Discussion about Low Voltage Concentric Power Cable

In the China wire and cable products, there is no very small structure with a concentric cable. Some developed countries abroad are committed to research in this area and put it into practice. In the British Standard, BS4553-92 copper core PE insulated cable for power supply, the same core layer can be used as The sexual conductor and the grounding conductor are made of unplated annealed copper wire in accordance with BS4109.If the number, diameter and resistivity of the copper wires in each specification cable are the same as the core layer, the right winding should be adopted.
So what does concentric neutral mean?
The Concentric Neutral is the current flowing from one end to the other, then into the ground and back into the wire at the starting position.

The wrapping pitch and the number of threads should make the minimum coverage rate reach 90%, and the wrapping tape that does not absorb moisture should be overlapped immediately outside the same core layer. The NYCY type in the German standard DIN57271-83 adopts the same core layer conductor and can be used as a neutral wire, a ground wire or a neutral wire, and it can be used as a shield wire at the same time. The number, diameter and resistivity of unplated annealed copper wires are also specified. Compared with the same type of cable, it has a compact structure and a round shape, which can save about 10% of the sheath cable material. Therefore, according to the above structure.
The characteristics, cross-sectional area requirements, equipment and production technology used to formulate the structural design principles of low power cable concentric conductors.

  1. In order to facilitate production and management, the wire diameter of the same core conductor should be unified as much as possible.
  2. According to the production equipment, the minimum wire diameter shall be specified under the condition that it is evenly distributed on the stranded insulating core around the cladding and the spacing is not greater than 4mm. The number of concentric conductors is as small as possible.
  3. In order to ensure the uniformity of the same-core conductors without relative sliding, and to ensure that the

The cable appearance is round. Using layered non-woven fabric or PVC wrapping method, that is, insulating core into cable wrapping tape + concentric conductor twisted spiral copper tape wrapping + non-woven fabric tape overlapping wrapping.

  1. There are three types of structures. The structure of three-phase four-wire cable is a structure, and the structure of three-phase five-wire cable is b and c.

 In the structure, the concentric conductor is wrapped outside the cable core. It can be N wire (neutral wire), PE wire (protective earth), or PEN (neutral wire and protective earth shared), and the material is copper Wire or copper tape, or copper wire plus copper tape. The carrying surface is the same or half of the electric phase core

  1. In the structure, the N wire is in the cable core, and the neutral wire is wrapped around the cable core as a PE wire. The neutral wire can be copper wire or copper wire plus copper tape. The cross section is half of the cable phase core
  2. In the structure, the N wire is wrapped outside the copper core, and the PE wire composed of copper wire is wrapped outside the N wire, and the copper wire is composed of copper tape to form a double-layer shielding structure. The cross-section is the same as the phase core of the cable.

Advantages of the same-core conductor and shielded neutral power cable

  1. When transmitting power, it will not cause interference to surrounding electrical equipment; it also eliminates the influence of external electromagnetic interference on electrical equipment connected to this cable
  2. It can avoid breakdown or damage caused by strong lightning electromagnetic effect voltage, and improve the lightning protection effect and reliability of high-rise buildings
  3. It can improve the sensitivity of the overcurrent protection device at the head end of the line, so that the cable and the electrical equipment connected to it can be effectively protected
  4. It has low and uniform forward (reverse) sequence and zero sequence impedance, which is conducive to improving the quality of copper electricity
  5. TN-C and TN-C-S system power supply suitable for coreless cables
  6. It can eliminate the ground fault caused by the insulation breakdown of one phase as soon as possible
  7. Convenient processing, high flexibility and easy construction
  8. Can reduce costs

How to Ensure Outdoor Cable Performance ?

Many users and installations are faced with the problem of cheap and efficient data transmission between buildings in the park environment. The choice of routing, transmission distance and application environment will all affect the choice of cable medium. Incorrect or inappropriate choice will result in a shortened period of wiring investment, and reinstallation will also cause the network system to stop running.
If it is an outdoor application, the fiber optic system is usually the choice for campus network connection. The real cost of optical fiber lies in the termination of optical fiber cabling system and optoelectronic equipment. When users only need to transmit 10Mbps or 100Mbps within a distance of 50 meters between buildings, optical fibers are generally not used.

