Brief Introduction to Lightning Protection of CCTV Monitoring System

A. Closed-circuit television monitoring system:

CCTV system structure:

Closed Circuit Television (CCTV) is generally composed of the following three parts:

Front end:

Mainly by the black and white (color) camera, lens, head, protective cover, brackets and other components.

Transmission section:

Use coaxial cables, wires, multi-core cables to take overhead, buried or wall-mounted, etc. to transmit video, audio, or control signals.

Terminal part:

Mainly by the screen divider, monitors, control equipment and other components.

Second, lightning damage route:

Zhongda Dentsu believes that if a TV monitoring system such as CCTV is struck by lightning, it will be possible to damage the system in the following ways.

1, direct lightning:

The thunder and lightning directly hit the camera in the open air to cause equipment damage; lightning strikes directly on the overhead cable and causes the cable to blow.

2, lightning wave intrusion:

When a CCTV power line, signal transmission, or metal pipe entering a monitoring room is struck by a lightning or induced by lightning, lightning waves invade the device along these metal lines, causing a potential difference to damage the device.

3, lightning induction:

When a lightning strikes a lightning rod, a strong transient electromagnetic field is generated 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. Studies have shown that the number of surges caused by electrostatic induction is several times that of electromagnetic induction.

Electromagnetic induction and electrostatic induction are called induced lightning, also called secondary lightning strikes. Its damage to the equipment did not directly strike the thunder of the mine, but it was much greater than the probability of a direct strike. The lightning accident caused by the statistics of the original Ministry of Posts and Telecommunications accounted for about 80% of the total number of lightning accidents.

Third, CCTV system lightning protection measures

According to the structural analysis of the CCTV television monitoring system and possible intrusion of lightning, China Delta has designed the following lightning protection solutions for the CCTV television monitoring system.

1. Lightning protection of front-end equipment
Typical monitoring architecture

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, in principle, in order to prevent the transient high potential of the lightning rod and the deflector, 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 Φ8.

In order to prevent electromagnetic induction, the camera power cable and signal cable led along the telephone pole should be put in the metal pipe to achieve shielding effect. Both ends of the shielding metal pipe should be grounded.

In order to prevent lightning waves from invading the front-end equipment along the line, appropriate surge arresters such as power cables (DC24V or 220V), video lines, signal lines, and PTZ control lines should be installed on each line in front of the device. Zhongda Denki developed a multi-function surge protector SV3-24DC for surveillance cameras. If the DC power transmission distance is greater than 15 meters, the camera should also be connected in series with a low-voltage DC arrester.

Front-end power input should also be equipped with B, C-level lightning protection device

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 to the ground, the signal overvoltage protection device must respond quickly. When designing the signal transmission line, the protection must be considered. Situation, according to the signal transmission rate, signal level, starting voltage and lightning flux and other parameters to choose the right lightning protection equipment. Zhongda SV series AC/DC power supply surveillance camera surge protector is an integrated function SPD. It can be divided into the power supply, video/audio, and PTZ control circuit of the camera for surge protection. The maximum flow capacity can reach 10000A. . The limit voltage can be as low as 15V and the reaction speed is in picoseconds, which fully protects the monitoring equipment with the latest technology.

2. Lines in lightning protection CCTV systems of transmission lines are 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 are generally selected from reinforced core shielded soft wires, which are erected (or laid) between the front end and the terminal, and both the reinforcing core and the shielding layer should be well grounded.

GB50198-1994 stipulates that when the transmission part of the line is laid in the suburbs and villages of the city, it can be laid directly. When the conditions are not allowed, communication channels or overhead can be used.

From the point of view of lightning protection, the direct burial laying method has the best lightning protection effect, and the overhead line is most vulnerable to lightning strikes, and has a large destructive effect and has a wide spread range. To avoid damage to the head and tail end equipment, the overhead transmission line 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 strikes. A large number of facts show that lightning strikes cause buried cable faults, which account for about 30% of the total faults. Even if lightning strikes a long distance, there will still be some lightning current flowing in. cable. Therefore, the use of a shielded cable or cable laid through the steel pipe buried to maintain the electrical connection of the steel pipe is very effective for shielding electromagnetic interference and electromagnetic induction. This is mainly due to the shielding effect of the metal pipe and the skin effect of the lightning current. If the cable is difficult to pass through the entire metal pipe, the metal pipe can be introduced before the cable enters the terminal and the front-end equipment. However, the length of the buried 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. .

The distance between the electronic information system cable and other pipelines should meet the requirements in the following table.

When laying out the route of the signal cable of the electronic information system, the area of ​​the induction loop formed by the cable itself should be minimized.

