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Summary of Lightning Protection Technology of CATV System

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Lightning is a discharge phenomenon in the atmosphere, which often causes serious damage to cable TV equipment. In CATV systems, lightning protection design is a very important work. In actual projects, lightning protection has not attracted enough attention from technical personnel. Once a lightning strike occurs, the system without good lightning protection measures will be severely damaged or even paralyzed. For large systems with long trunk lines, lightning protection design is an urgent matter. This article describes the lightning from the mechanism of lightning strikes and the distribution of lightning, in order to readers have an overall understanding of lightning strikes, and then elaborate lightning protection measures and CATV equipment Anti-lightning performance.
 
There are two main types of lightning strikes: "direct lightning strike" and "induction lightning strike". Direct lightning is only about 10% of the lightning strike rate, and the damage range is generally small. You can use lightning rods, lightning wires and lightning protection nets to prevent it. The much more harmful "induction lightning" accounts for nearly 90% of the lightning strike rate, and the damage range is very wide. The electronic equipment of the CATV system is damaged by lightning strikes, mainly caused by induced lightning. Direct lightning strikes are caused by the discharge between the charged cloud layer and the earth. During the formation of the thundercloud, some clouds that accumulate positive charges from some clouds approach a certain level, and a sudden discharge occurs. A dazzling flash appeared. When the thundercloud is very low, and there is no other oppositely charged thundercloud, the oppositely charged will be induced on the ground or building, forming a charged cloud layer to discharge to the ground or building; the discharge current can reach tens or even hundreds of kiloamperes The discharge time is 50-100µS. This discharge is a direct lightning strike, which is very harmful to the safety of buildings, people and animals. After being installed in the lightning rod, even if the electronic equipment of the CATV system is within its protection range, it may still be damaged by lightning strikes, most of which are burned fuses, power transformers, rectifier components, three-terminal voltage regulators, and serious damage. The integrated circuit and other components indicate that the lightning strike is not introduced from the antenna, but from the power line. It can be seen that although the lightning rod protects the building, it cannot protect the CATV electronic equipment placed in it. This is caused by induced lightning.
Summary of Lightning Protection Technology of CATV System
 
Summary of Lightning Protection Technology of CATV System
 
Induced by lightning, electrostatic induction and electromagnetic induction caused by lightning current are caused by two reasons. When charged clouds (thunderclouds) are close to the transmission line, they will induce heterosexual charges on them. These heterosexual charges are bound by thundercloud charges. When Thundercloud discharges nearby targets or lightning receptors (the lightning rod is the earliest and most commonly used lightning receptor), its charge is quickly neutralized, and the charge trapped on the transmission line is free, forming a local induced high potential. This induced high potential can reach 100KV when it is on low-voltage overhead lines; it can reach 40-60KV on telecommunication lines. And it can pass into electronic equipment along the line, causing damage. After a lightning strike, a huge lightning current generates an alternating magnetic field in the surrounding space. Due to electromagnetic induction, high voltage is induced by nearby equipment, which damages the equipment.
Summary of Lightning Protection Technology of CATV System
DISTRIBUTION LAW OF LIGHTNING- Sky Shield-
The distribution law of lightning in China is: usually hot and humid areas have more lightning than cold and dry areas, and the frequency of lightning increases with the decrease of geographical latitude, that is, the closer to the equator, the higher the frequency. Specifically, South China >Southwest>Yangtze River Basin>North China>Northeast>Northwest. In addition, the frequency of lightning varies with the region, that is: mountainous area>plain>desert, land>lake and sea, the peak of lightning every year is July and August, active Most of the time is between 14-22 hours. The presence or absence of lightning in an area is determined by meteorological conditions, but the specific lightning strike point is greatly affected by local conditions. ① The influence of terrain is usually higher than the northwest slope of the mountain on the east and south slopes of the mountain. The probability of thunder on the flat ground of the mountain is greater than that of the canyon. The probability of thunder on the lake is very small, but if the lake has mountains, it depends on the water. There are many chances of thunder on mountain slopes; wind outlets or downwind valleys are prone to thunder. ② Influenced by ground objects, isolated buildings in the open ground and towering buildings in the group of buildings are vulnerable to lightning strikes, metal-structured roofs and towers are vulnerable to lightning strikes, large trees near the house, receiving antennas, mountain transmission lines, etc. Being struck by lightning, this is because these ground objects are very conducive to the establishment of a good discharge channel between thundercloud and the ground. ③ Due to geological influence, the place where the relative value of the soil and the resistivity are small and the charge accumulation is relatively fast is likely to fall. When there is a large soil with a large resistance, the local soil with a small resistivity is easy to fall; the hillside with a small soil resistivity or a mountain is easy to fall ; Areas with conductive mineral deposits in the soil are prone to thunder; areas with high groundwater levels or mineral springs are prone to thunder.
 
