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**2-1 Wide Area Information System Electromagnetic Compatibility (EMC) Concept**
The wide-area information system (WAIS) refers to a network that spans a large geographic area and involves direct electrical connections between its components. These systems are not capable of achieving equipotential bonding, which makes them vulnerable to electromagnetic interference. Security systems fall under the category of "wide area information systems."
The electromagnetic environment in such systems is complex, encompassing influences from the air down to underground structures. Lightning in the atmosphere, electromagnetic waves in free space, and potential differences from charged cables on the ground all contribute to the electromagnetic challenges faced by these systems. This external environment significantly impacts the operational quality and safety of weak systems.
Wide-area electromagnetic compatibility (EMC) for WAIS assumes that all equipment within the system has already met EMC standards and is functioning properly. The focus here is on how the system performs under the influence of the external electromagnetic environment, rather than on individual device-level EMC issues. Some people mistakenly apply concepts from equipment or PCB-level EMC to explain the behavior of security systems in a wide-area context.
Designing a secure operation for a security system falls under the broader scope of wide-area EMC design. This includes anti-jamming, anti-static, direct lightning protection, and induced lightning protection. The goal is to ensure the system's own safety and maintain its operational quality.
The design of lightning protection for camera poles and grounding devices analyzed earlier can create artificial safety hazards due to a lack of understanding of the earth's electromagnetic environment and the unique characteristics of wide-area security systems. This is the root cause of many hidden dangers in system design. Emphasizing and studying system EMC design is crucial for the safety of weak systems and should be a priority in security projects.
**The basic principle of 2-2 security system security design is "single point earthing"**
1. **Basic concept of "single point earthing"**: Single point earthing means that the main unit of the system is grounded at one point, while remote cameras and other equipment are insulated from the earth. This ensures that the system maintains a centralized grounding point, typically the host or subsystem host. For example, in an optical cable transmission system, the front-end optical transmitter is considered the host, and its casing is grounded. Cameras connected via optical cables must be isolated from the earth. This setup prevents unwanted electrical connections and ensures safe operation.
2. **Project requirements for "single point earthing"**: The system host must be grounded at a single point, and all remote equipment must be suspended from the earth. This allows static charges to be discharged through the grounding point, maintaining a stable potential with respect to the earth. The term "overvoltage" and "high potential" used in some professional lightning protection discussions often misrepresents the nature of lightning-induced voltages. In reality, even with zero grounding resistance, the clamping voltage at both ends of a lightning arrester remains equal in magnitude but opposite in polarity, making true equipotential bonding impossible. Grounding discharge circuits involve AC and DC impedance, as well as grounding resistance, and the idea of "effective lightning current discharge" is misleading.
3. **Analysis of the rationality of "single point earthing"**: Single point earthing effectively eliminates ground loops and blocks the path of lightning and grid surges from entering the system. It is the most effective method for lightning protection, anti-surge, and anti-interference. Multi-point grounding introduces interference and risks of lightning counterattack, leading to damage to equipment. The principle of single point earthing is not only compatible with anti-induction measures but is also essential for proper lightning protection design. It ensures that the system floats at the same potential as the grounding point, avoiding the need for complex grounding networks.
4. **Benefits of "single point earthing"**: Adhering to this principle helps avoid being misled by complex grounding solutions and reduces unnecessary investment. It serves as a key criterion for evaluating the safety and design of security systems.
**2-3 Questions about "Lightning protection of buildings and their power supply system" in security engineering**
When designing a security project, it is important to consider whether the building and its power supply system have already been protected against lightning strikes and surges. If they have passed safety inspections and are regularly maintained, additional lightning protection may not be necessary. However, some "professional lightning protection" companies emphasize the need for multi-stage protection, which can lead to overdesign and unnecessary costs.
Building and power supply lightning protection should be part of the initial infrastructure, not the responsibility of the security engineering company. Most security firms lack the necessary qualifications to handle such tasks. Therefore, it is important to distinguish between what is the responsibility of the building owner and what belongs to the security system design.
**2-4 Security system direct lightning protection design**
All security equipment should operate within the effective protection range of existing lightning rods and grounding systems. Outdoor cameras should be installed in locations where direct lightning strikes are unlikely, and if necessary, independent lightning rods should be used. The distance between the camera pole and the lightning rod should be at least 4.5 meters to prevent counterattacks. Camera poles should not be equipped with lightning rods and should be made of insulating materials like wood or cement to reduce the risk of step voltage.
Some suggest using camera poles as lightning rods and insulating the camera from the pole, but this is not advisable. Lightning strike voltages can exceed 1 meter in rainy conditions, making conventional insulation ineffective. Responsible manufacturers should address these issues and improve their designs.
**2-5 About Lightning Sensing**
The terms used in "professional lightning protection" such as "lightning electromagnetic pulse," "lightning impulse wave," and "surge voltage" are often misleading. They do not accurately describe the actual energy received by cables during lightning events. The so-called "leakage voltage" and "discharge capacity" of surge arresters are often based on incorrect assumptions and false data.
Lightning induction is a small fraction of the total energy released during a lightning strike. Factors such as frequency spectrum, antenna efficiency, directionality, and polarization greatly limit the amount of energy that can be coupled into cables. As a result, the energy received by cables is much less than commonly claimed. Using grounded surge protectors for weak systems is not only unnecessary but can also introduce new risks.
**2-6 Inductive Lightning Protection Design**
Inductive lightning protection focuses on limiting the voltage induced in cables rather than discharging lightning currents to the ground. A voltage-limiting protection circuit at the input and output ports of equipment can effectively protect against induced lightning voltages. This approach avoids the need for complex grounding systems and is more suitable for wide-area information systems.
For video transmission lines, a protection circuit can be designed to limit the voltage drop across the load to below the maximum safe level. Different transmission methods require tailored protection circuits, and embedded protection in equipment is the most effective solution. Grounded surge protectors should not be used, as they can introduce ground loops and increase the risk of damage.
**2-7 About Room Lightning Protection**
The security room is usually located within the building and is already protected by the building’s lightning protection system. The power supply for the security system should be electrically isolated from the grid’s zero and ground wires. Ensuring "single point grounding" is critical for preventing ground potential intrusion and minimizing the risk of damage.
**2-8 Lightning Protection Concept of Security Engineering**
The ultimate goal of lightning protection design for security systems is to ensure the safety of equipment and operations. This requires a clear understanding of both weak current technology and lightning protection principles. Anti-direct lightning protection should be handled by independent lightning rods, while induced lightning protection relies on effective line protection.
Misunderstandings about lightning strike probabilities can lead to unnecessary investments and poor design choices. For example, a 3–5-meter-high outdoor camera pole has a very low probability of being struck by lightning, making extensive lightning protection unnecessary. Overdesigning systems with expensive grounding networks and lightning rods can lead to costly failures and safety risks.
**Conclusion**
Currently, the field of security and lightning protection faces significant challenges. Many professionals lack a comprehensive understanding of both weak current systems and lightning protection technologies. This leads to poorly designed systems and unsafe practices. Industry standards should reflect a holistic approach to system-wide safety rather than focusing on isolated components.
Lightning protection is a practical science, and real-world experience should guide design decisions. It is time for the industry to move away from outdated and unproven methods and adopt more effective, scientifically sound approaches. Only then can we ensure the long-term safety and reliability of security systems.