2-1 Wide Area Information System Electromagnetic Compatibility (EMC) Concept
The wide-area information system (WAIS) refers to a weak system that spans a wide area and has direct electrical connections between system devices and cannot achieve equipotential bonding between systems. The security system belongs to the "wide area information system".
Wide-area electromagnetic environment - Influencing the external electromagnetic environment of the wide-area information system, from the air to the underground, lightning in the air, various electromagnetic waves in the free space, the coupling of charged cables on the ground, and various types of underground potential Affects the operating quality and system safety of weak systems.
Wide area information system electromagnetic compatibility (EMC): It is assumed that the quality, safety, electromagnetic compatibility (EMC) and other problems of the various equipment (products) used in the system have been solved and are all qualified. Only the system security under the influence of the external electromagnetic environment is considered. Operational design issues. It is different from the commonly used equipment, products, and circuit board-level electromagnetic compatibility issues. Some people tried to explain the “wide-area electromagnetic compatibility of security systems†using the concepts of equipment and PCB electromagnetic compatibility.
The design of safe operation of the security system belongs to the electromagnetic compatibility (EMC) design problem of the wide area information system, including system anti-jamming design, anti-static design, direct lightning protection and inductive lightning design; the goal is to ensure the system's own safety and ensure the system operation quality.
The lightning protection design of the cameras “pole lightning rod†and “ground lightning protection device†analyzed in the previous analysis constitutes an artificial safety hazard, which is due to the lack of understanding of the earth electromagnetic environment and the lack of basic characteristics of the wide-area work of the security system. Awareness, this is the root cause of system design hidden dangers. Concern and study of system electromagnetic compatibility design is a major issue related to the safety of weak systems. It should be mentioned on the agenda of security projects.
The basic principle of 2-2 security system security design is "single point earthing"
1. Basic concept of "single point earthing" Single point earthing means that the host of the system is grounded at one point, and the remote camera and all equipment must be insulated from the earth; specifically, "single point earthing" means that there is a direct electrical connection. The "system" should make the electrical connection "centralization point" equipment (system host or subsystem host) do a ground. For example, optical cable transmission system: the front-end multi-channel optical transmitter is the host of the subsystem, and the casing “does†connect to the ground. The cameras connected to the optical transmitter cable should be insulated from the earth. This is the “direct electrical connection†system. "Single-point grounding" does not replace the back-end system host ground because there is an optical cable in the middle that isolates the electrical connection.
2. The "single point earthing" project requires the host to "single point earthing". All equipment at the far end of the system is suspended to the ground. The electrostatic charge generated by the system is discharged through the host earthing point, and the static potential is maintained with the earth to ensure the safety of operation. . The (ground) potential of the system after single-point earthing refers to the “ground potential†of the system with respect to the zero potential of the earth, ie, the potential of the system earthing point; the “Professional Lightning Protection†in the Security Forum once received the cable. Lightning induced electromotive force is described by "overvoltage" and "high potential", claiming that "cables at both ends can be clamped to equipotential at both ends with a grounded lightning protector at both ends." The high-frequency analysis shows that for the alternating induced electromotive force on the cable, even if the grounding resistance of the lightning arrester is 0 and the ground potential at both ends is equal, the “clamping voltage†of the lightning arrester at both ends is always “clamping voltage†Equal in size, opposite in polarity, where is there any potential equalization? In addition, the grounding "discharge circuit" includes the AC and DC impedance sum of the cable and the grounding wire as well as the grounding resistance. The so-called "effective discharge of the lightning current" is merely an illusion. Lightning induced electromotive force has nothing to do with the earth. There is no problem of discharge of current to the earth; "single point earthing" is used to discharge the static charge of the system, and there is no high requirement for grounding resistance, and no special grounding net is required; "single point "Grounding" and the traditional lightning rod grounding, power grid grounding, anti-surge protection device ground leakage current is essentially different from the requirements of the difference, using ordinary wire to connect the building steel, water pipes can be connected.
3. Analysis of the rationality of "single point earthing" 1 "Single point earthing" cuts off all ground loops and effectively blocks the path of "lightning ground potential" and "grid ground potential" invading the weak current system. This is lightning protection. , Anti-surge, anti-interference is the most effective basic technical means; multi-point grounding introduces ground potential interference and grid surge, the introduction of lightning counter-attack voltage, burning of security equipment and lightning protection equipment case has been more and more security engineering damage Case confirmed.
2 The "single point earthing" of the security system is not only not inconsistent with the anti-induction mine, but also the basic concept that the lightning protection design of the security system should follow and the basic conditions of the lightning protection design; because the anti-radar does not need, it cannot be connected through any part of the system. Earth discharge; Induction thunder only needs to pass the protection circuit to suppress the voltage value when the lightning induced electromotive force reaches the device port, ensuring that it is lower than the “highest safety voltage†of the device, and the protection circuit used does not need to connect to earth.
3 The system "single point earthing" makes the entire system float with the earth point equipotential. Artificially created multi-point grounding and attempted to achieve "equipotential bonding", this is theoretically and practically impossible for the "wide area information system."
4 Adhere to the "single point grounding" safety design principle, which can avoid being misled by "grounding and lightning protection", and can avoid the investment in the complex grounding system.