Buying conventional Category 5 copper cables underground or laying overhead may cause transmission failure of a certain network along the wiring line. Therefore, choosing the existing outdoor direct-buried enhanced type cable will bring a cheap link. Before deciding to choose these outdoor LAN cables, you should fully understand their design.

Anti-moisture protection nets have been used in communication cables for many years. These aluminum polymer materials have overlapping seals as protection to reduce the penetration path of water vapor to prevent water from entering. However, an unprotected dry cable will need to suffer as long as six months to a year of liquefaction due to infiltration, and a dry cable with a moisture-proof protective net will be completely protected. The cable designed in this way is approximately similar to a foil-screened LAN cable, and it is easy to connect and use.

Therefore, the wiring system designer must consider the application environment, which includes the following environment and parameters that affect the cable:
1. Whether the cable is placed under the eaves; as long as the cable is not directly exposed to sunlight or ultra-high temperature, the standard LAN cable can be used. It is recommended to use pipes:
2. External walls; avoid direct sunlight on the walls and man-made damage;
3. In the pipe (plastic or metal); if in the pipe, pay attention to the damage of the plastic pipe and the heat conduction of the metal pipe;
4. For suspended applications/overhead cables, the sag and pressure of the cable should be considered. Which bundling method you intend to use. Whether the cable is directly irradiated by sunlight; laying directly in the underground cable trench, this environment is the smallest control range. The installation of the cable trench should be checked regularly for dryness or humidity;
5. Underground pipeline. In order to facilitate future upgrades, cable replacement, and isolation from surface pressure and the surrounding environment, laying pipes is a better method. But don’t expect that the pipe will always remain dry, which will affect the choice of cable types.

Factors affecting cable performance include:
1. Ultraviolet (UV)-Do not use cables without UV protection in direct sunlight. You should choose cables with black polyethylene or PVC sheaths, such as Brand-Rex’s 4 pairs of reinforced type 5 MegaOutdoor outdoor cable, with metal mesh moisture-proof protective layer and black polyethylene sheath, is suitable for most inter-building connections, whether it is overhead laying, ground installation or pipeline construction, it can be used:

2. Heat-the temperature of the cable in the metal pipe or trunking is very high. Many polymer materials will reduce the service life at this temperature. Black polyethylene or PVC sheathed power cable should be selected;

3. Water-Water is the real killer of LAN cables. The moisture in the twisted-pair cable of the local area network will increase the capacitance of the cable, thereby reducing the impedance and causing near-end crosstalk problems. If it is extremely effective to prevent moisture and water vapor, a protective layer of metal shielding net is required;

4. Mechanical damage (repair costs)-the repair of optical cables is very expensive, and at least two terminations are required at each discontinuity;

 

Grounding-if the shielding layer of the cable needs to be grounded, the corresponding standards must be followed;

The total length of the route (not only between the buildings)-Use outdoor-grade LAN twisted-pair cables between the buildings, and the total length should be limited to 90 meters.
For a network of 100Mbps or 1000Mbps, the laying distance cannot exceed this limit. If the laying distance is between 100 meters and 300 meters, optical cable should be selected.
The following simple experiments can be used to self-test whether the wiring investment is safe: use a 20-meter enhanced category 5 UTP cable to terminate at both ends; carefully remove the cable sheath at the midpoint of the cable to expose a small section of copper cable (1 cm ); Test the cable according to AN/NZSD standard; soak the cut part of the cable in water for 1-2 minutes, and then retest.

Damage Caused By Chemical Corrosion to Directly Buried Cables

1. Chemical corrosion of laying environment
If the accumulated water in the cable trench or the directly buried soil contains corrosive components, such as sulfuric acid or nitric acid, the cable surface will be in contact with these corrosive substances for a long time, and severe chemical corrosion will occur. If the cable sheath is damaged, the moisture will spread left and right longitudinally after entering the cable. In some areas, the groundwater quality and soil are severely polluted by chemicals. If the cable path is not properly selected, the cable trench is poorly constructed, and the backfill is too corrosive, the molecules of the cable insulation and sheath organic materials will be chemically degraded. It will cause the cable to be corroded, and the insulation resistance of the cable will decrease or even lose the insulation resistance.

