Expertise: lightning protection measures for security monitoring systems

The importance of lightning protection for video surveillance systems has not been taken seriously by the project's developers. They always feel that there is no problem in not preventing lightning. In some places, there are not several lightning accidents every year. They may not be done, but they are often installed in areas where thunder often occurs. Video surveillance systems must focus on lightning protection.

In order to adopt effective lightning protection measures for the security monitoring system to ensure the normal and reliable operation of the monitoring system, it is first necessary to specify the main reasons for the damage to the monitoring system from lightning strikes and the possible intrusion paths for lightning, especially the outdoor monitoring equipment with serious damage caused by lightning strikes. Based on the analysis of the causes of damage, properly select and use lightning protection devices for monitoring system equipment, as well as research and discussion of signal, power line layout, shielding and grounding methods.

First, the composition of the video surveillance system and causes lightning damage

1, the composition of the monitoring system

Front part: camera, lens, head, shield, bracket and other components.

Transmission part: use coaxial cable, network cable, optical fiber, multi-core cable to take overhead, buried or laid along the wall, etc. to transmit video, audio or control signals.

Terminal section: It is mainly composed of storage devices, monitors, and control devices.

2. Causes of Lightning Damage in Monitoring System

Direct lightning: lightning strikes the open camera directly to cause equipment damage; lightning strikes the overhead cable and causes the cable to blow.

Lightning wave intrusion: When monitored power lines, signal transmissions, or metal lines entering a monitoring room are struck by lightning or induced by lightning, lightning waves intrude into the equipment along these metal wires, causing a potential difference to damage the equipment.

Lightning induction: When a lightning strikes a lightning rod, there will be a strong transient electromagnetic field around the deflector. Monitoring equipment and transmission lines in the electromagnetic field will induce a large electromotive force. This phenomenon is called electromagnetic induction. When a charged lightning cloud emerges, the charge opposite to the thundercloud is 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. Electromagnetic induction and electrostatic induction are called induction mines, and they are called secondary mines. Its damage to the equipment is not a sudden shock, but it is much more likely to occur than direct lightning.

Second, the integrated lightning protection monitoring system

1. Lightning protection of front-end equipment

There are two types of front-end equipment, outdoor and indoor installations. Equipment installed in the room is generally not subject to direct lightning strikes, but it is necessary to consider the protection against lightning over-voltage damage to the equipment. Outdoor equipment must also be considered to prevent direct lightning strikes.

The front-end equipment such as the camera should be placed within the effective protection range of the lightning receptor (lightning rod or other lightning conductor). When the camera is set up independently, the lightning rod is preferably 3-4 meters away from the camera. If there is difficulty, the lightning rod can also be set up on the support rod of the camera. The downlead can use the metal rod itself or use the galvanized round steel with Φ 8. To prevent electromagnetic induction, the power cables and signal cables led along the pole should be shielded by metal pipes.

Outdoor camera head lightning protection requirements: Outdoor type camera gun (with pan/tilt) needs to install lightning protector at the front of the encoder to protect the outdoor front-end equipment;

In order to prevent lightning waves from invading the front-end equipment along the line, appropriate surge arresters such as power cables (220V or DC12V), video cables, signal cables, and PTZ control cables should be installed on each line in front of the device.

The power supply of the camera generally uses AC220V or DC12V. The camera is powered by a DC transformer, and the single-phase power surge arrestor should be connected in series or in parallel with the front end of the DC transformer. If the transmission distance of the DC power supply is more than 15 meters, the camera end should also be connected with a low-voltage DC lightning arrester.

The signal line has a long transmission distance and a low withstand voltage level. It easily induces lightning current and damages the equipment. In order to conduct the lightning current from the signal transmission line into the ground, the signal overvoltage protector must respond quickly. The signal must be considered when designing the signal transmission line protection. The transmission rate, signal level, starting voltage, and lightning flux and other parameters.

If the POE power supply is used outdoors, the selected switch should use a lightning protection switch as much as possible.

Outdoor front-end equipment should have good grounding, grounding resistance is less than 4Ω, and high soil resistivity areas can be relaxed to <10Ω.

