Do you need a lightning rod? This question is asked by more than one owner of a private house, because a lightning discharge can cause failure of the entire household appliances Or worse, fire. If the house is located in a village or city surrounded by its own kind, then there is no need for a lightning rod. On the contrary, it can attract electrical discharges. If the house stands alone in a field or on a large plot, rises on a hillock, and the climate is hot and dry in summer, with frequent thunderstorms, then a lightning rod is simply necessary.
lightning rod device
The first lightning rod was designed by Benjamin Franklin, who, concurrently, was not only the president of America, but also an inventor. Since then, the design of this device has not changed much, as it copes with its task well. The lightning rod consists of three parts interconnected.
- Lightning rod- the most noticeable element, which is a long rod made of aluminum, copper, steel or other metal that conducts electricity well. It is attached or outside it in such a way that the top point rises above the roof. The thickness of the lightning rod depends on the metal, for steel it is 50 mm2, for copper - 35 mm2. A design in the form of a cable stretched over the ridge along its entire length is also possible; it is considered safer. Both the cable and the pin must rest on wooden supports. Metal roof without protective polymer coating itself can be a lightning rod, but in this case it must be well isolated from the inside. Such a roof device is negotiated at the design stage, since materials of sufficient thickness are selected, and the design itself has a number of features.
- By down conductor the charge from lightning goes to the ground. In fact, this is a wire connecting the lightning rod to the ground electrode. Its thickness depends on the material and length, since it must briefly cope with a load of 200 thousand amperes. Best fit copper wire with a cross section of at least 6 mm square.
- grounding conductor- a circuit through which the discharge voltage is transmitted to the ground. Usually it is made of copper or steel rods, the diameter of which depends on their length, dug into the ground. Do not use as a grounding conductor for a lightning rod a water pipe or other communications, or a ground loop from the electrical wiring of the house itself.
DIY lightning rod
Before installing a lightning rod, it is necessary to determine the place of its placement - whether it will be the roof of the house or a site on the site. A freestanding structure will require more material consumption, but, when installed at the border of plots, it can protect two or more households. Such a lightning rod should exceed the highest point of the roof by 2 meters.
The lightning rod is mounted on a tower, which can be made of a pipe of a suitable diameter. A down conductor will pass inside it, so the material of the pipe must serve as an insulator, a copper, steel or aluminum rod is attached to the top with clamps. The conductor is welded to the receiver.
The wire in those areas where it will not be protected by a pipe can be hidden in a corrugation to protect it from corrosion. The tower is dug into the ground to a depth of 2 meters; additionally, it can be fixed with supports fixed on a clamp.
If the lightning rod is located on the roof, then it should rise above it top point 30 cm. In this case, the conductor is laid so that it does not pass near windows or doors, the nearest metal structures (stairs, drains) should be at least 30 cm. The cable should not have sharp bends or right angles, since in these areas where sparks are likely to occur. It is attached to the wall with plastic clamps on dowels.
It is necessary to choose the location of the grounding, taking into account the fact that the nearest entrance to the house or other buildings should be at least 3 meters, and from the walls at least a meter. In this place, they dig a trench 3 meters long and 1-1.5 meters deep. At its ends, copper rods with a cross section of 50 mm2 are hammered to a depth of 2 meters. or steel with a section of 80 mm square. (unpainted reinforcement is suitable), connect them by welding a rod of the same material. A down conductor wire is welded to the circuit and the trench is again covered with earth.
Building a lightning rod on a site or on a roof will take time, welding skills and material costs. However, the losses that can occur in a fraction of a second when lightning strikes a house are significantly more serious.
It is worth remembering that a properly designed and installed lightning rod will be effective only when installing an RCD and voltage limiters in the house.
