Knowledge about lightning protection [Daily Question 090328]

1. Lightning strikes can be classified into: direct lightning strikes, induced lightning strikes, electromagnetic pulse radiation, intrusion of lightning overvoltage, and backstroke lightning. 2. Different lightning strikes can cause varying degrees of damage. Direct lightning strike: The electrical shock phenomenon that occurs when a thundercloud discharges electricity directly to the ground through a person, building, or equipment. At this time, the main destructive force of lightning lies in the electrical current characteristics rather than the high voltage generated by the discharge. The main destructive effects include thermal effects and mechanical forces. Induced lightning strike: The hazards caused to electrical equipment and human safety by electrostatic induction and electromagnetic induction that occur in the area affected by lightning discharge. Electromagnetic pulse radiation: The harm caused to modern electronic devices such as computers and communication equipment by electromagnetic pulse radiation that occurs in the active area during lightning discharges. Intrusion of lightning overvoltage: When a direct lightning strike or induced lightning occurs, it can generate overvoltage in wires or metal conduits. This overvoltage travels along these wires or conduits from distant areas or outside the lightning protection zone, entering buildings or equipment, thereby causing damage to the building structure and equipment components or resulting in injuries to people. Counterstrike: When lightning strikes the lightning receptor of a building, the high amplitude and steep waveform of the lightning current cause the potential of the grounding conductors and grounding systems to rise to hundreds of kilovolts. This can lead to discharge between the grounding conductors of one building and those of adjacent buildings, as well as between various metal wires, pipes, or the ground wires of electrical equipment. As a result, counterstrike currents are induced in these metal wires, pipes, and electrical equipment’s ground wires, which can cause lightning-related injuries to people and damage to equipment. 3. Modern lightning protection measures include: Modern lightning protection techniques take into account the specific characteristics of various industries and facilities, and with the aim of protecting buildings, structures, equipment, and people from lightning damage, they make use of five different types of technologies: (1) Conduction – known as “lightning conductors,” whose function is to channel lightning down into the ground. (2) Lap joint – referred to as “equalization connection” or “equipotential connection”. It is an extremely important lightning protection measure that works in close coordination with the conduction measure. (3) Grounding – in combination with conduction and bonding, it forms a complete system for protecting against direct lightning strikes. Good grounding is necessary to effectively dissipate the energy of lightning into the ground, thereby reducing the voltage on the down conductor. (4) Shunting – a key measure to protect various electrical and electronic devices. It involves installing lightning arresters at all wires coming from the outside, at various wires that enter the equipment, and along the internal wiring, in order to achieve multi-level current diversion. (5) Shielding – Enclose the object to be protected with conductive materials such as metal mesh, foil, shells, or tubes, thereby blocking all pathways for lightning electromagnetic pulse waves to invade from outside, so that lightning has no chance to get through. As China’s modernization process accelerates, there is an increasing number of electronic communication devices. On the one hand, system equipment such as large-scale computer networks and program-controlled switching systems is becoming increasingly vulnerable to excessive currents and lightning voltages. On the other hand, with more paths through which signals can travel, these systems are more susceptible to lightning strikes than before, resulting in frequent lightning-related disasters. The damage caused by lightning strikes to communication systems and network information systems can range from the destruction of cables to the damage to equipment, leading to system failures and incalculable losses. Therefore, the importance, urgency, and complexity of modern lightning protection have **increased**; lightning defense has evolved from protecting against direct strikes to providing system-wide protection, with an emphasis on comprehensive protection, integrated management, and multiple layers of defense. Therefore, modern lightning protection requires higher standards in both design and construction compared to traditional methods; it is necessary to employ five different types of technology in order to enhance the overall capacity to defend against lightning disasters.

