A Brief Analysis of Key Considerations for Lightning Protection Design of Security Monitoring Systems

I. Overview

With social development and progress, the continuous improvement of people's living standards, and the increasingly active social economy, the demand for "security" is rising, leading to the wider application of security monitoring systems. Their use is becoming increasingly common in various industries, including highways, financial systems, military units, traffic monitoring, important locations, residential communities, public places, and warehouse management. At the same time, the security of the security monitoring systems themselves has become a new and important issue.

Modern security monitoring products are all microelectronic products, characterized by high density, high speed, low voltage, and low power consumption. They are highly sensitive to various electromagnetic interferences such as lightning overvoltages, power system switching overvoltages, electrostatic discharge, and electromagnetic radiation. This makes monitoring system equipment extremely vulnerable to damage from lightning strikes/overvoltages, which could lead to the malfunction of the entire monitoring system and cause incalculable economic losses and security risks.

To provide accurate and effective lightning protection solutions for security monitoring systems, we must first accurately understand the system composition. Then, we must accurately analyze the main causes of lightning damage and possible intrusion paths of lightning overvoltage. Based on this, we can select appropriate lightning protection devices, study and discuss the rational layout of signal and power lines, and clarify shielding and grounding methods. Only then can we provide an accurate and systematic lightning protection solution, effectively improving the security monitoring system's resistance to lightning overvoltage interference and optimizing the overall lightning protection level of the system.

II. Overview of Security Monitoring System Composition, Classification, and Lightning Protection

2.1 Composition of Security Monitoring System

A security monitoring system generally consists of the following three parts:

Front-end section: mainly consists of black-and-white (color) cameras, pan-tilt units, protective housings, and brackets. Transmission section: uses coaxial cables, electrical wires, and multi-core cables, employing overhead, underground, or wall-mounted installation methods to transmit audio, video, control signals, and AC/DC power supplies. Terminal section: mainly consists of control equipment, video splitters, monitors, and video storage devices.

2.2 Classification of Security Monitoring Systems

Based on the transmission method, security monitoring systems can be mainly classified into the following categories: A. Coaxial cable transmission monitoring system: Lightning protection focuses on the protection of the line interfaces at both ends of the transmission cable and the protection of the transmission cable itself; B. Twisted pair transmission monitoring system: Lightning protection focuses on the power supply protection of the front end and the terminal, as well as the protection of the twisted pair interface; C. Optical fiber transmission monitoring system: Lightning protection focuses on the power supply protection of the front end and the terminal, as well as the protection of the optical fiber cable's own shielding armor and reinforcing ribs; D. Microwave transmission monitoring system: Protection focuses on the direct lightning strike protection of the wireless equipment at both the front and rear stations.

1.2.3 Main reasons for lightning damage to security monitoring systems

2.2.3.1 Direct lightning strike

A. Lightning struck the outdoor camera directly, directly damaging the equipment;

B. Lightning strikes the cable directly, causing it to melt and break.

2.3.2 Lightning Intrusion Wave

When the power lines, signal transmission lines, or other metal cables entering the monitoring room of a security monitoring system are struck by lightning or induced by lightning, the lightning surge enters the equipment along these metal wires/conductors, causing a high potential difference that damages the equipment.

2.3.3 Lightning Induction

Electromagnetic induction: When a lightning strike occurs in the vicinity, a powerful transient electromagnetic field is generated around the lightning strike path. Monitoring equipment and transmission lines located in this electromagnetic field will induce a large electromotive force, potentially causing damage or destruction to the equipment.

Electrostatic induction: When a charged thundercloud appears, bound charges opposite to those of the thundercloud are induced on buildings and transmission lines below the thundercloud. These induced charges can reach a static potential of 100kV on low-voltage overhead lines and 40-60kV on signal lines. Once the thundercloud discharges, the bound charges spread rapidly, causing induced lightning strikes.

