Lightning protection. Occupational Safety and Health

OCCUPATIONAL SAFETY AND HEALTH
Free library / Occupational Safety and Health

Lightning protection. Occupational Safety and Health

Occupational Safety and Health

Occupational Safety and Health / Legislative basis for labor protection

The possibility of hitting an object lightning is largely determined by the intensity of thunderstorm activity in the area where it is located, and depends on the size and configuration of the object, its location, as well as on the geological characteristics of the territory.

There are two types of lightning effects: primary, associated with a direct strike, and secondary, caused by electromagnetic and electrostatic induction. With a direct impact, fires, explosions, destruction of structures, injury to people, overvoltage on the wires of the electrical network can occur. The current strength in the lightning channel reaches 200 kA, the voltage is 150 MB, the length of the lightning spark is hundreds and thousands of meters, the temperature rises to 6-000°C.

Linear lightning It is characterized by very large values of currents, voltages and discharge temperatures, so the impact of lightning on a person, as a rule, ends with very serious consequences, usually death. On average, about 3000 people die every year from a lightning strike in the world, and cases of simultaneous defeat of several people are known. The lightning discharge follows the path of least electrical resistance. Since there is less distance and electrical resistance between a tall object and a thundercloud, lightning tends to strike taller objects. A lightning strike into the ground or into an object located on it depends on the electrical conductivity of the surface and underlying soil layers. Lightning strikes clay and wet areas much more often than dry and sandy areas, since the former have a higher electrical conductivity.

Lightning, striking tall plants, often affects deciduous trees (oak, poplar, willow, ash), because they contain a lot of starch. Linden, walnut, beech, coniferous trees - spruce, fir, larch - contain a lot of oils, so they have more electrical resistance and are less likely to be struck by lightning. The frequency of lightning strikes into a tree depends on its surface, the quality of the crown, bark, and their humidity. Statistics show that out of 100 trees, 27% of poplar, 20% of pear, 12% of linden, 8% of spruce and only 0,5% of cedar are affected by lightning.

When a lightning strikes, a tree splits according to the following mechanism: tree sap and moisture in the area where the discharge passes instantly evaporate and expand; this creates huge pressures that tear the wood. A similar effect, accompanied by scattering of chips, can occur when lightning strikes the wall of a wooden structure. Therefore, being under a tall tree during a thunderstorm is dangerous.

A person can be struck by lightning not only with a direct hit, which is always fatal. Dangerous is the step voltage also p. 2.7), which occurs when the lightning discharge current spreads in the ground. The radius of the damaging action of the step voltage reaches 30 meters. Jumps of lightning discharges and induced charges are also dangerous. Jumps of discharges occur from objects that are hit by lightning to objects located nearby. For example, a discharge may jump from a tall tree to a person, the wall of a house, etc., if the latter are located next to the tree. Charges are induced on well-conducting objects (for example, metal trusses, fences, etc.) under the action of the electric field of a thundercloud. Thus, the presence of a person during a thunderstorm near objects that are often struck by lightning (tall trees, masts, large metal objects, clay and wet areas of the earth) is dangerous.

Being in a city during a thunderstorm is less dangerous than in open areas, since steel structures and tall buildings act as lightning rods. Lightning often strikes people working in the field, tourists. It is dangerous to be on or near water during a thunderstorm, as water and land near water have high electrical conductivity and are often struck by lightning.

At the same time, being inside reinforced concrete buildings, metal structures (for example, metal garages) during a thunderstorm is safe for humans. A completely or partially closed electrically conductive surface forms the so-called Faraday chamber, inside which a significant and dangerous potential for humans cannot form. Therefore, passengers inside a car with an all-metal body, a tram, a trolleybus, a train car are safe during a thunderstorm until they go outside and open the windows. Linear lightning is quite often the cause of fires. Forest fires, fires of residential and industrial buildings, especially in rural areas, cause great material damage and can be the cause of death. In this regard, it is necessary to take special measures to protect against damage by linear lightning.