2. Lightning protection of transmission lines

The monitoring system is mainly transmission signal lines and power lines. The power of the outdoor camera can be introduced from the terminal equipment or from the power source near the monitoring point.

The control signal transmission line and the alarm signal transmission line generally use a core shielded flexible cord, and is erected (or laid) between the front end and the terminal.

When the transmission line is laid in the suburbs and villages of the city, it can be laid directly. When the conditions are not allowed, a communication pipe or overhead mode may be adopted. At this time, the minimum distance between the transmission cable and other lines of other lines and the minimum vertical distance between the common line and other lines are specified.

Minimum distance between cables and other lines Common trenches (tunnels) Type Min. spacing Pitch 220V AC wires 0.5 Communication cables 0.1, Minimum vertical spacing between cable and other lines Common rod erection (m) Type Minimum vertical spacing 1-10KV Power line 2.51KV The following power line 1.5 broadcast line 1.0 communication line 0.6

From the point of view of lightning protection, direct burial laying has the best lightning protection effect, and overhead lines are most vulnerable to lightning strikes. They are also destructive and have a wide range of spread. To avoid damage to the head and tail equipment, overhead transmission lines should be on each pole. Ground handling is required. The overhead cable overhead line and overhead line cable should be grounded. The signal source and the power supply at the input of the intermediate amplifier should be respectively connected to the appropriate arresters.

The laying of transmission lines does not prevent the occurrence of lightning strike equipment. A large number of facts show that lightning strikes cause buried cable faults, which account for about 30% of the total faults. Even if the lightning strikes a relatively long distance, there will still be some lightning current flowing in. cable. Therefore, the use of shielded cables or cables laid through the buried pipe, to maintain the electrical connection of the steel pipe. It is very effective for shielding electromagnetic interference and electromagnetic induction, which is mainly due to the shielding effect of the metal tube and the skin effect of the lightning current. If the cable is difficult to pass through the entire metal pipe, it can be introduced through the metal pipe before the cable enters the terminal and the front-end equipment, but the buried length shall not be less than 15 meters. The metal sheathing of the cable and the steel pipe shall be connected to the lightning protection grounding device at the entrance end. Connected.

3, lightning protection of terminal equipment

In the monitoring system, the lightning protection of the monitoring room is the most important and should be carried out in various ways from direct lightning protection, lightning wave intrusion, equipotential bonding and surge protection.

The building where the monitoring room is located shall have lightning rods, lightning protection strips or lightning protection nets that prevent direct lightning strikes. The various metal pipelines that are required to enter the monitoring room for direct lightning protection shall be connected to grounding devices that prevent lightning induction. When the overhead cable line is directly introduced, an arrester should be installed at the entrance to the house, and the metal outer sheath of the cable and the self-supporting steel cable should be connected to the grounding device.

An equipotential bonding bus (or metal plate) should be set up in the monitoring room. The equipotential bonding bus should be connected to lightning protection grounding, PE lines, equipment protection grounds, and anti-static grounds of buildings to prevent dangerous potential differences. The grounding wires of various surge protectors (arresters) should be electrically connected to the equipotential bonding busbars at the straightest and shortest distances.

Because 80% of the lightning high voltage is invaded from the power line, in order to ensure the safety of the equipment, three levels of lightning protection should be set on the general power supply. In the video transmission line, signal control line, intruder alarm signal line before entering the front-end equipment or before entering the center console should be equipped with a corresponding lightning protection device.

Good grounding is a crucial part of lightning protection. The smaller the value of the grounding resistance, the lower the overvoltage value. When the monitoring center uses a special grounding device, its grounding resistance must not exceed 4Ω. When using an integrated grounding grid, the grounding resistance must not exceed 1 Ω.

Third, the monitoring room shielding measures

1. Metal lines, PE lines, and information line metal skins of buried lines should be well grounded at the home end. If it is overhead or unshielded before entering the home, it is advisable to shield the front 20 meters of the metal wire in the entrance and to connect the shielding layer to the lightning protection area.

2. In the monitoring room, reliable connection shall be made between the metal housing of the metal computer table, computer equipment and control equipment and the lightning protection grounding device.