2.Acid rain chemical corrosion
The chemical corrosion factors that are serious to power cable, in addition to the water quality and soil condition of the laying environment, there are also serious effects of modern acid rain.
The so-called acid rain is due to the burning of fossil fuels (coal, oil, natural gas) or biomass fuels, which discharges acidic compounds (such as sulfur dioxide, carbon dioxide, and nitrogen dioxide, mainly sulfur dioxide) into the air, causing sulfuric acid, Phenomenon of acidic substances such as nitric acid. The main component of acid rain is sulfur dioxide. It is generally believed that if the pH value of rainwater is less than 5.6, it can be considered as acid rain. The main reason for the formation of acid rain is the excessive emission of sulfur dioxide from factories. At present, the “energy saving and carbon reduction” and “energy saving and emission reduction” being implemented in the world are mainly aimed at reducing the emission of sulfides and carbides in order to protect a clean atmosphere.

here have been acid rain disasters in more than 20 provinces and cities in my country, mainly in the area south of the Yangtze River. Acid rain not only causes severe extinction hazards to crops, forests, grasslands, fish, etc., but also severe corrosion to metal objects, such as wires and cables, railway tracks, ships, vehicles, transmission lines, bridges, houses, electromechanical equipment, etc. Will cause serious damage.
The Journal of Sichuan University published a research report “Corrosion behavior of copper during acidification of acid soil under the action of acid rain”. Experiments show that acid rain will increase the corrosion rate of copper. The corroded surface of copper is mainly cuprous oxide (Cu2O) and copper oxide (CuO).
The harm route of acid rain to directly buried cables is: sulfur dioxide in the air reacts with rainwater to form sulfurous acid, which is oxidized to sulfuric acid:
SO2 + H2O = H2SO3
2H2SO3 + O2 = 2H2SO4
Rainwater containing sulfuric acid enters the cable from the damaged point of the power cable pvc sheath or the cable joint in a high temperature environment, and it will corrode the insulation layer, the sheath and the copper cable. Sulfuric acid corrodes the cable sheath and insulation layer, degrades and damages the molecular structure, severely reduces the insulation resistance, and even loses insulation and protection. Sulfuric acid reacts with copper to form blue copper sulfate (CUSO4) crystals, which become a blue copper sulfate solution in contact with water. .
CU + 2H2SO4 = CUSO4 + SO2↑+ 2H2O

In the past few years, a certain power supply department in Chongqing, where the soil is corrosive and acid rain is hardest hit by acid rain, discovered blue liquid and damage to the insulation layer at the end of the cable. If the cable sheath is seriously damaged, especially in the season of high temperature, high humidity, and strong sunlight, if acid rain occurs, or the sulfuric acid content in the soil is large, the cable enters a lot of water, this blue copper sulfate solution will quickly follow the length of the cable Spread upward until it overflows from the damaged part of the cable and the end of the cable. Copper sulfate solution can conduct electricity. After it penetrates into the insulation layer, the conductivity of the insulation layer has been strengthened, and the resistance of the insulation layer will drop sharply, the insulation effect will be lost, and the cable short circuit accident will occur.

Multi-core Cable Parallel Connection Problem

In the actual parallel use of cables, there are more single-core cables in parallel. During the actual parallel use of single-core cables, due to the influence of the laying method, the actual current carrying capacity may not meet the actual load needs, and it may appear in actual use. Overload phenomenon. In fact, when 6 cables are laid in the air and laid side by side with no gaps, the actual reflow can only reach about 60% of the theoretical carrying capacity. If the cable load is added, the theoretical selection is not carried out according to the actual installation. Correct the situation. It is very likely that the cable will be in a full-load operating state during the actual power-on process, causing the cable to generate heat during power-on operation. Therefore, in the process of parallel laying of cables, the actual current carrying capacity is not simply a relationship of “1+1=2”. It is very likely that “1+1=1.5” or even “1+1=1” will appear, causing the cable Severe heating occurred during actual operation.
Now let’s give a simple example, such as a three-phase asynchronous motor load with a capacity of 570KW and a rated current of about 1140A. Two YJV-0.6/1KV(low voltage)-1*300 cables are used for power supply in parallel, and the given value is calculated according to the theoretical design. , YJV-0.6/1KV-1*300 single cable is laid in the air, and the theoretical calculation current carrying capacity is about 750A. The theoretical parallel current carrying capacity of two cables can reach about 1500A, which can fully meet the actual needs of the equipment. We now assume that there are 32 cables that are all concentrated on a bridge, stacked side by side, stacked and laid randomly, and the two YJV-0.6/1KV-1*300 powering in parallel are also located in it. After consulting related materials, it is found that when 6 cables are stacked in the air without gaps, the actual current carrying capacity of the cable will drop to 60% of the theoretically calculated value. Then the actual current carrying capacity of the original cable is 1500×60%=900A, and the actual carrying capacity assigned to each cable is about 450A, which is nearly 300A different from the theoretically calculated carrying capacity of 750A, so that the cable will have serious overload and heat during actual use. phenomenon.