If the nature of linear lightning is sufficiently studied and its behavior is predictable, then the nature ball lightning is still not completely clear, and her behavior does not always find an explanation. Although the probability of hitting a person with ball lightning is small, it poses a serious danger, since there are no reliable methods and rules for protecting against it. It can suddenly appear anywhere, including indoors. It often enters buildings through pipes, open windows and doors. The size of ball lightning can be from a few centimeters to several meters. Usually ball lightning easily floats or rolls above the ground, sometimes jumps. Ball lightning reacts to wind, draft, ascending and descending air currents. Ball lightning can appear and disappear without causing harm to a person or a room. Any contact with a person leads to severe injuries, burns, and in most cases to death. Ball lightning often explodes. The resulting air wave can injure a person or lead to destruction. It is believed that ball lightning has a temperature of about 5000°C and can start a fire.

Lightning protection is a set of protective measures against discharges of atmospheric static electricity, ensuring the safety of people, the safety of buildings and structures, equipment and materials from fires, explosions and destruction. The probability of a lightning strike into a ground object is greater, the higher the object.

One of the main lightning protection measures is a device lightning rods. Rising above the objects, they take on the discharges of a thundercloud. Lightning rods create a protection zone - a space inside which lightning does not occur. The lightning rod consists of lightning rod, down conductor, ensuring the passage of the discharge current through it to the grounding device, and the grounding device. There are several types of lightning rods: rod, mesh, cable; single, double, multiple; stand-alone; isolated from the object and non-isolated. Rod and cable lightning rods are installed either on separate supports or on supports associated with the structure of the facility. Mesh lightning rods are laid on the roof of the building (Fig. 7).

Lightning protection is based on the property of lightning to strike the highest and well-grounded metal structures. The lightning rod consists of three main parts (Fig. 8): a lightning rod (1) that perceives a lightning strike, a down conductor (2) connecting the lightning rod to a ground electrode (3), through which the lightning current flows into the ground. The lightning rod is placed on the mast (4).

Fig.7. Arrangement of lightning rods: 1 - cable, 2 - cable-rod, 3 - mesh

Fig.8. Rod lightning rod 1 - lightning rod, 2 - down conductor, 3 - grounding, 4 - mast

The most common are rod and cable lightning rods. According to the number of lightning rods, lightning rods are divided into single, double and multiple.

In the vicinity of the lightning rod, a protection zone is formed, i.e. a space within which a building or any other object is protected from a direct lightning strike with a high degree of reliability. The degree of protection in this area is more than 95%. This means that out of 100 lightning strikes to the protected object, less than 5 cases of a direct lightning strike are possible, the rest of the strikes will be received by the lightning rod. The protection zone of a single rod lightning rod is limited by the generators of two cones, one of which has a height h equal to the height of the lightning rod, and the radius of the base R= 0,75k, and the other - a height of 0,8k and a base radius of 1,5k (with the base radius of the second cone R=k provides 99% protection efficiency).

Lightning rod lightning rods are made of steel of any profile, usually round, with a cross section of at least 100 mm2 and a length of at least 200 mm. They are painted to protect against corrosion. Lightning rods of wire lightning rods are made of metal wires with a diameter of about 7 mm.

The down conductors must be able to withstand the heat of very high lightning currents for a short period of time, so little resistance is needed. The cross section of down conductors in the air must be at least 48 mm2, and in the ground - 160 mm2.

If the lightning rod is fixed on the roof of the building, then metal structures and fittings of the building, for example, metal stairs located on the outside of the building and leading to the roof, can be used as down conductors. Down conductors must be securely connected to the lightning rod and ground electrode.

Grounding conductors are the most important element in the lightning protection system. They provide a sufficiently low resistance to the spreading of the lightning current into the ground. As a grounding conductor, you can use metal pipes buried in the ground to a depth of 2-2,5 m, plates, coils of wire and mesh, pieces of metal fittings.

Lightning rods are installed on hills to shorten the lightning path and increase the size of the protection zone. Lightning rods protect all public buildings, buildings for the storage of material values, single buildings located on hills, historical and cultural values. Particular attention is paid to lightning protection of storage facilities for flammable and explosive materials, flammable liquids and gases. To do this, multiple lightning rods are used by installing a plurality of lightning rods along the contour of the protected space.