The lightning rod directly perceives a direct lightning strike. Therefore, it must reliably withstand the mechanical and thermal effects of the current and the high-temperature lightning channel. The supporting structure carries a lightning rod and a down conductor, combines all elements of the lightning rod into a single, rigid, mechanically solid construction. In electrical installations, lightning rods are installed near live parts that are under operating voltage. The fall of the lightning rod on the current-carrying elements of the electrical installation causes a severe accident. Therefore, the supporting structure of the lightning rod must have a high mechanical strength, which would exclude in operation cases of a lightning rod falling on the equipment of power plants and substations. The lightning rod must have a reliable connection with the ground with a resistance of 5-25 ohms to the spreading of the impulse current. Protective property of rod lightning rods lies in the fact that they orient the leader of the emerging lightning discharge towards themselves. The discharge occurs necessarily at the top of the lightning rod, if it is formed in a certain area located above the lightning rod. This area has the form of an upward expanding cone and is called the 100% lesion zone.
It has been established by experimental data that the lightning orientation height H depends on the height of the lightning rod h. For lightning rods up to 30 meters high:
and for lightning rods with a height of more than 30 meters H=600 m.
where is the active part of the lightning rod, corresponding to its excess over the height of the protected object:
Figure 1.1 Protection zone of a single rod lightning rod: 1 - boundary of the protection zone; 2 - section of the protection zone at the level.
To calculate the protection radius at any point of the protective zone, including at the height of the protected object, the following formula is used:
where - correction factor, equal to 1 for lightning rods less than 30 meters high and equal to higher lightning rods.
Protection zones of extended objects in which several lightning rods are used, it is advisable that the zones of their 100% defeat close over the object or even overlap each other, excluding the vertical lightning breakthrough to the protected object. The distance (S) between the axes of the lightning rods should be equal to or less than the value, determined from the dependency:
The protection zone of two and four rod lightning rods in the plan at the height of the protected object has the outlines shown in Figure 1.3, a, b.
The smallest width of the protection zone, the protection radius shown in the drawing is determined in the same way as for a single lightning rod, but is determined by special curves. Figure 1.2 shows the designs of lightning rods. If lightning rods with a height of up to 30 meters located at a distance, the smallest width of the protection zone is equal to zero.
Figure 1.2 Structures of rod lightning rods on reinforced concrete supports: a - from vibrated concrete; b - centrifuged concrete
Figure 1.3 Rod lightning rods on metal supports: a - wire lightning rod (supporting structure); b - rod lightning rod (supporting structure)
Figure 1.3 shows the designs of lightning rods on metal supports. The protection radii are determined in this case in the same way as for single lightning rods. The size is determined from the curves for each pair of lightning rods. The diagonal of a quadrilateral or the diameter of a circle passing through the vertices of a triangle formed by three lightning rods, according to the conditions of protection of the entire area, must satisfy the following dependencies:
For lightning rods less than 30 m high:
For lightning rods with a height of more than 30 m:
Free-standing rod lightning rods with metal supports are installed on reinforced concrete foundations. Down conductors for such lightning rods are load-bearing structures. On metal and reinforced concrete structures of outdoor switchgear, as a rule, lightning rods with metal bearing parts are installed. The design of their fastening is determined by the features of the outdoor switchgear design to which the rod lightning rod is attached. Typically, the design of lightning rods installed on outdoor switchgear structures is steel pipe, often consisting of pipes of several diameters. Lightning rods with a height of more than 5 m at the base have a lattice structure made of angle steel. The potential at the lightning rod at the moment of discharge is determined by the dependence:
where is the impulse grounding resistance of the lightning rod 5-25 Ohm;
Lightning current in a well-grounded object.
The potential at the lightning rod is determined by:
where is the steepness of the current wave front;
- - lightning rod point at the height of the object;
- - specific inductance of the lightning rod.
To calculate the minimum allowable approach of an object to a lightning rod, one can proceed from the dependence:
where is the permissible impulse electric field strength in the air, assumed to be 500 kV / m.
Guidelines for surge protection recommend that the distance to the lightning rod be taken equal to:
This dependence is valid for a lightning current of 150 kA, a current slope of 32 kA/μs and a lightning rod inductance of 1.5 μH/m. Regardless of the calculation results, the distance between the object and the lightning rod must be at least 6 meters.
