1. Overview

With the development and progress of society and the continuous improvement of people’s living standards, the social economy has become more and more active, and the demand for “safety” has become higher and higher. Security monitoring systems have been 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, warehouse management and so on. 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. This makes the monitoring system equipment extremely vulnerable to lightning/overvoltage damage, and the consequences may cause the entire monitoring system to operate Failure, and cause incalculable economic losses and safety risks. In order to accurately and effectively provide lightning protection solutions for the security monitoring system, we should first accurately understand the system composition of the security monitoring system, and then accurately analyze the main reasons for lightning damage to the security monitoring system and possible lightning overvoltage intrusion paths. On this basis, select appropriate lightning protection devices, research and discuss the reasonable layout of signal and power lines, and clarify the shielding and grounding methods, so that accurate and systematic lightning protection solutions can be given. Effectively improve the anti-lightning overvoltage interference capability of the security monitoring system, and optimize the overall lightning protection level of the system.

2. Security monitoring system composition, classification and lightning protection overview

2.1 Security monitoring system composition security

Monitoring system generally consists of the following three parts: Front-end part: mainly composed of black and white (color) cameras, PTZ, protective cover, brackets, etc. Transmission part: Use coaxial cables, wires, and multi-core wires to transmit audio, video, control signals, and feed AC and DC power sources by overhead, buried or laying along walls. Terminal part: mainly composed of control equipment, screen splitter, monitor, video storage equipment, etc.

2.2 Classification of security monitoring systems

According to the transmission mode of the transmission part, security monitoring systems are mainly divided into the following categories:
A. Coaxial cable transmission monitoring system: Lightning protection focuses on line interface protection at both ends of the transmission cable and protection of the transmission cable itself;
B. Twisted-pair transmission monitoring system: Lightning protection focuses on front-end and terminal power protection and twisted pair Line interface protection;
C. Optical cable transmission monitoring system: Lightning protection focuses on the power protection of the front and terminal and the protection of the optical cable’s own shielding armor and reinforcement;
D. Microwave transmission monitoring system: The protection focuses on the wireless equipment of the front and rear stations. Direct lightning protection.

2.3 The main reason for the lightning damage of the security monitoring system

2.3.1 Direct lightning 

A. The lightning directly hits the camera in the open air and directly destroys the equipment;
B. The lightning strikes the cable directly, causing the cable to fuse and damage.

2.3.2 lightning invasion wave when the security monitoring system further metal cable power supply line or signal transmission line into the control chamber lightning strike or lightning induction, lightning conductors along the metal / conductor invasive devices, resulting in high electric potential damage errand .

2.3.3 Lightning induction
Electromagnetic induction: When there is a lightning flashover in the nearby area, a strong transient electromagnetic field will be generated around the lightning strike. Monitoring equipment and transmission lines in the electromagnetic field will induce a large electromotive force, which will damage or destroy the equipment. Electrostatic induction: When a charged thundercloud appears, the buildings and transmission lines under the thundercloud will induce the opposite bound charge to the thundercloud. This kind of induced charge can reach 100kV electrostatic potential on low-voltage overhead lines, and 40-60kV electrostatic potential on signal lines. Once the thundercloud is discharged, the bound charge spreads rapidly, causing an induced lightning strike. The lightning phenomenon caused by electromagnetic induction and electrostatic induction is called induction thunder, also known as secondary thunder. Its damage to equipment is not as violent as direct lightning strikes, but it has a much greater chance of occurrence than direct lightning strikes. Statistics show that induced lightning strikes account for more than 80% of modern lightning strikes.

2.3.4 Ground potential counterattack
When the direct lightning protection device (air-termination rod) guides a strong lightning current into the earth, it generates a very high instantaneous voltage on its down conductor, grounding body and the metal conductors connected to them, and is close to them. There is a huge potential difference between metal objects, equipment, circuits, and human bodies that are close but not connected to them. The electric shock caused by this potential difference is the ground potential counterattack. This kind of counterattack is not only sufficient to damage electrical appliances and equipment, but may also cause personal injury or fire and explosion accidents.