Lightning distribution law
 
Lightning protection system measures LIGHTNING PROTECTION SYSTEM MEASURES- Sky shield-
The most favorable measure for the system's lightning protection is that the system has a good grounding. A good grounding can not only discharge the voltage generated by the induced lightning as early as possible, but also discharge the ground voltage caused by the leakage of the equipment to protect the equipment and the person. For safety purposes, the specific system lightning protection also starts from grounding. At the same time, lightning protection devices and lightning arresters are installed where necessary, but the anti-lightning performance of the selected equipment is not neglected. When selecting equipment, it is also an important part to check the anti-lightning performance of the equipment. At present, China produces and uses surge arresters in large quantities. There are various valve arresters with electrical silicon carbide valve discs as basic components; there are zinc oxide arresters with zinc oxide discs as basic components. Compared with valve arresters, zinc oxide arresters have action Quick, large flow capacity, low residual pressure, no continuous flow, simple structure, high reliability, long life and easy maintenance. The following focuses on the zinc oxide surge arrester. The zinc oxide pressure-sensitive arrester is effective in preventing the intrusion of lightning and lightning waves. Its working principle is the non-linearity of its volt-ampere characteristics. At normal voltage, the arrester is in a high-impedance state, with only a small leakage current (in the order of µA), and the power loss is very small. When an overvoltage occurs in the line, the arrester is in a low-impedance state (time effect is on the order of 10-9). In the form of discharge current, it flows into the ground through the arrester, and the overvoltage is suppressed. After the surge voltage passes, when the line voltage returns to normal, the arrester is in a high-impedance insulation state, so the arrester must have a good grounding device to cooperate with it.
 
The current of lightning overvoltage is a pulse that rises quickly to the peak and then decreases slowly. The time value is on the order of 10-6S. The national general standard is 8/20µS, that is, the time to rise to the peak in 8µS, and then It takes 20μS to fall to half peak, and the response time of zinc oxide surge arresters is on the order of 10-9, so the response time is too late, followed by the amplitude problem. The power supply of the cable TV passes through the multi-level lightning protection of the power system, so lightning The probability that the pulse current is greater than 5 KA will not be 15%. As long as the lightning arrester with a flow capacity of 5 KA is selected, the residual voltage can also be tolerated by the equipment, and the effect of lightning protection can be achieved by adding the required grounding.
 
Lightning protection system measures
 
Lightning protection grounding type LIGHTNING PROTECTION GROUNDING TYPE- Sky shield-
Wireless lightning protection grounding
The receiving antenna and the vertical pole of the cable TV are generally erected on the top of the building. All the receiving antennas, including the satellite receiving wireless ground, should be welded together. The vertical pole (rack) connected to the antenna should be equipped with a lightning rod. The height of the lightning rod It should be able to meet the protection of the antenna facilities. When installing an independent lightning arrester, since the protection scope of a single lightning rod is tent-shaped and the boundary line is hyperbolic, the length above the top of the antenna outside the lightning arrester should be greater than the maximum size of the antenna. The minimum horizontal distance between the antenna and the antenna should be greater than 3M. When the building has a lightning protection grounding system, the grounding of the lightning rod and the antenna pole should be connected to the building's lightning protection grounding system; the building does not have a special lightning protection grounding. When using it, a special grounding device should be set up. Two down conductors are used from the air-termination device to the grounding device. The grounding resistance should be less than 4 ohms from different directions with the shortest distance along the building. Whether it is newly made The grounding system is the grounding wire of the original building. The grounding resistance should be less than 4 ohms. In addition to the antenna should have a good lightning protection grounding, the following measures should also be taken: ① A special CATV security device should be installed at the wireless output end; Grounding requires grounding; ③ Use an antenna amplifier or channel amplifier equipped with a gas discharge tube and a fast-response protection diode.
 