And the actual number of cables laid is far more than 6, so the actual cable reflow may be smaller than 900A. How to solve this problem, some people have proposed to connect another YJV-0.6/1KV-1*120 cable in parallel to reduce the distributed current of the other two cables. Now we theoretically assume and calculate that after the three cables are connected in parallel, the load current In the actual distribution situation, assuming that the length of 3 cables used in parallel is 1 km, the laying temperature is all calculated at 20°C. Moreover, it is assumed that the conductor resistances of two YJV-0.6/1KV-1*300 cables connected in parallel for 1 km are exactly the same. In fact, due to manufacturing process problems, it is impossible to achieve complete consistency, and there is still a slight difference in conductor resistance. In the actual calculation process, we ignore the above influence. The maximum DC resistance of copper conductor at 20℃ is 0.0601Ω/km for copper core 300mm2, 0.153Ω/km for 120 mm2, and the actual distribution of 1140A current is calculated as 120 mm2 cross-section distribution current is (0.0601*0.0601/0.153*0.0601+0.153*0.0601+ 0.0601*0.0601)=187A, the current distributed on the remaining 300 mm2 cross-section is 953A, and the actual load current flowing on each 300 mm2 cable is about 477A. ​​In this case, the actual power of the cable still has an overload phenomenon. . In this case, the actual current capacity of the cable 120 is 435*60%=261A, which still has a large margin, but the current distribution law does not distribute the current to the 120-section cable. In fact The original problem remains unresolved. And our assumption is that there are only 6 cables, which does not meet our established requirements. Imagine adding another cable with a cross-section of 300 mm2. The actual current carrying capacity distribution law is 1140*1/3=380A. Therefore, in the actual parallel cable process, the cross-section of the cable must be calculated strictly before proceeding. Use in parallel, otherwise the problem may not be solved by adding cables in time. The best case is to use cables of the same specifications and ensure the same length, so as to ensure that the current distribution is basically even. In fact, it is very difficult to re-install and rework the on-site cable after all on-site installation is completed. Therefore, the formal design, laying and installation of the cable in the early stage is very important, and the method adopted in the later stage is often only a remedial measure, and it is difficult to solve the problem fundamentally.

In addition, there are some problems in the parallel use of multi-core cables. For armored cables, the main core A, B, and C of each cable should be staggered and used in parallel. All wires of the armored multi-core cable cannot be used in parallel. The new parallel connection is used as a single-core cable on one phase. If this is done, eddy current effects will be generated in the armored steel tape of the cable, which will cause the cable to heat up and cause thermal breakdown. Although this is a very simple electrical principle, in the process of the author’s many visits to users, sometimes users still raise similar questions and practices. In the three-phase four-wire unbalanced lighting load, the wiring and distribution method of our load should ensure that the load is distributed as evenly as possible, and the three-phase current is balanced as much as possible, otherwise it may be caused by the serious imbalance of the three-phase current. Alternating induced current is generated in the shaped steel strip, which causes heating of the cable.

The parallel use of cables should also pay attention to the tightness of the lugs at the end of each line, because the load capacity of the parallel cables is generally relatively large, and the conductor resistance per kilometer is below 0. Loose wire noses and poor contact will double the conductor resistance of the line, causing uneven current distribution and even bypassing. This will cause individual cables connected in parallel to generate heat and cause malfunctions.

At the same time, it is possible that the conductor resistance of the actual circuit of the cable may not be completely the same. Therefore, the current distribution of cables of the same type and specification cannot be absolutely evenly distributed, and there may be some differences in the actual current distribution process.

Therefore, during the actual parallel use of multiple single-core cables, corrections should be made according to their actual laying conditions, otherwise it may cause heating during the parallel use of the cables and affect the normal use of the cables.

