Calculate the duration of the lightning if through the cross section. How to determine at what distance lightning struck from you. Accounting and saving electricity

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

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

So, there are almost no thunderstorms beyond the Arctic Circle, and in the southern regions of the North Caucasus, the Krasnodar Territory, in the subtropical zone or in some regions of Siberia and the Far East, thunderstorms are a frequent phenomenon. 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 cards are far from perfect. Nevertheless, they are suitable for indicative 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 according to 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 fall into the protected object.
An estimate can be made using the contraction radius (Rst). Experience shows that an object of height h, on average, attracts all lightning to itself from a distance up to: Rst ≈ 3h.

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

Such an estimate 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 partially overlap each other. Only the area bounded 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 to the 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 implications 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, near a power line) - proportional to the height to the first degree. Other configuration objects occupy an intermediate position.

Secondly, when many objects accumulate in a limited area, when their areas of constriction partially overlap each other (urban development), the number of lightning strikes into each of the objects will be noticeably less than into the same object in an open area.
In conditions of dense development, when the free space between objects is much 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.

The degree of danger of lightning

When assessing the degree of danger of lightning, there is one nuance that is best explained with an example. Assume that the number of impacts on an antenna mast 30 m high is estimated. 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\u003d 2, then the mast should annually, on average, take on itself Nm \u003d 0.025 x 2 \u003d 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: it can happen with equal probability at any time, both in the first year and in the twentieth year of operation.

If we evaluate the degree of lightning danger for a particular antenna mast from the standpoint of mobile phone owners, then we can probably put up with a break in communication, which can occur once in 20 years of operation. The telephone company itself may have a fundamentally 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 100/20 = 5 antenna units.

It should also be said that the estimate of 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 probability of possible destructive consequences from them, which makes it possible to determine the feasibility of certain lightning protection measures. Read about this blog article:

Buildings and structures or parts thereof, depending on the purpose, the intensity of lightning activity in the area of ​​location, 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 action of a lightning rod in the form of a grid applied to the protected structure is similar to the action of a conventional lightning rod.

The protective action of a lightning rod is based on the property of lightning to strike the highest and well-grounded metal structures. Due 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 protection zone of the lightning rod. The protection zone of a lightning rod is considered to be a part of the space around the lightning rod, which provides protection for buildings and structures from direct lightning strikes with a certain degree

reliability. The surface of the protection zone has the least and constant degree of reliability; as you move inside the zone, the reliability of the protection increases. Type A protection zone has a degree of reliability of 99.5% and above, and type B - 95% and above.

The general scheme for solving the problem: a quantitative assessment is made of the probability of a lightning strike of a protected object located on a flat area with fairly homogeneous 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 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 are calculated at a given height from the earth's surface.

Depending on the type, number and mutual arrangement of lightning rods, protection zones can have a wide variety of geometric shapes. The assessment of the reliability of lightning protection at different heights is carried out by the designer, who, if necessary, specifies the parameters of the lightning protection device and decides on the need for further calculation.

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 thunderstorm activity in the area of ​​​​their location, are divided into three categories according to the lightning protection device: I - industrial buildings and structures with explosive premises of classes B-1 and B-2 according to PUE; it also includes buildings of power plants and substations; II - other buildings and structures with explosive premises that do not belong to category I; III - all other buildings and structures, including fire hazardous premises.

To assess thunderstorm activity in different parts 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 lightning 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 lesions is determined by the formula

where S and L - accordingly, the width and length of the protected building (structure), which has a rectangular shape in plan, m; h - the largest

height of the protected object, m; P- the average annual number of lightning strikes per 1 km 2 of the earth's surface at the location of the object, values P at equal intensity of thunderstorm activity are determined by the tables. For buildings of complex configuration when calculating as S and L the latitude and length of the smallest rectangle in 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 degree of reliability 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 explosive and fire hazardous. For all other objects of this category, regardless of the value N protection zone type is adopted B.

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

h
she is

h
she is

(12.16)

where h 0 - top 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 the plan is graphically represented by a circle of the corresponding radius. The center of the circle is at the installation point of the lightning rod.

The 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. It can be seen from the figure that the protection zone between two rod lightning rods is much 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 the rest of its parts are called end.

The definition of the outlines of the end parts of the protection zone is carried out according to the calculation formulas used to build 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 according to 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 the 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 built 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. 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 a double rod lightning rod have the following dimensions:

(12.17)

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

(12.18)

Zone B exists at L  5h, otherwise the 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 the lightning rods, m; r with - width of the joint protection zone in section A-A(Fig. 12.1) at ground level, m; d - the width of the horizontal section of the joint protection zone in the 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 is two isosceles trapezoids with 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 a r cx = 0, the joint protection zone in the plan is 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 protection zone is not built.

Objects located on a sufficiently large area are protected by several lightning rods (multiple lightning rod). To determine the outer boundaries of the protection zone of multiple lightning rods, the same methods are used as for single or double rod lightning rods. At the same time, for the calculation and construction of the external contours 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 protection zones BUT 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 wire lightning rods are used.

The protection zone formed by the interaction of the 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 lightning rod are equated to rod lightning rods with a height of A and the radius of the base of the protection zone r, depending on the type of protection zone.

Questions for self-examination

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

List the types of grounding used.

Describe the grounding device and the design of the grounding switches.

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

5. What is the calculation of simple ground electrodes?

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.

Perform the calculation of the protection zone of a single rod lightning rod.

Perform the calculation of 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 SAVING ELECTRICITY

Lightning current parameters

Lightning parameter

Protection level

Peak value of current, kA

Full charge, C

Charge per pulse, C

Specific energy kJ/Ohm

Average slope kA/µs

3.1.3. Lightning and atmospheric electricity

Lightning is one of the common causes of unwanted surges, faults and failures in automation systems. The charge accumulated in clouds has a potential of about several million volts relative to the Earth's surface and is often negative. The direction of the lightning current can be both from the ground to the cloud, with a negative charge of the cloud (in 90% of cases), and from the cloud to the ground (in 10% of cases). The duration of a lightning discharge is on average 0.2 s, rarely up to 1 ... 1.5 s, the duration of the leading edge of the pulse is from 3 to 20 μs, the current is several thousand amperes, up to 100 kA, the temperature in the channel reaches 20,000 ˚С, a powerful magnetic field and radio waves appear [ Vijayaraghavan]. Lightning can also be formed during dust storms, snowstorms, volcanic eruptions. During a lightning discharge, several pulses appear ( rice. 3.64). The steepness of the front in subsequent pulses is much greater than in the first ( rice. 3.65).

The frequency of lightning damage to buildings with a height of 20 m and dimensions in terms of 100x100 m is 1 time in 5 years, and for buildings with dimensions of the order of 10x10 m - 1 hit in 50 years [ RD]. The number of direct lightning strikes to the 540 m high Ostankino television tower is 30 strikes per year.

,

where is the maximum current; - correction factor; - time; - front time constant; is the decay time constant.

The parameters included in this formula are given in tab. 3.23. They correspond to the strongest lightning discharges, which are rare (less than 5% of cases [ Vijayaraghavan]. Currents of 200 kA occur in 0.7 ... 1% of cases, 20 kA - in 50% of cases [ Kuznetsov ]).

Dependences of the first impulse of the lightning current and its derivative on time, built according to the formula (3.2), are shown in rice. 3.65. Note that the time scales on the graphs differ by a factor of 10 and that the scale is logarithmic. The maximum slew rate (first derivative) of the first pulse is 25 kA/μs, of subsequent pulses - 280 kA/μs.

The current slew rate is used to calculate the magnitude of the induced pulse in automation cables.