The distance between the electronic information system cable and the power cable shall 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 and should be carried out in various aspects 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. Its direct lightning protection measures shall comply with the provisions of GB50057-94 for direct lightning protection.

Various metal pipelines entering the monitoring room shall be connected to grounding devices for lightning protection. When the overhead cable line is directly introduced, an arrester shall be installed at the entrance to the house, and the cable outer sheath and self-supporting steel cable shall be connected to the grounding device.

Because 80% of the lightning high potential 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, and the MP3-10/2 lightning protection module should be installed in the last stage. The lightning protection module is nominal. Flow capacity is 10KA, limit voltage is <1200V (equipment pressure value is 1.2KA).

Install the SCTA-1B lightning arrester on the front of the center console of the video transmission line.

Install the 24VAC/2SP surge arrester on the front of the center console of the signal control line.

Install the MPDC-24 lightning protection module in the front of the center console of the power cable.

4. Closed-circuit monitoring room equipotential bonding

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 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.

GB 50057-94 (2000 edition) states Article 3.1.2 paragraph 3.1.1: "To reduce the risk of fire, explosion and danger in the space where lightning protection is required, isoelectricity is an important measure." And Paragraph 3.3: "The most important measure to prevent the danger of life in the space where lightning protection is needed is to use equipotential bonding."

The equipotential bonding is part of the internal lightning protection device and its purpose is to reduce the potential difference caused by lightning currents. The equipotential is the use of connecting wires or overvoltage (surge) protectors to connect lightning protection devices in spaces requiring lightning protection, metal structures, metal devices, external wires, electrical devices, telecommunication devices, etc. of buildings. An equipotential bonding network is formed to achieve equal potential equipotentiality to prevent fire, explosion, life hazards and equipment damage in the lightning protection space.

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 LEMP in different parts of the space and to specify the boundaries of each zone. The location of the equipotential bonding point.

A 400×100×10 mm copper plate is placed in the equipment room of the monitoring equipment room as the local equipotential bonding row of the equipment grounding. All indoor electronic equipment and all metal pipes, metal doors and windows, signal cable outer shield, power cable outer armor, computer device itself (including exposed conductive parts), PE wire, anti-static floor, power line (through SPD) Connections, etc. are connected to the equipotential terminal block and grounded.

5, closed-circuit monitoring system grounding

GB50198-94 "Civil Closed-circuit Television Monitoring System Engineering Technical Specifications":

Article 2.5.8: "The lightning protection grounding device should be connected to the grounding device of the electrical equipment and the buried metal pipe. When not connected, the distance between the two should not be less than 20m."

Article 2.5.4: "When the system uses a special grounding device, the grounding resistance must not exceed 4 ohms. When using the integrated grounding grid, the grounding resistance must not exceed 1 ohm."

GB50343-2004 "Lightning Protection Technical Specifications 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 work grounding, DC work grounding, and safety protection grounding. The grounding resistance value of the grounding device shall be in accordance with the access equipment. The required minimum value is determined."

Article 5.2.6 (This article is mandatory): "The grounding device should use the natural grounding body of the building. When the grounding resistance of the natural grounding body does not meet the requirements, an artificial grounding body must be added.

According to the analysis above, the lightning protection grounding of the monitoring system shall share a set of grounding devices with the AC working ground, DC working grounding, and safety protection grounding of the system. The grounding resistance shall not be greater than 1Ω.

Surveillance System Protection Features

Lightning damage to the safety monitoring system is manifold. This paper mainly analyzes the main reasons of the damage caused by lightning to the CCTV surveillance system and the possible intrusion methods. At the same time, it also introduces the lightning protection technology of the security monitoring system. It should be noted that lightning protection is a relatively complex issue. The design of lightning protection protection for the security monitoring system not only depends on the performance of the lightning protection device, but more importantly, it must be considered before the design and construction of the monitoring system. The geographical environment in which the system is located is designed with appropriate cable deployment methods, shielding, and grounding methods. In short, lightning protection design should be considered comprehensively to achieve good results.

Precautions:

1. Lightning strike risk assessment should be carried out on the object to be protected before the scheme is formulated, and the lightning protection level should be determined.

2. Design of lightning protection program should carefully investigate geographical, geological, geomorphological, soil, meteorological, environmental and other conditions and the characteristics of the lightning activity and the characteristics of the protected object, etc., and study in detail the form and arrangement of lightning protection devices, and adapt them to local conditions. Take lightning protection measures to ensure safety, reliability, advanced technology and economical efficiency.

3. This plan only provides a brief introduction to the system's lightning protection. Refer to related regulations for other regulations and construction requirements.