I. How is lightning formed? Answer: Lightning is a phenomenon of discharge in the atmosphere that occurs during thunderstorms. As thunderclouds form, some of these clouds carry a positive charge while others carry a negative charge. Their electrostatic induction on the ground causes opposite charges to be generated on the surface of the ground or buildings. When these charges accumulate to a certain level, the electric field strength between different charge clusters, or between those clusters and the ground, can break down the air (usually 25-30 KV/cm), leading to free discharge – what we refer to as \"leader discharge\". The leading discharge from the cloud to the ground represents the gradual progression of the cloud toward the ground; when it reaches the ground (such as buildings or overhead power lines), a main discharge occurs in the reverse direction, from the ground back to the cloud. During the main discharge phase, due to the intense neutralization of opposite charges, very large lightning currents are generated (typically ranging from several dozen kiloamperes to several hundred kiloamperes), accompanied by intense lightning flashes and loud noises – this is what constitutes lightning. II. What is step voltage? Answer: Step voltage occurs when lightning strikes a surface object; the lightning current flows into the ground and spreads throughout the soil. Due to the varying resistivity of the soil, a potential difference arises between different points on the ground. The closer to the point where lightning struck, the greater the current density, and consequently the greater the potential difference. If a person stands or walks near the location where lightning strikes, the potential difference between their feet can allow the lightning current to flow through their feet and the lower part of their torso, causing injury. The potential difference between these two feet is called the \"step voltage\". III. Why, in one type of lightning protection, must the distance between the installed standalone lightning rod (including its lightning protection grounding system) and the building to be protected be at least 3 meters? Answer: To prevent backflow to the protected object when the independent rod is struck by a direct lightning strike. IV. What is a voltage equalizing ring? What are the requirements for the design of equipotential rings in building lightning protection design? Answer: A equipotential ring is a horizontal lightning protection belt surrounding a high-rise building, designed to protect it from side lightning strikes. In architectural design, when the height exceeds the rolling sphere radius (30 meters for Category 1, 45 meters for Category 2, and 60 meters for Category 3), a equipotential ring is installed every 6 meters. In terms of design, the equalizing ring can be formed by welding the two main rebar bars inside the ring beam into a closed loop, and this closed loop must be connected to all down conductors. It is required to install a equalization ring every 6 meters, with the purpose of facilitating the connection of the metal doors and windows on the upper and lower floors within a 6-meter height range to the equalization rings. V. What are the requirements for down conductors and roof grids in various types of lightning protection? Answer: Down conductors and roof grids are usually made of galvanized round steel with a diameter of not less than φ8. For categories I, II, and III, the spacing between the down conductors shall not exceed 12 meters, 18 meters, and 25 meters respectively ； The roof grids corresponding to categories one, two, and three are 5*5 square meters (4*6 square meters), 10*10 square meters (8*12 square meters), and 20*20 square meters (16*24 square meters) respectively. VI. In areas with high soil resistivity, what methods are suitable for reducing the grounding resistance of lightning protection systems? Answer: According to Clause 4.3.4 of Standard P26, in areas with high soil resistivity, one of the following methods can be used to reduce the grounding resistance: (1) Use a multi-wire external grounding system, where the length of the external wires should not exceed the effective length, that is, le=2 ρ. (2) The grounding electrode is buried in deeper soil with low resistivity. (3) Use a resistivity reducer. (4) Soil replacement. VII. What is the phenomenon of lightning backstroke? How to eliminate the counterattack phenomenon? Answer: The backflow phenomenon of lightning usually refers to the situation where a metal object struck by direct lightning (including lightning receptors, grounding conductors, and grounding electrodes) experiences a very high voltage between itself and the ground at the moment of impact. This voltage causes discharge (also known as flashover) to other metal objects that are connected to the ground. Furthermore, when lightning strikes a tree, the high voltage on the tree can also cause backflow to nearby houses and metal objects. To eliminate the backflow phenomenon, two measures are usually taken: one is to make an equipotential connection, using metal conductors to link two metal parts together so that their potentials are equal when lightning strikes them ； Second, maintain a certain distance between the two. VIII. What are the requirements for metal oil tanks in terms of protection against direct lightning strikes? Answer: Requirements for metal oil tanks regarding protection against direct lightning strikes: (1) For oil tanks storing flammable and combustible substances, if the thickness of their metal walls is less than 4 millimeters, measures to protect them against direct lightning strikes must be implemented (such as installing lightning rods) ； (2) For oil tanks storing flammable and combustible materials, if the thickness of their metal walls is ≥ 4 millimeters, lightning protection devices are not required; however, in areas with frequent lightning strikes, such devices may still be considered. (3) Flame arresters must be installed on the breather valves and safety valves of fixed-roof metal oil tanks. (4) All metal oil tanks must be equipped with circular lightning protection grounding systems, with no fewer than two grounding points; the arc distance between these points shall not exceed 30 meters, and the distance between the grounding electrodes and the tank walls should be greater than 3 meters. (5) When the tank is equipped with a lightning rod or used as a lightning receptor, the grounding impulse resistance shall not exceed 10 ohms. 9. What materials are commonly used for cathodic protection systems? Why? Answer: Cathodic protection devices usually use magnesium alloys or zinc alloys. Since magnesium alloys or zinc alloys are metal elements that are more reactive than iron, when specially processed blocks of these alloys are connected to the metal tank (iron) that is to be protected, the negative ions from the magnesium or zinc alloys move continuously through the connecting conductors toward the metal tank buried underground. This allows the metal tank to acquire a certain amount of negative ions from the magnesium or zinc alloys, thus becoming a cathode, while the magnesium or zinc alloys continue to lose their negative ions, exhibiting anodic properties. It is precisely because of these more active magnesium or zinc anions that continuously move toward the metal storage tank, thereby compensating for the corrosion of the tank. Over the years of use, magnesium or zinc alloys lose their ability to resist corrosion and sacrifice themselves; hence, this type of device is also known as a sacrificial magnesium (zinc) anode, a device used to protect the cathode (the tank body). 10. What components are included in lightning protection measures? Answer: It mainly includes three aspects: protection against direct lightning strikes, protection against side lightning strikes, and protection against induced lightning strikes. Technical measures such as lightning arresters, current diversion, shielding, voltage equalization, equipotential connection, and grounding are employed. Only by understanding any application in detail can we take precautions; everyone should learn more!

