**I. Introduction**
In recent years, as urbanization has rapidly expanded across China, banks have been increasing the number of new branches while simultaneously upgrading their existing ones. Major financial institutions have significantly invested in constructing comprehensive network security surveillance systems, with lightning protection device testing being an essential component of these systems. Drawing upon issues identified during the inspection of lightning protection devices in bank network security systems, this paper primarily examines the causes of lightning strikes in such systems and proposes effective lightning protection strategies.
**II. Composition of Security Monitoring Systems and Lightning-Related Challenges**
1. **Basic Components of Security Monitoring Systems**
A security monitoring system typically consists of three primary components: front-end equipment, transmission equipment, and terminal equipment (which includes control equipment, display/recording devices, and power console equipment). Front-end components include cameras, lenses, heads, and protective covers. Transmission equipment involves cables or optical fibers, along with signal modulation/demodulation equipment, if applicable. Terminal control equipment encompasses video switchers, lens controllers, various communication interfaces, power consoles, monitor cabinets, etc., while display and recording devices consist of monitors and multi-screen splitters.
2. **Causes of Lightning Strikes in Security Monitoring Systems**
- **Direct Lightning**: Cameras exposed to open spaces may be struck directly by lightning, causing severe equipment damage. Overhead cables, including power lines and signal transmission lines, are also prone to destruction when struck.
- **Lightning Wave Intrusion**: Lightning surges or induced currents can enter the monitoring room via power lines, signal transmission lines, or metallic pipelines. These intrusions propagate along these lines, creating a potential difference that damages connected equipment.
- **Electromagnetic Pulse**:
- **Electromagnetic Induction**: When lightning strikes a lightning rod, a strong transient electromagnetic field forms around the down conductor. Equipment and transmission lines within this field may experience significant induced electromotive forces, known as electromagnetic induction.
- **Electrostatic Induction**: Below a charged thundercloud, buildings and transmission lines induce charges opposite to those of the cloud. This can result in up to 100kV on low-voltage overhead lines and 40-60kV on others, termed electrostatic induction. Together, electromagnetic and electrostatic induction are collectively referred to as lightning induction. While less severe than direct lightning strikes, these events are more frequent and cause broader damage.
- **Ground Potential Counterattacks**: Direct lightning protection devices, like lightning rods, generate immense instantaneous voltages on down conductors and grounding bodies. Significant potential differences exist between nearby metallic objects, equipment, and human bodies not bonded for equipotentiality. Such potential differences can lead to electric shocks, termed ground potential counterattacks. These counterattacks can not only damage equipment but also pose risks of personal injury or fires.
Current lightning protection practices in bank network security systems often fall short:
- Protection against lightning wave intrusion is either absent or relies solely on single-stage SPDs.
- Electromagnetic pulse protection measures are inadequate, with improper equipment room locations and poor electromagnetic shielding.
- Counterattacks of ground potential are inadequately addressed, with insufficient spacing between lines and grounding leads, and failure to make equipotential connections near lightning protection devices.
These deficiencies frequently result in equipment damage due to lightning strikes, significantly diminishing the practical effectiveness of security systems.
**III. Lightning Protection Measures for Security Monitoring Systems**
Security monitoring systems within bank outlets are embedded within buildings, so their lightning protection focuses on electromagnetic pulse mitigation and direct lightning protection for outdoor front-end equipment.
1. **Front-End Equipment Protection**
Front-end equipment can be installed indoors or outdoors. Indoor equipment avoids direct lightning strikes but requires protection against lightning wave intrusion. Outdoor equipment, however, necessitates direct lightning protection. To prevent lightning waves from infiltrating via transmission lines, surge protection devices (SPDs) must be installed on each line leading to the equipment. Signal SPDs must respond quickly and match the transmission rate and other parameters. Outdoor equipment should have proper grounding, using copper wires with a cross-sectional area of at least 25mm², marked with grounding symbols. The grounding resistance should meet the lowest value in the connected equipment’s specifications, exceeding 4 ohms.
2. **Transmission Equipment Protection**
Transmission equipment includes signal and power lines. Grounding should be implemented on poles for overhead lines to protect head and tail-end equipment. Overhead wires and metallic pipes in cable lines should be grounded. Surge protectors should connect the signal and power sources at the input of intermediate amplifiers. Shielded cables or cables housed in steel pipes should maintain electrical continuity, with both ends of the pipe grounded. If passing through metal pipes is impractical, cables should enter terminals and front-end equipment through a metal pipe, with the buried section being at least 15 meters long. The cable's metallic sheathing, steel pipes, and lightning protection grounding devices should be equipotentially bonded at the entrance.
3. **Terminal Equipment Protection**
Terminal equipment is housed in monitoring rooms. Comprehensive lightning protection involves direct lightning protection, electromagnetic pulse shielding, equipotential bonding, and surge protection. Direct lightning protection measures for the building housing the monitoring room must comply with GB50057 regulations. The monitoring room should be located in the LPZ1 and LPZ2 zones, at the center of the building’s bottom floor, and away from external lightning deflectors. Proper shielding measures should be applied in the room. All metal pipelines entering the monitoring room must connect to the lightning protection grounding device. For overhead cable lines, appropriate surge protectors should be installed at the entrance, with the cable's metallic outer sheath connected to the grounding device. The monitoring room should feature an equipotential bonding bus and shared lightning protection grounding for building lightning protection, PE line protection, equipment protection, and anti-static grounding. This prevents ground potential counterattacks from damaging equipment. All installed surge protectors should connect their grounding wires to the equipotential bonding bus via the shortest possible path. The specifications of the connecting leads and lengths should adhere to Table 1, with connection lines not exceeding 0.5 meters.
**IV. Conclusion**
The security monitoring system of bank outlets relies heavily on the building as a foundation. Effective building lightning protection serves as the prerequisite for safeguarding the monitoring system. As security systems are often installed during the second phase of renovations, lightning protection should focus on electromagnetic pulse mitigation and direct lightning protection for outdoor equipment. Prior to designing and implementing lightning protection solutions, it is crucial to consider the geographical, environmental, and specific characteristics of the monitored areas. Detailed analysis of the form and arrangement of lightning protection devices is essential to achieve optimal protection outcomes.
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