Thread Content
:) 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 appear 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 at 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 a 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 pin 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 rebars within 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 aim 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 phenomenon of backstroke in 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 occur 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 in terms of protection against direct lightning strikes: (1) For oil tanks storing flammable and combustible materials, 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, it may not be necessary to install lightning protection devices; however, in areas with frequent lightning strikes, such devices can still be considered for installation. (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 in the ground. This allows the metal tank to acquire a certain amount of negative ions from the magnesium or zinc alloys, thereby becoming a cathode, while the magnesium or zinc alloys lose their negative ions and exhibit 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 bonding, and grounding are employed. XI. What is the purpose of protection against direct lightning strikes? According to the requirements of modern lightning protection technology, what measures are taken for protecting against direct lightning strikes? Answer: Direct lightning protection serves to safeguard the building itself from damage caused by lightning, as well as to reduce the various effects that occur within the building when the massive lightning current flows along the building into the ground following a lightning strike. 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, meshes, down conductors, equipotential rings, equipotential bonding, and grounding electrodes. XII. What is induced lightning? What is the purpose of protecting against induced lightning? What protective measures should be taken? Answer: The protection measures against induced lightning involve restricting the lightning pulses generated in various metal pipes and wires that lead into buildings when there are intra-cloud discharges, inter-cloud discharges, or cloud-to-ground discharges, thereby safeguarding the safety of people inside the buildings 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 lightning arresters 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 employed. 13. What are the types of grounding? Answer: In addition to lightning protection grounding, there are also types of grounding such as AC operating grounding, protective grounding, DC grounding, overvoltage protection grounding, anti-static grounding, shielding grounding, and so on. 14. What are the grounding methods and requirements for grounding resistance of electronic devices? Answer: The grounding methods for electronic devices include independent grounding and shared grounding. Unless otherwise specified, the independent ground resistance value is generally not greater than 4 ohms, and a single-point grounding method is adopted. The grounding of electronic equipment should be integrated with the lightning protection grounding system, but its grounding resistance should not exceed 1 ohm. If separated from the lightning protection ground, the distance between the two grounding systems should not be less than 20 meters. 15. What is a lightning electromagnetic pulse? Answer: Lightning current and lightning electromagnetic fields as interference sources. 16. How should equipotential treatment be carried out at the boundaries between lightning protection zones? Answer: For equipotential bonding at the boundaries of the lightning protection zones LPZOA, LPZOB, and LPZO1, all external conductive elements that enter the building must be connected to each other at equal potential. When external conductive objects and power lines, as well as communication lines, enter the building at different locations, several equipotential bonding strips must be installed. These strips should be connected to the ring grounding electrode as closely as possible, and they should also be connected to the rebar and metal facades. If no ring grounding electrode is installed, these equipotential bonding strips shall be connected to their respective grounding electrodes, and interconnected by an internal ring conductor (or by a pair of ring conductors). For conductive objects that enter from above the ground level, the equipotential bonding strip should be connected to a horizontal ring conductor located on the inside or outside of the wall; when there are down conductors and rebar present, this horizontal ring conductor must be connected to those down conductors and rebar. When external conductive materials enter a building via power lines or communication lines from the ground, it is recommended to make an equipotential connection at the same location; this is particularly important for buildings with little or no shielding. The equipotential bonding strip located at the point of entry into the building should be connected as closely as possible to the grounding electrode; when there are rebar bars present, it should be connected to those rebar bars. For the equipotential bonding at the junctions between subsequent lightning protection zones, the aforementioned principles also apply to the equipotential bonding at the junctions of these zones. All conductive objects entering the boundary of the lightning protection zone, as well as power lines and communication lines, are equipotentially connected at that boundary. A local equipotential bonding strip should be used for equipotential bonding; various shielding structures or other local metal components (such as equipment enclosures) should also be connected to this local equipotential bonding strip for equipotential bonding. 