There are mainly four types of lightning: direct lightning, induced lightning, lightning wave intrusion, and ball lightning. What harms can lightning cause? 1. Lightning generates a strong electric current; when this current passes through objects in an instant, it creates high temperatures that can lead to burning and melting ； When it comes into contact with humans or animals, it can cause injuries or deaths. 2. The effects of lightning explosions and static electricity can cause trees, utility poles, buildings, and other structures to be split in half and collapse. 3. During thunderstorms, air at temperatures of tens of thousands of degrees expands rapidly, generating shock waves that possess considerable destructive power. 4. Lightning current creates a strong electromagnetic field in the surrounding space. Electromagnetic induction can cause spark discharge at the openings of conductors, and if there are flammable or explosive materials present, this can lead to explosions or fires. 5. Various power lines, telephone lines, and broadcast lines generate high voltage due to lightning strikes, resulting in damage to electrical equipment. Preventive measures mainly include three categories: protection against direct lightning strikes, protection against side lightning strikes, and protection against induced lightning strikes. Technical measures such as lightning arresters, current diversion, shielding, voltage equalization, equipotential connection, and grounding are employed. Article 19 of the “Measures for Lightning Protection and Disaster Reduction Management” (Order No. 8 of the Meteorological Bureau): “Regular inspection systems shall be implemented for lightning protection devices once they are put into use.” The inspection of lightning protection devices should be carried out once a year, while those in areas with explosion hazards should be inspected every six months. ” Lightning protection focuses on safeguarding the building itself from damage caused by lightning, as well as reducing the various effects that occur within the building’s interior when the massive lightning current flows along the building’s structure toward the ground. Protection against direct lightning strikes primarily relies on individual rods (for low-rise buildings). Measures to protect buildings from direct lightning strikes should include lightning rods, strips, nets, down conductors, equipotential rings, equipotential connections, and grounding electrodes. 　　The purpose of protecting against induced lightning is to limit the lightning pulses generated in various metal pipes and wires that enter a building when there are intra-cloud discharges, inter-cloud discharges, or cloud-to-ground discharges, thereby safeguarding the safety of people inside the building as well as various electrical equipment. The measures to be taken should be based on the specific conditions of various devices. In addition to having a proper grounding and wiring system as well as appropriate safety distances, corresponding surge protectors must be installed according to the characteristics of the power supply lines, power cables, signal lines, communication lines, and feed lines, and shielding measures must also be adopted.

