One of the lightning damage phenomena: the system is equipped with lightning rod and grounding grid, and adopts common grounding.
When lightning current is introduced into the grounding grid through the lightning rod, the ground potential rises instantly. Suppose the lightning current I=50KA, the grounding resistance R = 1ω, and the ground potential △U = I×R = 50KA× 1ω= 50KV.
Due to the adoption of common grounding, the ground potential of the equipment is increased by 50KV, while the power supply line and signal transmission line are led out from a distance, while the ground potential in the distance is still at zero potential, so there will be a huge potential difference of 50KV between the ground potential of the equipment shell and the power supply input terminal and signal input terminal of the equipment (called "counterattack" or "negative potential introduction" in lightning protection technology), thus causing damage to the corresponding parts of the equipment.
If the grounding grid is not well grounded, the counterattack voltage will be higher and the harm may be more serious.
The second phenomenon of lightning damage: the equipment in the monitoring center computer room is damaged by lightning in the distance.
The transmission line of the front-end equipment camera of CCTV monitoring system is led out from the remote computer room. Assuming that there is lightning near the transmission line, instantaneous overvoltage will be induced on the transmission line. The induced high voltage extends to both ends along the transmission line. If the monitoring center computer room is not too far from the lightning point, the induced voltage may still have a certain intensity when it reaches the computer room, which may damage the equipment in the computer room.
The third phenomenon of lightning damage: power supply is damaged when there is no lightning.
For the power supply line, we always mistake it for a clean sine wave. In fact, due to the switching of power grid and the start and stop of inductive load in the same power grid, various voltage disturbances will appear in the power supply line. In the harmful interference of power grid, the occurrence rate of instantaneous oscillation and instantaneous pulse is very high.
This kind of transient voltage lasts for a short time, only a few microseconds, which is not easy to be found by non-dedicated instruments, but their harm to equipment cannot be underestimated. Because modern monitoring and security devices are microelectronic products, these monitoring devices have the characteristics of high density, high speed, low voltage and low power consumption, and the more advanced the devices, the higher the circuit integration.
The distance between devices is only a few tenths of a micron, so as long as there is a slight pulse impact in the power supply, the devices in the circuit may be damaged. It's just that they are hidden and gradually cause damage. The damage is not as serious as lightning, mainly reflected in the shortened life span, so it is not easy to attract attention.
Extended data:
Lightning protection of monitoring system;
1, front-end equipment lightning protection
The front camera is usually installed indoors and outdoors. Cameras installed indoors generally do not suffer from direct lightning, but lightning overvoltage may also cause damage to cameras, so outdoor equipment needs to be protected from direct lightning. Faced with this situation.
Usually, we will put the front-end equipment, such as cameras, within the effective protection range of lightning arresters (lightning rods or other lightning wires). When the camera is erected independently, the lightning rod zui should be 3-4 meters away from the camera to prevent the transient high potential on the lightning rod and the down lead.
In addition, in order to prevent electromagnetic induction, the power line and signal line of the camera led out along the telephone pole should pass through the metal tube to achieve shielding effect, and both ends of the shielding metal tube should be grounded.
In order to prevent the lightning wave from invading the front-end equipment along the line, we usually install suitable lightning arresters on each line in front of the equipment, such as power lines (DC24V or 220V), video lines, signal lines, pan-tilt control lines, etc. The power input front end should also be equipped with B and C lightning arresters.
The signal line has long transmission distance and low withstand voltage level, which is easy to induce lightning current and damage the equipment. In order to transmit lightning current from signal transmission line to ground, signal overvoltage protector must respond quickly. When designing the protection of signal transmission line, we must consider the actual situation and choose the correct lightning protection equipment according to the parameters such as signal transmission rate, signal level, starting voltage and lightning flux.
2. Lightning protection of transmission lines
For the analog system, its lines are mainly transmission signal lines and power lines. The power supply of the outdoor camera can be imported from the terminal equipment or from the power supply near the monitoring point. Control signal transmission lines and alarm signal transmission lines are generally shielded flexible wires with reinforced cores, which are erected (or laid) between the front end and the terminal, and both ends of the reinforced core and the shielding layer should be well grounded.
