Calculate the duration of lightning if. We count the frequency of lightning strikes into a building. Electricity metering and saving

Buildings and structures or parts thereof, depending on their purpose, the intensity of lightning activity in the area of ​​location, and the expected number of lightning strikes per year, must be protected in accordance with the categories of lightning protection device and the type of protection zone. Protection against direct lightning strikes is carried out using lightning rods of various types: rod, cable, mesh, combined (for example, cable-rod). Rod lightning rods are most often used; cable lightning rods are used mainly for protecting long and narrow structures. The protective effect of a lightning rod in the form of a mesh applied to the structure being protected is similar to the action of a conventional lightning rod.

The protective effect of a lightning rod is based on the ability of lightning to strike the highest and well-grounded metal structures. Thanks to this, the protected building, which is lower in height compared to the lightning rod, will practically not be struck by lightning if all its parts are included in the lightning rod’s protection zone. The protection zone of a lightning rod is considered to be the part of the space around the lightning rod that provides protection of buildings and structures from direct lightning strikes to a certain extent

reliability. The surface of the protection zone has the least and constant degree of reliability; As you move deeper into the zone, the reliability of the protection increases. Type A protection zone has a reliability level of 99.5% or higher, and type B has a reliability level of 95% or higher.

General scheme for solving the problem: a quantitative assessment is made of the probability of a lightning strike to a protected object located on a flat area with fairly uniform soil conditions on the site occupied by the object, i.e., the expected number of lightning strikes per year of the protected object is determined. Depending on the category of the lightning protection device and the obtained value of the expected number of lightning strikes per year of the protected object, the type of protection zone is determined. The mutual distances between lightning rods taken in pairs are calculated and the parameters of protection zones at a given height from the ground are calculated.

Depending on the type, number and relative position of lightning rods, protection zones can have a wide variety of geometric shapes. The reliability of lightning protection at various heights is assessed by the designer, who, if necessary, clarifies the parameters of the lightning protection device and decides on the need for further calculations.

Industrial, residential and public buildings and structures, depending on their design characteristics, purpose and significance, the likelihood of an explosion or fire, technological features, as well as the intensity of lightning activity in the area of ​​their location, are divided into three categories according to lightning protection: I - industrial buildings and structures with explosive premises of classes B-1 and B-2 according to the PUE; it also includes buildings of power plants and substations; II - other buildings and structures with explosive premises not classified as category I; III - all other buildings and structures, including fire hazardous premises.

To assess thunderstorm activity in different areas of the country, a map of the distribution of the average number of thunderstorm hours per year is used, on which lines of equal duration of thunderstorms or data from the corresponding local meteorological station are plotted.

The probability of an object being struck by lightning depends on the intensity of thunderstorm activity in the area of ​​its location, the height and area of ​​the object and some other factors and is quantified by the expected number of lightning strikes per year. For buildings and structures not equipped with lightning protection, the number of damage is determined by the formula

Where S And L - respectively, the width and length of the protected building (structure), which has a rectangular shape in plan, m; h - greatest

height of the protected object, m; P- average annual number of lightning strikes per 1 km 2 of the earth’s surface at the location of the object, values P with equal intensity of thunderstorm activity are determined from tables. For buildings of complex configuration when calculating as S And L the latitude and length of the smallest rectangle into which the building can be inscribed in the plan are considered.

The category of lightning protection device and the expected number of lightning strikes per year of the protected object determine the type of protection zone: buildings and structures belonging to category I are subject to mandatory lightning protection. The protection zone must have a reliability degree of 99.5% or higher (type A protection zone); protection zones for buildings and structures belonging to category II are calculated according to type A, if N> 1, and type B otherwise; zones belonging to category III are calculated according to type A, if N > 2, and type B otherwise. This applies only to buildings and structures that are classified as explosive and fire hazardous. For all other objects of this category, regardless of the value N protection zone type is accepted B.

Calculation of lightning protection of buildings and structures involves determining the boundaries of the lightning rod protection zone, which is the space protected from direct lightning strikes. Protection zone of a single lightning rod height h  150 m is a circular cone, which, depending on the type of protection zone, is characterized by the following dimensions:

h
she

h
she

(12.16)

Where h 0 - apex of the protection zone cone, m; r 0 - radius of the base of the cone at ground level, m; r x - radius of the horizontal section of the protection zone at height h x from ground level, m; h x - height of the protected structure, m.

