Thurder Protection in Security Video Surveillance System
Thunder protection is a systematic project. In addition to various devices themselves, they need to have certain thunder protection and surge protection capabilities. In the engineering design and installation of security systems, attention should also be paid to the thunder protection of equipment and systems.
Thunder and lightning is a very spectacular sound, light and electrical phenomenon in nature. At the same time, a thunder disaster is also a very serious natural disaster. The thunder damage to the weak current security system and electronic information system is mainly through the following types of thunder:
Direct thunder refers to thunder strikes directly on buildings or thunder protection devices, causing electromagnetic, thermal and mechanical effects. About 3.1 billion thunder strikes occur on the earth every year, and direct thunder strikes account for 1/5–1/6.
The peak voltage of direct thunder can reach tens of thousands of volts or even millions of volts, and the peak current can reach tens of KA or even hundreds of KA, but its duration is usually only a few μs to hundreds of μs, which is huge in terms of instantaneous power. Yes, so it is very destructive.
Inductive thunder refers to all objects on the ground when a thundercloud comes, especially conductors, due to electrostatic induction, will gather a large amount of thunder with the opposite polarity of the bound charge. After the thundercloud strikes the ground or discharges another thundercloud, The charge in the cloud becomes a free charge, which generates a very high electrostatic voltage (induced voltage), and its overvoltage amplitude can reach tens of thousands to hundreds of thousands of volts. This overvoltage often causes damage in buildings. Wires, poorly grounded metal conductors and large metal equipment discharge and cause electric sparks, which can cause fires, explosions, endanger personal safety or cause harm to various electronic information systems such as security. This is an electrostatic induction mine.
In another case, when a thunder strike occurs, a strong induced electromagnetic field is formed near the passage due to the large rate of change of the thunder current, which causes interference and damage to the electronic equipment in the building. It may also cause the surrounding metal components to generate induced current, generate a lot of heat and cause a fire. This is an electromagnetic induction mine.
When the outdoor overhead line is directly struck by thunder or induced thunder, the high potential will invade along various cable lines and be transmitted into equipment or buildings. This kind of thunder intrusion will cause harm to electronic information equipment or cause damage in the building. Metal equipment discharges, causing damage.
Thunder protection standard
The main organizations for the formulation of thunder protection standards are the Thunder and thunder Protection Technical Committee (TC81) under the International Electrotechnical Commission (IEC), and the corresponding National thunder Protection Standardization Technical Committee (SAC/TC258) under the Chinese Association for Standardization (CAS), and International Telecommunications Union (ITU).
The main thunder protection standards of IEC/TC81 are:
IEC-61024 (thunder protection of buildings)
IEC-62305 (thunder Protection)
IEC-61643 (low-voltage surge protection device)
IEC-1312 (protection of thunder electromagnetic pulse)
IEC 61000-4-2 (Electromagnetic compatibility test and measurement technology, electrostatic discharge immunity test). Corresponding to GB/T-17626.2
IEC61000-4-5 (Electromagnetic compatibility test and measurement technology, surge immunity test)
ITU-T (International Telecommunication Union Telecommunications Part) K series (anti-interference protection):
ITU-T K.21 (the ability of terminal telecommunication equipment to withstand overvoltage and overcurrent). Corresponding to GB/T-17626.5
ITU-T K.20 (Electromagnetic Compatibility Technical Standard for Telecommunications Switches)
IUT-T K.27 (connection structure and grounding in the telecommunication building)
GB/T-17626 (Electromagnetic compatibility test and measurement technology)
GB50057-2016 (Code for thunder protection design of buildings)
GB500174-2017 (Data Center Design Specification)
GA173-2002 (thunder Protection Device for Computer Information System)
GB50343-2012 (Technical Specification for thunder Protection of Building Electronic Information System)
GA267-2018 (Computer Information System Thunder and thunder Electromagnetic Pulse Safety Protection Specification)
GB50311-2016 (Code for Design of Generic Cabling System Engineering)
YD5078-98 (Technical Regulations for thunder Protection of Communication Engineering Power System)
GB/T-9361-2011 (Safety Requirements for Computer Sites)
DL/T621-1997 (Grounding of AC electrical equipment)
GB/T-19856-2005 (thunder Protection Communication Line). Corresponding to IEC 61663:2001
GB50689 (Code for thunder Protection Grounding and Engineering Design of Communication Bureau (Station))
Security cameras, data transmission equipment, back-end management equipment, etc. generally have a certain degree of anti-thunder and anti-surge and anti-surge capabilities.
For example, a certain model of Hikvision camera indicates the protection level: TVS 6000V thunder protection, anti-surge, and anti-surge, in line with GB/T17626.5 level four standards. A certain PTZ dome camera of Dahua indicates that it supports TVS 8000V thunder protection, anti-surge and anti-surge protection. A certain PTZ dome camera of Univision is marked with a 6KV anti-surge design for the network port.
thunder protection for buildings and computer rooms
According to the requirements of GB50057-2016 (Code for thunder Protection Design of Buildings), buildings should meet thunder protection requirements.
For the newly-built security management computer room, it should be placed within the protection range of thunder rods, or be installed with thunder rods to reduce the probability of being struck by thunder. At the same time, it should be grounded and grounded.
There are many calculation methods for the protection range of thunder rods. For details, please refer to the article on the protection range of thunder rods. It should be noted that the protection range of the thunder rod and its height do not increase proportionally. For security cameras installed outdoors, we can calculate the range of thunder rod protection according to the 45° broken line method, that is, the radius of the protection range is the height of the thunder rod.