Rope lightning rod. The values of the coefficients k and z are taken depending on the allowable probability of a lightning breakthrough into the protection zone. The probability of a lightning breakthrough into the protection zone is equal to the ratio of the number of lightning discharges into the protected structure to the total number of lightning discharges into the lightning rod and the protected structure. If the probability of a lightning breakthrough into the protection zone is 0.01, then the coefficient is 1, and with an acceptable probability of 0.001, i.e., the protective zones of lightning rods are somewhat smaller than the protective zones of rod lightning rods. The shape of the protection zone of two parallel wire lightning rods up to 30 m high. The outer boundaries of the protection zone of each wire are determined in the same way as for a single wire lightning rod. Depending on the design of the supports, one or two cables can be used, tightly attached to metal support or to grounding metal descents wooden poles. To protect the cable from overburning by lightning current and to control grounding, the support of the cable is made using one suspension insulator shunted with a spark gap. The efficiency of cable protection is the higher, the smaller the angle formed by the vertical passing through the cable and the line connecting the cable with the outermost of the wires. This angle is called the protective angle, taking its value within
The protection zone of two wire lightning rods with a height of more than 30 m. The method of constructing a protection zone for this case is the same as for wire lightning rods up to 30 m high, but at a distance from the top, the zone is truncated in the same way as for single wire lightning rods. The width of the protective zone, which excludes direct damage to the wires at the level of their suspension height, is determined by the dependence:
This dependence is valid for a cable suspension height of 30 m and below.
3.1. Supports of rod lightning rods must be designed for mechanical strength as freely standing structures, and supports of cable lightning rods - taking into account the tension of the cable and the effect of wind and ice loads on it.
3.2. Supports of free-standing lightning rods can be made of steel of any grade, reinforced concrete or wood.
3.3. Rod lightning rods must be made of steel of any grade with a cross section of at least 100 mm 2 and a length of at least 200 mm and protected from corrosion by galvanizing, tinning or painting.
Rope lightning rods must be made of steel multiwire ropes with a cross section of at least 35 mm 2.
3.4. Connections of lightning rods with down conductors and down conductors with grounding conductors should be carried out, as a rule, by welding, and if hot work is unacceptable, it is allowed to perform bolted connections with transition resistance not more than 0.05 Ohm with mandatory annual control of the latter before the start of the thunderstorm season.
3.5. Down conductors connecting lightning rods of all types with grounding conductors should be made of steel with dimensions not less than those indicated in Table. 3.
3.6. When installing lightning rods on the protected object and the impossibility of using metal structures of the building as down conductors (see clause 2.12), the down conductors must be laid to the ground electrodes along the outer walls of the building in the shortest possible ways.
3.7. It is allowed to use any structures of reinforced concrete foundations of buildings and structures (pile, strip, etc.) as natural grounding lightning protection (taking into account the requirements of clause 1.8).
Permissible dimensions of single structures of reinforced concrete foundations used as ground electrodes are given in Table. 2.
ANNEX 1
BASIC TERMS
1. Direct lightning strike (lightning strike) - direct contact of a lightning channel with a building or structure, accompanied by the flow of lightning current through it.
2. The secondary manifestation of lightning is the induction of potentials on the metal elements of the structure, equipment, in open metal circuits, caused by close lightning discharges and creating the danger of sparking inside the protected object.
3. High potential drift - transfer to a protected building or structure along extended metal communications (underground, ground and overhead pipelines, cables, etc.) electrical potentials arising from direct and close lightning strikes and creating the danger of sparking inside the protected object.
4. Lightning rod - a device that perceives a lightning strike and diverts its current to the ground.
In general, a lightning rod consists of a support; lightning rod that directly perceives a lightning strike; a down conductor through which the lightning current is transmitted to the ground; grounding conductor, which ensures the spreading of the lightning current in the ground.
In some cases, the functions of a support, lightning rod and down conductor are combined, for example, when using metal pipes or trusses as a lightning rod.
5. Lightning rod protection zone - the space inside which a building or structure is protected from direct lightning strikes with a reliability not lower than a certain value. The surface of the protection zone has the least and constant reliability; In the depth of the protection zone, reliability is higher than on its surface.
Type A protection zone has a reliability of 99.5% or more, and type B - 95% or more.