Power Cable Damage Caused by Natural Conditions

1. Termite damage
Termites are the enemy of underground power cables, especially in Southeast Asia and the hot and humid areas of southern my country, where termites often erode the plastic sheath of cables. When termites encounter a cable, in addition to biting, they also secrete formic acid, which severely corrodes the cable insulation and sheath, causing the cable insulation performance to decline or even short-circuit. Therefore, in the laying regulations formulated by the cable use department, there are clear provisions on anti-termite measures for cable lines.
There are three kinds of test methods for anti-termite performance of cables, namely, the knockdown method, group hair and ant nest method specified in the national standard GB2951.38 and the machinery industry standard JB/T10696.9-2011. In the past many years, the group approach has been used the most. However, after years of experience and lessons in the prevention and control of cable ant pests, the knockdown method and the colony method test can not truly reflect the ant resistance of cables in different environments. Therefore, in the power cable bidding of Guangdong Power Grid Corporation from 2009, it is stipulated that the anti-ant cable must pass the ant nest test, and the corrosion condition of the cable sample must reach the level I.

2. Rodent injury
The damage of rodents to underground cables is mainly the mechanical damage caused by biting. When the hardness of the cable sheath material is lower than that of the mouse’s incisor, the cable is likely to be bitten by the mouse. There is no uniform cable rat-proof test standard in the world, but every country has its own test method. my country JB/T10696.10-2011 stipulates the rat bite test method. In addition, the national standard “General Rules for Anti-rodent and Ant-proof Wires and Cables” developed by Henan Huaxing Wire & Cable Co., Ltd. has already held a preparation meeting on March 19, 2016. Soon my country will have a formal anti-rodent Ant cable product standard.

3. Mold damage
As early as the late 1950s, some countries had stipulated that electrical products used in humid tropical regions should have anti-mold properties. my country has formulated relevant standards for the mildew resistance of wires and cables used in humid tropics for the wires and cables exported to these regions. In some parts of southern my country, the degree of mildew on directly buried cables varies due to the degree of dampness and heat in each year, the difference in duration, the region and the use environment of wires and cables.

According to research reports on the reproduction of microorganisms and molds, the main conditions for mold growth are temperature and humidity. The general temperature suitable for mold growth is 15°C to 35°C, and the most suitable temperature is 25°C to 30°C. When the temperature is lower than 0°C or higher than 40°C, the mold actually stops growing. The relative humidity suitable for mold growth is 80% to 90%, and when the relative humidity exceeds 95%, it is the most vigorous condition for mold growth. Therefore, when the ambient temperature is 30°C±2°C and the relative humidity is greater than 95%, it is most suitable for the proliferation of molds. The hot and humid climate of Hainan Island is just suitable for the growth of molds.

If a large number of molds grow on the surface of the cable, it will have a greater impact on the performance of the cable, which will cause: discoloration, pitting, and rot on the cable surface; decrease in insulation resistance, volume resistivity, and dielectric strength, causing leakage or even insulation breakdown; insulation Chemical degradation occurs with the sheath material molecules, the mechanical properties of the material are obviously reduced, and its protective effect is lost; moisture enters the cable, causing serious electrical performance failures.

4. The impact of lightning
When a thunderstorm occurs, if the quality of the lightning arrester used on the line is poor or the grounding protection is improper, the lightning will hit the arrester, causing the line load to suddenly increase and over-voltage, resulting in over-voltage surge in the cable, causing cable insulation damage wear. In southern my country, including areas with frequent thunderstorms on Hainan Island, lightning strikes on cable lines are not uncommon.

Environmental Requirements of PVC wires and cables

The use of non-environmental protection cables is strictly prohibited in developed countries, and China also attaches great importance to this field. Relevant Chinese laws and regulations clearly require important buildings to prohibit the use of polyvinyl chloride wires and cables, and halogen-free and low-smoke cross-linked PVC wires and cables must be used to avoid a large amount of smoke and chlorine that may cause casualties in the event of a fire.