Pupils and students should be familiar with the precautions against being struck by lightning. First of all, you need to be able to predict the approach of a thunderstorm. In the presence of thunderstorm signs, it is better to refrain from traveling to the forest, to the field or to the reservoir, it is advisable not to go far from home. If distant peals of thunder are heard and flashes of lightning are visible, then it is possible to determine the approximate distance to the place of thunderstorm activity by the time interval between the flash of lightning and the first peal of thunder. We see a flash of lightning almost immediately, as light travels at a speed of 300 km/s. The speed of sound propagation in air is 000 m / s, i.e., in about 344 seconds, sound travels 3 kilometer. Thus, dividing the time in seconds between the flash of lightning and the first clap of thunder that followed it by 1, we determine the approximate distance in kilometers to the thunderstorm. If these time intervals decrease, then a thunderstorm is approaching and protective measures must be taken. Lightning is dangerous when a flash of thunder immediately follows, i.e. a thundercloud is above you and the danger of a lightning strike is greatest.

Actions before and during a thunderstorm:

1. Do not leave the house, close windows, doors and chimneys, take care that there is no draft that could attract ball lightning. During a thunderstorm, you should not heat the stove, since the smoke coming out of the chimney has a high electrical conductivity and the likelihood of a lightning strike into the chimney rising above the roof increases.

2. During a thunderstorm, keep away from electrical wiring, antennas, windows, doors and other objects related to the external environment. You should not stay near the wall, on the outer side of which there is a tall tree at a distance of less than 3 m, since when a lightning strikes a tree, a lightning discharge can jump to the wall of the house.

3. Radios and TVs must be disconnected from the network, electrical appliances and telephones should not be used.

4. If a thunderstorm caught you while walking, you need to hide in the nearest building. Thunderstorms are especially dangerous in the field. When looking for shelter, opt for a large or metal-framed metal structure, a residence, or other structure that is protected by a lightning rod.

5. If it is not possible to hide in a building, you should not hide in small sheds, under lonely trees.

6. You can not be on hills and open unprotected places, near metal or mesh fences, large metal objects, power lines, wet walls, lightning rod grounding, etc. objects that are highly likely to be struck by lightning.

7. In the absence of shelter, you need to lie on the ground; in this case, preference should be given to dry sandy soil, remote from the reservoir.

8. Dense forest without individual tall trees - good protection during a thunderstorm. If a thunderstorm caught you in the forest, you need to take cover in a patch of undersized forest. You can not hide under tall trees, especially pines, oaks, poplars. It is better to be at a distance of more than 30 m from a single tall tree. It is necessary to pay attention to the trees - are there any trees previously affected by a thunderstorm. A characteristic sign of a lightning strike is a split tree. It is best to stay away from an affected tree or an area of forest with a large number of affected trees. The abundance of trees struck by lightning indicates that the soil in this area has a high electrical conductivity and a lightning strike in this area is very likely.

9. During a thunderstorm, you can not be on the water and near the water - swim, fish. You need to move away from the coast.

10. In the mountains, you must stay away from mountain ridges, sharp towering rocks and peaks. When approaching a thunderstorm in the mountains, you need to go down as low as possible. Metal objects - climbing hooks, ice axes, pots, knives, etc. should be collected in a backpack and lowered on a rope 20-30 m down the slope from the location.

11. During a thunderstorm, do not play outdoor sports, do not run, because it is believed that fast movement "attracts" lightning.

12. If you are on a bicycle or motorcycle in a thunderstorm, stop moving and wait out the thunderstorm at a distance of about 30 m from the vehicle.

13. If a thunderstorm caught you in a car, then you do not need to leave it. It is necessary to close the windows and lower the car antenna. Driving during a thunderstorm is not recommended, as a thunderstorm is usually accompanied by a downpour that impairs visibility on the road, and a flash of lightning can blind and cause fear and, as a result, an accident.

14. When meeting with ball lightning, one should not show any aggressiveness towards it, if possible, one should remain calm and not move. No need to approach her, touch her with anything, as an explosion may occur. You should not run away from ball lightning, as you can drag it along with the air flow that has arisen.

In case of injury, the victim must immediately receive the same assistance as burns and electric shock.

Authors: Volkhin S.N., Petrova S.P., Petrov V.P.

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