With the development of the economy and the growth of the urban population, the high-rise and intelligent construction of buildings has become a trend of urban development. According to the formula of the expected number of lightning strikes in the building year N=KNgAe (see GB50057-94 for details), the high-rise building is much more likely to be struck by lightning than the general building, and in the event of a lightning disaster, the loss will be very serious and the consequences will be unimaginable. It can be seen that the reliability of the lightning protection system of high-rise buildings is extremely important.
There are many reasons for the formation of lightning, and the negative-lead downlink pilot is mainly used for detonation. In general, there are three types of lightning damage: direct lightning, that is, the mechanical and thermal effects of lightning directly hitting buildings and equipment. The parts of buildings that are susceptible to direct lightning are mostly eaves, ridges, corners, and rafts. Corners, parapet walls, and lightning strikes on high-rise buildings; inductive lightning, that is, electromagnetic and electrostatic effects generated by lightning currents; lightning waves invading, lightning currents are introduced into the interior of buildings along electrical lines and pipelines, endangering equipment safety.
In lightning protection design, three levels of protection are generally used:
(1) Directly introduce most of the lightning current into the underground foundation grounding device to vent;
(2) Blocking the overvoltage introduced along the power line or data and signal lines;
(3) Limit the surge amplitude of the protected equipment.
2. Lightning protection system for high-rise buildings Lightning protection system is a lightning-receiving device consisting of lightning rods, lightning protection nets (belts) or a mixture of conductors, columns, beams, slabs or external down conductors of the main structure. And the grounding device composed of the basic natural grounding body (pile foundation, ground beam, cap or floor steel bar) or artificial grounding body is synthesized, and the whole building forms a Faraday cage to introduce lightning current into the earth. The following specific lightning protection measures for high-rise buildings are discussed.
2.1 Grounding device The lightning protection grounding of modern high-rise buildings, the protective grounding of electrical equipment and the working grounding are all combined to form an integrated grounding system. The grounding resistance is usually required to be less than 4Î© (in view of the increasing number of intelligent buildings in high-rise buildings, design time) The grounding resistance should not be greater than 1 Î©). Because the reinforced concrete foundation of the high-rise building is buried deep, and the contact area with the earth is large, the grounding resistance is much lower than that obtained by the general artificial grounding, and it is easy to meet the above requirements. Considering that the foundation of most high-rise buildings has been waterproofed, resulting in increased grounding resistance, it should be grounded around the building as much as possible. The circumferential grounding can avoid the waterproof treatment layer, and the grounding body is buried in the foundation. Outside, it also has the effect of balancing the potential, which improves safety. A high-rise complex with a hotel and office is encountered in this situation. In addition, the soil resistivity of the earth is too high, and the grounding resistance of the foundation completion test cannot meet the design requirements. After the above method is used, the effect is remarkable. In areas where the earth's soil resistivity is high, when the grounding resistance of the joint grounding body of the general practice is difficult to meet the requirements, the outwardly extending grounding body, the improved soil (changing soil, using a resistance reducing agent), the deep buried electrode, and the outside may be used. Lead the way.
2.2 Light-receiver The lightning receptor has lightning rods, lightning protection belts (net) and lightning arresters. The method should be determined according to the shape of the building and the lightning protection effect. At present, the general high-rise buildings are mostly combined with lightning rods, surface-mounted lightning protection belts and concealed lightning protection nets. The arrangement of the lightning receptors should meet the following requirements.
The building is more than 30m in length, and each layer is made of flat steel with a horizontal horizontal lightning protection belt (can be used as a pressure equalizing ring and a metal embedded part); the roof can be welded to each other by a beam or a steel bar to a size of not more than 10 *10m concealed lightning protection net. In addition, metal flagpoles, billboards, steel ladders, winds, vent pipes, water pipes, equipment, etc. on the roof of high-rise buildings must be welded to the nearest lightning protection belt and lightning protection net. The roof wall of the upper man's roof using the stainless steel railing stairs can be welded with the flat steel and the bracket and the down conductor. The railings and brackets should also be welded.
When the lightning protection pin is used on the top of the tower, in order to reduce the height of the lightning rod and enhance the lightning protection effect, a semiconductor less long needle lightning protection device (usually used for buildings of 35 m or more) and a lightning rod can be used.
2.3 Down-conductor and equalizing ring High-rise building columns The main ribs and beam-slab steel bars can be directly used as the down-conducting line and the equalizing ring, but it should be noted that the down-line, grounding device, equalizing ring and lightning receptor must be firm and reliable. Ground connection. When the height of the building exceeds 30m, a pressure equalizing ring is arranged along every four layers of the building. Large metal objects such as outer wall railings and metal doors and windows of more than 30m are connected to the pressure equalizing ring or the down conductor through the embedded parts; Each of the three vertical metal pipes shall be connected to the equalizing ring once every three layers, and the parallel or intersecting pipes shall also be connected.
The use of outdoor glass curtain walls, large composite metal sheets and stainless steel metal surfaces in high-rise buildings enhances the artistic effect of the building, but it also imposes requirements on lightning protection. Since the panel of the curtain wall is mounted on the metal dragon skeleton and fixed on the embedded part of the main structure by the support, the support is welded with the embedded part, so only the girders or columns and the embedded parts of the support are needed. A reliable connection of the down conductor can meet the requirements.
2.4 Electrical part Since the high-rise building has formed an equipotential structure on the structure, the damage caused by lightning strikes to electrical equipment is mainly caused by inductive lightning. Inductive mines are invaded by:
(1) The ground potential counterattack voltage of the lightning strikes through the grounding body;
(2) Intrusion by communication signal lines;
(3) Intrusion by AC power supply line.
In order to prevent the intrusion of lightning waves, surge protectors are installed step by step in all parts of the power supply lines in the building to eliminate lightning overvoltage; various lines and metal pipes entering the building should be buried in the whole line, and will be introduced at the household end. The metal sheath, steel skin and metal pipe of the cable shall be connected to the grounding device; the grounding devices of various metal pipes and electrical equipment entering and leaving the building shall be connected to the lightning protection grounding device at the entrance and exit.
For high-rise residential buildings with air-conditioning mainframes and their supports, a well-sealed metal junction box should be pre-buried 30 cm to 50 cm below the window opening, and galvanized flat iron should be placed inside the box. One end of the flat iron is welded to the equalizing ring or the steel reinforcing wire in the main body, and the other end is connected with the multi-strand wire of 10 mm2 or more with the copper connecting terminal. When it is used, the wire in the box is taken out to the outdoor unit of the air conditioner and Its bracket.
The lights, aviation obstacle lights and other electrical equipment fixed on the building are protected by measures such as being placed in the lightning protection, the steel pipe is placed outside the line, and the voltage protection device is installed in the distribution box.
3. Conclusion Lightning protection of high-rise buildings is a complex system engineering. It plays a vital role in the safe use of buildings and the normal operation of electrical equipment. In addition to strict compliance with specifications, it should be actively adopted in design and construction. New technologies to eliminate lightning more effectively.
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