Monitoring system lightning protection grounding FAQ
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One, grounding resistance is too large or no grounding. The grounding resistance of the lightning protection system of the monitoring system is not clearly defined in the "Engineering Design Specification for Video Security Monitoring System" and "Technical Specifications for Lightning Protection in Building Information Systems." Generally, the grounding resistance of the design monitoring system is designed according to the resistance value of the computer room and the grounding resistance of the information system: the independent grounding is not greater than 4 ohms, and the joint grounding resistance is not greater than 1 ohm. Specifically speaking, the more different geological conditions can appropriately increase the grounding resistance of the monitoring system. For example, the forest fire protection unit must be installed on the ridge. The address is bluestone, there is no river ditches within 200 meters, and no excavation is possible. Large gravel, soil zone. At this time, if the ground of the monitoring system in this particular place is required to be no more than 4 ohms, and the cost is ten times or even several ten times of the monitoring system itself, then the appropriate increase in the grounding resistance should be selected, and the cable with the large flow rate should be used as the connection. Ground line.
Second, no direct lightning protection measures. This situation is more common is the bolt camera, some people think that the three-in-one signal lightning arrester has been installed, grounding is also done, there should be no problem, but do not know the lightning arrester is a lightning conduction, lightning induction tool, There is no effect on direct lightning. Lightning damage to the electronic information system generally has several means: 1. Mechanical effects, such as a lightning strike on a tree, sometimes knock off the support of a big tree, mainly relying on the mechanical effect of a lightning strike. 2. Thermal effect: The instantaneous temperature of the lightning strike point can reach more than 3,000 degrees, and the metal can be instantly melted. 3, electromagnetic effect, this is an invisible killer, it has contributed to the instantaneous high current electromagnetic induction equipment burned out. As long as the lightning protection is made by relying on the equipment of the lightning rod, the direction of operation of the lightning is changed so that the lightning is unlikely to hit the monitor directly. There is also a situation in which a lightning rod is installed on a curved ball machine. In this case, the metal rod itself will protect the dome, and there is no need to increase the cost and install the lightning rod.
Third, the signal arrester is too far away from the equipment to be protected. If the arrester is installed too far away from the device, there is no way to eliminate the lightning currents induced on the communication lines and power lines after the arrester reaches the front of the device. Therefore, it is recommended that the signal lightning arrester should be installed directly to the front end of the monitoring equipment, especially where the signal power lines go.
Fourth, the problem of grounding cables. In order to save costs, the metal rod is often used as the down conductor of the working grounding system. Regardless of whether it is "right" or not, according to the design, the metal rod is used as the direct lightning protection system. At this time, the work place is connected to the metal rod. On the other hand, if there is a direct lightning strike, there is no voltage difference between the front end of the signal arrester and the moment of the metal rod body. That is to say, your direct lightning will directly hit the signal lightning arrester or the power lightning arrester, which will cause the arrester to age prematurely or be damaged on the spot. Therefore, it is recommended that the downline of work should be singled out.
V. Three-in-one surge protector (or BNC surge protector) selection. According to the specific situation of the market and the experience of time installation, the BNC signal electrophoretic protector is set for a long period of time in the range of lightning protection. This is not enough for the protection of the monitor, and it is generated for the monitor. There are also surges, spikes, and burrs that can cause surges, spikes, and burrs to start or close to large nearby power tools, and changes in nearby electromagnetic fields. These need more than just pressure-sensitive or Discharge tube protection also requires a logic filter circuit to provide a safer environment for the monitor. The latter factor was ignored.
Lightning protection is a complex issue. The lightning protection design of the safety monitoring system not only depends on the performance of the lightning protection device, but more importantly, the geography of the monitoring system must be taken into account before the design and construction of the monitoring system. Environment, design suitable cable deployment methods, shielding and grounding methods. In short, lightning protection design should be considered comprehensively to achieve good results.
Security monitoring system Lightning protection 1. Front-end equipment lightning protection Front-end camera installation location is often divided into indoor and outdoor, installed in the indoor camera is generally not subject to direct lightning strikes, however, lightning over-voltage may also cause damage to the camera , And outdoor equipment needs to be considered to 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 located 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, and 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 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 method has good lightning protection effects, the overhead line is vulnerable to lightning strikes, and it is destructive and has a wide range of spread. To avoid damage to the head and tail equipment, the overhead line should be grounded on each pole. For handling, overhead wires for overhead cables and metal conduits for overhead cable lines should be grounded. The signal source and the power supply at the input of the intermediate amplifier should each be connected to a suitable arrester.
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 will still be 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 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 sheathing 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. Its direct lightning protection measures shall comply with Article 3.1.1 of Article 3.1.2 of the provisions of GB 50057-94 (GB 50057-94 (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 important measure to prevent the danger of life in a space requiring lightning protection is the use of equipotential bonding".
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 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 The flow capacity is 10KA, and the limit voltage is <1200V (the pressure resistance of the equipment is 1.2KA).
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