The building suffered a direct lightning strike or nearby suffered a direct lightning strike in the case of lightning electromagnetic pulse, in the center of the lightning strike within the range of 1.5km ~ 2km, may produce dangerous over-voltage, damage to the equipment along the line, under the action of it, the line and the equipment due to the over-voltage and over-current will produce a surge, in order to the lightning electromagnetic pulse on the safety of the security system to a minimum degree of infringement, the need for the security system to suffer from lightning strike. Need to explore the particularity and universality of security systems subjected to lightning strikes.
The past system was struck by lightning to analyze the system was struck by lightning, most of the system is not in accordance with the relevant lightning protection standards for design and construction and caused by the students should master the knowledge of lightning protection and the relevant norms, in the design and construction of strict control, in strict accordance with the relevant lightning protection standards for the design, construction and maintenance of the system.
1) direct lightning - flash strike directly on the building, other objects, the earth or lightning protection devices, producing electrical effects, thermal effects and mechanical forces.
In the security system lightning directly into the outdoor camera, alarm detector caused by equipment damage; lightning directly struck in the overhead cable caused by cable fusion, etc., these phenomena may be due to the damage caused by direct lightning.
2) lightning induction - lightning discharge, electrostatic induction and electromagnetic induction on nearby conductors, it may make the sparks between metal parts.
In the security system when the lightning rod is struck by lightning, in the lead wire around a strong transient electromagnetic field. In the electromagnetic field in the monitoring equipment and transmission lines will induce a large electric potential, this phenomenon is called electromagnetic induction. When a charged thundercloud appears, in the thundercloud below the building and transmission lines will be induced with the thundercloud opposite charge, this induction charge in the low-voltage overhead lines up to 100KV, signal lines up to 40 ~ 60KV, this phenomenon is called electrostatic induction. Electromagnetic induction and electrostatic induction is called induction lightning, also called secondary lightning. It is not as serious as direct lightning damage to the equipment, but the chances of it occurring is much greater than direct lightning; security system power lines, signal lines, control line transmission or access to the monitoring center of the metal pipeline was struck by lightning or by lightning induction, lightning waves along these metal wires into the equipment, the formation of a huge potential difference so that the equipment is damaged;
3) Lightning Surge -- Electromagnetic radiation associated with lightning discharges, the resulting electric and magnetic fields are capable of coupling into electrical (electronic) systems to produce damaging inrush currents or voltages.
Surge protection of security system equipment: for security systems, must be installed in each in and out of the cable, over-voltage protector, once the cable on the induction of over-voltage. Due to the role of the overvoltage protector, the system's ports voltage roughly equal level (i.e., equipotential), thus protecting the system from damage.
4) lightning activity area classification: according to the annual average number of days of thunderstorms, lightning activity area is appropriate to be divided into: fewer mines, more mines, high thunderstorms and strong thunderstorms.
Little minefield: the average annual thunderstorms in the area of less than 20 days;
Multi-minefield: the average annual thunderstorms greater than 20 days, not more than 40 days of the area;
High minefield: the average annual thunderstorms greater than 40 days, not more than 60 days of the area;
Strong minefield: average annual thunderstorms more than 60 days of the area.
5) the division of lightning protection zone (see the figure below):
LPZ0A zone: each object in the zone may be directly struck by lightning and conduct away all the lightning current; the electromagnetic field strength in the zone is not attenuated.
LPZ0B zone: objects in this zone are unlikely to be directly struck by lightning currents larger than the radius of the selected ball; however, there is no attenuation of the electromagnetic field strength in this zone.
LPZ1 zone: objects in this zone are unlikely to be directly struck by lightning, and the current flowing through each conductor is smaller than in LPZ0n; the magnetic field strength in this zone may be attenuated, depending on shielding measures.
LPZn +1 subsequent lightning protection zone: when there is a need to further reduce the inflow of current and electromagnetic field strength, should be added to the subsequent lightning protection zone, and in accordance with the need to protect the object required by the environmental zone to select the required conditions of the subsequent lightning protection zone.