6. Structurally, lightning rods are divided into the following types:
rod - with a vertical arrangement of the lightning rod;
cable (extended) - with a horizontal arrangement of the lightning rod, fixed on two grounded supports;
grids - multiple horizontal lightning rods intersecting at right angles and laid on the protected object.
7. Stand-alone lightning rods are those whose supports are installed on the ground at some distance from the protected object.
8. A single lightning rod is a single design of a rod or wire lightning rod.
9. Double (multiple) lightning rod - these are two (or more) rod or cable lightning rods that form a common protection zone.
10. Lightning protection grounding conductor - one or more conductors buried in the ground, designed to divert lightning currents into the ground or limit overvoltages that occur on metal cases, equipment, communications in case of close lightning discharges. Grounding conductors are divided into natural and artificial.
11. Natural grounding conductors - metal and reinforced concrete structures buildings and structures.
12. Artificial grounding - specially laid in the ground contours of strip or round steel; concentrated structures consisting of vertical and horizontal conductors.
APPENDIX 2
CHARACTERISTICS OF THE INTENSITY OF LIGHTNING ACTIVITY AND THE LIGHTNING PROBLEM OF BUILDINGS AND STRUCTURES
The average annual duration of thunderstorms in hours at an arbitrary point on the territory of the USSR is determined from a map (Fig. 3), or from regional maps of the duration of thunderstorms approved for some regions of the USSR, or from average long-term (about 10 years) data from a weather station closest to the location of the building or structures.
The calculation of the expected number N of lightning strikes per year is made according to the formulas:
for concentrated buildings and structures ( chimneys, towers, towers)
for buildings and structures rectangular shape
where h - highest altitude buildings or structures, m; S, L - respectively the width and length of the building or structure, m; n is the average annual number of lightning strikes per 1 km of the earth's surface ( specific gravity, lightning strikes to the ground) at the location of a building or structure.
For buildings and structures complex configuration as S and L are considered the width and length of the smallest rectangle in which a building or structure can be inscribed in the plan.
For an arbitrary point on the territory of the USSR, the specific density of lightning strikes to the ground n is determined based on the average annual duration of thunderstorms in hours as follows:
Rice. 3. Map of the average annual duration of thunderstorms in hours for the territory of the USSR
APPENDIX 3
LIGHTNING PROTECTION ZONES
1. Single rod lightning rod.
The protection zone of a single rod lightning rod with a height h is a circular cone (Fig. A3.1), the top of which is at a height h 0 1.1. The protection zones of single rod lightning rods with a height of h £ 150 m have the following overall dimensions. Zone A: h 0 = 0.85h, r 0 \u003d (1.1 - 0.002h)h, r x \u003d (1.1 - 0.002h) (h - h x / 0.85). Zone B: h 0 = 0.92h; r x \u003d 1.5 (h - h x / 0.92). For zone B, the height of a single rod lightning rod at known values h and can be determined by the formula h = (rx + 1.63hx)/1.5. Rice. P3.1. Protection zone of a single rod lightning rod: I - the boundary of the protection zone at the level h x , 2 - the same at ground level 1.2. Protection zones of single rod lightning rods of skyscrapers 150< h < 600 м имеют следующие габаритные размеры. 2. Double rod lightning rod. 2.1. The protection zone of a double rod lightning rod with a height of h £ 150 m is shown in fig. P3.2. The end areas of the protection zone are defined as zones of single rod lightning rods, the overall dimensions of which h 0 , r 0 , r x1 , r x 2 are determined by the formulas of clause 1.1 of this appendix for both types of protection zones. Rice. P3.2. Protection zone of a double rod lightning rod: 1 - the boundary of the protection zone at the level h x 1 ; 2 - the same at the level h x 2, 3 - the same at ground level The internal areas of the protection zones of a double rod lightning rod have the following overall dimensions. at 2h< L £ 4h When the distance between the lightning rods L > at h< L £ 6h With a distance between rod lightning rods L > 6h, to build zone B, lightning rods should be considered as single ones. With known values of h c and L (at r cx = 0), the height of the lightning rod for zone B is determined by the formula h \u003d (h c + 0.14L) / l.06. 2.2. Protection zone of two rod lightning rods different heights h 1 , and h 2 £ 150 m is shown in fig. PZ.Z. dimensions end areas of protection zones h 01 , h 02 , r 01 , r 02 , r x 1 , r x 2 are determined by the formulas of clause 1.1, as for protection zones of both types of a single rod lightning rod. The overall dimensions of the inner area of the protection zone are determined by the formulas: where the values h c 1 and h c 2 are calculated according to the formulas for h c p. 2.1 of this appendix. For two lightning rods of different heights, the construction of zone A of a double rod lightning rod is carried out at L £ 4h min , and zone B - at L £ 6h min . With corresponding large distances between lightning rods, they are considered as single ones. Rice. ПЗ.