There are mainly four types of lightning: direct lightning, induced lightning, lightning wave intrusion, and ball lightning. What harms can lightning cause? 1. Lightning generates a strong electric current; when this current passes through objects in an instant, it creates high temperatures that can lead to burning and melting ； When it comes into contact with humans or animals, it can cause injuries or deaths. 2. The effects of lightning explosions and static electricity can cause trees, utility poles, buildings, and other structures to be split in half and collapse. 3. During thunderstorms, air at temperatures of tens of thousands of degrees expands rapidly, generating shock waves that possess considerable destructive power. 4. Lightning current creates a strong electromagnetic field in the surrounding space. Electromagnetic induction can cause spark discharge at the openings of conductors, and if there are flammable or explosive materials present, this can lead to explosions or fires. For relevant standards, refer to the Code for Design of Lightning Protection of Buildings

1. (1) Lightning occurs when thunderclouds approach the ground, causing the surface to develop opposite charges. When these charges accumulate to a certain level, discharge takes place between the clouds themselves as well as between the clouds and the ground, resulting in light and sound. Lightning can be divided into direct lightning, induced lightning (including electrostatic induction and electromagnetic induction), and ball lightning. 　　Direct lightning strike: When lightning hits a building directly, it generates electrical effects, thermal effects, and mechanical forces. In areas affected by thunderstorms, the electrical discharge that occurs when thunderclouds discharge energy to the ground through people, buildings, or equipment is referred to as a direct lightning strike. At this time, the main destructive force of lightning lies in the electrical current characteristics rather than the high voltage generated by the discharge. When lightning strikes a person, building, or equipment, the intense lightning current is converted into heat energy. The charge amount of a lightning discharge is approximately 25~100 C. Based on this estimation, the heat generated at the lightning strike point is approximately 500–2000 J. This energy is sufficient to melt 50–200 mm3 of steel; therefore, the high-temperature effect of lightning current can burn human bodies, cause buildings to catch fire, and melt equipment components. The water in objects located along the path taken by the lightning current vaporizes due to the heat, resulting in intense expansion and the generation of powerful mechanical forces. This mechanical force can reach 5,000 to 6,000 N, enabling it to break human tissues, building structures, and equipment components, thereby causing injuries and deaths, damage to buildings, and destruction of equipment. 　　Induced lightning: The damage caused by induced lightning is also known as secondary damage. The rapid changes in lightning current create a strong alternating magnetic field, which induces currents in the surrounding metal components. These currents can discharge into nearby objects; if there are flammable materials present, this can lead to fires and explosions. Meanwhile, when such currents affect wires that are connected to equipment, it can cause severe damage to that equipment. (2) Hazards of lightning The hazards of lightning arise from its thermal effects, mechanical effects, overvoltage effects, and electromagnetic effects. The thermal effect of lightning: The lightning current generates considerable Joule heat within the object struck, causing a very rapid rise in its temperature, which in turn leads to the burning or melting of that object. Normally, although the peak value of lightning current is very high, its short duration results in only localized instantaneous high temperatures, which cause the metal in a small volume at the point of impact to melt. For large volumes of metal, the melting capacity resulting from the thermal effects of lightning current is quite limited. If the overhead conductor struck by lightning is of thin diameter, it may break. After being hit by lightning, small dents appear on the surface of the lightning rod, but this does not cause any significant damage to the rod as a whole. If a lightning strike occurs in a flammable or explosive area, it can cause a fire due to the high temperatures. Especially spherical bombs; wherever they passed, almost everything was burned to ashes. The fire in the Daxinganling region of Heilongjiang in May 1987, the fire in the Daxinganling region of Inner Mongolia in August 2002, and the forest fire in Dali, Yunnan this April were all caused by lightning, resulting in direct economic losses of nearly 10 billion yuan. Mechanical effects of lightning An isolated conductor carrying an electric current is subjected to an inward radial self-compression force. When the magnetic field strength at the surface of the conductor becomes very high, severe mechanical distortion occurs. Radial self-compression force also causes the temperature of the object struck by lightning to rise, but the main factor contributing to this temperature increase is the Joule heat generated by the lightning current. When lightning strikes an object, the yield point of the material decreases due to Joule heating, and the radial self-compression force may exceed this yield point. As a result, the material of the struck object deforms, or different materials that are combined together may separate, delaminate, or come apart. At the same time, self-compression force is also the cause of spherical lightning. During a lightning strike, the current path is filled with hot air molecules as well as positive and negative ions. The lightning current generates a strong magnetic field, and the positive and negative ions in this path are compressed due to the Lorentz force; this is what is known as self-compression force. Due to some differences in the channels, the more fragile parts of them will break apart, and the shiny channels turn into individual fireballs. The particularly hot fireballs last longer than