Lightning strikes caused by electromagnetic induction and electrostatic induction are both called induced lightning, also known as secondary lightning. While they are not as damaging to equipment as direct lightning strikes, they are much more frequent. Statistics show that induced lightning strikes account for more than 80% of modern lightning accidents.

2.3.4 Ground potential backflash

When a lightning rod guides a powerful lightning current into the ground, it generates a very high instantaneous voltage on its down conductor, grounding electrode, and connected metal conductors. This creates a huge potential difference between the lightning rod and nearby but unconnected metal objects, equipment, wiring, and people. The electric shock caused by this potential difference is called ground potential backflash. This backflash can not only damage electrical appliances and equipment but may also cause personal injury or fire and explosion accidents.

III. Key Points/Common Issues/Precautions for Lightning Protection Solutions for Security Monitoring Systems

3.1 Direct lightning strike protection

Direct lightning strike protection is an indispensable and important foundation of lightning protection and an essential component of lightning protection.

3.1.1 Direct lightning protection for front-end equipment

Security monitoring system front-end equipment comes in two types: outdoor and indoor. Equipment installed indoors is generally not subject to direct lightning strikes, while equipment installed outdoors is mostly located in relatively open areas, where the risk of direct lightning strikes is greater, so direct lightning protection must be considered.

Front-end equipment of security monitoring systems, such as cameras, should be placed within the effective protection range of lightning rods or other lightning conductors. For front-end equipment already within the protection range of other lightning rods or the existing lightning protection system of high-rise buildings, direct lightning strike protection is generally not required. However, for front-end equipment not within the protection range of any lightning protection system, direct lightning strike protection should be considered. From a technical and economic perspective, installing independent lightning rods for direct lightning strike protection of front-end equipment is not feasible. Generally, the lightning rod is mounted on the camera's support pole, and the down conductor can directly utilize the metal pole itself (or Φ8 galvanized round steel or 30×3 galvanized flat steel). However, to prevent electromagnetic induction, the power and signal cables leading up the camera to the pole should be laid in metal conduits, and the conduits should be reliably grounded.

3.1.2 Direct lightning protection for transmission lines

To protect transmission lines from direct lightning strikes, overhead installations should be avoided as much as possible. Ideally, they should be buried underground in metal conduits, with both ends of the conduits reliably grounded.

2.3.1.3 Direct Lightning Protection for Terminal Equipment The building where the terminal equipment room (generally referred to as the monitoring room) is located should take measures to protect against direct lightning strikes. A lightning protection strip can be constructed on the roof using φ10 round steel (painted with silver powder). In addition, φ10 round steel can be used as supports for the lightning protection strip, with a support height of 15cm and a support every 1m. A 40×4mm galvanized flat steel can be used as a down conductor to connect to the grounding grid (the resistance of the grounding grid should be less than 10Ω). The spacing between the down conductors should not exceed 25 meters.

Lightning rods can also be used as a measure to protect against direct lightning strikes. 40×4mm galvanized flat steel is used as the down conductor and connected to the grounding grid. The height and installation position of the lightning rod should be calculated according to the rolling sphere method.

3.2 Lightning protection grounding system

All lightning protection systems should have reliable and effective grounding. The grounding system is also an essential component of lightning protection.

All front-end and terminal equipment of a security monitoring system should have good lightning protection grounding, and the corresponding grounding system should meet the requirements of the specifications. Generally, front-end equipment that is independent of the building where the monitoring room is located must have independent grounding. However, it should be noted that if the distance between the grounding systems of both front-end and terminal equipment is less than 20 meters, the two grounding systems should be equipotentially connected.

3.3 Issues to be aware of when laying along a wall

Many wiring workers, due to limited knowledge of lightning protection or for the sake of simplicity and convenience, habitually bundle outdoor wiring with building lightning protection strips and down conductors. While this facilitates construction and enhances aesthetics, it also creates significant lightning safety hazards. This is a point that deserves serious attention. To reduce the risk of lightning damage, all conductors/metal lines should avoid being bundled parallel to direct lightning protection systems whenever possible, and should be laid out according to relevant specifications.