Lightning is a natural phenomenon that occurs in the atmosphere, often associated with thunderstorms. It carries an immense amount of energy, and although the discharge lasts only a fraction of a second, the current can reach tens of thousands or even hundreds of thousands of amperes. This makes lightning extremely destructive along its path. Even if a structure is not directly struck by lightning, it can still suffer damage due to electromagnetic induction. The induced voltage can reach several thousand volts or even tens of thousands, which can travel rapidly through the building or along conductors, causing damage to electrical equipment and posing a risk to human safety.
In the context of CATV (Community Antenna Television) systems, lightning strikes can cause severe damage if proper lightning protection measures are not in place. For large systems with long trunk lines, effective lightning protection becomes a critical concern. This paper aims to explain the mechanism of lightning formation and its impact on CATV systems, helping readers gain a better understanding of lightning and ultimately improve the lightning protection strategies for such systems.
First, how does lightning form? Lightning is essentially an electrical discharge that occurs within cumulonimbus clouds. As these clouds move and interact with temperature and air currents, friction between particles generates static electricity, resulting in charged cloud layers—some positive, some negative. Below the cloud, objects like buildings and trees can become oppositely charged due to electrostatic induction. As the charge accumulates, the voltage within the cloud increases until the electric field between the cloud and the ground exceeds 25–30 kV/cm. At this point, a powerful discharge occurs, producing both a bright flash and a loud thunderclap. The temperature during the discharge can reach up to 2000°C, causing rapid expansion of the surrounding air and creating the sound of thunder. The frequency and intensity of lightning depend on factors such as terrain, weather conditions, and geographic location. Generally, mountainous areas and coastal regions experience more lightning than flatlands, and taller structures are more likely to be struck.
Second, what are the types of lightning and their effects? There are two main types: direct lightning and induced lightning. Direct lightning accounts for about 10% of all lightning strikes and typically affects a small area. It can be mitigated using lightning rods, protection lines, and mesh systems. However, even with these protections, electronic equipment in CATV systems may still be damaged by lightning, especially when the strike occurs near the system. Common damages include blown fuses, damaged power transformers, and destroyed rectifying components. This indicates that lightning often enters the system through power lines rather than antennas. Induced lightning, which accounts for nearly 90% of all lightning strikes, is far more dangerous. It can affect a much larger area and is the primary cause of damage to CATV equipment.
Third, how does lightning enter the CATV system? Lightning can enter through various points, including lightning rods, antennas, down cables, and overhead cables. The tip of a lightning rod has a small radius of curvature, which causes the electric field to concentrate there, forming an ionization zone. When lightning strikes, it usually hits the rod first. Although lightning rods are typically higher than antennas, they may not always provide sufficient protection. High voltage can be induced on the outer conductor of the antenna and connected coaxial cable. If the outer conductor is not properly grounded, a high voltage difference can develop between the inner and outer conductors, potentially damaging equipment and televisions. Additionally, when lightning strikes nearby, strong electromagnetic fields can induce high voltages on exposed overhead cables, leading to similar damage.
Fourth, what are the key points and measures for lightning protection in CATV systems?
1. **Antenna Lightning Protection and Grounding**: Receiving antennas and vertical rods are usually installed at the top of buildings. All antennas, including satellite dishes, should be grounded together. A lightning rod should be placed on the antenna’s vertical rod, and its height must ensure adequate protection. When installing a separate lightning rod, the protection range is cone-shaped, so the rod should extend beyond the antenna's maximum size. The horizontal distance between the rod and the antenna should be at least 3 meters. If the building already has a grounding system, the lightning rod and antenna rod should be connected to it, with a grounding resistance below 4 ohms.
2. **Front-End Equipment Protection**: Lightning strikes near the front-end equipment can induce high voltage on metal cases and outer enclosures, endangering both equipment and personnel. Proper grounding of all equipment, shielding layers, and metal pipes is essential. The grounding of AC power systems should be connected at the main ground. The grounding device of electrical equipment and buried metal pipes must be linked to the lightning protection grounding. If not, the distance between them should be at least 3 meters. Some front-end devices lack over-voltage protection, so installing a surge arrester at the main power supply is recommended for better protection.
3. **Mainline System Lightning Protection**: Underground cables in open areas should have their shields or metal sheaths grounded every 2 km if the region experiences more than 20 thunderstorm days annually and soil resistivity exceeds 100 ohms. Overhead cables should be grounded every 250 meters. At junction boxes, the shield and steel strand should share a grounding device. During optical cable installation, care must be taken to connect the wire ends in the splice box to avoid sparks that could damage the fiber. Proper grounding of steel strands and shielding layers is also crucial.
4. **Distribution System Lightning Protection**: When cables enter a building, the outer shield should be grounded near the entrance. For overhead cables, a surge arrester should be installed, and the outer conductor connected to the electrical grounding. For buried cables, the metal sheath should be grounded at the entrance. Cables should not be directly laid between buildings; instead, they should be lowered along walls into a protected area. Steel wires should be grounded, and the shielding mesh of coaxial cables and overhead support wires should be well grounded. Discharge protectors should be installed at equipment input and output terminals, and overvoltage protection devices should be used at the power supply end of 220V amplifiers. Alternatively, switching to a centralized 60V power supply can enhance system independence and reduce the risk of direct lightning intrusion.
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