Monitoring lightning protection equipment voltage protection Raiders

The arrival of June means that the summer has already begun. There is no doubt that there are more and more thunderstorms. As security users, we have to start worrying that some monitoring equipment in the outdoors will receive a lightning “door-to-door visit”.

This is not, afraid of what to do, on June 8th from 1:30 pm to 2:30 pm, the original clear skies of the coast of Quanzhou Wanli in the afternoon, suddenly ushered in an hour of thunderstorms and torrential rain, only the downtown area of ​​Quanzhou has 14 Twist light control, 21 monitoring and 22 10 kV power lines failed after the thunderstorm and coincided with the college entrance examination day. To ensure the smooth travel of the college entrance examination candidates, more than 400 police officers from Quanzhou were on the road to maintain traffic order.

However, this is not an accidental situation, and monitoring systems are often susceptible to "injury" during thunderstorms. The data of lightning protection annual inspections released by the Beijing Lightning Protection Device Safety Testing Center shows that the pass rate of direct lightning strike detection in the city’s building reaches 90% or more. However, the detection rate of indirect lightning strikes in electronic systems in buildings is less than half. In recent lightning disasters, more than 80% of internal equipment including electrical appliances, monitoring systems, and probes have been damaged. Some high-tech electronic devices have weak anti-jamming capabilities. For example, monitoring probes, intercom systems, and other facilities in residential areas do not have lightning protection devices. Inductive lightning is easy to take advantage of, and destroy systems.

In general, the front-end cameras of surveillance systems are mostly installed outdoors, in relatively open areas, and have a relatively high risk of lightning strikes. They are very sensitive to lightning overvoltages, overvoltages in power system operation, and electrostatic discharges, making monitoring systems extremely hazardous. It is vulnerable to lightning overvoltage damage, causing the entire closed-circuit television monitoring system to collapse. Below, we will analyze the lightning strikes that the surveillance system may face.

One of the lightning damage phenomena is that the system is equipped with a lightning rod, a grounding net, and a joint grounding. When the lightning current passes through the lightning rod and is introduced into the ground net, the ground potential rises instantaneously. Assume that the lightning current I = 50KA, grounding resistance R = 1Ω, ground potential △ U = I × R = 50KA × 1Ω = 50KV.

Because joint grounding is used, the ground potential of the equipment is also increased by 50KV. The power supply line and the signal transmission line are led from a distance. The remote ground potential is still at zero potential. Therefore, the ground potential of the device housing will be connected to the power input terminal of the device. There is a large potential difference of 50 kV between the signal input terminals (called "counterattack" or "negative potential introduction" in lightning protection technology), resulting in damage to the corresponding parts of the equipment.

If the grounding network is not well grounded, the counter-attack voltage is even higher, and the damage may be more serious.

The second one is lightning damage: remotely destroying and destroying monitoring center equipment room CCTV front-end equipment The transmission line of the camera is led by the remote equipment room. Suppose there is a lightning near the transmission line, and transient overvoltage is induced on the transmission line. Inductive high voltage along the transmission line extends to both ends. If the monitoring center room is not too far away from the lightning point, this induced voltage may still have a certain intensity when transmitted to the equipment room, resulting in damage to the equipment in the equipment room.

The third phenomenon of lightning damage: The power supply is damaged when there is no lightning. For the power supply line, we always mistakenly believe that the sine wave that flows through the line is very clean. In fact, due to the switching of the power grid and the start and stop of inductive loads in the same power grid, various voltage disturbances may occur in the power supply line. In the harmful interference of the power grid, the incidence of momentary oscillations and transient pulses is very high.

This kind of transient voltage duration is very short, only a few microseconds short, non-special instrument is not easy to find, but their harm to the equipment can not be underestimated. Because modern monitoring and security devices are all microelectronic products, these monitoring devices have features such as high density, high speed, low voltage, and low power consumption. The more advanced the device is, the higher the degree of circuit integration is, and the distance between devices is only At a few tenths of a micron, as long as there is a slightly larger pulse impulse in the power supply, device damage in the circuit may be caused. However, they are more subtle and gradually cause damage. The damage is not as serious as lightning. It is mainly reflected in shortened life span, so it is not easy to attract attention.

For such momentary overvoltages in the power supply, the UPS and the voltage regulator can not only not play a protective role, but on the contrary, they themselves become the objects to be protected.

We have found that not only are outdoor cameras damaged by lightning, but the equipment in the monitoring center room is also “damaged” by lightning strikes. So, what areas can we start from "to get" lightning to protect the monitoring system? In general, the monitoring system is divided into front-end camera, intermediate transmission, and back-end monitoring center. From the above analysis of several major problems, it can be seen that the lightning protection of the monitoring system can also be started from three places.

1. The lightning protection front of the front-end equipment The installation location of the camera is often divided into indoor and outdoor. The cameras installed in the room are generally not subject to direct lightning strike. However, the lightning over-voltage may also cause damage to the camera, and the outdoor equipment needs to be considered. Prevent direct lightning strikes. Faced with this situation, how do we conduct lightning protection?

The usual method is that we will place the front-end equipment such as the camera within the effective protection range of the lightning receptor (lightning rod or other lightning conductor). When the camera is set up independently, in order to prevent the transient high potential of the lightning rod and the down conductor, the lightning rod is preferably 3-4 meters away from the camera.

In addition, in order to prevent electromagnetic induction, the camera power cord and signal line led along the telephone pole should be put in the metal tube to achieve the shielding effect. Both ends of the shielding metal tube should be grounded.

