Pay attention to common safety issues of lightning protection grounding system
The grounding system consists of several key components, including the grounding electrode, grounding conductor, grounding plate or busbar, and the grounding wire. The primary factor affecting the resistance of the grounding system is the resistivity of the surrounding soil. If the soil has a high resistivity and the grounding resistance cannot meet the standard of less than 4 ohms, artificial methods must be employed to reduce it. In terms of lightning protection for surveillance systems, this is a complex issue that involves not only outdoor equipment but also the protection of transmission cables and the monitoring center itself.
**Frequently Asked Questions About Lightning Protection in Surveillance Systems**
a) **Grounding Resistance Issues:**
In some cases, the grounding resistance may be too high or even non-existent. While the "Engineering Design Specification for Video Security Monitoring System" and the "Technical Specifications for Lightning Protection in Building Information Systems" do not clearly define the grounding resistance for surveillance systems, it is generally designed based on the standards for computer rooms and information systems. Typically, an independent grounding system should have a resistance of no more than 4 ohms, while a shared grounding system should not exceed 1 ohm. However, in certain geological conditions, such as rocky areas or regions with large gravel, the grounding resistance can be appropriately increased. For example, in forest fire monitoring units located on ridges without nearby water sources, achieving a resistance below 4 ohms could be costly. In such cases, it's practical to adjust the grounding resistance and use high-current cables for the grounding connections.
b) **Lack of Direct Lightning Protection Measures:**
A common mistake is assuming that installing a three-in-one signal surge protector and grounding it is sufficient. However, surge protectors are effective against induced lightning and voltage surges, not direct lightning strikes. Lightning damage can occur through mechanical, thermal, or electromagnetic effects. To prevent direct lightning from hitting the camera, a lightning rod is essential. In some cases, if a dome camera is mounted on a curved ball machine, the metal rod itself can act as a natural lightning rod, eliminating the need for additional installation.
c) **Surge Arrester Placement Issues:**
If the surge arrester is placed too far from the equipment, it may not effectively suppress the induced lightning currents on power and communication lines. It is recommended to install the signal surge arrester directly at the front end of the monitoring equipment, especially near the signal and power lines.
d) **Improper Grounding Cable Use:**
To save costs, some people use metal rods as down conductors for grounding systems. However, this can lead to dangerous situations if a direct lightning strike occurs. If the metal rod is used as a down conductor, there may be no voltage difference between the signal arrester and the metal rod during a lightning strike, which can cause the arrester to fail prematurely. Therefore, it is advisable to use a dedicated grounding wire instead of a metal rod.
e) **Selection of Surge Protectors:**
While BNC surge protectors are commonly used, they may not provide adequate protection for long-term exposure to various types of surges, such as those caused by nearby electrical equipment or changes in electromagnetic fields. A better solution would include a logic filter circuit to enhance the protection level and ensure a safer electrical environment for the monitoring system.
Lightning protection is a multifaceted challenge. The design of a security monitoring system’s lightning protection should consider not only the performance of the protective devices but also the geographical and environmental factors. Proper cable routing, shielding, and grounding techniques must be implemented. In short, a comprehensive approach to lightning protection is essential for optimal results.
**Twelve Key Points on Lightning Protection in Surveillance Systems**
1. **Types of Lightning Overvoltage:**
- Direct Lightning: A direct strike from a cloud to the ground or an object.
- Induction Lightning: Caused by electromagnetic induction from nearby lightning strikes.
2. **Role of the Lightning Receptor:**
A metallic component designed to intercept lightning strikes. It must be connected to the grounding system via a grounding conductor.
3. **Function of Lightning Rods:**
Lightning rods are used to attract and safely divert direct lightning strikes away from sensitive equipment.
4. **Use and Function of Surge Arresters:**
Common types include valve-type, exhaust-type (pipe-type), and zinc oxide arresters. These devices protect against overvoltage by redirecting lightning energy away from critical equipment.
5. **Common Lightning Protection Measures in Power Distribution Rooms:**
Lightning rods, conductors, and nets are typically used to protect against direct lightning strikes. Surge arresters are also installed to guard against voltage surges.
6. **Concept of Safe Current:**
The maximum current a person can withstand without serious injury. In China, the safe current limit is 30 mA for 50Hz AC.
7. **Concept of Safe Voltage:**
The voltage that does not cause fatal injuries. In China, the safe low voltage is 36 volts.
8. **Definition of Ground:**
A point where the electrical potential is considered zero. This is usually about 20 meters away from a fault location.
9. **Contact Voltage and Step Voltage:**
Contact voltage is the voltage between a grounded device and the earth. Step voltage is the potential difference between two points on the ground when someone walks near a fault.
10. **Working Grounding vs. Protective Grounding:**
- Working grounding ensures the normal operation of electrical systems.
- Protective grounding safeguards against electric shocks.
11. **Common Grounding Methods in China:**
TN system (TN-C, TN-S, TN-CS), TT system, and IT system are widely used.
12. **Equipotential Bonding:**
Total equipotential bonding connects all conductive parts to the PE wire at the main service entrance. Auxiliary bonding is required in high-risk areas like bathrooms to further reduce the risk of electric shock.
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