17. What is the working principle of zinc oxide arresters? Answer: The zinc oxide ZnO arrester is a new type of arrester that was developed in the 1970s; it is primarily composed of zinc oxide varistors. Each varistor has a specific switching voltage inherent to it from the time it is manufactured (this is known as the varistor voltage). At normal operating voltages (that is, below the varistor voltage), the value of the varistor is very high, resulting in an insulating state. However, when subjected to surge voltages (above the varistor voltage), the varistor assumes a low value and breaks down, resulting in a short-circuit condition. However, the varistor can be restored to its normal state after being damaged ； When the voltage higher than the pressure-sensitive voltage is removed, it returns to its high-resistance state. Therefore, when zinc oxide arresters are installed on power lines, during a lightning strike, the high voltage of the lightning wave causes the varistors to break down. The lightning current then flows through these varistors into the ground, thereby keeping the voltage on the power lines within a safe range and protecting electrical equipment from damage. 18. What is a lightning protection zone? How to divide the space that needs protection into different lightning protection zones (LPZ)? What are the characteristics of each lightning protection zone? Answer: An lightning protection zone (LPZ) is those areas where the electromagnetic environment caused by lightning needs to be defined and controlled. Based on the severity of lightning electromagnetic pulses in different areas of the space, as well as the location of equipotential connection points at the boundaries between various zones, the space to be protected is divided into different lightning protection zones labeled OA, OB, 1, and 2. The characteristics of each lightning protection zone are as follows: LPZOA zone: All objects within this zone may be struck by lightning directly, and therefore all of them may conduct the entire lightning current. The electromagnetic field in this area has not attenuated. LPZOB zone: Objects within this zone are not likely to be struck by lightning directly, but the electromagnetic field in this zone does not attenuate. LPZO1 zone: Objects in this zone are not likely to be struck by lightning directly. The current flowing through the conductors is further reduced compared to the LPZOB zone, and the electromagnetic field in this zone may also diminish, depending on the shielding measures in place. LPZO2 zone (subsequent lightning protection zone): At the points where a cable passes from one lightning protection zone to another, equipotential bonding must be carried out at each junction. LPZO2 is constructed in this way to prevent lightning current from entering or passing through this space. 19. What are the main types of lightning arresters? Answer: Lightning arresters can be basically divided into three main types. The first type is power supply lightning arresters, which are installed either in parallel or in series; depending on the voltage, they come in 22V single-phase versions and 380V three-phase versions. The second type is signal arresters, which are mostly used in computer networks and communication systems, and are installed in series. Third is the antenna feedline arrester, which is suitable for systems with transmitter antenna systems and equipment for receiving radio signals, and it is also connected in series. 20. Discuss the design of comprehensive lightning protection for buildings (it should include the scope, objectives, and steps) Answer: The comprehensive lightning protection for modern buildings includes three main aspects: protection against direct lightning strikes, side lightning strikes, and induced lightning. The three main components of building lightning protection facilities are distinguished based on their protective functions against different forms of lightning hazards. To achieve its purpose of protecting against various types of lightning damage, a comprehensive lightning protection system must employ technical measures such as lightning arresters, current diversion, shielding, voltage equalization, and grounding. Therefore, the lightning protection facilities of a building should include eight technical components: grounding electrodes, down conductors, lightning protection grids, lightning protection strips, lightning rods, equipotential rings, equal potential connections, and lightning arresters. From design to construction, it should be carried out in two phases. The first phase involves the installation of direct (side) lightning protection systems that are constructed together with the building itself. The purpose of this design is to protect the building from damage caused by lightning, to minimize the electromagnetic effects resulting from lightning strikes inside the building, and to provide the necessary foundation for protecting the equipment within the building from induced lightning. Its characteristic is that it is carried out simultaneously with the civil engineering work of the building. The purpose of the design in the second phase is to protect the low-voltage equipment inside buildings, such as communication systems, computer systems, and household electrical appliances; it refers to the part of the building’s lightning protection system that deals with induced lightning. Its characteristic is that its installation takes place simultaneously with the installation of the building’s mechanical and electrical equipment. In the second phase, it is particularly important to emphasize that before installing electronic devices such as computers and communication equipment, which have relatively low resistance to interference (or overvoltage), it is necessary first to understand the basic characteristics of the lightning protection systems in the building where these devices will be installed. This includes information on lightning receptors, grids, the type and power frequency resistance value of lightning protection grounding electrodes, equipotential bonding, the distribution of down conductors, the type of power supply connections, and the installation of high- and low-voltage surge protectors ； For high-rise buildings, it is also necessary to understand basic design parameters such as the form of equipotential rings and the grounding of glass curtain walls, as well as the value of transition resistance. Only then can technical solutions such as the location of machine rooms, the layout of cables, the form of the grounding system, and voltage limiting and current distribution be determined. Otherwise, a design that is detached from reality will be highly arbitrary.