The hazards of lightning to Construction machinery and preventive measures

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Abstract: From April to September every year is the peak season for lightning disasters in South China, which cause losses to people's lives and property. This article introduces the causes of lightning, its hazards to construction machinery, as well as the prevention methods and techniques for lightning disasters. These methods and techniques have universal reference significance for peers in preventing lightning disasters during the construction process.

1. The generation of lightning

Lightning is a common discharge phenomenon in nature. There are various theories explaining the formation of lightning. Generally, we believe that hot air in the atmosphere rises and rusts against cold air at high altitudes, thus forming small water droplets with positive and negative charges. When positive and negative charges accumulate to a certain charge value, a strong electric field will be formed between cloud masses with different polarities and between cloud masses and the ground, thereby generating the discharge processes of cloud masses to cloud masses and cloud masses to the ground. This is what is commonly referred to as lightning and thunder. Specifically, the friction of ice crystals, the breaking of raindrops, the freezing of water droplets, and the collision of cloud bodies can all cause cloud particles to become electrified. Generally, the top of a cloud is positively charged and the bottom is negatively charged. The two charges of different polarities will create a strong electric field within the cloud or between the cloud and the ground, causing a sudden and intense discharge that bursts out into a powerful electric spark, which is what we see as lightning. In a lightning channel, the current is extremely strong, the temperature can suddenly rise to 20,000 degrees Celsius, the air pressure surges, and the air expands violently. People will then hear sound waves oscillating like explosions, which is thunder.

2. Types of lightning hazards

The main hazards of lightning strikes are threefold: The first is direct lightning strikes. It refers to the intense discharge that occurs at a certain point on the earth by a thundercloud. It can directly strike equipment. Lightning strikes overhead lines, such as power lines, telephone lines, etc. The lightning current then enters the equipment along the wires, thereby causing damage. The second is induced lightning. It can be divided into electrostatic induction and electromagnetic induction. When a charged thundercloud (usually negatively charged) appears above a conductor, due to electrostatic induction, a large amount of opposite charges are bound to the conductor. Once a thundercloud discharges to a certain target, the negative charges on the thundercloud disappear instantly. At this time, a large amount of positive charges on the wire still exist and travel along the wire in the form of lightning waves through the equipment to the ground, causing damage to the equipment. When lightning current flows into the ground along a conductor, due to its high frequency and intensity, a very strong alternating electromagnetic field is generated near the conductor. If equipment is in this field, it will induce a very high voltage, which may cause damage. Special attention should be paid to sensitive electronic devices. The third is the increase in ground potential. When A 10kA lightning current passes through a lower conductor and enters the ground, we assume the grounding resistance is 10Ω. According to Ohm's Law, we know that the voltage at point A of entry is 100kV. Since point A is connected to points B, C and D, the voltage at all these points is 100kV. When point E is grounded, its voltage value is 0. There is a 100kV voltage difference between point D and point E of the equipment, which is sufficient to damage the equipment. According to relevant statistics, the damage caused by direct lightning strikes accounts for only 15%, while the damage caused by induced lightning and increased ground potential makes up 85%. At present, the disasters caused by direct lightning strikes have significantly decreased, but with the development of urban economy, the hazards caused by induced lightning and lightning wave intrusions have greatly increased. The lightning rods on ordinary buildings can only prevent direct lightning strikes, but the induced lightning and pulse voltage generated by strong electromagnetic fields can seep into the room and endanger weak electrical equipment such as televisions, telephones and networked microcomputers.

3. Methods and techniques for lightning protection

Today, with the rapid development of science and technology, although it is impossible for humans to completely control the violent lightning, through long-term exploration and practice, a lot of knowledge and experience related to lightning protection have been accumulated, and a series of effective methods and technologies for lightning protection have been formed.

(1) Catching lightning: Catching lightning means allowing the lightning energy that occurs within a certain range to be discharged into the earth through channels designed by people. Capture the lightning discharge within a certain protection range and incorporate it into the pre-designed reasonable path for ground discharge. A lightning rod is an active lightning receiving device, whose function is to guide the lightning current into the ground. Lightning protection wires and lightning protection belts have developed on the basis of lightning rods. The use of lightning rods is the most primary and fundamental lightning protection measure.

(2) Grounding Grounding is to discharge the lightning energy that has been incorporated into the lightning protection system into the ground. Only with good grounding can the voltage on the down conductor be effectively reduced and counterattacks be prevented. In the past, some regulations required electronic devices to be grounded separately, with the aim of preventing stray currents or transient currents in the power grid from interfering with the normal operation of the devices. Grounding is the most fundamental link in a lightning protection system. If the grounding is poor, the lightning protection effect of all lightning protection measures cannot be fully exerted. Lightning protection grounding is the most fundamental safety requirement in the installation and acceptance standards for lightning protection facilities.

(3) When the equalizing connection lightning arrester captures lightning, the down conductor immediately rises to a high potential, which will cause side flashover to the conductors around the lightning protection system that are still at the ground potential, and increase their potential, thereby posing a threat to personnel and equipment. To reduce this flashover risk, the simplest approach is to use a voltage equalizing ring to connect the conductors at the ground potential at the same potential all the way to the grounding device. Metal facilities, electrical installations and electronic equipment, if the distance between them and the conductors of the lightning protection system, especially the lightning arrester, does not meet the specified safety requirements, should be connected to the lightning protection system at the same potential with thicker wires. In this way, when lightning current passes through, all facilities immediately form an "equipotential island", ensuring that no harmful potential difference is generated between conductive components and no side flashover discharge occurs. A perfect equipotential bonding can also prevent the counterattack caused by the increase in ground potential resulting from lightning current entering the ground.