З The zone of protection of two rod lightning rods of different heights. The designations are the same as in Fig. P3.1 3. Multiple rod lightning rod. The protection zone of a multiple lightning rod (Fig. A3.4) is defined as the protection zone of pairwise taken adjacent lightning rods with a height h £ 150 m (see paragraphs 2.1, 2.2 of this appendix). Rice. P3.4. Protection zone (in plan) of a multiple rod lightning rod. The designations are the same as in Fig. P3.1 The main condition for the protection of one or more objects with a height h x with a reliability corresponding to the reliability of zone A and zone B is the fulfillment of the inequality r cx > 0 for all lightning rods taken in pairs. Otherwise, the construction of protection zones must be performed for single or double rod lightning rods, depending on the fulfillment of the conditions of clause 2 of this appendix. 4. Single wire lightning rod. The protection zone of a single wire lightning rod with a height of h £ 150 m is shown in fig. P3.5, where h is the height of the cable in the middle of the span. Taking into account the sag of the cable with a cross section of 35-50 mm 2 with a known height of the supports h op and the span length a the height of the cable (in meters) is determined by: h \u003d h op - 2 for a< 120 м; h = h op - 3 at 120< а< 15Ом. Rice. P3.5. Protection zone of a single wire lightning rod. The designations are the same as in Fig. P3.1 The protection zones of a single wire lightning rod have the following overall dimensions. For a type B zone, the height of a single wire lightning rod with known values of h x and r x is determined by the formula 5. Double wire lightning rod. 5.1. The protection zone of a double wire lightning rod with a height of h £ 150 m is shown in fig. P3.6. Dimensions r 0 , h 0 , r x for protection zones A and B are determined according to the corresponding formulas of clause 4 of this appendix. The rest of the zone sizes are determined as follows. Rice. PZ.6. Protection zone of a double wire lightning rod. The designations are the same, 410 and in fig. P3.2 at h< L £ 2h at 2h< L £ 4h When the distance between the wire lightning rods is L > 4h, for the construction of zone A, the lightning rods should be considered as single ones. at h< L £ 6h When the distance between the wire lightning rods is L > 6h, for the construction of zone B, the lightning rods should be considered as single ones. With known values of h c and L (at r cx = 0), the height of the lightning rod for zone B is determined by the formula h \u003d (h c + 0.12L) / 1.06. Rice. P3.7. Protection zone of two wire lightning rods of different heights 5.2. The protection zone of two cables of different heights h 1 and h 2 is shown in fig. P3.7. Values r 01 , r 02 , h 01 , h 02 , r x1 , r x 2 are determined by the formulas of clause 4 of this appendix as for a single catenary wire lightning rod. To determine the dimensions r c and h c, the formulas are used: where h c 1 and h c 2 are calculated by the formulas for h c A.5.1 of this appendix. APPENDIX 4 MANUAL TO "INSTRUCTIONS FOR LIGHTNING PROTECTION OF BUILDINGS AND STRUCTURES" (RD34.21.122-87) This manual aims to clarify and specify the main provisions of RD 3421.122-87, as well as to familiarize specialists involved in the development and design of lightning protection of various objects with the existing ideas about the development of lightning and its parameters that determine the hazardous effects on humans and material values. Examples of lightning protection of buildings and structures of various categories are given in accordance with the requirements of RD 34.21.122-87. 1. BRIEF DATA ON LIGHTNING DISCHARGE AND THEIR PARAMETERS Lightning is an electrical discharge several kilometers long that develops between a thundercloud and the ground or any ground structure. A lightning discharge begins with the development of a leader - a weakly glowing channel with a current of several hundred amperes. In the direction of the leader's movement - from the cloud down or from the ground structure up - the lightning is divided into descending and ascending. Downward lightning data has been accumulating for a long time in several regions the globe. Information about ascending lightning appeared only in recent decades, when systematic observations of the lightning resistance of very tall structures, such as the Ostankino television tower, began. The leader of a descending lightning appears under the action of processes in a thundercloud, and its appearance does not depend on the presence of any structures on the earth's surface. As the leader moves towards the ground, counter leaders directed towards the cloud can be excited from ground objects. The contact of one of them with the descending leader (or the contact of the latter with the surface of the earth) determines the location of the lightning strike to the ground or some object. Rising leaders are excited from high grounded structures, at the tops of which electric field increases dramatically during thunderstorms. The very fact of the emergence and sustainable development of an ascending leader determines the place of defeat. On flat terrain, ascending