the others, and almost always only one such highly hot fireball is visible to us. This is the spherical bomb we see. If most of the fireballs are extremely hot, we will see many spherical bolts arranged in a row, like a string of beads. Since the channel itself also expands due to heat, spherical lightning cannot be seen frequently. Two adjacent conductors carrying current exert a force on each other; the magnitude of this force is proportional to the product of the currents in the two conductors, and inversely proportional to the distance between them. For the same bent conductor or metal component carrying lightning current, the smaller the bending angle, the greater the force exerted on it, making it easier for the wire to break. Therefore, the routing of lightning protection ground wires should follow a straight path as much as possible. Where bends are necessary, obtuse angles should be used with curved paths downward, avoiding sharp angles and right angles when routing downward. 　　During a ground flash, there is intense air ionization as well as strong charge neutralization in the channel; the instantaneous temperature in this channel is also very high, causing the air surrounding it to expand rapidly and spread out at ultrasonic speeds, thereby generating shock waves. Where the shock wave front arrives, the density, pressure, and temperature of the air increase suddenly, resulting in intense vibrations. This type of shock wave is similar to those generated by explosions, and it can cause damage or injury to nearby buildings, people, and animals. The overvoltage effect of lightning: Before a ground flash occurs, thunderclouds appear in the sky. Due to electrostatic induction, the ground (buildings or other objects) directly beneath a thundercloud develops a positive charge of opposite sign. If there are large areas of metal buildings beneath thunderclouds and they are insulated from the ground, then under the high voltage generated by electrostatic induction, these metal structures can cause spark discharges to certain grounded objects below them, leading to damage to equipment and personnel as well as injuries or deaths; fires may also be triggered. If the grounding lead of the top metal body is broken or has excessive resistance in certain areas, high voltages will appear in those areas as well, causing localized spark discharge and posing a risk to the safety of equipment and personnel inside the building. To reduce the severity of the hazards caused by lightning-induced electrostatic induction, it is necessary to properly ground the metal components on the roof of a building, so as to discharge the induced charges into the ground as quickly as possible. Even with a grounding wire, due to the high magnitude and rapid variation of lightning current, the inevitable inductance and resistance of the grounding wire result in extremely high voltages being generated across it. When lightning strikes a large tree or other objects, the lightning current passing through them also generates overvoltage. 　　When lightning strikes a power line, the lightning current must flow through the line into the ground. Even if lightning does not strike the power lines, after a lightning strike, the opposite-charged particles induced on the wires lose their restraint and flow toward both ends of the wires. These currents enter the substation through the lines or attack electrical equipment, causing overvoltage on the equipment. When the overvoltage exceeds the device’s rated lightning impulse withstand voltage, the device gets damaged. Electromagnetic effects of lightning: As the lightning current changes from 0 to tens of thousands of amperes within a time period of 50–100 microseconds, and then back from tens of thousands of amperes to 0, a strong transient electromagnetic field is generated in the space surrounding it. An object placed in a spatially varying strong electromagnetic field develops a very high induced electromotive force within it due to electromagnetic induction. Previously, it did not attract attention due to its extremely low likelihood of causing problems; however, with significant advances in microelectronics and the emergence of very large-scale integrated circuits, their characteristics such as low power consumption and high sensitivity made them prone to damage, which drew people’s attention ； At the same time, lightning can emit electromagnetic waves with frequencies ranging from a few hertz to several thousand hertz, covering a wide frequency band, with the electromagnetic radiation intensity being highest at 5–10 kilohertz. [17] These electromagnetic waves can cause serious damage to communication equipment; in mild cases they interfere with television broadcast signals, while in more severe cases they disrupt command systems and damage instruments and equipment. During a lightning strike, in areas close to the site of the strike, the hazards caused by electrostatic induction are the primary concern ； At a distance from the location of the lightning strike, the hazards caused by electromagnetic induction are the primary concern. (3) Protection measures against lightning 　　Given the immense destructive power of lightning, it inevitably affects people’s production and daily life. To enable people to produce and live better, we must implement proper protective measures. Artificial induction of lightning: There are many methods for artificially influencing lightning, which can be broadly divided into three categories. First, when the clouds reach a certain volume, frozen nuclei are scattered into them – usually AgI is used – to increase the weight of the clouds rapidly, thereby inducing precipitation. This shortens the time during which the thunderclouds accumulate charge, preventing lightning from forming. In reality, it is a process of artificial precipitation. 　　II. When thunderclouds appear, metal foil is scattered to increase the conductivity within the clouds, keeping the electric field there below the level required for lightning to occur, thereby suppressing the formation of lightning. 　　III. Use a laser or rocket to guide a wire connected to the ground into the clouds, allowing the energy of lightning to be released into the ground through this wire, which functions like a horse’s tail. 　　