Grounding is an important method of thunder protection for buildings and various electrical equipment, especially grounding resistance. The larger the grounding resistance, the more unfavorable the discharge of overvoltage and overcurrent, so the grounding resistance should be strictly controlled within the required range.
GB50174 (Code for Design of Electronic Computer Room) stipulates: AC grounding should be less than 4 ohms, and safety grounding should be less than 4 ohms.
GB50057 (Code for Design of thunder Protection for Buildings) stipulates that the grounding resistance for thunder protection should be less than 10 ohms.
YD2011 (Code for Design of thunder Protection and Grounding for Microwave Stations) pointed out: It is strictly forbidden to make zero protection. The power frequency grounding resistance should not be greater than 10 ohms.
YDJ 26-89 (Interim Technical Regulations on Grounding Design of Communication Bureau (Station)): It is strictly forbidden to use the neutral wire as the AC protective ground wire. The grounding resistance value of the comprehensive communication building should not be greater than 1 ohm.
The thunder protection design in YD5003 (Code for Design of Special Telecommunication Buildings): The impulse grounding resistance of the thunder protection grounding device of the telecommunications building should not be greater than 10 ohms, and the three-in-one grounding (joint grounding) should meet the working grounding resistance requirements.
According to the requirements of the above national regulations, the grounding resistance of general buildings should be less than 10 ohms, and the grounding resistance of computer rooms should be less than 4 ohms. Switches, small computers and other equipment generally require DC working grounding resistances of less than 1 ohm.
Grounding resistance measurement method
The arrangement of the electrodes is shown in the figure below. The distance d1 between the current electrode and the edge of the grounding grid is generally 4 to 5 times the maximum diagonal length D of the grounding grid, so that the potential distribution between them appears in a flat section. In general, the distance d2 between the voltage electrode and the edge of the grounding grid is about 50% to 60% of the distance d1 from the current electrode to the grounding grid.
Ground resistance measurement
When measuring, move the voltage electrode along the connection line between the grounding grid and the current electrode three times, each time the moving distance is about 5% of d1, if the resistance value measured three times is close.
For example, it is difficult to take d1 from 4D to 5D. In areas with relatively uniform soil resistivity, d1 can be 2D and d2 is D; in areas or regions with uneven soil resistivity, d1 can be 3D and d2 is 1.7D.
The voltage pole and current pole can also be arranged in a triangle as shown in the figure below. Generally take d2=d1≥2D, and the included angle is about 30 degrees.
Ground resistance triangulation
The current pole and voltage pole should be arranged in a direction perpendicular to the line or underground metal pipeline. Avoid measuring grounding resistance immediately after rain.
When the AC ammeter-voltmeter method is adopted, the arrangement of the electrodes should adopt a triangular arrangement.
Selection of protective grounding wire diameter
According to the selection method of the wire diameter of the equipment protective grounding wire in IEC60950 “Safety of Information Technology Equipment”, the wire diameter of the protective grounding wire is selected as follows:
Rated current of the equipment A Nominal cross-sectional area MM2
Equipment installation thunder protection
Cameras and other equipment installed outdoors should be placed within the effective protection range of thunder rods.
The camera should be equipped with corresponding thunder protection devices such as power cables, video cables, control signal cables and communication cables.
The grounding wire of the camera protective cover, the equipment grounding wire, the thunder arrester grounding wire, the general grounding wire, etc. must be equipotentially connected to form an equipotential body.
The camera is installed outdoors with a pole with a thunder rod. The thunder rod is likely to cause induced thunder. One is the introduction of large current, but electromagnetic induction. Be sure to pay attention to the equipment grounding and equipotential connection, otherwise the thunder rod will be counterproductive and bring destructive effects.
When the camera is installed outdoors, when the existing poles (such as street light poles, traffic light poles, and telephone poles) are used for installation, if there is no grounding body, a new grounding body needs to be built.
Use angle steel or steel pipe with a length of not less than 2.5m and drive directly into the ground. The distance between the upper end and the ground should not be less than 0.7m. The section of the angle steel should be no less than L×W×H = 50×50×5mm, the wall thickness of the steel pipe should be no less than 3.5mm, and the material should be galvanized steel.
The protective grounding cables of cameras and other equipment should be connected to the angle steel by electric welding, and the surface of the welding points should be coated with anti-rust paint for anti-rust treatment. The cross-sectional area of the protective grounding cable shall not be less than 16 square meters, and the cable shall be as short as possible during construction and cannot be coiled.
Schematic diagram of a typical outdoor pole-mounted camera thunder protection
Transmission line thunder protection
Transmission equipment (such as switches, fiber optic transceivers, etc.) should be installed with the same thunder protection measures as cameras.
It is recommended to lay the cable through the metal pipe and bury it in the ground. At the same time, ensure effective grounding at both ends of the metal pipe.
If the metal pipe cannot be worn all the way, the metal pipe must be buried in the ground before the cable is connected to the monitoring room and the network camera. The buried length should not be less than 15m. The metal sheath of the cable and the metal pipe should be effectively connected to the thunder protection ground at the access end. . Finally, corresponding thunder protection devices should be installed at both ends of all transmission cables.
Use shielded transmission cables, and use thunder protection devices at both ends of each cable.
Optical fiber cables will not conduct electricity and are not susceptible to electromagnetic interference. The use of optical fibers to transmit signals can better prevent thunder hazards.
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Tags: safety, surge, thunder protection
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