The protection zone of a single rod lightning rod in plan is graphically depicted by a circle of the corresponding radius. The center of the circle is at the point where the lightning rod is installed.

Protection zone of a double rod lightning rod up to 150 m high with a distance between lightning rods equal to L, shown in Fig. 12.1. The figure shows that the protection zone between two lightning rods is significantly larger than the sum of the protection zones of two single lightning rods. Part of the protection zone

between the rod lightning rods in the section passing through the axes of the lightning rods is joint (Fig. 12.1), and its remaining parts are called end parts.

Determination of the outlines of the end parts of the protection zone is carried out according to the calculation formulas used to construct the protection zone of single lightning rods, i.e. dimensions h 0 , r 0 , r x 1, r x2, are determined depending on the type of protection zone using formulas (12.15) or (12.16). In plan, the end parts are semicircles with a radius r 0 or r x, which are limited by planes passing through the axes of lightning rods perpendicular to the line connecting their bases.

The joint part of the protection zone is limited from above by a broken line, which can be constructed using three points: two of them lie on lightning rods at a height h 0, and the third is located in the middle between them at a height h c. Cross-sectional outline of the protection zone A-A(Fig. 12.1) are determined according to the rules and formulas adopted for single rod lightning rods.

The protection zones of the double rod lightning rod have the following dimensions:

(12.17)

Zone A exists when L  3 h , otherwise, lightning rods are considered as single;

(12.18)

Zone B exists when L  5h, otherwise lightning rods are considered as single. In formulas (12.17), (12.18) L - distance between lightning rods, m; h c - height of the protection zone in the middle between lightning rods, m; r With - cross-sectional width of the joint protection zone A-A(Fig. 12.1) at ground level, m; d - width of the horizontal section of the joint protection zone in section A-A on high h x from ground level, m.

The main condition for the presence of a joint protection zone of a double rod lightning rod is the fulfillment of the inequality r cx > 0. In this case, the configuration of the joint protection zone in plan consists of two isosceles trapezoids having a common base of length 2 r cx, which lies in the middle between the lightning rods. The other base of the trapezoid has length 2 r X. The line connecting the installation points of lightning rods is perpendicular to the bases of the trapezoid and divides them in half. If r cx = 0, the joint protection zone in plan represents two isosceles triangles, the bases of which are parallel to each other, and the vertices lie at one point, located in the middle between the lightning rods. If the construction of a protection zone is not carried out.

Objects located over a fairly large area are protected by several lightning rods (multiple lightning rod). To determine the external boundaries of the protection zone of multiple lightning rods, the same techniques are used as for single or double lightning rods. In this case, to calculate and construct the external outlines of the zone, lightning rods are taken in pairs in a certain sequence. The main condition for the protection of one or a group of structures with a height h x with reliability corresponding to the protection zones A And B, is the fulfillment of the inequality r cx > 0 for all lightning rods taken in pairs.

To protect long and narrow structures, as well as in some other cases, single cable lightning rods are used.

The protection zone formed by the interaction of cable and rod (single or double) lightning rods is determined in the same way as the protection zone of a multiple rod lightning rod. At

In this case, the supports of the catenary lightning rod are equal to rod lightning rods of height A and the radius of the base of the protection zone r, depending on the type of protection zone.

Self-test questions

1. Give a classification of electrical installations regarding electrical safety measures.

List the types of grounding used.

Describe the grounding arrangement and the design of the grounding conductors.

4. List the features of grounding devices in installations up to and above 1 kV.

5. What is the calculation of simple grounding conductors?

6. Calculate the specific equivalent electrical resistance of the earth.

Describe the protective effect of a lightning rod and categorize buildings and structures known to you.

Calculate the protection zone of a single lightning rod.

Calculate the protection zone of a double rod lightning rod and depict the protection zone for different heights of the protected building.

CHAPTER THIRTEEN

ACCOUNTING AND ENERGY SAVINGS

Lightning- a huge electric spark discharge in the atmosphere, as usual accompanied by a flash of light and thunder. There is a small delay between the flash and the audible discharge of thunder, the duration of which can be used to calculate the distance to the lightning strike.

You will need

• Stopwatch, calculator

Instructions

1. It turns out, wait for the lightning with a stopwatch in your hand. At the moment of the flash, start the stopwatch, when you hear thunder, turn off the stopwatch. As a result, you will get the thunder delay time - that is, the time it takes for the air vibration to travel from the point of discharge to you.

