locate, compare and confirm. The types of these fault devices mainly include various cable fault detectors designed, manufactured and developed based on the principles of bridging method and wave method. In this way, it may take several days or even longer to locate the fault. In this way, if you are lucky, you can determine the location of the fault point. If you are unlucky, the location of the fault point is still uncertain.
In northern China, the ground freezes in winter, and the directly buried cables fail at this time. The actual fault detection and handling process is actually a difficult task. First of all, the cable fault locator used must have high accuracy, and secondly, the corresponding personnel must have a clear understanding of the actual cable laying route. Although some cable fault detection instruments are now equipped with cable path testers, they must also be equipped with cable path testers. Only on-site personnel who understand the approximate laying path of the cable can cooperate to improve the positioning accuracy. The actual handling of cable faults sometimes depends on man-made three points and machine-made seven points.


At present, there are many manufacturers selling cable fault detection instruments on the market, and there are many types of detection instruments, but in fact, it is impossible to locate all cable faults. In actual use, the instrument usually can only effectively locate one or several types of faults, but still cannot do anything about some faults. The current electric power department hopes to spend a lot of money to purchase a universal cable fault tester with complete functions and high positioning accuracy (including rough and precise measurement points) to quickly and effectively solve all actual cable faults. But it’s actually hard to buy. There are various updated cable fault detectors on the market. However, the actual on-site inspection will still encounter some technical problems that cannot be located using the cable fault table. I think the reasons are mainly from two aspects: First, the various insulation, filling and wrapping materials used in the cable and its accessories are constantly being developed and updated, which leads to continuous changes in the types of cable failures. The other is that the market demand for cable fault detectors is limited, and related R&D personnel are scarce, resulting in a delay in the start of portable, high-precision, intelligent and multi-functional cable fault detectors. It is believed that with the advent of the smart grid era and the rapid development of Aerial Bundled Cable ASTM B231 Standard fault detection technology, the location of cable faults will become very simple and easy.

Comprehensive performance comparison of copper and copper clad aluminum wire

Copper-clad aluminum is formed by concentrically cladding a copper layer on the surface of aluminum or aluminum/steel alloy core material, and the thickness of the copper layer is 0.55mm or more. Due to the skin effect characteristics of high-frequency signal transmission on the conductor, the cable TV signal is transmitted on the surface of the copper layer above 0.008mm. The copper-clad aluminum inner conductor can fully meet the signal transmission requirements, and its signal transmission characteristics are the same as those of copper wires with the same diameter. The body is consistent.

Copper-clad aluminum and pure copper can be compared in the following three aspects:

Mechanical characteristics:

The strength and elongation of pure copper conductors are larger than copper-clad aluminum conductors, which means that pure copper is better than copper-clad aluminum in terms of mechanical properties. From the perspective of cable design, pure copper conductors have the advantage of better mechanical strength than copper-clad aluminum conductors, which are not necessarily required in the actual application process. Copper-clad aluminum conductors are much lighter than pure copper, so the overall weight of copper-clad aluminum cables is lighter than pure copper conductor cables, which will bring convenience to cable transportation and cable erection. In addition, copper-clad aluminum is a little softer than pure copper, and cables produced with copper-clad aluminum conductors are a little better than pure copper cables in terms of flexibility.

Electrical performance:

Because the conductivity of aluminum is worse than that of copper, the DC resistance of copper-clad aluminum conductors is larger than that of pure copper conductors. Does this affect the main 2014 high school entrance examination sprint comprehensive review guidance Beijing area test questions Guangdong area test questions Jiangsu area test questions to see if the cable will be damaged Using power supply, such as providing power to the amplifier, if it is used for power supply, the copper-clad aluminum conductor will cause additional power consumption, and the voltage will drop more. When the frequency exceeds 5MHz, there is no obvious difference in the AC resistance attenuation under these two different conductors. Of course, this is mainly due to the skin effect of high-frequency current. The higher the frequency, the closer the current flows to the surface of the conductor. The surface of the copper-clad aluminum conductor is actually pure copper. When the frequency is high, the entire current is plated. It flows in the copper material. In the case of 5MHz, the current flows in a thickness of about 0.025 mm near the surface, and the thickness of the copper layer of the copper-clad aluminum conductor is about twice this thickness. For coaxial cables, because the transmitted signal is above 5MHz, the transmission effect of copper-clad aluminum conductor and pure copper conductor is the same. The attenuation of the actual test cable can prove this point.