The first and second methods are mainly used to prevent forest fires caused by lightning, while the third method is primarily intended to provide a clear path for aerial vehicles to pass safely through charged objects. [17] Building on the concept of \"artificial lightning induction,\" scientists further developed the \"directional lightning induction vehicle,\" which is equipped with a \"lightning monitor,\" \"lightning induction rockets,\" rocket launchers, a remote control igniter, and communication equipment. As needed, it can be driven to the designated location, where instruments are used for monitoring. Once thunderclouds form and pose a threat to the \"protected area,\" the command center immediately orders the launch of \"thunder induction rockets\" to trigger lightning at high altitudes, thereby preventing it from occurring in the protected zone. Lightning protection for buildings: Artificial lightning induction can be an effective method for protecting against lightning, but it is very costly, and not everyone is capable of carrying out this task. Therefore, we must take other lightning protection measures. For buildings, the most common and simplest lightning protection measure is to install lightning rods. A lightning rod actually consists of three parts: a lightning receptor, downconductors, and a grounding electrode. The basic principle is as follows: when a thundercloud is above the ground, electrostatic induction causes the ground to develop charges of opposite sign to those in the cloud. Since charges tend to concentrate at sharp points, the induced charges accumulate at the tip of the lightning rod. The air around it is ionized under the influence of this strong electric field, and lightning then flows along this ionized area and the lightning rod. Therefore, more accurately, a lightning rod should be called a “lightning conductor”. A lightning rod is a metal rod with a pointed or forked tip. If a metal wire is used in place of this metal rod, it becomes what we commonly call a \"lightning conductor\" or \"lightning protection strip\". If a metal frame that covers the entire building is used in place of this metal rod, it becomes what we commonly call a lightning protection mesh. There is also a lightning protection system that uses the isotope americium placed around electrodes; these electrodes emit charges under the influence of spatial electric fields, attracting thunderclouds to discharge onto the electrodes, thereby protecting buildings. Various lightning protection devices are essentially just variations of lightning rods. Since a voltage drop is generated when current flows through a resistor, and lightning current must reach the ground via lightning conductors, downconductors, and grounding electrodes, the resistance of the entire lightning protection system must not exceed 10 ohms; ideally, it should be below 5 ohms. Therefore, the down conductor cannot be too thin; on one hand, the thinner the wire, the greater its resistance, and on the other hand, overly thin wires are prone to breaking, which could otherwise lead to lightning entering the building. To reduce the resistance of the lightning protection system, the depth at which the grounding electrode is buried in the ground must be no less than 1 meter. Due to the voltage drop, it is also required that the grounding wires of all lightning protection systems must not be shared with those of electrical appliances. The protection range of a single lightning rod is similar to that of a conical tent with a pointed roof, or like an umbrella that has been opened. The diameter of the cone at ground level is generally 1 to 1.5 times the height of the lightning rod; the space inside this cone constitutes the safe area around the lightning rod. [5] Therefore, to ensure the lightning-conducting capacity of the lightning rod, it is necessary to have sufficient height. To protect a large building from lightning strikes, multiple lightning rods are often used. The protection area covered by these rods is equal to the total protection area resulting from the combination of the individual protection areas of each rod. Lightning rods are primarily used to prevent direct lightning strikes. To prevent the high voltage generated by induced lightning, the structural rebar of the building should be grounded. In addition, the roofs of buildings should also be properly grounded: for reinforced concrete roofs, the rebar on the roof should be welded together to form a network spanning 6–12 meters, creating a continuous path that can be grounded ； For non-metallic roofs, a metal mesh with a side length of 6 to 12 meters should be installed on the roof to form a continuous network, which must then be grounded. To prevent the effects of electromagnetic induction, when parallel pipes are less than 0.1 m apart, metal wires must be used for bridging every 20–30 m. Metal wires should also be used for bridging when the distance between the cross wires is less than 0.1 m. By utilizing the shielding effect of a metal frame on electromagnetic fields, what is essentially created is the so-called \"lightning protection net\" mentioned above. Forest lightning protection: Forest fires caused by lightning occur around the world, and many forest fires are triggered by lightning strikes. Forest fires caused by lightning strikes usually occur in remote areas. Once a fire breaks out, it is difficult to detect it in time, often resulting in significant economic losses for people. For the Dushan Forest, we should install lightning rods on the mountaintops to prevent lightning strikes from causing fires in the forest. For mountainous forests, we should lay a \"lightning protection belt\" on the mountaintops. As a precaution, reservoirs should be built in appropriate locations in the forest. It facilitates fire extinguishing in case of forest fires. Lightning protection for electrical equipment: Inside buildings, a large number of various conductive circuits are installed. When a building is struck by lightning or when lightning strikes in its vicinity, the lightning current generates transient magnetic field