2. Further, distance, according to the famous formula, is the product of speed and time. You have time. As for the speed of sound in the atmosphere, for daring calculations it is enough to remember the value of 343 meters per second. If you want to calculate the distance more or less correctly, then you should remember that sound travels faster in humid air than in dry air, and faster in hot air than in cold air. Let's say, in a cold autumn with a torrential downpour, the speed of sound in the air will be 338 m/sec, and in a hot and dry summer - 350 m/sec.

3. Now count. Let's say 8 seconds passed from the flash of lightning to the sound of thunder. Take the speed of sound - 343 m/s, then the distance to the lightning will be 8 * 343 = 2744 meters, or (rounded) 2.7 kilometers. If the air temperature is 15 degrees Celsius with a humidity of 80% (moderate rainfall), then the speed of sound will be 341.2 m/sec, and the distance will be 2729.6 m (it can be rounded to 2.73 km).

4. You can enter a tolerance for wind direction. If the wind blows in the direction from the lightning towards you, the sound will travel this distance somewhat more quickly, and if the wind is directed from you to the lightning, it will travel somewhat more leisurely. For daring calculations, it is enough to remember that in the first case (wind to lightning) the distance must be reduced by 5%, and in the second (wind from lightning) increased by 5%. Thus, with a thunder delay of 8 seconds and a speed of sound of 343 m/sec and the wind direction from the lightning towards you, the distance of 2744 meters must be increased by 137.2 m.

There are sports that directly depend on the direction wind. For example, kiteboarding. An athlete who is interested in it needs to be able to positively determine direction wind before going out on the water.

You will need

• – a flag, scarf or handkerchief.

Instructions

1. Take a closer look to see if I have a flag. By looking at it, you can easily determine not only direction, but also approximate strength wind. If you don’t find a flag nearby, then try other methods, fortunately there are plenty of them.

2. Likewise, look at the smoke. It’s possible that somewhere nearby there is a factory with smokestacks, or someone is grilling shish kebab on the grill.

3. Take a flag, scarf or long scarf. Get out onto a flat surface. Raise your hand with the object up. If there are no obstacles on the sides, then you can easily determine direction wind .

4. Turn your head from side to side. Once it is positioned directly into the wind, you will hear an identical noise in both ears.

5. Look at the water, or rather at the waves. They invariably move in a downwind direction.

Video on the topic

Note!
If the wind blows perpendicular to a high hill, forest, etc., then it can change direction. This is permissible due to the result of reflection on these original walls. Then the wind will not only blow in the opposite direction, but may also decrease in strength or even subside altogether. When engaging in water sports, it is not enough just to determine the direction of the wind, you also need to be able to calculate its strength. Without special equipment at hand, you can do this visually.

Helpful advice
When determining the direction of the wind, it is worth considering such a concept as turbulence. It’s easiest to explain it using the example of water. Its flow, encountering an obstacle, cannot flow around it without interruption, due to inertia. Therefore, when twisting, it forms seething, foam and even funnels. The same thing happens with the wind, which encounters an obstacle in its path, say, a building. That is why, when you are in the courtyard of a building, it is sometimes difficult to determine the direction of the wind. This chaotic movement of wind currents is called turbulence. And those vortices that they create behind the obstacle are rotors.

Lightning- this is a powerful electrical discharge, the one that appears when clouds are strongly electrified. Lightning discharges can flow both inside a cloud and between neighboring clouds that are highly electrified. Occasionally, a discharge occurs between the ground and an electrified cloud. Before a lightning flash, electrical potential differences appear between the cloud and the ground or between neighboring clouds.