lightning strikes objects more than 150 m high, and in mountainous areas they are excited from peaked relief elements and structures of lower height and therefore are observed more often. Let us first consider the development process and the parameters of downward lightning. After the establishment of a through leader channel, the main stage of the discharge follows - the rapid neutralization of the leader charges, accompanied by a bright glow and an increase in current to peak values ranging from a few to hundreds of kiloamperes. In this case, an intense heating of the channel (up to tens of thousands of kelvins) and its shock expansion occur, which is perceived by ear as a thunderclap. The main stage current consists of one or more successive pulses superimposed on the continuous component. Most current pulses have a negative polarity. The first pulse, with a total duration of several hundred microseconds, has a front length of 3 to 20 μs; the peak value of the current (amplitude) varies widely: in 50% of cases ( average current) exceeds 30, and in 1-2% of cases 100 kA. Approximately in 70% of downward negative lightning, the first pulse is followed by subsequent ones with lower amplitudes and front length: the average values are 12 kA and 0.6 μs, respectively. In this case, the steepness (rate of rise) of the current at the front of subsequent pulses is higher than for the first pulse. The current of the continuous component of downward lightning varies from a few to hundreds of amperes and exists throughout the entire flash, lasting an average of 0.2 s, and in rare cases 1-1.5 s. The charge carried during the entire lightning flash varies from a few to hundreds of coulombs, of which 5-15 coulombs fall on the share of individual impulses, and 10-20 coulombs on the continuous component. Downward lightning with positive current pulses are observed in about 10% of cases. Some of them have a shape similar to the shape of negative pulses. In addition, positive pulses with significantly larger parameters were recorded: a duration of about 1000 μs, a front length of about 100 μs, and a transferred charge of 35 C on average. They are characterized by variations in current amplitudes over a very wide range: with an average current of 35 kA, in 1-2% of cases, amplitudes of more than 500 kA may appear. The accumulated actual data on the parameters of downward lightning do not allow us to judge their differences in different geographical regions. Therefore, for the entire territory of the USSR, their probabilistic characteristics are assumed to be the same. Ascending lightning develops as follows. After the ascending leader has reached the thundercloud, the discharge process begins, accompanied in approximately 80% of cases by currents of negative polarity. Currents of two types are observed: the first is continuous pulseless up to several hundred amperes and a duration of tenths of a second, carrying a charge of 2-20 C; the second is characterized by the superimposition of short pulses on the long pulseless component, the amplitude of which is on average 10–12 kA and exceeds 30 kA only in 5% of cases, and the transferred charge reaches 40 C. These impulses are similar to the subsequent impulses of the main stage of the downward negative lightning. In mountainous areas, ascending lightning is characterized by longer continuous currents and larger transferred charges than in the plains. At the same time, the variations in the pulse components of the current in the mountains and on the plain differ little. To date, no relationship has been found between ascending lightning currents and the height of the structures from which they are excited. Therefore, the parameters of ascending lightning and their variations are estimated to be the same for any geographic regions and object heights. In RD 34.21.122-87, data on the parameters of lightning currents are taken into account in the requirements for the designs and dimensions of lightning protection equipment. For example, the minimum allowable distances from lightning rods and their grounding conductors to objects of category I (clauses 2.3-2.5 *) are determined from the condition of lightning rods being struck by downward lightning with the amplitude and steepness of the current front within the limits of 100 kA and 50 kA / μs, respectively. This condition corresponds to at least 99% of downstream lightning strikes. During a thunderstorm, discharges of atmospheric electricity with a voltage of up to 150,000,000 V and a current of up to 200,000 A can cause explosions, fires and destruction of ground objects. In order to ensure the safety of people, the safety of buildings and structures, equipment and materials from the electrical, thermal and mechanical effects of lightning, lightning protection is performed. Lightning protection is a complex protective devices provided for by SN 305-77. The standards establish three categories of lightning protection devices, depending on the explosive and fire hazard, capacity, fire resistance and purpose of protected objects, as well as taking into account the average thunderstorm activity per year in the geographical area of the object's location. Objects of categories I and II are protected from direct lightning strikes, from