To prevent lightning waves from entering the front-end equipment along the line, we usually install appropriate lightning arresters on each line in front of the device, such as power lines (DC24V or 220V), video lines, signal lines, and PTZ control lines; Should also install B, C-level lightning protection device.

The signal line has a long transmission distance and a low withstand voltage level. It easily induces lightning current and damages the equipment. In order to conduct the lightning current from the signal transmission line to the ground, the signal overvoltage protection device must respond quickly. When designing the signal transmission line, the protection must be considered. Situation, according to the signal transmission rate, signal level, starting voltage and lightning flux and other parameters to choose the right lightning protection equipment.

2. Lightning protection for transmission lines For analog systems, the lines are mainly transmission signal lines and power lines. The power of the outdoor camera can be introduced from the terminal device or from the power source near the monitoring point. The control signal transmission line and the alarm signal transmission line are generally selected from reinforced core shielded soft wires, which are erected (or laid) between the front end and the terminal, and both the reinforcing core and the shielding layer should be well grounded.

According to the regulations, when the transmission line is laid in the suburbs and villages of the city, it can be laid directly. When the conditions are not allowed, communication channels or overhead can be used. From the point of view of lightning protection, the direct burial laying method has the best lightning protection effect, and the overhead line is most vulnerable to lightning strikes, and is destructive and has a wide spread range. To avoid damage to the head and tail end equipment, the overhead line should be transmitted on each pole. Ground handling is required. The overhead cable overhead line and overhead line cable should be grounded. The signal source and the power supply at the input of the intermediate amplifier should be respectively connected to the appropriate arresters.

However, it should be noted that the laying of the transmission line does not prevent the occurrence of a lightning strike. A large number of facts show that the failure of a buried cable caused by a lightning strike accounts for about 30% of the total failure. Even if it is far from the lightning strike, it still will Some lightning current flows into the cable. Therefore, if a shielded cable or cable is laid through a buried steel pipe, we need to pay attention to maintaining the electrical continuity of the steel pipe. This is very effective for shielding electromagnetic interference and electromagnetic induction. This is mainly due to the shielding effect of the metal pipe and the thunder. The skin effect of the current. Of course, if the cable is difficult to pass through the metal pipe, it can be introduced through the metal pipe before the cable enters the terminal and the front-end equipment. However, the length of the buried shall not be less than 15 meters. The metal sheath of the cable and the steel pipe shall be grounded to the lightning protection at the entrance end. The device is connected.

3. Lightning protection of the monitoring center In the monitoring system, the monitoring center is often the place where video surveillance is handled. Therefore, its lightning protection is also very important. In simple terms, the lightning protection of the monitoring center should be carried out in various ways from direct lightning strike protection, lightning wave intrusion, equipotential bonding and surge protection.

From the standpoint of direct lightning protection, the building where the monitoring center is located should have lightning rods, lightning conductors, or lightning protection nets that prevent direct lightning strikes. The measures for direct lightning protection shall comply with Article 3.1.2 of Article 3.1.2 of the provisions of GB50057-94 (2000 Edition): "To reduce the risk of fire, explosion and death in the space where lightning protection is needed, Equipotentiality is a very important measure"; Section 3.3: "The most important measure to prevent the danger of life occurring in a space requiring lightning protection is the use of equipotential bonding" in the provisions concerning direct lightning protection.

From the point of view of lightning wave intrusion protection, various metal pipelines entering the monitoring center should be connected to grounding devices that prevent induction lightning. When the overhead cable line is directly introduced, an arrester should be installed at the entrance to the house, and the cable metal outer sheath and the self-supporting steel cable should be connected to the grounding device.

Because 80% of the lightning high potential is invaded from the power line, in order to ensure the safety of the equipment, the general power supply should be set to three levels of lightning protection, the last stage installed in the MP3-10/2 lightning protection module, the lightning protection module nominal Flow capacity is 10KA, limit voltage is <1200V (equipment pressure value is 1.2KA).

To explain the equipotential bonding problem of the monitoring center, in general, the monitoring center should be equipped with an equipotential bonding bus (or metal plate). The equipotential bonding bus should be connected to the building lightning protection grounding, PE line, and equipment protection ground. , ESD ground, etc. are connected together to prevent dangerous potential differences. The grounding wires of various surge protectors (arresters) should be electrically connected to the equipotential bonding busbars at the straightest and shortest distances. The equipotential bonding is part of the internal lightning protection device and its purpose is to reduce the potential difference caused by lightning currents. The equipotential is the use of connecting wires or over-voltage (surge) protectors to connect lightning protection devices in spaces requiring lightning protection, metal structures, metal devices, external wires, electrical devices, telecommunication devices, etc. of buildings. An equipotential bonding network is formed to achieve equal potential equipotentiality to prevent fire, explosion, life hazards and equipment damage in the lightning protection space.

It should be said that the lightning protection of the surveillance system is a more complex issue, not only the protection of outdoor monitoring equipment, but also consider the lightning protection issues of transmission cables and monitoring centers. The lightning protection design of the safety monitoring system not only depends on the performance of the lightning protection device, but more importantly, before the design and construction of the monitoring system, the geographical environment of the monitoring system must be taken into consideration, and a suitable cable deployment method should be designed. Shielding and grounding methods. In the face of thunder and lightning attacks, can your monitoring system shout loudly: Will storms be more violent?