Talking about the Lightning Protection Scheme of CCTV Video Surveillance System

With the development and progress of society, the continuous improvement of people's living standards, the more active social economy, the higher and higher demand for "security", the security monitoring system has been more widely used. It is more and more common in various industries such as highways, financial systems, military units, traffic monitoring, important premises, various communities, public places, and warehouse management. At the same time, the security of the security monitoring system itself has also become a new and important issue. Modern security monitoring products are all microelectronic products. These monitoring devices have the characteristics of high density, high speed, low voltage and low power consumption. It is very sensitive to various electromagnetic interferences such as lightning overvoltage, power system operating overvoltage, electrostatic discharge, electromagnetic radiation, etc., which makes the monitoring system equipment extremely vulnerable to lightning strike/overvoltage damage, and the consequences may cause the entire monitoring system to run. malfunction and cause inestimable economic losses and safety risks. In order to provide lightning protection solutions for security monitoring systems accurately and effectively, we should first accurately understand the system composition of the security monitoring system, and then accurately analyze the main reasons for the lightning strike damage to the security monitoring system and the possible intrusion paths of lightning strike overvoltage. . On this basis, selecting appropriate lightning protection devices, researching and discussing the reasonable layout of signal and power lines, and clarifying shielding and grounding methods, can provide accurate and systematic lightning protection solutions. Effectively improve the anti-lightning overvoltage interference ability of the security monitoring system, and optimize the overall lightning protection level of the system.

The monitoring system is generally composed of front-end part, transmission part and terminal part. Video, audio or control signals are transmitted between them by using coaxial cables, wires, and multi-core wires overhead, buried or along the wall.
An introduction to lightning
Direct lightning strike: Lightning directly strikes the camera in the open air, causing equipment damage; lightning strikes the overhead cable directly, causing the cable to fuse.
Lightning wave intrusion: When the power line, signal transmission or metal pipeline entering the monitoring room of CCTV is struck by lightning or is induced by lightning, the lightning wave invades the equipment along these metal wires, causing potential difference to damage the equipment.
Lightning induction: When lightning strikes the lightning rod, a strong transient electromagnetic field will be generated around the down conductor. Monitoring equipment and transmission lines in the electromagnetic field will induce a large electromotive force. This phenomenon is called electromagnetic induction. When a charged thundercloud appears, a 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, also known as secondary mines. Its damage to equipment is not as sudden as a direct lightning strike, but it is much more likely to occur than a direct lightning strike.
2. Comprehensive lightning protection of CCTV system
1. Lightning protection of front-end equipment
Front-end equipment can be installed outdoors or indoors. Equipment installed indoors is generally not subject to direct lightning strikes, but it is necessary to consider preventing the damage of lightning overvoltage to the equipment, while outdoor equipment also needs to be considered to prevent direct lightning strikes.
Front-end equipment such as cameras should be placed within the effective protection range of air-termination devices (lightning rods or other air-termination conductors). When the camera is erected independently, the lightning rod should preferably be 3-4 meters away from the camera. In order to prevent electromagnetic induction, the power and signal lines leading to the camera along the pole should be shielded by metal pipes. In order to prevent lightning waves from invading the front-end equipment along the line, appropriate lightning arresters should be installed on each line in front of the equipment, such as power lines (220V or DC12V), video lines, signal lines and PTZ control lines. Now there are also three-in-one (power + video + control) and two-in-one lightning arresters for cameras (protecting power lines and video lines)

The signal line has a long transmission distance and a low withstand voltage level. It is easy to induce lightning current and damage the equipment. In order to transmit the lightning current from the signal transmission line to the ground, the signal overvoltage protector must respond quickly, and the signal must be considered when designing the protection of the signal transmission line. parameters such as transmission rate, signal level, starting voltage and lightning flux. The outdoor front-end equipment should be well grounded, and the grounding resistance should be less than 4Ω, which can be relaxed to <10Ω in areas with high soil resistivity.
2. Lightning protection of transmission lines
The CCTV system mainly transmits signal lines and power lines. The power supply of the outdoor camera can be introduced from the terminal equipment or from the power supply near the monitoring point.
The control signal transmission line and the alarm signal transmission line generally use the core shielded flexible wire, which is erected (or laid) between the front end and the terminal. Gb50198-1994 stipulates that when the transmission part of the line is laid in the suburban and rural areas, the direct buried laying method can be adopted. When conditions are not sufficient, communication pipelines or overhead methods can be used.
From the perspective of lightning protection, the direct burial laying method has the best lightning protection effect. The overhead line is most likely to be struck by lightning, and it is destructive and has a wide range of influence. For grounding, the suspension wires of the overhead cables and the metal pipes in the overhead cable lines should be grounded. The signal source and power supply at the input end of the intermediate amplifier should be connected to appropriate arresters respectively.
Buried laying of transmission lines can not prevent the occurrence of lightning strike equipment. A large number of facts show that lightning strikes cause buried cable failures, accounting for about 30% of the total failures. Even if lightning strikes are far away, there will still be some lightning currents flowing into them. cable. Therefore, cables with shielding layers or cables are buried through the steel pipe to maintain the electrical connection of the steel pipe. It is very effective to protect against electromagnetic interference and electromagnetic induction, mainly due to the shielding effect of metal pipes and the skin effect of lightning current. If it is difficult for the cable to pass through the metal tube, the cable can enter the terminal and front end.
The equipment shall be introduced into the ground through a metal pipe, but the buried length shall not be less than 15 meters, and the metal sheath of the cable and the steel pipe shall be connected to the lightning protection grounding device at the entrance end.
3. Lightning protection of terminal equipment
In the CCTV system, the lightning protection of the monitoring room is the most important, which should be carried out from the aspects of direct lightning protection, lightning wave intrusion, equipotential connection and surge protection.
The building where the monitoring room is located should have lightning rods, lightning strips or lightning protection nets to prevent direct lightning strikes. The measures for preventing direct lightning strike shall comply with the provisions on direct lightning protection in GB50057-94.
Various metal pipelines entering the monitoring room should be connected to the grounding device against induction lightning. 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.
An equipotential bonding busbar (or metal plate) should be set up in the monitoring room, and the equipotential bonding busbar should be connected with the building lightning protection ground, PE line, equipment protection ground, anti-static ground, etc. to prevent dangerous potential difference. The ground wire of various surge protectors (surge arresters) should be electrically connected to the equipotential bonding busbar at the straightest and shortest distance.
Since 80% of the high potential of lightning strikes is intruded from the power line, in order to ensure the safety of the equipment, three-level lightning protection should be set on the general power supply. Corresponding lightning protectors should be installed before the video transmission line, signal control line and intrusion alarm signal line enter the front-end equipment or before entering the central console.
Good grounding is a vital part of lightning protection. The smaller the grounding resistance value, the lower the overvoltage value. When the monitoring center adopts a special grounding device, its grounding resistance shall not be greater than 4Ω. When a comprehensive grounding grid is used, its grounding resistance shall not be greater than 1Ω.
Summary: The lightning protection of the monitoring system is mainly divided into the lightning protection of the front-end equipment and the lightning protection of the back-end equipment. The way to induce lightning intrusion is mainly the wires connected between the two devices, so there is no need to do lightning protection at the two ends. Oversight, all video cables, control cables and power cables must be protected.