Note: n=1, 2, .......
6) lightning protection classification of buildings: GB 50057 "Building Lightning Protection Code" according to the importance of the building, the nature of the use of the building, the possibility of the consequences of the occurrence of lightning accidents will be divided into three categories of buildings, see GB 50057 Chapter II.
GB 50057 Chapter 5 of the selection and arrangement of the flashback using the rolling ball method to determine the scope of protection of the flashback, the rolling ball method is to hr for the radius of a sphere, rolling along the need to protect against direct lightning parts, when the sphere only touches the flashback (including being used as a flashback of the metal), or only touch the flashback and the ground (including contact with the earth and can withstand the lightning strikes on the metal) , and does not touch the part to be protected, then that part is protected by the flashover.
Specification settings: the first category of buildings rolling radius hr 30m; the second category of buildings rolling radius hr 45m; the third category of buildings rolling radius hr 60m.
7) building electronic information systems, lightning protection level of choice
Lightning protection level according to the GB 50343 specification for the classification of lightning protection level is divided into A, B, C, D four levels:
A level: large computing centers, large communications hubs, national financial centers, banks, airports, large ports, train hubs; national cultural relics, archives of video security monitoring systems and alarm systems. Level B: medium-sized computing centers, medium-sized communication hubs, mobile communication base stations, large sports stadiums (stadiums) monitoring systems, securities centers; provincial cultural relics, archives, video security monitoring systems and alarm systems, radar stations, microwave stations, highway monitoring and toll collection systems; medium-sized electronic medical equipment; four-star hotels. Level C: small communication hubs, medium and large cable television systems, three-star hotels. Level D: In addition to the above A, B, C level other than general-purpose electronic information systems equipment.8) external lightning protection device: by the flashover, lead wire and grounding devices, mainly used to prevent direct lightning protection device.
The main purpose of installing the flashover is to protect the building from lightning caused by fire accidents and personal safety accidents, but also to avoid damage to electrical equipment in the building.
Must be installed in the direct lightning non-protection zone (LPZ0A) of the front-end equipment, should be taken to prevent direct lightning protection measures. Provide lightning rods on camera poles.
9) internal lightning protection device: by the equipotential connection system, *** with the grounding system, shielding system, reasonable wiring system, surge protector, etc., mainly used to reduce and prevent the electromagnetic effect of lightning current in the space to be protected.
Internal lightning protection system is to prevent lightning and its form of overvoltage intrusion into the equipment caused by the destruction, is the external lightning protection system can not guarantee, in order to realize the internal lightning protection, the need to enter and exit the various protected areas of the cable, metal piping, etc. Connected to the lightning arrester and over-voltage protector, and the implementation of equipotential grounding.
10) equipotential connection: equipment and devices exposed conductive part of the potential is basically equal to the electrical connection.
In order to eliminate lightning-induced destructive potential difference, the need to implement equipotential grounding, power lines, signal lines, metal piping, etc. should be equipotential connection, the interface of each inner layer of the protected area, the same should be according to local equipotential connection, the local equipotential connectors are connected to each other, and finally with the main equipotential connectors connected.
11) equipotential connection tape: the metal device, foreign conductive material, power lines, communication lines and other cables connected to it in order to be able to do with the lightning protection device equipotential connection of the metal tape.
12) equipotential connection network: a system of exposed conductive parts of the conductor to make equipotential connection of the network.
13)*** with grounding system: parts of the lightning protection device, building metal components, low-voltage power distribution protection line (PE), equipotential connection band, equipment protective ground, shielding body grounding, anti-static grounding and grounding devices, etc., connected to the grounding system.
14) natural grounding body: with the function of both grounding but not for this purpose and specifically set up with the earth have good contact with a variety of metal components, metal well pipe, reinforced concrete in the steel, buried metal piping and facilities of the collective term.
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