One of the first to establish the interaction of electrical discharges in the sky was an overseas scientist, the one who also held the main government post - Benjamin Franklin. In 1752, he performed a fascinating skill with a paper kite. The tester attached a metal key to its cord and launched the kite in time for a thunderstorm. After a while, lightning struck the key, emitting a sheaf of sparks. Since then, lightning has begun to be studied in detail by scientists. This amazing natural phenomenon can be extremely dangerous, causing significant damage to power lines and other tall buildings. The main reason for the origin of lightning lies in the collision of ions (impact ionization). The electric field of a cloud has a very high intensity. In such a field, free electrons experience great acceleration. When they collide with atoms, they ionize them. The final output produces a stream of rushing electrons. Impact ionization forms a plasma channel through which a rod current pulse passes. An electrical discharge occurs, the one that we track in the form of lightning. The length of such a discharge can reach several kilometers and last up to several seconds. Lightning invariably accompanied by a brilliant flash of light and thunder. Lightning often appears during a thunderstorm, but there are exceptions. One of the most unexplored natural phenomena associated with electrical discharges by scientists is ball lightning. All we know is that it appears unexpectedly and can cause significant damage. So why is lightning so brilliant? The strength of the electric current during a lightning strike can reach 100,000 Amperes. In this case, a large amount of energy is released (about a billion Joules). The temperature of the main channel reaches approximately 10,000 degrees. These collisions give rise to a brilliant light, which can be seen during a lightning strike. After such a strong electrical discharge, a pause occurs, which can last from 10 to 50 seconds. During this time, the rod channel approximately goes out, the temperature in it drops to 700 degrees. Scientists have found that the bright glow and heating of the plasma channel propagate from the bottom up, and the pauses between the glows are tens of fractions of seconds each. It is therefore that a person perceives several strong shocks as a single bright flash of lightning.

Video on the topic

Lightning, as usual, appears as a brilliant zigzag flash in thunderclouds and is accompanied by thunder. Its electrical discharge reaches 100,000 amperes, and its voltage reaches several hundred million volts. In order to determine distance before lightning, it is necessary to calculate the time in seconds from the flash to the first rumble of thunder.

You will need

• – stopwatch or watch$
• - calculator.

Instructions

1. Lightning is a natural phenomenon unsafe for human life. However, ironically, it is precisely because of people that they are becoming more and more numerous. This happens due to a very irresponsible attitude towards the environment: pollution of the surrounding air in megacities increases the heating of the air and the rise of steam-condensate into the atmosphere. This increases the electrical intensity in the clouds and provokes lightning strikes.

2. The need to determine distance before lightning is caused not only by the need to expand one’s horizons, but also by the elementary instinct of self-preservation. If it is too close, and you are in an open space, then it is better to run away from there as quickly as possible. Electric current chooses the shortest path to the ground, and the skin curtain is a good conductor for it.

3. Start counting the seconds as soon as you see a light flash in the sky, use a watch or stopwatch. As soon as the 1st clap of thunder is heard, stop counting, this will give you time.

4. In order to discover distance, you need to multiply time by speed. If accuracy is not very important to you, then it can be taken equal to 0.33 km/s, i.e. multiply the number of seconds by 1/3. Let's say, according to your calculations, the time until lightning was 12 seconds, after dividing by 3 you get 4 km.

5. In order to determine distance before lightning is more correct, take the average speed of sound in air to be 0.344 km/s. Its true value depends on many factors: humidity, temperature, type of terrain (open space, forest, urban high-rise buildings, water surface), wind speed, etc. Let's say, in rainy autumn weather, the speed of sound is approximately 0.338 km/s, in dry summer heat - about 0.35 km/s.

6. Dense forests and tall buildings slow down the speed of sound much more. It is reduced due to the need to go around countless obstacles and diffraction. It is quite difficult to make an accurate calculation in this case, and the main thing is impractical: despite the fact that lightning will not strike the ground, it can hit a tall tree next to you. So wait it out between low-growing trees with a dense crown, the best of each is squatting, and if you find yourself on a city street, then take cover in a nearby building.

7. Pay attention to the wind. If it is quite powerful and blows towards you in the direction from lightning, which means the sound comes faster. Then its average speed can be taken to be approximately 0.36 km/h. When the wind direction is away from you lightning the movement of sound, on the contrary, slows down and the speed is approximately 0.325 km/h.

8. Average length lightning reaches 2.5 km, and the discharge extends to distance up to 20 km. Therefore, you should move as quickly as possible from the open space to the nearest building or structure. Remember that when approaching lightning You need to close all windows and doors and turn off electrical appliances, otherwise damage may occur through the antenna and cause damage to your equipment via the network.

9. Lightning is not only ground-based, but also intra-cloud. They are not dangerous for those on the ground, but they can damage flying objects: airplanes, helicopters and other vehicles. In addition, a metal object caught in a cloud with a strong electric field that can support, but not create a charge, can become a starter lightning and provoke its occurrence.

Video on the topic

Note!
Fascinating fact: among some Indian peoples, a lightning strike is considered, so to speak, an initiation necessary for a shaman to achieve the highest level of abilities.

Average annual duration of thunderstorms. Specific density of lightning strikesn M.. Contraction radius Rst.. Number of direct lightning strikes into an object.. Degree of lightning danger.