electrostatic and electromagnetic induction, from the introduction of high potentials through above-ground and underground metal communications. Objects of category III protect against direct lightning strikes and from the introduction of high potentials through above-ground metal communications, and installations with reinforced concrete or synthetic materials and floating roofs - and from electrostatic induction. The most dangerous is a direct lightning strike, when there is a direct contact of lightning with an object, accompanied by the flow of lightning current through it. Protection of buildings and structures from direct lightning strikes is carried out by lightning rods that perceive lightning and divert its current to the ground. The protective effect of a lightning rod is based on the fact that lightning strikes the highest and well-grounded metal structures. Therefore, the structure will not be struck by lightning if it is located in the protection zone of the lightning rod. Lightning rod protection zone - a part of the space adjacent to the lightning rod, which provides protection for the structure from direct lightning strikes with a sufficient degree of reliability (99%). Rapid changes in the lightning current generate electromagnetic induction - induction of potentials in open metal circuits, creating the danger of sparking in places where these circuits approach each other. This is called the secondary manifestation of lightning. It is also possible to bring high electrical potentials induced by lightning into the protected building along external metal structures and communications. Protection against electrostatic induction is achieved by attaching metal enclosures of electrical equipment to protective earth or to a special grounding conductor. To protect against the drift 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 bearing part (support), a lightning rod, a down conductor and a ground electrode. There are two types of lightning rods: rod and cable. They can be free-standing, isolated and not isolated from the protected building or structure (Fig. 86, a-c). Rice. 86. Types of lightning rods and their protective zones: a - single rod; b - rod double; 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. Rope lightning rods - one or two horizontal cables, each fixed on two supports, along which a down conductor is laid, connected to a separate ground electrode; lightning rod supports 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 mm 2. All parts of lightning rods and down conductors are connected by welding. Earthing switches are surface, deep and combined, made of steel of various sections or pipes. Surface earthing(strip, horizontal) are laid at a depth of 1 m or more from the surface of the earth in the form of one or more beams up to 30 m long. in-depth earth electrodes (rod vertical) 2-3 m long are driven into the soil to a depth of 0.7-0.8 m (from the upper end of the earth electrode to the surface of the earth). Ground electrode resistance for each stand-alone lightning rod should not exceed for lightning protection of buildings and structures of I and II categories - 10 Ohm and category III - 20 Ohm. (Instruction for lightning protection of buildings, structures and industrial communications. CO-153-34.21.122-2003) Lightning rod (lightning rod) - a structure installed on buildings and structures and serving to protect against lightning strikes. Lightning rod- part of the lightning rod, designed to intercept lightning. The fundamental Rule for grounding is the head of the PUE 1.7. Some points from the instructions of category III - RD 34.21.122-87: 3.2.1.1. General Considerations 3.2.1.2. Natural lightning rods Lightning rod - minimum sections: Table 3.2 The thickness of the roof, pipe or tank body that performs Attention. The lightning rod is mounted above the house on the top of a special mast or on a roof structural element (pipe, pediment, etc.), the end of the peak of the lightning rod works the more efficiently, the sharper it is sharpened. However, a point that is too thin can melt when struck by lightning, and its resistance to weathering small - quickly rusted. Therefore, you have to compromise and make the end thin enough, but also durable. The options for designing the working end of the lightning rod used in practice are shown in Fig. Now about the roof. If it is all placed in a cone, then there is no problem. But if, say, the roof is gable, its gables will not fit into the protective cone. It would be possible to raise the lightning rod higher, but this is too frontal, defeatist decision. Better problem bypass. For example, if you put two lightning rods (lightning rod), their cones will cover the entire roof. By the way, for a long narrow house, this is also good decision: it will reduce the height of the structure compared to the case of a single mast. You can create a separate protection of the corners of the roof with small lightning rods (lightning rod). Generally speaking, metal roof itself can serve as an lightning rod (CO-153-34.21.122-2003. - 3.2.1.2. Natural lightning rods). If you use it in this capacity (taking into account the requirements - 3.2.1.2. Natural lightning rods), then both slopes must be connected by magnetic conductors to ground electrodes.