Copper-clad aluminum is softer than pure copper conductors, and it is easy to straighten during the production process. Therefore, to a certain extent, it can be said that cables with copper-clad aluminum have better return loss indicators than cables with pure copper conductors.

Economy:

Copper-clad aluminum conductors are sold by weight, and pure copper conductors are also sold by weight. The price of copper-clad aluminum conductors is more expensive than pure copper conductors of the same weight. However, the copper clad aluminum of the same weight is much longer than the pure copper conductor, and the cable is calculated according to the length. The copper clad aluminum wire of the same weight is 2.5 times the length of the copper wire, and the price is only a few hundred yuan per ton. Taken together, copper-clad aluminum is very advantageous. Because the copper-clad aluminum cable is relatively light, the transportation cost and installation cost of the cable will be reduced, which will bring a certain degree of convenience to the construction.

Ease of maintenance:

The use of copper-clad aluminum can reduce network failures and prevent network personnel from “cutting the core in winter and cutting the skin in summer” during maintenance (aluminum strip longitudinal packaging or aluminum tube products). Due to the large difference in thermal expansion coefficient between the copper inner conductor and the aluminum outer conductor of the cable, in the hot summer, the aluminum outer conductor stretches greatly, and the copper inner conductor is relatively retracted, and cannot fully contact the elastic contact piece in the F header; in severe cold In winter, the aluminum outer conductor shrinks greatly, causing the shielding layer to fall off. When the coaxial cable uses a copper-clad aluminum inner conductor, the difference in thermal expansion coefficient between it and the aluminum outer conductor is small. When the temperature changes, the cable core-pulling failure is greatly reduced, which improves the transmission quality of the network.

In general, the overall performance of copper-clad aluminum conductors is better than pure copper conductors, which will save users’ costs.

Insiders believe that the use of copper-clad aluminum wire in the wire and cable industry is also a good way to relieve the current pressure on enterprises. The aluminum wire is covered with a layer of copper and made of bimetallic wire. It has the advantages of small specific gravity and good transmission performance. It is especially suitable for the inner conductor of radio frequency coaxial cable. Compared with pure copper wire, its density is About 40% pure copper. The transmission characteristics are better than pure copper wire, and it is the most ideal inner conductor of radio frequency coaxial cable branch line.

How does the submarine cable work?

Many people are full of curiosity about submarine cables. Submarine cables are actually a kind of long-distance optical fiber. Generally, long-distance optical cables in the world use submarine cables. So how do submarine cables be laid?
Compared with submarine cables, the laying of terrestrial cables is much simpler. You only need to dig the cable trench and bury the cables in it. Submarine cables are not that simple. To lay the submarine cables, you must first have a ship. The submarine optical cable laying ship puts the submarine cable on the ship, and then the ship slowly sinks the cable into the seabed according to the prescribed route. It sounds not difficult, but the actual operation is actually very difficult. At present, the international long-distance submarine cable laying is almost unfavorable. Developed countries such as Japan and Europe have a monopoly. However, the domestic cable industry has developed rapidly in recent years. Leading companies such as Huawei have the technology and ability to lay long-distance submarine cables.