pulses within the building’s interior. These magnetic field pulses interfere with the aforementioned circuits, inducing transient overvoltages in them, which can endanger the devices connected to those circuits. Although buildings are equipped with lightning protection measures, gaps will inevitably appear in them. For example, large areas of doors and windows can allow electromagnetic waves to enter and exit freely, and power transmission lines can draw lightning into the building. Some electrical appliances are even used outdoors, so it is necessary to take measures to protect them from lightning. Power systems, being located at high altitudes and exposed, are the most susceptible to lightning strikes. At the beginning of the 20th century, when electric power systems were applied to transmission lines, lightning conductors were invented. A lightning conductor is one or two grounded steel wires installed above the transmission lines, which are what are referred to as “lightning conductors” earlier on. Due to its simplicity and effectiveness, this type of lightning conductor has also been adopted in building construction; it is installed in areas of buildings that are prone to lightning strikes to serve as a means of protecting against lightning. Now, power supply lines use underground cables, which greatly helps with lightning protection. However, high-voltage transmission still requires overhead lines, so lightning protection wires must be used. 　　People usually use an overvoltage protector to protect electrical appliances. Its lightning protection principle is as follows: This type of protector has two ports; the live wire and the neutral wire of the electrical supply are connected to separate ports of the protector, while the other end of each protector is grounded. Discharge to the ground is achieved through gap breakdown. At the normal operating voltage, the arrester gap does not break down. However, when overvoltage travels along the wires and poses a threat to the protected equipment, the arrester gap breaks down rapidly, allowing discharge to the ground; this enables a large amount of charge to be released into the earth, thereby limiting the input voltage to the protected equipment and serving to protect it. After the overvoltage passes, the gap can recover quickly, allowing the electrical equipment to function properly. Some high-end electrical appliances come equipped with protectors, but many others do not have such a feature. Therefore, a protector should be installed on the incoming household wire. During thunderstorms, if there are no special power requirements, it is best to turn off the main circuit breaker at the entrance. 　　With the development of microelectronics technology, integrated circuits are widely used in electrical appliances. The electromagnetic effects that occur during lightning strikes are a menace to integrated circuits. And today, computers are the devices that make the most use of integrated circuits. To prevent damage caused by electromagnetic induction, the casings of computer hosts are generally made of metal. Utilizing the electrostatic shielding of metals and the skin effect on electromagnetic waves provides excellent protection against induced lightning. Human protection against lightning: Indoors, if a building does not have lightning protection measures, one should avoid touching any metal objects during thunderstorms, as lightning tends to discharge into metals. At this time, power supply is usually cut off, so do not use a flashlight. Even if there are lightning protection facilities indoors, do not touch metal pipes or wires; for example, avoid using the phone, touching the TV antenna cables that come from outside, or electrical wiring. Because when a lightning strike occurs, it induces high voltages on nearby metal pipes and wires, so it is best to keep a distance from them. Especially the grounding wire far from the lightning rod. If inside a pavilion, one should also stay away from the pillars, as when lightning strikes the pavilion, the lightning current will inevitably flow along the pillars into the ground, generating high voltage. When a thunderstorm occurs, close the doors and windows to prevent ball-shaped lightning from entering the house. Even if ball-shaped lightning does get inside, there’s no need to panic; do not run around, as it is actually just a mass of hot air that will dissipate over time. If you have something in your hands, you can throw it to distract the ball-shaped lightning. Outside, in the event of a thunderstorm, one should quickly find a building equipped with lightning protection or a building with a large metal frame to take shelter in. You can run, but definitely not in big steps, as this increases the risk of electric shock from step voltage. But it’s best not to run; instead, squat with your feet together, place your hands on your knees, and lean forward. When walking, one should stay away from large trees, utility poles, towers, chimneys, metal buildings, etc., as lightning current passing through these objects creates voltage on them. One should also not stand next to metal structures. Do not stand in open areas such as fields, parking lots, sports fields, swimming pools, lakes, or beaches; especially avoid being alone on high mountains, as this can turn a person into a kind of \"lightning rod.\" If a person is alone in a thunderstorm area and feels their hair standing on end, it is a sign that thunderclouds are above them; in such cases, they should immediately lower their head, squat with their feet together, bend forward, and wrap their arms around their knees. The purpose of doing this is to prevent the human body from becoming a site where ground-induced charges accumulate. But in this situation, one cannot lie down, because if a lightning strike occurs nearby, it can cause electric shock due to step voltage. It is best not to use a mobile phone during thunderstorms, as its antenna is a metal tip that can act as a \"lightning rod.