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§ 7. Lightning protection. Types of lightning rods and their protective zones: single rod, double rod, antenna.
Lightning rod - part of the lightning rod (lightning rod).
Ground electrode for lightning rod.
When installing grounding (artificial ground electrodes) for lightning rod/s, one should ADDITIONALLY be guided by category I-II-III lightning protection - RD 34.21.122-87.
in which the permissible design features- minimum number, location and length of vertical and horizontal earthing switches.
For example, if there is protection (category III) against lightning in a private house, it will be necessary to install at least two vertical ground electrodes with a length of at least 3 m, spaced at a distance of at least 5 meters and connected by a horizontal conductor together with the electrical installation ground electrode.
If in simple words- v country house electrical wiring and lightning protection must have a common ground, consisting of at least two vertical electrodes.
...In all possible cases the grounding conductor of protection against direct lightning strikes must be combined with the grounding conductor of the electrical installation specified in Ch. 1.7 PUERequirements for lightning rods (CO-153-34.21.122-2003).
Lightning rods can be specially installed, including at the facility, or
their functions are performed structural elements protected object in the last
case they are called natural lightning rods.
Lightning rods can consist of any combination of the following elements:
rods, stretched wires (cables), mesh conductors (grids).
The following structural elements of buildings and structures can be considered as
natural lightning rods:
a) metal roofs of protected objects, provided that: electrical
continuity between different parts secured on long term;
the thickness of the roofing metal is not less than the value of t given in Table. 3.2 if
it is necessary to protect the roof from damage or burns;
the thickness of the roof metal is at least 0.5 mm, if it is not necessary to protect it from
damage and there is no danger of ignition of combustibles under the roof
materials;
the roof is not insulated. At the same time, a small layer of anti-corrosion
paint or layer 0.5 mm asphalt pavement, or a layer of 1 mm plastic coating is not
considered isolation;
non-metallic coatings on/or under metal roofing do not go beyond
protected object;
b) metal constructions roofs (trusses, interconnected steel
fittings);
c) metal elements of the type downpipes, decorations, fencing along the edge
roofs, etc., if their cross section is not less than the values prescribed for ordinary
lightning rods;
d) technological metal pipes and tanks, if they are made of metal
not less than 2.5 mm thick and penetration or burn through of this metal will not lead to
dangerous or unacceptable consequences;
e) metal pipes and tanks, if they are made of metal with a thickness not
less than the value of t given in table. 3.2, and if the temperature rise from the inside
side of the object at the point of lightning strike does not pose a danger.
functions of a natural lightning conductor
The diagrams below are illustrative and cannot be used during installation without a preliminary analysis of the actual requirements and calculations:
A legitimate question arises - where is the guarantee that lightning will strike exactly at the lightning rod (lightning rod), and not next to it, into the building? If you mentally imagine a cone with the apex at the tip of the lightning rod and with an angle at the apex of about 90°, then everything inside the cone is protected by a lightning rod (lightning rod).
Approximately, it can be considered that if the diameter of the house fits into a circle of radius R, then the lightning receiver should rise above the walls of the house to a height h(m) = R(m), which means, from the ground - to a height H = h + ho. So , for a square log house 10 x 10 m, the diameter of the house will be about 14 m, the radius of the protection zone R = 7 m.