The laying of submarine optical cables is usually done by a cable laying machine that digs the seabed. The working principle is a bit like the plow used in plowing the field. During operation, the rows of water jet holes at the bottom of the cable burial machine simultaneously spray high-pressure water jets to the seabed to flush out the seabed sediment to form an optical cable trench. At the same time, the fairlead on the upper part of the equipment can guide the optical cable to the bottom of the optical cable trench. The cable laying machine is towed forward by the submarine optical cable laying vessel, and transmits various instructions through the working optical cable through the laying vessel.
In order to avoid damage to the optical cable due to too small bending radius or excessive tension, the laying of the optical cable-laying ship must use underwater monitors and underwater remote control vehicles to continuously monitor and adjust to control the sailing speed of the laying ship and the release speed of the optical cable. Control the angle of entry of the optical cable into the water and the laying tension and avoid uneven places and reefs. Of course, after the fiber optic cable is laid, there will be landfill work. In the past, sea currents were used to allow sand to naturally cover the trenches to save time for burying cables, but it was also vulnerable to sea currents and other unstable factors (such as sharks). Therefore, it is now common to use an underwater robot equipped with a high-pressure water pump to flush a ditch and then put it in and bury the soil.

Rubber insulation of cables

1. Preparation of raw materials
The preparation of raw materials requires warm glue and glue cutting. The weighing of raw materials is a crucial process of preparation.
2. Plasticizing
Rubber has high elasticity at room temperature and is not easy to process. Therefore, it is necessary to change its high elasticity to make it have a certain degree of plasticity. The process of converting rubber from a highly elastic state to a plastic state is called plasticizing.
Factors affecting mastication; roll distance, temperature, mastication time, speed ratio and number of revolutions
Natural rubber must be masticated, while synthetic rubber may not be masticated.
Three, mixing
Mixing is to scoop all kinds of compounding agents into the rubber to make 2 rubber materials with a certain degree of plasticity. One strand is divided into open mixer mixing, internal mixer mixing, and continuous mixing.


Fourth, filter rubber
The requirements for the mixed rubber materials used for insulation and thin sheath products are relatively high. In the actual production process, various rubber materials inevitably contain impurities, and impurities will also be mixed in the transportation and mixing processes. The purpose is to eliminate the above-mentioned various impurities.
There are two methods for filtering rubber: raw rubber after plastic refining and rubber filtering during mixing. Under normal circumstances, raw rubber does not need to be filtered.
l Extrusion process and equipment
Extrusion is a process that uses a rubber extrusion machine to squeeze the insulation layer on the conductor or squeeze the rubber jacket layer on the cable core.
Extrusion process: warm glue, selection and adjustment of molds, control of extruding temperature, cold feed extrusion process
The equipment used to squeeze insulating rubber or sheath rubber has become the main extruder and is divided into eight types: 30.45, 60.90, 120, 150, 200, and 250.
The squeezing machine is composed of a main engine, a traction device, a take-up device, a meter counter, a cooling device and a transmission system, etc.
Main technical parameters: screw, outer diameter, screw length-to-diameter ratio, screw speed range, glue output per hour, main motor model power, equipment center height and overall dimensions


l Vulcanization process
Under heating conditions, the raw rubber in the rubber compound chemically reacts with the vulcanizing agent, so that the rubber is cross-linked from linear structure macromolecules into three-dimensional network structure macromolecules, resulting in a significant improvement in the physical and mechanical properties and other properties of the rubber compound. This process is called vulcanization.
The entire vulcanization process is divided into four stages: vulcanization induction stage (scorch), pre-vulcanization stage (low sulfur), normal vulcanization stage (normal sulfur), and over-bowling stage (over)
The scorch time refers to the time when the rubber begins to harden and the Men’s viscosity increases, and the thermoplastic flow cannot be carried out from then on. The stage between the scorch time and the positive vulcanization is called under-sulfur, which seriously affects the aging performance. Rubber should strictly control the lack of sulfur phenomenon: normal sulfur means that the degree of vulcanization cross-linking meets the process requirements, and the physical and mechanical properties meet the vulcanization time required by the use. The positive vulcanization time is called the vulcanization flat zone, and the vulcanization flatness refers to the wide area in the positive vulcanization zone. Flat vulcanization curve: Excessive sulfur indicates that the vulcanization crosslinking temperature is too high, at this time the tensile strength and fixed extension are reduced.
Vulcanization process: fixed vulcanization of vulcanized tube, continuous vulcanization
Vulcanization conditions include vulcanization temperature, vulcanization time, and vulcanization pressure. Correctly setting vulcanization conditions is a decisive factor to ensure quality.
The most basic principle of continuous vulcanization: use the method of increasing the temperature to speed up the vulcanization rate. According to the relationship between the vulcanization speed and the vulcanization temperature, it can be seen that for every 10 degrees increase in the vulcanization temperature, the vulcanization speed can be doubled on average, that is, the vulcanization time can be reduced by half.
According to the position of the vulcanizing tube, it can be divided into four types: horizontal, inclined, catenary and vertical. In addition to saturated steam, there are superheated steam, low melting point metal salts and infrared rays.
Continuous vulcanization has many advantages compared with tank line vulcanization: high production efficiency, good product quality, product length is not limited by equipment, easy to operate, continuous vulcanization has become the most important vulcanization method in the wire and cable industry, vulcanization tube vulcanization Be in a secondary position.

