Introduction to Lightning Protection of CCTV Monitoring System
1. Brief introduction of CCTV monitoring system:
CCTV system structure:
Closed Circuit Television (CCTV) is generally composed of the following three parts:
It is mainly composed of black and white (color) camera, lens, gimbal, protective cover, bracket, etc.
Use coaxial cables, wires, and multi-core wires to transmit video, audio, or control signals by overhead, underground, or laying along the wall.
It is mainly composed of screen divider, monitor, control equipment, etc.
2. The way of lightning damage:
â‘ , direct lightning:
Lightning strikes directly on the camera in the open air causing damage to the equipment; lightning strikes directly on the overhead cable causing the cable to melt.
â‘¡, Lightning wave invasion:
When the power cable of CCTV, the signal transmission, or the metal pipeline entering the monitoring room is struck by lightning or induced by lightning, the lightning wave invades the equipment along these metal wires, causing potential differences to damage the equipment.
â‘¢ Lightning induction:
When the lightning strikes the lightning rod, a strong transient electromagnetic field will be generated around the down conductor. Monitoring equipment and transmission lines in an electromagnetic field will induce a large electromotive force. This phenomenon is called electromagnetic induction.
When a charged thundercloud appears, the charges opposite to the thundercloud will be induced on the buildings and transmission lines below the thundercloud. This induced charge can reach 100kv on low-voltage overhead lines and 40-60kv on signal lines. This phenomenon is called electrostatic induction. Studies have shown that surges caused by electrostatic induction are several times greater than those caused by electromagnetic induction.
Electromagnetic induction and electrostatic induction are called induction lightning, also called secondary lightning strike. Its damage to equipment is not as violent as a direct lightning strike, but it is much more likely to occur than a direct lightning strike. According to the statistics of the Ministry of Posts and Telecommunications, lightning strikes caused by induced lightning accounted for about 80% of the total lightning strikes.
3. Lightning protection measures of CCTV system According to the analysis of the structure of CCTV television monitoring system and the possible way of lightning intrusion, Zhongda Dentsu designed the following lightning protection solutions for CCTV television monitoring system.
1. Typical lightning protection monitoring architecture for front-end equipment The front-end equipment has both outdoor and indoor installations. The equipment installed indoors is generally not subject to direct lightning strikes, but it is necessary to consider preventing the equipment from being damaged by lightning overvoltage, while the outdoor equipment is At the same time, consideration should be given to preventing direct lightning strikes.
Front-end equipment such as cameras should be placed within the effective protection range of the lightning receptor (lightning rod or other lightning conductor). When the camera is installed independently, in principle, in order to prevent the lightning rod and the transient high potential on the down conductor, the lightning rod is preferably 3-4 meters away from the camera. If there is difficulty, the lightning rod can also be erected on the support rod of the camera, and the downstroke can directly use the metal rod itself or choose Î¦ 8 galvanized round steel.
In order to prevent electromagnetic induction, the camera power cord and signal line led along the telephone pole should be worn in the metal tube to achieve the shielding effect, and both ends of the shielded metal tube should be grounded.
In order to prevent lightning waves from invading the front-end equipment along the line, proper lightning arresters should be installed on each line in front of the equipment, such as power lines (DC24V or 220V), video lines, signal lines and PTZ control lines. Zhongda Dentsu specially developed the SV3-24DC multi-function surge protector for surveillance cameras. If the transmission distance of the DC power supply is greater than 15 meters, a low-voltage DC surge arrester should also be connected in series at the camera end.
The front end of the power input should also be installed with B and C class lightning protection devices.
The transmission distance of the signal line is long, the withstand voltage level is low, and it is easy to induce lightning current to damage the device. In order to transfer the lightning current from the signal transmission line to the ground, the signal overvoltage protector must respond quickly. When designing the protection of the signal transmission line, you must consider the actual In case, choose the correct lightning protection equipment according to the signal transmission rate, signal level, starting voltage and lightning flux and other parameters. Zhongda SV series AC / DC power supply surveillance camera surge protector is an integrated function lightning protector, which can be divided into surge protection for the camera's power supply, video / audio, and PTZ control lines. The maximum flow capacity can reach 10000A . The limited voltage can be as low as 15V, the reaction speed is picosecond level, which can fully protect the monitoring equipment using the latest technology.
2. Lightning protection of transmission lines The lines in CCTV systems are mainly transmission signal lines and power lines. The power supply of the outdoor camera can be introduced from the terminal equipment or the power supply near the monitoring point.
The control signal transmission line and the alarm signal transmission line generally use a reinforced core shielded flexible wire, which is erected (or laid) between the front end and the terminal. The reinforced core and the shielding layer should be well grounded at both ends.
GB50198-1994 stipulates that when the transmission part of the line is laid in urban suburbs and villages, it can be laid directly. When conditions are not sufficient, communication channels or overhead methods may be used.
From the perspective of lightning protection, the direct-buried laying method has the best lightning protection effect, and the overhead line is most susceptible to lightning strikes, and is destructive and has a wide range. To avoid damage to the head and tail equipment, the overhead line should be transmitted on each pole For grounding, the suspension wires of overhead cables and metal pipes in overhead cable lines should be grounded. The signal source and power supply at the input of the intermediate amplifier should be connected to suitable lightning arresters respectively.