3. Lightning protection solutions for monitoring systems

3.1 Direct lightning protection.

Direct lightning protection is an indispensable and important basis for lightning protection, and it is an integral part of lightning protection that cannot be ignored.
3.1.1 Direct lightning protection of front-end equipment. The security monitoring system has two front-end equipment, outdoor and indoor. Equipment installed indoors generally will not suffer direct lightning strikes, while equipment installed outdoors are mostly located in relatively open areas and are at risk of direct lightning strikes. Larger, you must consider the problem of direct lightning protection. The front-end equipment of the security monitoring system, such as cameras, should be placed within the effective protection range of air-termination devices (air-termination rods or other air-termination conductors). For front-end equipment that is already within the protection scope of other air-termination systems or high-rise buildings, it is generally no longer necessary to consider direct lightning protection; for front-end equipment that is not within the protection scope of any air-termination system, both The problem of direct lightning protection should be considered. From a technical and economic point of view, it is not feasible to install an independent air-termination rod for direct lightning protection of front-end equipment. Generally, the air-termination rod is erected on the support rod of the camera, and the downline can directly use the metal rod itself (also can use Φ8 galvanized round steel or 30×3mm galvanized flat steel), but in order to prevent electromagnetic induction, the power line and signal line of the camera along the pole should be laid through a metal pipe, and the metal pipe should be reliably grounded.
3.1.2 Protection against direct lightning strikes for transmission lines. In order to protect transmission lines from direct lightning strikes, the transmission lines should be laid overhead as far as possible. It is best to lay through metal pipes and bury them in the ground. Both ends of the metal pipes should be reliably grounded.
3.1.3 Direct lightning protection of terminal equipment. The building where the terminal equipment room (generally called the monitoring room) is located should take measures to prevent direct lightning strikes. φ10 round steel (brushed with silver paint) can be used to construct an air-termination zone on the roof, and a 40×4mm galvanized flat steel can be used as a guide. The offline is connected to the grounding device. Air-termination rods can also be used as a measure to prevent direct lightning strikes. The height and installation position of the air-termination rods should be calculated according to the rolling ball method.

3.2 Lightning protection grounding system.

All lightning protection systems should have reliable and effective grounding. The grounding system is also one of the necessary components of lightning protection. The front end and terminal equipment of the security monitoring system should have good lightning protection grounding, and the corresponding grounding system should meet the requirements of the specification.

3.3 Selection of AC power surge protector   

Effective lightning protection is provided for all AC power inlets of the security monitoring system. And should ensure that the building where the equipment is located has a good grounding system, and further confirm whether the lightning protection device of the building where it is located is properly used. The corresponding power surge protector should be installed at the AC power inlet of the front-end equipment. Three-level protection should be considered for the power cord entering the monitoring room. Level 1 lightning arrester can be installed at the power inlet of the main power distribution room of the building, and second-level lightning protection can be installed at the power inlet of the power distribution box on the floor where the monitoring room is located. Class-level power surge protector, install a three-level power surge protector at the power inlet of important equipment in the monitoring equipment room, and all surge protectors should be reliably grounded.

3.4 Transmission line protection of security monitoring system

Statistics show that more than 80% of lightning accidents in the security monitoring system are caused by lightning intrusive wave overvoltage induced on the lines connected to the system. Therefore, protecting the lines connected to the system is a link that cannot be ignored in the overall lightning protection. The safest wiring method should be buried through metal pipes throughout the entire process, and both ends of the metal pipe must be effectively grounded. A transmission line buried in the ground through a metal pipe can attenuate the amplitude of lightning intrusive waves to a considerable extent, thereby reducing the probability of equipment being damaged by lightning intrusive waves. In actual engineering, in many cases, when conditions do not permit, the metal pipe can be used for overhead wiring; or the metal pipe is not used throughout, but the cable must be buried in the ground through the metal pipe before entering the monitoring room and front-end equipment. The buried length should be Not less than 15 meters, effectively connect the cable metal sheath and metal tube to the lightning protection ground at the entrance end. Corresponding lightning protection devices should be installed at both ends of all transmission lines.

3.5 Protection of optical fiber communication lines

Generally speaking, optical fiber lines do not need to be equipped with lightning and surge protection devices, because optical fiber lines themselves are not conductors and will not induce/transmit over-voltage surges. But what is often overlooked is the lightning protection of optical fiber cables, which leads to some lightning overvoltage flashovers and damage to equipment. The main reason for this is that optical fiber cables generally have metal reinforcing ribs and metal armor to protect the optical fiber cable. Although the optical fiber itself does not induce and transmit overvoltage, its metal reinforcing ribs and metal armor are very easy to sense. , The transmission of lightning overvoltage must be properly handled, that is, grounding protection must be done at the fiber entrance end.