3. The exposed signal cable between the outdoor camera gun and the decoder shall be covered with stainless steel or copper metal pipe, and the metal shield shell of the camera gun and the metal shield shell of the decoder shall be reliably connected with the pole of the down conductor.

4. Shielding is a basic measure for reducing electromagnetic interference. The following measures should be taken: External shielding measures, laying of circuits on suitable paths, and shielding of lines, these measures should be used jointly.

5. To improve the electromagnetic environment, all large-sized metal parts combined with the building should be equipotentially connected and connected to the grounding device. Metal surfaces for roofs, metal surfaces for facades, steel bars in concrete, and metal window frames must all be equipotentially grounded.

6. In the space where protection is required, shielded cables shall be shielded at least at both ends and should be connected equipotentially at the junction of the lightning protection zone. When the microelectronic equipment system requires only equipotential bonding at one end, the shielded cable can be introduced through the metal pipe, and the metal pipe is connected equipotentially at one end.

7. The connection cables between the buildings shall be laid in metal pipes. The metal pipes shall be electrically connected from one end to the other and shall be connected to the equipotential connection belt of each building. Cable shields should also be connected to these strips.

8. The large space shielding of buildings or rooms in practice consists of natural components such as metal supports, metal frames or reinforced concrete reinforcement. These components form a grid-shaped large space shield. The conductive metal that penetrates this type of shield should be connected to the ground with its equipotential bonding.

IV. Monitoring room equipotential bonding and common grounding

1. The shields and machines of various types of pipelines that will enter the monitoring center building will be grounded after being equipotentially connected before entering the building. Ground the device after the second equipotential bonding before entering the device. The outer layer of the coaxial cable output from the outdoor camera and the outer layer of the other pipeline are grounded before being connected to the building by equipotential bonding.

2. The separate external conductive device is grounded with an equipotential bonding conductor to reduce the potential difference between the metal components of the building where the system equipment is located and the equipment or between the device and the device due to lightning strikes. Using a multiple connection of all metal components in a building with a reinforced concrete structure to establish a three-dimensional connection network is the best choice for equipotential bonding. In order to facilitate the construction of equipotential bonding, equipotential bonding preparations should be embedded in some places.

3. The metal outer layer of all types of water pipes, heating pipes, and air-conditioning pipes entering the system shall be equipotentially connected when entering the building. After the gas pipe enters the home, an insulation shall be inserted at the joint of the flange plate. After the terminal is connected with the switch type SPD, the indoor metal pipe can participate in the equipotential bonding, and the large-sized metal parts combined with the building are connected together, and after the equipotential bonding according to the requirements of GB50054, the total equipotential bonding tape is connected. , And reliably connected to ground.

4, at the entrance of the building, that is LPZ0B and LPZ1 area at the junction of the total equipotential connection after grounding, in the subsequent lightning protection zone at the junction of the total equipotential connection method for local equipotential connection, the main body of the connection should contain the system equipment itself (including exposed conductive parts), PE wires, cabinets, racks, enclosures of electrical and electronic equipment, DC work grounds, anti-static grounding, metal shielded cable outer layers, pipes, shielded slots, surge protection SPD grounding, etc. Both should be connected directly to this equipotential bonding tape at the shortest distance. The basic method of connection should be either mesh (M) or star (S). The ring-shaped equipotential bonding strips of the net structure shall be connected to the grounding system through the internal steel bars and metal facades of the building walls every 5 meters. When the S-type equipotential bonding network is used, all metal components of the system except for the connection at the ground reference point (ie, ERP) should be sufficiently insulated from the components of the common grounding system (greater than 10KV, 1.2/50μs).

5, arrester connection wire should be short and straight, SPD connection wire should not be greater than 0.5 meters, when the length is greater than 0.5 meters should be properly thickened wire diameter.

6. The use of optical cables containing metal parts, such as reinforced metal cores for tensile strength, metal tide layers, outer layers of rodents, or metal communication facilities for repairs and maintenance should be reliably grounded, and all cables along the cable should be connected. Junctions, regenerators, etc., at the tidal layer (metal layer), and directly earthed at the end of each end of the cable length.

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