The buried laying of transmission lines cannot prevent the occurrence of lightning strikes. A large number of facts show that lightning strikes cause buried cable faults, which account for about 30% of the total failures. Even if the lightning strikes are far away, some lightning current will still flow in. cable. Therefore, the cables with shielding layer or cables are laid underground through the steel pipe to maintain the electrical connection of the steel pipe, which is very effective for protecting electromagnetic interference and electromagnetic induction, mainly due to the shielding effect of the metal pipe and the skin effect of lightning current. If it is difficult for the cable to pass through the metal pipe in the whole process, it can be introduced through the metal pipe buried before the cable enters the terminal and the front-end equipment, but the buried length should not be less than 15 meters. Connect the metal sheath and steel pipe of the cable to the lightning protection grounding device at the entrance .
The distance between the electronic information system cable and other pipelines should comply with the following table.
When arranging the routing of the signal cables of the electronic information system, the area of â€‹â€‹the induction loop formed by the cable itself should be minimized.
The distance between the electronic information system cable and the power cable should comply with the following table.
3. Lightning protection of terminal equipment In the CCTV system, the lightning protection of the monitoring room is the most important. It should be carried out from direct lightning protection, lightning wave intrusion, equipotential connection and surge protection.
The building where the monitoring room is located shall have lightning rods, lightning belts or lightning nets for preventing direct lightning. The direct lightning protection measures shall comply with the provisions of GB50057-94 on direct lightning protection.
Various metal pipelines entering the monitoring room should be connected to the grounding device to prevent lightning induction. When the overhead cable is directly introduced, a lightning arrester should be installed at the entrance, and the metal outer sheath of the cable and the self-supporting steel cable should be connected to the grounding device.
Since 80% of the high lightning strike potential is invaded from the power line, in order to ensure the safety of the equipment, the general power supply should be equipped with a three-level lightning protection. The flow capacity is 10KA, and the limiting voltage is <1200V (equipment withstand voltage is 1.2KA).
Install Zhongda SCTV-1B lightning arrester at the front of the video transmission line entering the center console.
Install a 24VAC / 2SP arrester at the front of the signal control line entering the center console.
Install the Zhongda MPDC-24 lightning protection module at the front of the power line entering the center console.
4. The equipotential connection monitoring room of the closed-circuit monitoring equipment room should be equipped with an equipotential connection bus (or metal plate). The equipotential connection bus should be connected to the building lightning protection grounding, PE line, equipment protection ground, antistatic ground, etc. Together to prevent dangerous potential differences. The grounding wires of various surge protectors (arrester) should be electrically connected to the equipotential bonding busbar with the shortest and shortest distance.
The provisions of GB50057-94 (2000 edition) explain Article 3.1.2, paragraph 3.1.1: "In order to reduce the risk of fire, explosion, and life hazard in spaces where lightning protection is required, equipotential is a very important measure." And Section 3.3: "The most important measure to prevent the danger of life in a space where lightning protection is required is to use equipotential bonding."
Equipotential bonding is part of the internal lightning protection device, and its purpose is to reduce the potential difference caused by lightning current. Equipotential is to connect the lightning protection device, metal structure of the building, metal device, foreign wire, electrical device, telecommunication device, etc. in the space that needs lightning protection with connecting wire or overvoltage (surge) protector, Form an equipotential connection network to achieve equalized equipotentiality to prevent fires, explosions, life hazards, and equipment damage in spaces that require lightning protection.
In order to implement the installation of equipotential and surge protectors, the IEC standard divides the space to be protected into different lightning protection zones (LPZ) to specify the severity of different lightning electromagnetic pulses (LEMP) in each part of the space and indicate the boundary of each zone At the point of equipotential bonding.
A 400 Ã— 100 Ã— 10mm copper plate is laid in the equipment room of the monitoring equipment room as a local equipotential bonding bar for equipment grounding. Connect all the electronic equipment in the room and all the metal pipes, metal doors and windows, the outer shielding layer of the signal cable, the outer armor of the power cable, the computer device itself (including the exposed conductive parts), PE wire, anti-static floor, power line (through SPD) Connect) to the equipotential terminal block and ground.
5. Grounding of closed circuit monitoring system GB 50198-94 "Technical Specifications for Civil Closed Circuit Television Monitoring System Engineering":
Article 2.5.8: "The lightning protection grounding device should be connected to the grounding device of the electrical equipment and the buried metal pipeline. When not connected, the distance between the two should not be less than 20m."
Article 2.5.4: "When the system uses a dedicated grounding device, its grounding resistance shall not be greater than 4Î©, and when a comprehensive grounding network is used, its grounding resistance shall not exceed 1Î©."
GB50343-2004 "Lightning Protection Technical Specification for Building Electronic Information System":
Article 5.2.5 (This article is a mandatory clause): "Lightning protection grounding shall share a set of grounding devices with AC working grounding, DC working grounding, and safety protective grounding. The minimum value required is determined. "
Article 5.2.6 (This article is a mandatory clause): "The grounding device shall use the natural grounding body of the building. When the grounding resistance of the natural grounding body fails to meet the requirements, an artificial grounding body must be added.
According to the analysis of the above clauses: the lightning protection grounding of the monitoring system should share a set of grounding devices with the AC working ground, DC working ground, and safety protection ground of the system, and the grounding resistance should not be greater than 1Î©.
6. Lightning protection features of monitoring system Lightning damages the security monitoring system in many ways. This article mainly analyzes the main causes of lightning strike damage of CCTV monitoring system and possible intrusion routes, and introduces the lightning protection technology of the security monitoring system accordingly. It should be noted that lightning protection is a relatively complicated issue. The design of lightning protection for a safety monitoring system depends not only on the performance of the lightning protection device, but more importantly, the monitoring must be considered before the design and construction of the monitoring system. For the geographical environment where the system is located, design appropriate cable layout, shielding, and grounding methods. In short, the lightning protection design should be considered comprehensively to obtain good results.
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