Environmental protection cables will strongly promote the development of the cable industry, enhance the competitiveness of cable manufacturers, and make the cable industry more stable on the road of sustainable development through green practice and innovation. In recent years, my country’s cable companies have gradually worked towards environmental protection cables, low-smoke and halogen-free cables, etc., as far as possible to be free of heavy metals such as lead, cadmium, mercury, bromine-based flame retardants, and no harmful halogen gases. There is no corrosive gas, less heat when burning, no pollution to the soil, etc., environmentally friendly cables will be more favored by the market, and it will be easier to obtain high orders in the bidding of power grid companies.

Users’ requirements for environmentally friendly wires and cables can be summarized as follows:

1. Low lead and heavy metal content is required

It is well known that lead and certain heavy metals will have adverse effects on human health. In PVC formulations, additives such as lead, cadmium, barium and other heavy metal compounds are usually mixed. However, since the 1990s, some developed countries and large electric companies have successively formulated regulations, and in the PVC wires and cables used, the content of eight heavy metals must not exceed the regulations. These new regulations not only bring new business opportunities to wire and cable manufacturers, but also bring serious challenges to PVC cable manufacturers.

2. Low smoke and low hydrogen oxide are required

Good flame-retardant performance is a very obvious advantage of PVC wires and cables. However, when ordinary flame-retardant PVC cables catch fire, they will release thick black smoke and a large amount of HCI gas, which will cause the “secondary” after the fire. “Disasters” seriously affect the evacuation of personnel and the development of fire rescue work. Therefore, while putting forward flame-retardant requirements for wires and cables, “low smoke” and “low HCI” requirements will be further proposed in certain applications.

3. Low toxicity or non-toxicity is required

For some cables used in household appliances, medical and health appliances, and children’s electric toys that may come into contact with food and blood products, in addition to lead-free and heavy-metal-free, they are also non-toxic (or low-toxic). This means that 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.

4. For other prohibited items

Currently, the European Union is reviewing regulations that restrict the following items to ensure that wires and cables can meet environmental protection requirements. For example: 1) asbestos; 2) mostly biphenyl and its ethers, polybrominated phenol; 3) polychlorinated biphenyl; 4) short-chain (C10~C13) chlorinated paraffin.

As environmental issues are getting more and more attention, many countries have formulated various environmental protection policies, so people are more inclined to use green cables. Low-smoke halogen-free flame-retardant cable material is one of the most popular environmental protection cables. Although the demand for green cables in the Chinese market is increasing, compared with developed countries, my country still has a long way to go in terms of relevant policy formulation and mandatory application. my country should improve environmental protection cable standards as soon as possible.

How to Prevent Wires and Cables from Catching Fire ?

How to prevent wires and cables from catching fire due to overload of wires?

During the operation of the wire and cable, it will generate heat due to the existence of resistance. The resistance of the wire is generally very small, and its heating power can be expressed by the formula q=I^2R. q=I^2R indicates: for a piece of wire in actual use (R is basically constant), the larger the current passing through the wire, the greater the heating power; if the current is constant, the heating power of the wire is also constant . The heat released during operation will be absorbed by the wire itself and cause the temperature of the wire to rise. Although the wire is constantly absorbing the heat released by current work, its temperature will not rise indefinitely.

Because the wire is constantly radiating heat to the outside while absorbing heat, the facts show that the temperature gradually rises after the wire is energized, and finally the temperature is constant at a certain point. At this constant point, the wire has the same heat absorption and heat dissipation power, and the wire is in a thermal equilibrium state. The ability of the wire to withstand higher temperature operation is limited, and operation exceeding a certain maximum temperature will be dangerous. This maximum temperature naturally corresponds to a certain maximum current, and running a wire that exceeds this maximum current is an overload. The overload of the wire directly causes the temperature of the wire itself and nearby objects to increase. Temperature rise is the most direct cause of this type of fire.

Overload damages the insulation layer between the twin wires and causes a short circuit, which burns down the equipment and causes a fire. The double-stranded wires are separated by the insulating layer between them, and the overload will soften and destroy the insulating layer, which will cause the two-stranded wires to directly contact and cause a short circuit and burn the equipment. At the same time, the high temperature generated by the large current at the moment of short-circuit causes the line to catch fire and fuse, and the resulting molten beads fall to combustible materials to cause a fire. Overload temperature rise can also directly ignite nearby combustibles. The heat transfer of the overloaded wire increases the temperature of nearby combustibles. For nearby combustibles with low ignition point, it is possible to ignite them and cause a fire. This danger is particularly prominent in warehouses for storing flammable materials and buildings that are easy to use and combustible decorations.

Overload also causes the connections in the circuit to be overheated, which accelerates the oxidation process. Oxidation produces a thin layer of oxide film that is not easy to conduct electricity at the connection points. The oxide film increases the resistance between the contact points, resulting in ignition and other phenomena, causing a fire.
So, how to prevent fire due to wire and cable overload?

1. During the circuit design process, the capacity of the site should be accurately verified, and the possibility of adding capacity in the future should be fully considered, and the appropriate type of conductor should be selected. For large capacity, thicker wires should be selected. Circuit design and reasonable selection are key steps to prevent overload. If the design is not properly selected, it will leave inherent 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. New electrical appliances and electrical equipment should fully consider the capacity of the original lines. If the original line does not meet the requirements, it should be redesigned and modified.

2. The line should be constructed and laid by qualified electricians in accordance with relevant specifications. The wiring conditions directly affect the heat dissipation of the wires. Generally speaking, the wiring should not pass through flammable and combustible materials and stacks, which will cause poor heat dissipation of the wires, heat accumulation, and the possibility of igniting the surrounding combustible materials, which increases the risk of fire under overload conditions; Lines laid in the decoration ceiling of public entertainment places should be protected by steel pipes to separate the ceiling from the lines. Even if there are molten beads in overload, short circuit, etc., they will not fall to avoid fire.

 

3. Strengthen power management, avoid random wiring and wiring, and use mobile sockets with caution. Random wiring, random wiring, and use of mobile sockets are actually adding electrical equipment to a certain section of the line, which increases the amount of current and may cause overload. There are obviously more mobile socket jacks than fixed wall sockets. If too many electrical equipment are used on the mobile sockets, the original wiring must be unbearable. For larger power equipment and electrical appliances, separate lines should be provided, and mobile sockets should not be used as wiring sources.

4. Speed ​​up the renewal and transformation of old lines to eliminate fire hazards. Old enterprises, old residential areas and other units have been used for a long time, and many lines have been aging and have exceeded the service life. Even if the current-carrying capacity of some lines is not large, the aging circuit can hardly bear such current-carrying capacity, and it also has the danger of overload. Especially in 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 electricity consumption is still rising year by year, which is really worse. For old lines, timely supervision and coordination should be carried out, and rectification should be promoted as soon as possible to eliminate fire hazards and ensure safety.

General Rubber Plastic Wire Structural Characteristics

Requirements of General rubber plastic wire:
1. Scope of application: connection and internal installation of power, lighting, electrical equipment, instrumentation and telecommunication equipment with AC rated voltage of 450/750V low voltage cable and below.
2. Laying occasions and methods: indoor open laying and communication channels, along the wall or overhead laying in tunnels; outdoor overhead laying, laying through iron pipes or plastic pipes, electrical equipment, meters and radio installations, all fixed laying; plastic The sheathed wire can be directly buried in the soil.
3. General requirements: economical and durable, simple structure.
4. Special requirements:
1) When laying outdoors, due to the influence of sunlight, rain and freezing conditions, it is required to be resistant to the atmosphere, especially sunlight aging; cold resistance requirements are required in severe cold areas;

2) In use, it is easy to be damaged by external force or flammable, and it should be worn in the occasions where it is in contact with oil. When the pipe is worn, the wire is subject to greater tension and may be scratched, so lubrication measures should be taken;
3) When used internally for electrical equipment, when the installation location is small, it should have a certain degree of flexibility, and require clear color separation of the insulated wire core, and should be matched with the corresponding connector terminals and plugs to make the connection convenient and reliable; for occasions with electromagnetic protection requirements , Shielded wires should be used;

4) For high ambient temperature occasions, sheathed cables should be used; for special high temperature occasions, heat-resistant cables should be used.

Structural features
1. Conductive core: when used for internal installation of power, lighting and electrical equipment, copper core is preferred. For large cross-section wires, it is better to use compact core; bare conductors for fixed installation generally adopt category 1 or category 2 conductor structure .
2. Insulation: Insulation materials generally use natural styrene-butadiene rubber, polyvinyl chloride (PVC) cable, polyethylene, and nitrile polyvinyl chloride compound; heat-resistant wires use polyvinyl chloride with a temperature resistance of 90°C.
3. Sheath: There are generally five types of sheath materials: polyvinyl chloride, cold-resistant polyvinyl chloride, anti-termite polyvinyl chloride, black polyethylene, and neoprene rubber.