Burning test for ACSR Conductor production

There are two key links in the wire and cable manufacturing industry, namely theAluminum conductors steel-reinforced production link and the wire and cable testing link, especially the wire and cable testing link. This is the last stop for quality control and a comprehensive assessment of the performance of wires and cables.
The burning test of the cable is an extremely important one of its many tests. The national cable performance classification has special grade requirements for it, such as the number GB/T31248-2014. This standard divides the burning of the cable into 4 grades, A Class (non-flammable cable or optical cable, equivalent to metal sheathed cable or optical cable), Class B1 (flame-retardant B1 cable or optical cable, equivalent to halogen-free low-smoke flame-retardant cable or optical cable), Class B2 (flame-retardant B2 cable or Optical cable, equivalent to low-halogen, low-smoke flame-retardant cable or optical cable) and Class B3 (ordinary cable or optical cable).


Especially for fire-resistant and flame-retardant special cables, the burning test is a very important test for their professional performance. These fire-resistant and flame-retardant cables are required to have certain flame-retardant properties, and they must also have a certain flame-retardant performance. The characteristics of normal operation for a certain period of time, so the burning test is also a test of the performance of this type of cable.
In addition, there are a number of data for various combustion conditions. The values ​​need to be tested. For example, the heat release rate of the cable is the heat energy released by the combustion of the cable within a certain period of time under the specified conditions. This value is the normal combustion of the cable. Detect the data, and the value of the data will be affected by many factors, such as the combustion state, and the amount of non-metallic materials.


There is also an average heat release rate, which refers to the average heat release rate within 30 seconds. The average heat release rate during the combustion process is also a value that changes with the continuation of the combustion time. The peak heat release rate is the maximum value of the average heat release rate during the entire combustion period. The standard stipulates that the combustion time is 1200 seconds, which is the maximum value among the 40 average heat release rates. This value is related to the combustion state of Aerial Bundled Cable during combustion. If the flame is particularly large in a certain period of time within 1200 seconds, the peak heat release rate may not meet the requirements. The total amount of heat release refers to the integral value of the heat release rate within a specified time.
The various test data of the cable in the flame combustion can reflect the performance and influence of the cable in the flame, and can fully reflect whether the cable is qualified and up to the standard, and whether the fire performance of the fire-resistant cable meets the requirements, these are extremely important Therefore, the burning test is also one of the routine tests of wire and cable manufacturers.

The output of high-performance fireproof cables reaches 40,000 kilometers

ThePower Cable industry is an industrial basic industry, and its products are widely used in energy, transportation, communications, automobiles, petrochemicals and other fields, occupying an extremely important position in my country’s national economy, and the market demand is huge.
Since the 21st century, China’s economy has continued to grow at a high speed. In particular, large-scale projects such as power grid transformation and UHV have been successively invested in upgrading and construction, providing a huge market space for the development of the wire and cable industry. Among them, the market demand for special cables such as high-temperature superconducting cables, aerospace cables, and high-performance fire-resistant cables is increasing.
Despite the huge market space in the entire industry, high-performance special cables with increasing demand have always been the shortcomings of my country’s wire and cable industry, occupying a relatively low proportion in the entire industry, showing that foreign-funded enterprises are leading the way and domestic enterprises are closely following the situation. In terms of market segments, the high-end market for special cables has the characteristics of high technical content, high entry barriers, strict protection of intellectual property rights, and high added value of products. Therefore, how to upgrade the technical level of domestic cable manufacturers, pry open the high-end market, continuous research and development, strengthen innovation, and build brands is the key.


At present, economic construction is shifting from traditional infrastructure such as “Tie Gongji” to “new infrastructure”, which brings new opportunities and new challenges to the wire and cable industry. The cable industry has entered a reshuffle period, and the industry concentration will further increase; “One Belt One Road” The implementation of the strategy has led to a significant increase in the role of overseas markets in the market structure; industry consensus and restrictions on safety, environmental protection, and energy conservation force cable companies to accelerate their entry into the high-end specialOverhead Cable market.


During the “14th Five-Year Plan”, China’s wire and cable industry will usher in new developments, and the market for high-performance special cables will exceed 100,000 kilometers. Wire and cable companies must firmly seize major development strategic opportunities such as power investment, rail transit construction, high-voltage and ultra-high-voltage transmission and distribution network construction, aerospace industry, and deep-sea mining, relying on technological innovation and focusing on digital and intelligent manufacturing. , Focusing on the fields of ultra-high voltage and ultra-high voltage power cables, high-performance fireproof cables, high-temperature superconducting cables, aerospace cables, special cables for urban rail, special cables for nuclear power, etc., strengthen the research and development of key materials and equipment, and strengthen process research , Accelerate the research and development of various new products, strive to make new breakthroughs in basic technologies and common technologies, strengthen the foundation of industrial technology upgrading, seize the commanding heights of international industrial competition, and seek the initiative for future development to promote the optimization and upgrading of the entire industrial chain of the industry, and accelerate Build an industrial basic capability system for the cable industry in the new era, create a modern industrial chain with stronger innovation, higher added value, safer and more reliable, and better support the construction of a modern industrial system.

Recommendations for BTLY cable cross-section selection

1. Select the cross section according to the temperature rise of the core
When the load current is passed, the core temperature does not exceed the long-term working temperature allowed by the cable insulation. That is to say, select according to the allowable current-carrying capacity.
Suggestion: The cable passes through different heat dissipation conditions. The corresponding core operating temperature will be different, and the section should be selected according to the area with severe heat dissipation conditions (usually no more than 1 meter).


2. Select the cross section according to the allowable range of voltage loss
When the terminal voltage of the electrical equipment actually deviates from the rated value, its performance will be affected, and the extent of the impact will be determined by the magnitude and duration of the voltage deviation.
Suggestion: The voltage loss increases due to the sharp rise of the core temperature during fire. The voltage loss should be calculated according to the fire conditions to ensure the continuous operation of important equipment. Due to the excellent heat insulation and heat dissipation characteristics of BTLY products, it is only necessary to enlarge the cable selected according to the normal situation by one to two. Usually, it can meet the condition that the voltage deviation under fire conditions is not more than -10%.
Three, select the cross section according to the economic current
The total cost during the economic life is small. That is, the initial investment and the cost of line loss during the economic life are small.
The so-called economic current is the working cable (range) corresponding to the applicable cross-section (range) during the life of the cable, the sum of investment and conductor loss costs. For details, please refer to “Low Voltage Cable Economic Current Density Range Table”.