\" But if we are inside a building with lightning protection measures, we can use our mobile phones, because radio waves do not ionize the air, and thus do not attract lightning. If you encounter a spherical thunder bomb in the wild, do not panic or run around. If there are stones nearby, slowly pick one up and throw it to distract the thunder bomb. When we are in the car, we don’t have to worry about being struck by lightning, because the metal body of the car provides good protection against lightning; that’s why people in a car during a thunderstorm need not fear being struck by lightning. But never stick your head or hands out of the car window. This is because the head and hands act as the car’s \"lightning rods\". When a person is struck by lightning, the strong electric current flowing through the body often causes a state of apparent death, with breathing stopping and the heart ceasing to beat. In such a situation, artificial respiration should be used immediately to save the person; once the person falls to the ground, the charge on their body has flowed into the ground, so there is no need to worry about the person still being charged. 2. A comprehensive inspection should be carried out every year before the onset of the thunderstorm season. In areas with intense lightning activity, lightning protection devices should be visually inspected at all times. Check the electrical continuity of the external lightning protection devices; if any signs of desoldering, loosening, or rusting are found, appropriate actions should be taken. In particular, electrical continuity measurements should be conducted at the disconnect clips or grounding test points. Inspect the corrosion and mechanical damage of lightning rods, lightning conductors (nets, wires), poles, and downconductors, including damage caused by lightning discharges. If there is damage, it should be repaired promptly ； When the corroded area exceeds one-third of the cross-section, it should be replaced. Measure the grounding resistance of the grounding device; if the measured value is higher than the specified value, the grounding device and soil conditions should be inspected to identify the causes of the variation, and effective corrective measures should be taken. Check the electrical continuity of the equipotential connection of internal lightning protection devices and equipment such as metal enclosures and frames; if any looseness or disconnection is detected at the connections, it should be repaired promptly. Check the operation of the surge protector: look for issues such as poor contacts, excessive leakage current, overheating, poor insulation, or excessive dust accumulation; any faults that arise should be resolved promptly.

Types of lightning: (1) Direct lightning strike； (2) Induced lightning ； (3) Ball bomb ； Dangers of lightning: (1) Fires and explosions ； (2) Electric shock ； (3) Destruction of equipment and facilities ； (4) Large-scale power outages – Protection against direct lightning strikes: This is achieved primarily by installing lightning rods, lightning conductors, lightning grids, and lightning strips. Protection against secondary discharges: Whether in the air or underground, it is essential to ensure that there is sufficient safety distance between the lightning receptors, downconductors, grounding devices, and adjacent conductors. For buildings in different categories, it should be no less than 2m and 3m respectively. Protection against induced lightning: Connect non-energized metal equipment and metal structures together and ground them, and bridge parallel pipes or pipes that are less than 100 mm apart with metal wires. Lightning impulse wave protection: Valve-type arresters are installed at the line inlet, with the lower end grounded. Lightning protection for building power supply: (1) Grounding of the metal outer sheath of the incoming cable ； (2) Power supply via overhead transmission cables; valve-type arresters are installed at the connection point between the overhead lines and the cables, and these arresters are reliably grounded ； (3) Valve-type arresters or sufficient protection gaps shall be installed at the point where the overhead line supplies power to the premises, and reliable grounding shall be provided.

The topic is a bit too broad; it’s recommended that each daily question be solvable within about 15 minutes

I. There are mainly four types of lightning: direct lightning, induced lightning, lightning wave intrusion, and ball lightning. II. Hazards caused by lightning: 1. Lightning generates a strong electric current; when this current passes through objects in an instant, it creates high temperatures that can cause burning and melting ； When it comes into contact with humans or animals, it can cause injuries or deaths. 2. The effects of lightning explosions and static electricity can cause trees, utility poles, buildings, and other structures to be split in half and collapse. 3. During thunderstorms, air at temperatures of tens of thousands of degrees expands rapidly, generating shock waves that possess considerable destructive power. 4. Lightning current creates a strong electromagnetic field in the surrounding space. Electromagnetic induction can cause spark discharge at the openings of conductors, and if there are flammable or explosive materials present, this can lead to explosions or fires. 5. Various power lines, telephone lines, and broadcast lines generate high voltage due to lightning strikes, resulting in damage to electrical equipment. III. The main preventive measures include three main aspects: protection against direct lightning strikes, protection against side lightning strikes, and protection against induced lightning strikes. Technical measures such as lightning arresters, current diversion, shielding, voltage equalization, equipotential bonding, and grounding are employed. IV. Lightning protection inspection cycle: Article 19 of the “Measures for the Management of Lightning Protection and Disaster Reduction” (Order No. 8 of the Meteorological Bureau) states: “Regular inspections shall be carried out on lightning protection systems once they have been put into use.” The inspection of lightning protection devices should be carried out once a year, while those in areas with explosion hazards should be inspected every six months. ” 5# The answer is very comprehensive. Last edited by eleg on 2009-3-29 08:55.]