The designer’s task is to provide in the project a reliable and appropriate lightning protection system for the facility. To determine the sufficient amount of protective measures that provide effective protection against lightning, it is necessary to understand the predicted number of direct lightning strikes into the protected structure. INFirst of all, the frequency of direct lightning strikes depends on the frequency of thunderstorms at the location of the object.

Thus, there are almost no thunderstorms beyond the Arctic Circle, but in the southern regions of the North Caucasus, the Krasnodar Territory, in the subtropics or in some areas of Siberia and the Far East, thunderstorms are a frequent occurrence. To assess thunderstorm activity, there are regional maps of the intensity of thunderstorm activity, which indicate the average duration of thunderstorms in hours per year. Of course, these maps are far from perfect. However, they are suitable for rough estimates. For example, for the central part of Russia we can talk about 30–60 thunderstorm hours per year, which is equivalent to 2–4 lightning strikes per year per 1 km 2 earth's surface.

Specific density of lightning discharges

Average annual number of lightning strikes per 1 km 2 surface of the earth or the specific density of lightning discharges ( n M) is determined based on meteorological observations at the location of the object. If it is unknown, then it can be calculated using the following formula:

n M = 6.7*T d /100 (1/km 2 year)

Estimating the frequency of lightning strikes through the contraction radius

Having determined the specific density of lightning discharges, the designer needs to estimate what proportion of these lightning strikes will hit the protected object.
An assessment can be made using the contraction radius (Rst). Experience shows that an object with height h, on average, attracts all lightning from a distance up to: Rst ≈ 3h.

This is the contraction radius. In the plan, you need to draw a line that is spaced from the outer perimeter of the object at a distance Rst. The line will limit the contraction area (Sst). It can be calculated by any available methods (even using cells on graph paper).

This assessment is also suitable for objects of complex shape, individual fragments of which have fundamentally different heights. Near each of the fragments, based on their specific height, a curve is constructed that limits its own contraction area. Naturally, they will partially overlap each other. Only the area enclosed by the outer envelope should be taken into account, as shown in Fig. 1. This area will determine the expected number of lightning strikes.
Fig.1

The number of direct lightning strikes into a protected object is determined simply: the value of the contraction area, expressed in square kilometers, is multiplied by the specific density of lightning discharges:

N M = n M*Sst.

Practical conclusions

Several obvious conclusions follow from this methodology.
Firstly, the number of lightning strikes into a single concentrated object such as a tower or support, whose height is much greater than other overall dimensions, will be proportional to the square of its height (Sst=π(3h) 2 ), and for extended objects (for example, a power line) – proportional to the height to the first power. Other objects occupy an intermediate position in configuration.

Secondly, with the accumulation of many objects in a limited area, when their contraction areas partially overlap each other (urban development), the number of lightning strikes to each of the objects will be noticeably less than to the same object in an open area.
In conditions of dense buildings, when the free space between objects is significantly less than their height, then each of the objects will practically collect lightning only from the area of ​​its roof, and its height will cease to play any noticeable role. All this is convincingly confirmed by operating experience.

Lightning danger level

When assessing the degree of danger of lightning, there is one nuance that is better explained with an example. Suppose we estimate the number of impacts on an antenna mast 30 m high. With good accuracy we can assume that its contraction area is a circle with a radius Rst ≈ 3h = 90 m and is equal to Sst = 3.14*(90) 2 ≈25,000 m 2 = 0.025 km 2 .

If at the location of the mast the specific density of lightning discharges n M= 2, then the mast should annually on average take on Nm = 0.025 x 2 = 0.05 lightning strikes. This means that on average 1 lightning strike will occur every 1/Nm = 20 years of operation. Naturally, it is impossible to know when this will actually happen: with equal probability it can happen at any time, both in the first year and in the twentieth year of operation.

If we assess the degree of lightning danger for a specific antenna mast from the perspective of mobile phone owners, then we can probably put up with a communication interruption that can occur once in 20 years of operation. The telephone company itself may have a completely different approach. If it operates not one, but 100 antenna systems, then the company is unlikely to be satisfied with the prospect of annual repairs on average of 100/20 = 5 antenna units.

It should also be said that assessing the frequency of direct lightning strikes in itself says little. In fact, it is not the frequency of lightning strikes that is important, but the assessment of the likelihood of possible destructive consequences from them, which allows us to determine the feasibility of certain lightning protection measures. Read also blog articles about this: