Below is explained the approach to determining the protection zones of lightning rods, the construction of which is carried out according to the formulas applications 3 RD 34.21.122-87.
The protective effect of a lightning rod is based on the “property of lightning that is more likely to strike higher and well-grounded objects compared to nearby objects of lower height. Therefore, the lightning rod, rising above the protected object, is assigned the function of intercepting lightning, which in the absence of a lightning rod would strike the object. Quantitatively the protective effect of a lightning rod is determined through the probability of a breakthrough - the ratio of the number of lightning strikes into a protected object (the number of breakthroughs) to total number strikes on the lightning rod and the object.
There are several ways to assess the probability of a breakthrough, based on different physical concepts of the processes of lightning damage. RD 34.21.122-87 uses the results of calculations using a probabilistic method that relates the probability of damage to a lightning rod and an object with the spread of downward lightning trajectories without taking into account variations in its currents.
According to the adopted calculation model, it is impossible to create ideal protection against direct lightning strikes, completely excluding breakthroughs to the protected object. However, in practice it is feasible mutual arrangement object and a lightning rod, providing a low probability of a breakthrough, for example 0.1 and 0.01, which corresponds to a reduction in the number of damage to the object by approximately 10 and 100 times compared to an object without a lightning rod. For most modern facilities, such levels of protection ensure a small number of breakthroughs over their entire service life.
Above, we considered an industrial building with a height of 20 m and dimensions in plan of 100 × 100 m, located in an area with a thunderstorm duration of 40-60 hours per year; If this building is protected by lightning rods with a breakthrough probability of 0.1, no more than one breakthrough can be expected into it in 50 years. However, not all breakthroughs are equally dangerous for the protected object; for example, ignition is possible at high currents or transferred charges, which are not found in every lightning discharge. Consequently, one hazardous impact can be expected on this object over a period obviously exceeding 50 years, or for most industrial facilities Category II and III no more than one dangerous impact during the entire period of their existence. With a breakout probability of 0.01, the same building can expect no more than one breakout in 500 years—a period far longer than the lifespan of any industrial facility. Such a high level of protection is justified only for category I objects that pose a constant threat of explosion.
By performing a series of calculations of the probability of a breakthrough in the vicinity of a lightning rod, it is possible to construct a surface that is the geometric location of the vertices of protected objects, for which the probability of a breakthrough is a constant value. This surface is the outer boundary of the space called the lightning protection zone; for a single rod lightning rod this limit is side surface a circular cone, for a single cable - a gable flat surface.
Typically, a protection zone is designated by the maximum probability of a breakthrough corresponding to its outer boundary, although in the depths of the zone the probability of a breakthrough decreases significantly.
The calculation method makes it possible to construct a protection zone for rod and cable lightning rods with an arbitrary value of the probability of a breakthrough, i.e. for any lightning rod (single or double), you can build an arbitrary number of protection zones. However, for most commercial buildings, a sufficient level of protection can be ensured by using two zones, with a breakthrough probability of 0.1 and 0.01.
In terms of reliability theory, the probability of a breakthrough is a parameter characterizing the failure of a lightning rod as a protective device. With this approach, the two accepted protection zones correspond to a reliability degree of 0.9 and 0.99. This reliability assessment is valid when the object is located near the border of the protection zone, for example, an object in the form of a ring coaxial with a lightning rod. For real objects (ordinary buildings) on the border of the protection zone, as a rule, only the upper elements are located, and most of the object is located in the depths of the zone. Assessing the reliability of the protection zone along its outer border leads to excessively underestimated values. Therefore, in order to take into account the relative position of lightning rods and objects that exists in practice, protection zones A and B are assigned in RD 34.21.122-87 an approximate degree of reliability of 0.995 and 0.95, respectively.
Rice. 1. Nomograms for determining the height of single (a) and double equal height (b) lightning rods in zone A
The calculation method for the probability of a breakthrough has been developed only for downward lightning, predominantly striking objects up to 150 m high. Therefore, in RD 34.21.122 - 87, formulas for constructing protection zones for single and multiple rod and cable lightning rods are limited to a height of 150 m. To date, the volume of actual data on the incidence of damage from downward lightning to objects of greater height is very small and mainly applies to the Ostankino television tower (540 m). Based on photographic recordings, it can be argued that downward lightning breaks more than 200 m below its top and strikes the ground at a distance of about 200 m from the base of the tower. If we consider Ostankinskaya TV tower as a rod lightning rod, we can conclude that the relative sizes of the protection zones of lightning rods with a height of more than 150 m rarely decrease with increasing height of the lightning rods. Taking into account the limited actual data on the susceptibility of ultra-tall objects, RD 34.21.122 - 87 includes formulas for constructing protection zones only for lightning rods with a height of more than 150 m.
Rice. 2. Nomograms for determining the height of single (a) and double equal height (b) lightning rods in zone B
A method for calculating protection zones against rising lightning has not yet been developed. However, according to observational data, it is known that ascending discharges are excited from pointed objects near the top of tall structures and impede the development of other discharges with more low levels. Therefore, for such high objects as reinforced concrete chimneys or tower, protection is provided, first of all, from mechanical destruction of concrete when excited by ascending lightning, which is carried out by installing rod or ring lightning rods that provide the maximum possible, for design reasons, excess above the top of the object ( clause 2.31).
This manual provides nomograms for determining the heights of rods WITH and route T single and double lightning rods providing protection zones A and B (Fig. 1 and 2). Use of these nomograms, constructed in accordance with calculation formulas and notations applications 3 RD 34.21.122-87, allows you to reduce the amount of calculations and simplify the selection of lightning protection means during design.
The protective effect of a lightning rod is based on the fact that lightning strikes the highest and well-grounded metal structures. Consequently, a structure will not be struck by lightning if it is located in the protection zone of a lightning rod. Lightning rod protection zone - part of the space adjacent to the lightning rod, which provides protection of the structure from direct lightning strikes with a sufficient degree of reliability (99%)
Rapid changes in lightning current give rise to electromagnetic induction - the induction of potentials in open metal circuits, creating the danger of sparking in places where these circuits come together. This is called secondary lightning manifestation.
It is also possible for high electrical potentials induced by lightning to be carried into the protected building through external metal structures and communications.
Protection against electrostatic induction is achieved by attaching metal enclosures of electrical equipment to protective grounding or to a special grounding conductor.
To protect against the introduction of high potentials, underground metal communications, when entering the protected object, are connected to the ground electrodes for protection against electrostatic induction or electrical equipment.
Lightning rods consist of a load-bearing part (support), an air terminal, a down conductor and a grounding conductor. There are two types of lightning rods: rod and cable. They can be free-standing, isolated or not isolated from the protected building or structure (Fig. 86, a-c).
lightning rod: single rod lightning rod: double rod lightning rod: antenna
Rice. 86. Types of lightning rods and their protection zones:
a - single rod; b - double rod; c - antenna; 1 - lightning rod; 2 - down conductor, 3 - grounding
Rod lightning rods are one, two or more vertical rods installed on or near the protected structure. Cable lightning rods - one or two horizontal cables, each fixed to two supports, along which a down conductor connected to a separate grounding conductor is laid; The supports of the cable lightning rod are installed on the protected object or near it. Round steel rods, pipes, galvanized steel cable, etc. are used as lightning rods. Down conductors are made of steel of any grade and profile with a cross-section of at least 35 mm2. All parts of lightning rods and down conductors are connected by welding.
Grounding electrodes can be surface, deep and combined, made of steel of various sections or pipes. Surface grounding conductors (strip, horizontal) are laid at a depth of 1 m or more from the ground surface in the form of one or several beams up to 30 m long. Deep grounding conductors (vertical rods) 2-3 m long are driven into the ground to a depth of 0.7-0. 8 m (from the upper end of the ground electrode to the ground surface).
The grounding resistance for each individual lightning rod should not exceed 10 Ohms for lightning protection of buildings and structures of categories I and II and category III - 20 Ohms.
4. Grounding device.
The concept of the resistance of the grounding device of a power line support to lightning current. A grounding device is a structure made of electrically conductive materials that serves to drain current into the ground. Its main structural elements are grounding electrodes and grounding conductors. A grounding electrode is a conductor (electrode) or a set of interconnected metal conductors (electrodes) that are in contact with the ground. A grounding conductor is a conductor that connects the grounded parts to the ground electrode. The main function performed by the grounding device of a power line support is to drain lightning current into the ground, i.e., reducing the possibility (probability) of reverse flashovers when lightning strikes the support and the lightning protection cable. Unlike conventional flashovers caused by moisture or contamination of the insulation, the lightning current creates an electric potential on the support that is much greater than the potential of the phase wire, and thus the flashover occurs in the opposite direction. The lower the resistance of the grounding device, the lower the possibility of reverse flashover. The resistance of the grounding device is the ratio of the voltage on the grounding device to the current flowing from the grounding device into the ground. The resistance of the grounding device is not the only parameter that affects the likelihood of reverse flashovers. The following also have a significant influence: the length of the insulator string; height of lightning protection cable and phase wire; the distance between the cable and the wire, etc. As the length of the garland increases, for example, the electrical strength of the corresponding air gap increases and thereby the likelihood of reverse flashover decreases. This should happen as the line voltage class increases. However, for higher voltage lines, the height of the supports also increases, which leads to an increase in the number of lightning strikes on the supports and on the lightning protection cable. The inductance of the support also increases, which increases the likelihood of reverse flashovers. When a lightning strike strikes a support, the lightning current spreads along the lightning protection cable. The current in the cable induces currents in the wire and the support, which ultimately leads to an increase in the voltage applied to the insulating gap between the wire and the support. Thus, the probability of reverse flashover when a lightning strikes a support is a complex functional value that depends on a number of parameters. If all parameters, except the resistance of the grounding device, are considered constant, i.e., specified by a certain type of support, then the probability curve of reverse overlaps can be calculated. Below are the initial data for calculating the probability of reverse flashovers during a lightning strike into an intermediate support of type P220-2T: Maximum operating voltage, kV 252 50% discharge voltage of positive polarity: impulse strength of the air gap corresponding to the construction height of the insulator garland, kV 1248 Rope height on support, m 42 Height of top wire, m 33 Average span length, 400 Cable radius, 0.007 Wire radius, m 0.012 Horizontal distance between cable and top wire, 3 Distance between cables, m 1 Cable sag boom, 13 Wire sag boom, m 15 Equivalent radius of the support, m 3.2 Based on these data, calculations were made of the dependence of the probability of reverse overlap on the resistance value of the grounding device. This dependence is shown in Fig. 1. From the figure it can be seen that up to a resistance of R = 300 Ohms the curve rises quite steeply, then smoothly increases to R = 1000 Ohms. In the future, the probability of reverse overlaps slowly approaches the level of 0.3, without exceeding this value. A numerical probability value of 0.3 means that out of approximately 10 lightning strikes, reverse flashover will be observed in three cases. For other types of supports, this limit level may be different, it is only important to emphasize: if, due to the characteristics of the soil (sand, rock), the resistance of the grounding device turns out to be quite large, for example 5000 Ohms, then reducing the resistance to 1000 Ohms no longer makes sense. Thus, the probability of reverse flashovers and the associated number of lightning outages depend on the resistance of the grounding device of the support. This dependence manifests itself to a greater extent at low grounding resistances of the support: from units to hundreds of ohms. The grounding device of a power transmission line support is an electrical circuit with distributed parameters: metal resistance and inductance, soil conductivity and capacitance. If a sinusoidal voltage (or current) of a sufficiently high frequency is applied to the input of such a circuit, then at different distances from the source the ratio of voltage to current, i.e., the resistance at a given point, will be different. 
Rice. 1. Dependence of the probability of reverse flashovers on the resistance of the grounding device of the support. An even more complex type of relationship between voltage and current is observed when the ground electrode is exposed to a lightning current pulse. The pulse is characterized by two parameters: the largest value (amplitude) of the current and the rise time of the current (front duration). At low amplitudes, sparking does not occur in the soil. However, large lightning currents lead to electrical breakdown of the soil, which in the area adjacent to the ground electrode acquires zero electrical resistance: the ground electrode seems to increase in size. For a complete analysis of the processes in the grounding device when exposed to lightning current, it is necessary to take into account such factors as the length of the ground electrode, soil resistivity, amplitude and duration of the front of the lightning current pulse, and the moment of observation. All these factors are taken into account by impulse coefficients, which are denoted ai. Resistance of natural and artificial grounding conductors. Natural grounding electrodes are electrically conductive parts of communications, buildings and structures for industrial or other purposes that are in contact with the ground and are used for grounding. An artificial ground electrode is a ground electrode that is specially made for grounding. Rice. 2. Reinforced concrete footing (c) and its calculation model (b) The steel reinforcement of the foundations of metal supports and the buried part of reinforced concrete supports in many cases performs quite well the function of draining lightning currents into the ground, i.e., it plays the role of a natural grounding conductor. This is due to the fact that concrete, as a conductor of electric current, is a porous body consisting of large number thin channels filled with moisture and thus creating a path for electrical current. At a certain current strength and time of its flow, moisture evaporates, electric sparks and arcs appear in the concrete, which can destroy the material and burn out the reinforcement, which ultimately leads to a decrease in the mechanical strength of the reinforced concrete structure. In this regard, the reinforcement rods used for grounding are checked for thermal resistance when currents flow short circuit. It should also be borne in mind that in an environment with significant aggressiveness towards concrete, the use of reinforced concrete foundations as grounding conductors is not always possible. In networks with an isolated neutral, a long-term short circuit is dangerous for reinforced concrete foundations, and the construction of artificial grounding devices is necessary to unload the natural elements of the grounding device and protect them from destruction by flowing current. Below is the permissible electric current density established as a result of research for the reinforcement of reinforced concrete structures, depending on the type current and exposure time, A/m2: Continuous direct current 0.06 Continuous alternating current 10 Short-term alternating current (up to 3 s) 10000 Lightning current 100000 Artificial grounding electrodes are constructed, as a rule, in soils with a resistivity of more than 500 Ohm - m. This is due to the fact that natural grounding electrodes of BL35 - 330 kV supports in such soils have a resistance greater than the normalized ones . In lines of higher voltage classes with powerful foundations, artificial grounding conductors do not significantly reduce the resistance of the grounding device. Artificial ground electrodes, as a rule, are made in the form of two to four horizontal beams diverging from the support, laid at a depth of 0.5 m, and in plowing - 1 m. In the case of installation of supports in rocky soils, it is allowed to lay beam earth electrodes directly under the collapsible layer above the rock breeds In the absence of this layer (at least 0.1 m thick), it is recommended to lay grounding conductors along the surface of the rock and fill them with cement mortar. To reduce the corrosive effect from the soil, artificial grounding conductors must be round section with a diameter of 12-16 mm. 
Rice. 3. Location of natural a - tower intermediate support 35-330 kV; b - U-shaped intermediate support with guys 330-750 kV The indicated resistances of grounding devices also apply to supports without cables and other lightning protection devices, but with power or instrument transformers, disconnectors, fuses or other devices installed on these supports for overhead lines with a voltage of 110 kV and higher. Reinforced concrete and metal supports with a voltage of 110 kV and higher without cables and other lightning protection devices are also grounded if this is necessary to ensure reliable operation of relay protection and automation. The resistances of the grounding devices of such supports are determined when designing overhead lines. Reinforced concrete and metal supports with a voltage of 3 - 35 kV, which do not have lightning protection devices and other installed equipment, must be grounded, and in uninhabited areas for overhead lines 3 - 20 kV, the resistance of the grounding device is allowed: 30 Ohms at p less than 100 Ohms - m and 0, 3 р - at р more than 100 Ohm - m. Grounding devices of supports on which electrical equipment is installed. must meet the following requirements. In networks with a voltage of less than 1 kV with a solidly grounded neutral, the resistance of the grounding device should be 2, 4, 8 Ohms at line voltages of 660,380,220 V three-phase or 380,220,127 single-phase current. This resistance must be ensured taking into account the use of natural grounding conductors, as well as grounding conductors for repeated grounding of the neutral wire. In this case, the resistance of the grounding conductor located in close proximity to the neutral of the generator or transformer or the output of a single-phase current source should be no more than 25, 30, 60 Ohms for line voltages of 660, 380, 220 V three-phase or 380,220,127 V single-phase current. In networks with voltages above 1 kV with an isolated neutral, the grounded equipment installed on the overhead line support is connected to a closed horizontal grounding conductor (circuit) laid at a depth of at least 0.5 m. If the resistance of the grounding device is higher than 10 Ohms, then additional horizontal grounding conductors should be laid at a distance of 0.8 - 1 m from the support foundation. At p > > 500 Ohm-m, it is allowed to increase the resistance value by 0.002 p times, but not more than 10 times. Measurements of the resistance of grounding devices of overhead line supports should be carried out at industrial frequency current. On overhead lines with voltages below 1 kV, measurements are made on all supports with lightning protection grounding conductors and repeated neutral conductor grounding conductors. On overhead lines with voltages above 1 kV, measurements of the resistance of grounding devices are carried out on supports with arresters and protective gaps and with electrical equipment, and on supports of overhead lines 110 kV and above - with lightning protection cables when traces of insulator overlaps are detected electric arc. On the remaining reinforced concrete and metal supports, measurements are taken selectively at 2% of the total number of supports with grounding conductors: in populated areas, in areas with aggressive and landslide-prone soils and in poorly conductive soils.
20. The protection zone of a double catenary lightning rod is shown in Fig. 12. Dimensions r, h, r are determined by formulas (5) of this Instruction. The remaining dimensions of the protection zone are determined by the formulas:
At L h h = h, r = r r = r ; (6)
At L>h (7)
Fig. 12 Diagram of the protection zone of a double cable lightning rod:
1
, 2, 3-
boundaries of protection zones at the levels of the ground and the heights of the protected structure, respectively; 4 -
cable
The protection zone exists when L 3h.
Structural implementation of lightning rods
Supports, lightning rods and down conductors
21. Lightning rod supports should be made of steel of any grade, reinforced concrete or wood (Fig. 13). Metal tubular supports may be made from substandard steel pipes. Metal supports must be protected from corrosion. It is not allowed to paint contact surfaces in joints; wooden supports and stepsons must be protected from rotting by impregnation with antiseptics.
22. Supports of rod lightning rods must be calculated for mechanical strength as freely standing structures, and cable ones - taking into account the tension of the cable and the wind load on the cable, without taking into account the dynamic forces from lightning currents in both cases.
23. A lightning receiver is attached to the upper end of the support / 2,
protruding above the support by no more than 1.5 m (see Fig. 13). The lightning rod is connected by a down conductor 3
with grounding 4
and is attached to the pole with brackets 5. For large storage facilities, complex supports are used.
Fig. 13 Installation of rod lightning rods on wooden supports: A - two; b - one
To increase service life, wooden supports can be installed on rail or reinforced concrete attachments.
Dimensions of wooden supports
Lightning rod height, m...... 9 11 13 14 16 18 20 22
Composite height wooden parts supports m:
top A . . . . . . . . . . . . . 6 7 8 9 10 11 12 13
bottom b. . . . . . . . . . . . . 5,5 6,5 7,5 8,5 9,5 10,5 11,5 12,5
24. The use of trees as supports for lightning rods is not allowed.
25. The cross-sectional area of the steel lightning rod lightning rod must be at least 100 mm (Fig. 14). The length of the lightning rod must be at least 200 mm. Lightning rods should be protected from corrosion by galvanizing, tinning or painting.
Rice. 14. Designs of lightning rods made of round steel (A), steel wire with a diameter of 2-3 mm ( b), steel pipe (V), strip steel ( G), angle steel (d): 1 - down conductor
26. Lightning rods of cable lightning rods must be made of multi-wire galvanized steel cable with a cross-sectional area of at least 35 mm.
27. The connection of lightning rods with down conductors must be carried out by welding, and if welding is not possible - bolted connection with transitional electrical resistance no more than 0.05 Ohm. Compound steel roofing with down conductors can be performed using clamps (Fig. 15). The contact surface area in the connection must be at least twice the cross-sectional area of the down conductors.
Rice. 15. Clamp for connecting flat (A) and round (b) down conductors to metal roofing: 1 - down conductor; 2 - roof; 3 - lead gasket; 4 - steel plate; 5 -plate with a welded current conductor
| Down conductor location | ||
| View | outside the building in the air | in the ground |
| Round down conductors and jumpers with diameter, mm | - | |
| Round vertical electrodes with diameter, mm | - | |
| Round horizontal electrodes with diameter, mm *1 | - | |
| Rectangular (from square and strip steel): | ||
| cross-sectional area, mm | ||
| thickness, mm | ||
| Angle steel: | ||
| cross-sectional area, mm | - | |
| shelf thickness, mm | - | |
| Steel pipes with wall thickness, mm | - | 3,5 |
_____
*1 Only used for deep grounding and potential equalization inside buildings.
28. Down conductors, jumpers and grounding conductors must be made of 113 shaped steel with element dimensions not less than those indicated on page 217.
Grounding devices
29. Based on the location in the ground and the shape of the electrodes, ground electrodes are divided into:
A) recessed - made of strip (sectional area 40 X 4 mm) or round (diameter 20 mm) steel, laid on the bottom of the pit in the form of extended elements or contours along the perimeter of the foundations. In soils with an electrical resistivity of 500 Ohm m, reinforcement of reinforced concrete piles and other types of reinforced concrete foundations can be used as deep grounding conductors;
B) horizontal - from strip (sectional area 40 X 4 mm) or round (diameter 20 mm) steel, laid horizontally at a depth of 0.6-0.8 m from the ground surface or in several beams diverging from one point to which it is connected down conductor;
C) vertical - made of steel, vertically screwed rods (diameter 32-56 mm) or driven electrodes made of angle steel (40X40 mm). The length of screwed-in electrodes should be 3-5 m, driven ones - 2.5-3 m. The upper end of the vertical ground electrode should be buried 0.5-0.6 m from the ground surface;
D) combined - vertical and horizontal, combined into common system. The connection of down conductors should be made in the middle of the horizontal part of the combined ground electrode.
As combined meshes, meshes with a depth of 0.5-0.6 m or meshes with vertical electrodes should be used. The pitch of the grid cells must be at least 5-6 m;
E) plate - for ships with VMs, the hulls of which are made of non-conductive material.
30. All connections of grounding electrodes to each other and to down conductors must be carried out by welding. The length of the weld seam must be at least twice the width of the strips being welded and at least 6 diameters of the round conductors being welded,
Bolted contact is allowed only when installing temporary grounding conductors and in places where individual circuits are connected to each other, made in accordance with clause 11 of these Instructions. The cross-sectional area of the connecting strips of grounding conductors must be no less than specified in clause 28 of these Instructions.
31. The design of grounding conductors should be carried out taking into account the heterogeneity of the soil.
32. The design of grounding conductors is selected depending on the required impulse resistance, taking into account the structure and electrical resistivity of the soil, as well as the convenience of laying them. Typical designs grounding conductors and the values of their resistance to the spreading of industrial frequency current , Ohm, are given in table. 1P.
In soils with electrical resistivity less than 500 Ohm m, horizontal or vertical grounding electrodes should be used. For soils with non-uniform conductivity, horizontal grounding conductors should be used if the electrical resistivity the upper layer of soil is less than the lower one, and vertical grounding conductors if the conductivity of the lower layer is better than the upper one.
33. Each ground electrode is characterized by its impulse resistance, i.e., resistance to lightning current spreading R. The impulse resistance of the ground electrode can differ significantly from the resistance , obtained by generally accepted methods. Its value is determined by the formula:
R= (8)
Where - impulse coefficient, depending on the parameters of the lightning current, electrical resistivity of the soil and the design of the ground electrode.
Limit lengths of horizontal grounding conductors guaranteeing 1 at different soil resistivities R, are given below.
| , Ohm * m | Up to 500 | ||||
| l, m |
Table 1P
| Drawings | Type | Material | Resistance value (Ohm) to the spreading of industrial frequency current at various electrical soil resistivities, Ohm m | |||
| l00 | ||||||
| Vertical rod | Angle steel 40 X 40 X 4 mm: l = 2 ml = 3 m Round steel with a diameter of 10-20 mm: l = 2 ml = 3 ml = 5 m | 19 14 24 17 14 | 38 28 48 34 28 | 190 140 240 170 140 | 380 280 480 340 280 | |
| Horizontal strip | Strip steel 4 X 40 mm: l = 2 m l = 5 ml = 10 m l = 20 ml = 30 m | 22 12 7 4 3,2 | 44 24 14 8 6,5 | 220 120 70 40 35 | 440 240 140 80 70 | |
| Horizontal strip with current input in the middle | Steel strip 4 X 40 mm: l = 5 ml = 10 ml = 12 m l = 24 ml = 32 m l = 40 m | 9.5 5.85 5.4 3.1 Not applicable Same | 19 12 11 6.2 Not applicable Same | 95 60 54 31 24 20 | 190 120 110 62 48 40 | |
| Horizontal three-beam | Steel strip 4 X 40 mm: l = 6 m l = 12 m l = 16 m l = 20 ml = 32 ml = 40 m | 4.6 2.6 2 1.7 Not applicable Same | 9 5.2 4 3.4 Not applicable Same | 45 26 20 17 14 12 | 90 50 40 34 28 24 | |
| Combined two-rod | Angular steel 40 X 40 mm, strip steel 4 X 40 mm: C = 3 m; l = 2.5 mC = 3 m; l = 3 mС = 6 m; l = 2.5 mC = 6 m; l = 3 m C = 3 m; l = 2.5 mC = 3 m; l = 3 mС = 5 m; l = 2.5 mC = 5 m; l = 3 mC = 3 m; l = 5 mS = 5 m; l = 5 m | 7 6 5,5 4,5 7,5 6,8 6 5,5 5,5 4 | 14 12 11 9,1 15 14 12 11 11 8 | 70 60 55 45 75 70 60 55 55 40 | 140 120 110 90 150 140 120 110 110 80 | |
| Combined three-rod | Angular steel 40 X 40 X 4 mm, strip steel 4x40 mm: C = 3 m; l = 2.5 mC = 6 m; l = 7.5 mC = 7 m; l = 3 m Round steel with a diameter of 10-20 mm, strip steel 4 X 40 mm: C = 2.5 m; l = 2.5 mC = 2.5 m; l = 2 mS = 5 m; l = 2.5 mC = 5 m; l = 3 mС = 6 m; l = 5 m | 4 3 2,7 4,8 4,4 3,5 3,3 2,7 | 8 6 5,4 9,7 8,9 7,1 6,6 5,4 | 40 30 27 50 45 36 33 27 | 80 60 55 100 90 70 65 55 | |
| Combined five-rod | C = 5 m; l = 2 mC = 5 m; l = 3 mC = 7.5 m; l = 2 mС = 7.5 m; l = 3 m Round steel with a diameter of 10-20 mm, strip steel 4 X 40 mm: C = 5 m; l = 2 mC = 5 m; l = 3 mС = 7.5 m; l = 2 mС = 7.5 m; l = 3 mС = 5 m; l = 5 mC = 7.5 m; l = 5 m | 2,2 1,9 1,8 1,6 2,4 2 2 1,7 1,9 1,6 | 4,4 3,8 3,7 3,2 4,8 4,1 4 3,5 3,8 3,2 | 22 19 18,5 16 24 20,5 20 17,5 19 16 | 44 38 37 32 48 41 40 35 38 32 | |
| Combined four-rod | Angle steel 40 X 40 X 4 mm, strip steel 4 X 40 mm: C = 6 m; l = 3 m | 2,1 | 4,3 | 21,5 | 43 | |
| Horizontal with current input in the center | Steel strip 4 X 40 mm: D=4 m D = 6 mD = 8 mD = 10 mD = 12 m | 4,5 3,3 2,65 2,2 1,9 | 9 6 5,3 4,4 3,8 | 45 33 26,5 22 19 | 90 66 53 44 38 |
Grounding electrodes of longer length practically do not remove the pulse current over a section exceeding l.
The values of the impulse coefficient for different soil resistivities are given in Table. 2P.
Table 2P
The impulse coefficients are determined for lightning current amplitude values of 60 kA and slope of 20 kA/µs.
34. After installation of grounding conductors design resistance spreading should be clarified by direct measurement. Measurements should be carried out in summer in dry weather.
Connecting individual grounding conductors of lightning rods with a steel strip is allowed in soils with electrical resistivity > 500 Ohm m.
If the measured resistance of the grounding conductors exceeds the calculated one, then in soils with an electrical resistivity of 500 m m or more, it is necessary to connect the grounding conductors of lightning rods of adjacent storage facilities with each other at a distance between them no more than specified in paragraph 10 of this Instruction.
Buildings and structures are protected from direct lightning strikes by lightning rods of various designs. But any of the lightning rods includes four main parts: a lightning rod that directly perceives a lightning strike; down conductor connecting the lightning rod to the grounding conductor; ground electrode through which lightning current flows into the ground; load-bearing part (support or supports) designed to secure the lightning rod and down conductor.
Depending on the design of the lightning rod, rod, cable, mesh and combined lightning rods are distinguished. Based on the number of jointly operating lightning rods, they are divided into single, double and multiple. In addition, depending on their location, lightning rods can be free-standing, isolated or not isolated from the building being protected.
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 property, a protected building that is lower in height is practically not struck by lightning if it enters the lightning rod protection zone. The protection zone of a lightning rod is the part of the space adjacent to it and with a sufficient degree of reliability (at least 95%) providing protection of structures from direct lightning strikes.
Lightning rods are most often used to protect buildings and structures. An air terminal of a rod lightning rod is a vertically located steel rod of any profile with a length of 2... 15 m and a cross-sectional area of at least 100 mm2, mounted on a support located, as a rule, no closer than 5 m from the protected object. The lightning rod is connected to the grounding conductor by a down conductor made of steel wire with a diameter of at least 6 mm, and in the case of laying a down conductor in the ground - at least 10 mm. When installing lightning rods directly on the roof of a building, at least two down conductors are installed, and for a roof width of more than 12 m - four. If the length of the protected object is more than 20 m, then for each subsequent 20 m of length it is necessary to install additional down conductors; with a building width of up to 12 m - on both sides of the building. All connections (lightning rod - down conductor, down conductor - grounding conductor) should be welded.
As rod lightning rods, it is necessary to make maximum use of tall structures existing near the protected object: water towers, exhaust pipes etc. Trees growing at a distance of no more than 5 m from buildings of III...V degrees of fire resistance can also be used as a support for a lightning rod, if a down conductor is laid on the wall of the building opposite the tree to the entire height of the wall, welding it to the grounding conductor of the lightning rod .
Cable lightning rods are most often used to protect long buildings and high voltage lines. These lightning rods are made in the form of horizontal cables attached to supports, along each of which a down conductor is laid. Lightning rods of cable lightning rods are made of multi-wire galvanized steel cable with a cross-section of at least 35 mm2.
It should be noted that rod and cable lightning rods provide the same degree of protection reliability.