Especially cold-resistant and outdoor overhead laying should use black polyethylene and neoprene sheathed wires.
In environments with external forces, corrosion, humidity, etc., rubber or plastic sheathed wires can be used.

Cable selection method of generic cabling system

Generic cabling is one of the key links of information network system. In order to ensure the high-speed and stable operation of information network system, it is particularly important to select high-quality and high-performance cables.

The cable selection should be based on the system requirements, technical performance, investment estimates and other factors, but in the wiring system, it should first determine the type of cable used and the structure of wiring (unshielded cable, shielded cable, optical cable or mixed use). Cables usually use Aluminum conductors with insulation and one or more layers of plastic sheath. The cable usually consists of 2 to 3600 pairs. Large logarithmic cables are usually used in backbone cabling systems and are suitable for voice and low rate data transmission. The maximum lengths of these cables for trunk and horizontal (hub to desktop) cabling systems are specified in the latest GB 50311-2007. It should be noted that these maximum length limits apply to all media. They do not consider the impact of performance differences due to different cable types and protocol types used in the network. In fact, the maximum cable length will depend on the application of the system, the type of network and the quality of the cable.

Method of cable selection in generic cabling system

Since it is a building, it will definitely use a lot of communication cables. As the nervous system of intelligent building, generic cabling system is the key part and infrastructure of intelligent building. It has a very close relationship with the planning and design, construction, installation and maintenance of construction engineering. It is like an information superhighway in the building. People can communicate and communicate conveniently, quickly and effectively. It can be said that the generic cabling system connects the communication, computer and various facilities and equipment in the intelligent building to form a complete set of whole, so as to meet the requirements of highly intelligent.

But then we have to consider a problem: fire prevention. Once a fire breaks out in the building, the heat and gas released by these cables will become a major safety hazard. Looking at several large-scale fire accidents in China in recent years, many of them are due to the fact that the victims can’t escape. The burning of cables emits poisonous acid gas. In addition, a lot of heat and smoke are released from the combustion, which makes it difficult for the victims to breathe and lead to tragedies. Therefore, in addition to considering the performance, fire protection and environmental protection are also very important reference factors when we select cables for generic cabling system.

WTTEZ cable performance characteristics

First, fire prevention, fire resistance is very superior
In order to ensure the normal power supply of the cable in flame, that is, to keep the cable structure stable under fire conditions, a layer of coating with fire resistance and expansion performance was added in the cable structure design. The volume expansion of the coating layer under flame conditions makes the cable insulation structure more stable. Therefore, this kind of cable can continue to maintain power supply in the case of fire near the melting point of copper, which is a real sense of fire prevention cable.
Continuous production with long length, large section and good bending performance
1. Long continuous production length
The production length of the cable is not limited by the processing technology and production equipment, and completely meets the demand of power supply length. The longest can be produced up to 2000 meters.
2. Large section
The cable conductor is made of multi-strand single wire twisted, which can greatly increase the flexibility of the conductor. In addition, the twisted conductor has good production continuity and long processing length, which can reduce the number of joints. The section of large single-core cable can reach 800mm2.
3. Good bending performance
The copper sheath is processed to increase the softness and bending performance of the cable.


4. Heavy load flow
When the cable is normal, the conductor temperature can reach 180℃. Under the premise of the same conductor cross section, its carrying capacity is much higher than that of conventional cable.
5. Waterproof
The cable is protected by sealed non-magnetic metal tube, and its waterproof property is very superior.
6. Explosion-proof
Because the cable adopts copper sheath, and after the cable burns, the filler layer expands and compacts the cable core tightly, the flammable gas and flame cannot reach the electrical equipment connected with the cable, so the cable has explosion-proof characteristics.
7. Good corrosion resistance
Due to the good corrosion resistance of copper, no additional protection is required under normal use environment. In special environmental conditions, such as the environment with strong corrosion effect on copper, the corrosion resistance can be achieved by adding a layer of low-smoke halogen-free outer protection on the cable.
8. No electromagnetic interference
Under the shielding of copper sheath, the cable will not disturb the signal and control line when it is laid together with information and control line.