Suggestion: BTLY products should have good heat dissipation characteristics. If the cross section is selected according to the temperature rise of the core, the selection can be reduced by one level. But taking into account the economic current factor. Therefore, it is not recommended. If selected in accordance with the routine, the line loss will be reduced by 4-7%, which is of great economic significance.
Fourth, the selection of the cross section of the neutral power cable and the ground wire
(1) In a single-phase two-wire circuit, regardless of the size of the phase wire cross section, the neutral wire and ground wire should be the same cross section as the phase wire.
(2) In the three-phase five-wire power distribution system, the allowable current carrying capacity of the neutral wire and ground wire should not be less than the sum of the large unbalanced load current and harmonic current in the line. When the phase wire core is not larger than 16mm2, the neutral wire and ground wire should have the same cross-section as the phase wire. When the phase wire core is larger than 16mm2, if the neutral wire current is small, the cross section of the phase wire can be selected, but it should not be less than 50% of the phase wire cross section and not less than 16mm2.
Suggestion: Use the aluminum metal sheath in the BTLY cable as the grounding core. The aluminum pipe grounding wire is connected by a dedicated connector, and the connection is firm and reliable. The equivalent resistance of the aluminum tube section of the BTLY cable meets the grounding needs of the corresponding copper core

The basic structure of the wire

1. Single round wire The main varieties are copper single wire, aluminum single wire and copper-aluminum alloy wire. The diameter of the wire is used as the structural parameter, ranging from 0.01mm to 3.00mm. Except for some of the round single wires used directly as products, most of them are semi-finished products used by twisting single wires into multiple twisted wires.
In addition to the round single wire of a single material, there are also compound round single wires. Commonly used are bimetal round single wires, such as aluminum clad steel wire (high conductivity of aluminum, strong tensile strength of steel wire), copper clad aluminum wire (high conductivity of copper, light aluminum) and coated single wire, such as tinned copper wire, Nickel-plated copper wire.
2. The round stranded wire and cable are products of large length, which have to withstand many times of winding and unwinding during production, installation and use. If the wires with larger cross-sections are not used to twist small-diameter single wires into a certain cross-section Twisted wire, the product cannot be manufactured and used. Different use conditions and conditions require wires and cables to have different flexibility-that is, bendable or twistable. Therefore, the stranded wires used in various products have different requirements in terms of flexibility, etc. Therefore, 7 kinds of wire structures are specified in the wire standard. Table 1 is the wire structure specified by the standard.


Wires with the same material and the same cross-section can be composed of several structures, that is, the number and diameter of single wires are different. The more single wires in the same cross-section (the thinner the single wire diameter), the higher the flexibility of the wire.
The structural parameters of  round stranded wire are the cross-sectional area of ​​the wire (in mm2), the number of single wires and the diameter (mm), the stranded pitch diameter ratio, etc.
3. Derivative structure of round stranded wire
(1) Compact round stranded wire In the wire stranding process, the stranded wire is pressed tightly by means of a pressing wheel, etc., so that the conductor occupancy rate in the cross-section reaches about 90%. One is to flatten the protruding shape of the wire surface, which is beneficial to squeeze the thin (0.5-1mm), inner semiconductor layer (for uniform electric field) of the medium and high voltage XLPE cable, which is not easy to puncture. The second is to reduce the gap between the single lines.


(2) The diameter of the inner groove of the round stranded wire with inner hollow groove is generally 14-18mm. There are two kinds of product requirements, namely, oil-filled cables are used as oil grooves for oil flow, and high-current cables are used as channels for internal cooling through gas or liquid. (Can increase the energizing capacity). The conductor of the oil-filled cable is generally formed by twisting a Z-line or arc-shaped single wire to form a ring-shaped cross-section, and it can also be formed by twisting a round single wire into a spiral tube. As the internal cooling type forced cooling, the internal hole groove adopts a sealed metal tube.
(3) The sector conductor is the conductor structure used in the medium and low voltage 3-4 core oil-paper insulated power cable (1-10kv). It is used to make the insulated cores of the paper wrap insulation into three 120 shapes, which are twisted into a circle when the cable is formed. Shape (four-core cable is three 100, and the neutral wire 60 is sector-shaped), in order to save a lot of outer material. The line is rarely used.