Can be used as lightning rods metal roof, grounded at the corners and along the perimeter at least every 25 m, or a mesh of steel wire with a diameter of at least 6 mm placed on a non-metallic roof, having a cell area of up to 150mm2, with nodes secured by welding, and grounded in the same way as a metal roof . Metal caps are attached to the mesh or conductive roof over the smoke and ventilation pipes, and in the absence of caps - wire rings specially placed on the pipes.
Protective action lightning rod based on the “property of lightning with a greater probability of striking higher and well-grounded objects compared to nearby objects of lower height. Therefore, the lightning rod, rising above the protected object, is assigned the function of intercepting lightning, which in the absence of a lightning rod would strike the object. Quantitative protective effect of the lightning rod is determined through the probability of a breakthrough - the ratio of the number of lightning strikes into a protected object (the number of breakthroughs) to the total number of strikes into the lightning rod and the object.
According to the adopted calculation model, it is impossible to create ideal protection against direct lightning strikes, completely excluding breakthroughs to the protected object. However, in practice, the relative position of the object and the lightning rod is feasible, providing a low probability of a breakthrough, for example, 0.1 and 0.01, which corresponds to a reduction in the number of damage to the object by approximately 10 and 100 times compared to an object where there is no lightning rod. For most modern facilities, such levels of protection ensure a small number of breakthroughs over their entire service life.
Rice. 11.22. Lightning rod device.
The supports of overhead power lines are protected from destruction during direct lightning strikes by lightning rods, which are installed on input, cable, control, split, transition supports, as well as on supports that are replaced due to damage by lightning discharges. For a lightning rod, a steel linear wire with a diameter of 4 ... 5 mm is used, the lower end of which is retracted. This tap is called a ground electrode. The length of the grounding conductor wire (Fig. 11.22) depends on the nature of the soil and can be equal to 1 ... 12 m. The depth of the grounding conductor is 0.10 m. The greater the soil resistivity, the greater the length of the grounding conductor should be. On intermediate and corner supports, they usually do not make a tap, but bring the wires to the butt of the post.
The supports on which spark or gas-filled arresters are installed are also protected by lightning rods. According to safety regulations, on supports that intersect or approach overhead overhead lines, a gap is made on the lightning rod at a height of 30 cm from the ground, creating a spark gap 50 mm long.
The higher it is located, the greater the effectiveness of a lightning rod. The protective action zone of a lightning rod is determined approximately by the formula S=πh2, where h is the height of the lightning rod.
Lightning protection cable - grounded extended lightning rod stretched along overhead line power transmission over wires.
Depending on the location, the number of wires on the overhead line supports, soil resistance, voltage class of the overhead line, and the required degree of lightning protection, one or more cables are installed. The height of the suspension of lightning protection cables is determined depending on the protection angle, that is, the angle between the vertical passing through the cable and the line connecting the cable to the outermost wire, which can vary widely and even be negative.
On overhead lines with voltages up to 20 kV, lightning protection cables are usually not used. 110-220 kV overhead lines on wooden supports and 35 kV overhead lines (regardless of the material of the supports) are most often protected with cables only at the approaches to substations. Lines 110 kV and higher on metal and reinforced concrete supports protected with a cable throughout.
Steel ropes or sometimes steel-aluminum wires with a steel core of increased cross-section are used as lightning protection cables. Steel ropes conventionally designated by the letter C and numbers indicating their cross-sectional area (for example, C-35).
Rice. 21. Determination of the protection zone of a lightning rod on a model
Rice. 22. 100% damage zone of a rod lightning rod
Rice. 23. Protection zone of a single rod lightning rod up to 60 m high:
A is the height of the lightning rod; hx - height of a point on the border of the protected zone: h& -h-hx - active height of the lightning rod
This zone is called the zone of 100% damage to the lightning rod. Secondly, around the lightning rod with height h there is a zone that is not affected by discharges. This area is protected by lightning rod h. Minimum distance from the vertical BC, equal to r0=3.5/g, and is the radius of the lightning rod protection zone at ground level.
The radius of the protection zone at any height h by a lightning rod is also determined by experiments in the laboratory using a rod of height hx (see Fig. 21), simulating the protected object and located in the same plane with electrode A and lightning rod h. They move relative to each other. With their different locations, a certain number of discharges are produced.
Then the maximum distance rx between the rod of height hx and the lightning rod of height h is found, at which the rod is not affected by the discharge. This distance rx is the radius of the lightning rod protection zone at height hx.
The lightning rod protection zone of height h defined in this way is a “tent” (Fig. 23), radius rx, m, which the “Guidelines for calculating the protection zones of rod and cable lightning rods” for lightning rods with a height of up to 60 m recommend calculating
according to the formula