Lightning protection grounding of front-end equipment
If a lightning strike occurs near the front end, high voltage will be induced on the metal chassis and casing in the equipment room, endangering the safety of the equipment and personnel. The power leakage of the front-end equipment will also endanger the safety of personnel. Therefore, all equipment in the equipment room, the shielding layer of the input and output cables, and metal pipes need to be grounded. They cannot be connected to the grounding of the top-of-layer antenna. The grounding of the house lightning rod and the grounding of the power frequency AC power supply system should be connected together at the general grounding location. The electrical equipment grounding device and buried metal pipeline in the system should be connected to the lightning protection grounding device. When they are not connected, the distance between the two should be greater than 3 meters. The surface of the grounding busbar in the equipment room should be complete without obvious hammer marks and residual flux residue; Copper strip busbars should be smooth and free of burrs. The aging layer of the insulated wire should not have aging cracks. Some front-end equipment such as modulators and receivers do not have overvoltage protection, but only overcurrent protection. Once there is a lightning strike, the power supply will burn out and the insurance will continue. For this situation, a lightning arrester should be installed at the main power supply. To better protect the front-end equipment.
 
Lightning protection grounding for main lines and distribution systems
① For underground cables laid in open areas, when the number of days of thunderstorms in the area is greater than 20 days and the soil resistivity is greater than 100 ohms, the shield or metal sheath of the cable should be grounded every 2KM or so. ②The shielding layer, metal sheath and steel wire hanging wire of the overhead cable are grounded every 250M or so. The metal sheath of the overhead cable at the cable distribution box, the shielding layer and the stranded steel wire should be used together with the cable branching grounding device. . For underground cables buried in open areas, the shielding layer and sheath should be grounded every 2KM or so to prevent the influence of induced electricity. ③ When the cable enters the building, the outer conductive shielding layer of the cable should be grounded near the building. When the overhead cable is directly introduced, a lightning arrester should be added at the entrance and the outer conductor of the cable should be connected to the grounding device of the electrical equipment. When the cable is directly buried in the ground, the metal sheath of the cable should be connected to the grounding device at the household end. ④Do not lay cables directly between the roofs of the two buildings. The cables can be lowered to the lightning protection zone along the wall and should not hinder the operation of the vehicle. The suspension wire should be grounded. ⑤ The input and output terminals of the equipment in the system should have gas discharge protection tubes, and the power supply terminal of the 220V-powered amplifier should have overvoltage protection devices. At present, the amplifiers on the market are mixed, and in order to reduce costs, anti-overvoltage measures are even omitted, such as Input and output Ruiyuan gas overvoltage discharge tube, 220V power supply amplifier power supply end only has overcurrent protection, but no overvoltage protection. When selecting mains equipment, anti-overvoltage protection should be considered as an important prerequisite. ⑤The coaxial cable shielding net in the CATV system and the galvanized iron wire for overhead support cable are well grounded.
 
Lightning protection grounding of galvanized iron wire for coaxial cable shielding net and overhead supporting cable
The coaxial cable shielding net in the CATV system and the galvanized iron wire for overhead support cable are well grounded, and the chance of being induced by lightning is small. The lightning is most likely to enter the electronic equipment from the power cord and ground the power cord into the porcelain bottle iron foot. , Can play a certain role in protecting power equipment and personal safety, but because the ability of CATV and other electronic equipment to withstand overvoltage is much worse than power equipment, so in addition to the need to install a low-voltage surge arrester on the household line, you can put Light-shielded wires, cables, etc. are buried in buried metal pipes to allow lightning waves to enter the ground. The power line can be wound several times before entering the electronic device to form a small inductance, which has no effect on the 50HZ current, but has a certain effect on blocking the intrusion of lightning waves into the device.
 
Matters needing attention
When the system is grounded, we must pay attention to the minimization of the grounding resistance. If the grounding resistance is large, the lightning protection effect is poor. It is best to reduce the grounding resistance as much as possible and control it below 8 ohms.
 
The lightning protection of the cable TV system is a comprehensive technical project. If any loophole occurs, the lightning protection effect of the entire system will be affected. When doing the lightning protection design of the system, we must take a scientific and rigorous attitude and do a good job in the protection design of the system.
 

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