(4) Shunt: Shunt refers to the parallel connection of an appropriate lightning arrester between all outdoor conductors and the grounding wire. When the overvoltage waves generated by direct lightning or induced lightning on the line enter the room or equipment along these conductors, the resistance of the lightning arrester suddenly drops to a low value, nearly in a short-circuit state, and divides the lightning current into the ground. Diversion is a rapidly developing focus in modern lightning protection technology and a key measure for protecting various electrical and electronic equipment. Since a small portion of the lightning current still enters the equipment along the wires after being diverted, this remains very dangerous for microelectronic devices that are not resistant to high voltage. Therefore, for such equipment, multi-stage diversion should be carried out before the wires enter the casing. When adopting the lightning protection measure of diversion, special attention should be paid to the selection of the performance parameters of the lightning arrester, as the installation of additional facilities will more or less affect the performance of the system.

(5) Shielding Shielding involves enclosing the object to be protected with conductors such as metal mesh, foil, shell, or tube to block the passage through which the pulse electromagnetic field of lightning invades space. Shielding is the most effective way to prevent the influence of lightning electromagnetic pulse radiation on electronic equipment.

4. Hazards of Lightning to Construction Machinery and Prevention

Highway construction work is carried out in an open-air environment. The electrical control systems of construction machinery, especially the microelectronic control devices, are highly likely to be damaged by direct lightning strikes or lightning-induced overvoltage. The Qingyuan section of the Beijing-Zhuhai Expressway is located in a limestone mountainous area. During the thunderstorm season, it is a high-risk area for lightning strikes. Whenever the sky is overcast with dark clouds and heavy rain comes, lightning often strikes construction machinery directly. Sometimes, even when there are no rain clouds in the sky and it doesn't rain, induced lightning still occurs from time to time. The surge voltage it generates invades and damages the micro-electric control device of the road paver. Therefore, lightning protection work is imperative.

(1) Installing lightning rod devices in asphalt mixture mixing plants: As asphalt mixture mixing equipment and its supporting machinery are used in one production plant, it is relatively easy to carry out centralized lightning protection. Therefore, lightning rods are installed in the mixing plant. The height of the lightning rod is higher than the highest point of the mixing plant, reaching an effective protection radius to prevent direct lightning strikes on any working machinery. The grounding of the lightning rod must be reliable. Due to the relatively dry and hard geological soil in the limestone mountainous area, the soil resistance value is too high. Therefore, the method of laying the grounding grid is not exactly the same as that of the Guangzhou-Zhuhai East Line. The Angle steel pile points of the grounding grid should be buried in a pit about 1.0 meters deep. The Angle steel should be exposed about 20cm in the pit. The pit should be filled with charcoal and granular raw salt in proportion as the resistance-reducing medium. The weight ratio of raw salt to charcoal is 1:10, that is, 1kg of raw salt mixed with 10kg of charcoal, and then covered with soil. This can ensure that the grounding resistance value is below 4. When lightning strikes, the powerful lightning current is quickly introduced into the ground through the grounding grid by the lightning rod and its leads, preventing direct lightning strikes on the electromechanical system. In addition, the asphalt mixture mixing operation control room is shielded. The method is to connect the working grounding and protective grounding of the microelectronic control system in the operation room to the metal-structured control room shell with conductors, and then introduce them into the underground grounding grid through grounding leads to keep them at the same ground potential and prevent static electricity and lightning.

(2) Install overvoltage protectors on the electrical control devices of road paving machinery. As road paving machinery is constantly on the move for operation, it is impossible to centrally protect against lightning. The electrical control devices of mobile machinery in open-air environments are most vulnerable to the invasion of induced lightning surge voltages. For instance, the pressure sensors that control the road surface smoothness and mechanical positioning of asphalt pavers are severely affected. To protect these mechanical microelectronic control devices with extremely high control sensitivity from being invaded and damaged by induced lightning surge voltages, overvoltage protectors are installed on each mechanical control device according to its different structural characteristics.

5. Conclusion

Lightning damage to mechanical and electrical control devices, especially microelectronic devices, is a common natural disaster. To prevent lightning from damaging them, different lightning protection methods should be adopted according to the different structural characteristics of mechanical and electrical control equipment. As long as the lightning protection equipment is reasonably selected, the lightning protection facilities should be regularly inspected by professional lightning protection companies to assess whether they meet the national standards and requirements. Construction projects should appoint a person responsible for preventing lightning disasters, who is in charge of lightning protection safety work, establish various lightning protection safety measures, and conduct regular inspections, post-thunderstorm checks, and daily maintenance of various lightning protection facilities. When designing and constructing lightning protection facilities, construction units should comprehensively consider factors such as geology, soil, meteorology, environment, characteristics of the protected objects, and the laws of lightning activities, and adopt safe and reliable, technologically advanced, and economically reasonable design and construction methods. This will greatly reduce the losses caused by lightning disasters.