According to the regulations, when laying transmission lines in urban suburbs and rural areas, direct buried laying can be adopted. When the conditions are not sufficient, communication pipes or overhead methods can be used. From the point of view of lightning protection, the direct buried laying method has the best lightning protection effect. Zui is an overhead line, which is prone to lightning strike, destructive and wide in scope. To avoid damaging the head-end and tail-end equipment.
Overhead lines should be grounded on each pole during transmission, and the hanging wires of overhead cables and metal pipes in overhead cable lines should be grounded. The signal source and power supply at the input end of the intermediate amplifier should be connected to the appropriate lightning arrester respectively.
However, it should be noted that buried transmission lines cannot prevent lightning strikes. A large number of facts show that the faults of buried cables caused by lightning strike account for about 30% of the total faults, and even in places far away from lightning strike, some lightning current will still flow into the cables. So if cables or cables with shielding layers are buried through steel pipes.
We need to pay attention to the electrical connection of steel pipes, which is very effective for preventing electromagnetic interference and electromagnetic induction, mainly due to the shielding effect of metal pipes and the skin effect of lightning current.
Of course, if it is difficult for the cable to pass through the metal tube, it can be buried in the metal tube before the cable enters the terminal and front-end equipment, but the buried length should not be less than15m, and the metal sheath and steel tube of the cable should be connected to the lightning protection grounding device at the entrance end.
3. Lightning protection of monitoring center
In the monitoring system, the monitoring center is often the place where the monitoring video is processed, so its lightning protection is particularly important. Simply put, the lightning protection of the monitoring center should be carried out from direct lightning protection, lightning wave intrusion, equipotential connection, surge protection and other aspects.
From the point of view of direct lightning protection, the building where the monitoring center is located should have lightning rods, lightning belts or lightning protection nets to prevent direct lightning.
The measures to prevent direct lightning strike shall conform to the provisions in Article 3. 1.2 of GB50057-94(GB50057-94(2000 edition)): "Equipotential is very important to reduce the occurrence of fire, explosion and life danger in the space that needs lightning protection.
Article 3.3: "Equipotential bonding is an important measure for zui to prevent life danger in the space where lightning protection is needed").
From the perspective of lightning wave intrusion protection, all kinds of metal pipes entering the monitoring center should be connected with grounding devices to prevent induction lightning. When overhead cables are introduced directly, lightning arresters should be installed at the entrance, and the metal outer sheath of cables and self-supporting steel cables should be connected with grounding devices.
Because 80% of the lightning high potential intrudes from the power line, in order to ensure the safety of the equipment, three levels of lightning protection are set on the main power supply, and the last level is equipped with Zhong Da MP3- 10/2 lightning protection module. The nominal current capacity of the lightning protection module is 10KA, and the limit voltage is
Let's explain the equipotential connection of the monitoring center. Generally speaking, the monitoring center should be equipped with equipotential connection bus (or metal plate), which is connected with lightning protection grounding, PE line, equipment protection grounding and anti-static grounding of buildings to prevent dangerous potential difference.
The grounding wires of various surge protectors (lightning arresters) should be electrically connected with the equipotential bus in the straightest and shortest distance. Equipotential bonding is a part of internal lightning protection device, and its purpose is to reduce the potential difference caused by lightning current. Equipotential is a lightning protection device, which is the metal frame and metal device of a building. Equipotential is placed in the space that needs lightning protection by connecting wires or overvoltage (surge) protectors.
External wires, electrical equipment, telecommunication equipment, etc. Connect into an equipotential connection network to realize equal voltage equipotential and prevent fire, explosion, life danger and equipment damage in the space where lightning protection is needed.
Baidu Encyclopedia-Video Monitoring Lightning Protection
Baidu Encyclopedia-Security Monitoring System
Baidu Encyclopedia-Monitoring Lightning Protection