5 "Single point earthing" is the "touchstone and watershed" for testing and judging the security design and hidden danger design of the security system.
2-3 Questions about "Lightning protection of buildings and their power supply system" in security engineering
First of all, please consider a question: The security engineering company designed the security project for Party A. Party A was already working and using electricity in the building. Their buildings and power supply system passed the safety inspection and they all had a daily The detection and maintenance, how do you need to do when the security project to consider and design their buildings lightning protection, power distribution surge protection, engage in what three levels of protection? Even if the building and the power supply system of Party A are protected against lightning and unqualified surges, it is not a matter for the security engineering company to govern; the building is protected against direct lightning strikes and multi-stage lightning protection of the power supply is included in the relevant management documents of the security and lightning protection project. In the propaganda description of “Professional Lightning Protectionâ€, it occupies the main space, and the lightning protection design part of the real security system highlights two major potential safety hazards of “lightning rod lightning protection†and “lightning multi-point groundingâ€. It is a question worthy of our consideration.
Lightning protection of buildings and power supply systems is a basic condition that should be possessed before the construction of security systems. It should not belong to the jurisdiction of the security system design, or it would be superfluous. The vast majority of security engineering companies do not have the corresponding qualifications.
2-4 security system direct lightning protection design
First, all equipment of the security system should work within the effective protection scope of existing buildings' lightning protection and lightning protection systems and other independent lightning rods, and the transmission cables should be buried and routed as far as possible.
Second, outdoor isolated pole camera, if it is necessary to prevent direct lightning (prove the probability of lightning strike), should set up independent lightning rod protection pole camera, vertical pole and independent lightning rod distance should be greater than 4,5 meters (counterattack pole Distance), the top of the camera pole must not be installed lightning rod, the following will not be ground network, metal pole but also do a good job with the camera's high-level, all-weather insulation, in order to deal with the lightning rod when the "step voltage" through the pole Counter-attack cameras; the use of wood or cement insulation poles is more conducive to insulation, the camera bracket is best to use engineering plastic brackets to increase the level of insulation.
Third, there are people who propose to make the "camera poles a lightning rod and insulate the camera from the poles." This is also undesirable. Because the lightning strike voltage generated by the lightning arrester strikes the air at a distance of more than 30 to 40 centimeters, the rainy days can exceed a distance of more than 1 meter, and conventional insulation cannot be achieved.
A responsible manufacturer should take the initiative to correct its own "security hidden lightning protection project."
2-5 About Lightning Sensing
The description of lightning induction, "professional lightning protection" uses "thunder and lightning electromagnetic pulse", "lightning impulse wave", "lightning wave", "surge voltage", "surge current", "lightning overvoltage", "lightning over The words "current" and many other words have been described; but it is never clear what the "lightning sensor" on the cable is.
1) False Lightning Sensing Data The so-called "leakage voltage" of several tens of kilovolts on the monitoring system cable and the argument that the surge arrester "needs several thousand tens of kiloamperes of discharge capacity" should be derived from "professionalism." "Lightning protection" artificially created multi-point grounding, the introduction of ground potential and grid surge data, which is a false data with the lightning electromagnetic induction wind horse cattle unmatched. "Surge" and "earth surge protector" are originally terms and concepts of the power grid system. Now "professional lightning protection" confuses it with lightning induction. Moreover, it simply puts the "grounded surge protection" of the power system. The device is used directly as a "lightning arrester" for weak systems.
There is also a type of lightning-sensing calculation formula given by “professional lightning protectionâ€. The calculated data is also a “lightning shock wave†of several tens of kilovolts. This is the data that is put together in the absence of a basic understanding of the ground potential, and brings together the data. The lightning-sensing "hat" is to search for "reasonable arguments" for the sales of "earth lightning protectors" and "grounded surge protectors." Reflects these people have no basic understanding of the nature of lightning induction.
2) Qualitative Analysis of Lightning Electromagnetic Radiation and Electromagnetic Induction The process of thundercloud discharge is mainly the energy consumption of thermal energy, light energy, mechanical energy, and acoustic energy. The “electromagnetic radiation energy†of lightning current is only a small part of the total energy of lightning. Lightning induction refers only to the "electromagnetic radiation" energy actually received by a certain cable. There is a certain "electromagnetic coupling relationship" between "radiation" and "reception". This "electromagnetic coupling relationship" is affected and limited by many factors:
1. Spectrum effect: Only high frequency can form effective radiation, while high frequency radiation is just the higher harmonic component of lightning current, which is not the main energy spectrum range of lightning;
2. Antenna efficiency and electromagnetic radiation of lightning currents: Electromagnetic radiation is closely related to the size and wavelength (frequency) of the antenna structure. The antenna efficiency for strict design and precision machining is around 50 to 70%. Different harmonic frequencies, different lightning rod heights, and equivalent "antenna lengths" with different inter-cloud discharge currents are unlikely to have a good matching relationship with harmonic frequencies, so the actual lightning current electromagnetic radiation is of low efficiency and super Inefficient electromagnetic radiation; the same reason, "receiver antenna" also has the best frequency and size matching problem, the best match to achieve high efficiency reception, apparently monitoring cables as lightning radiation "antenna" is also inefficient, ultra-low Efficient "receiving antenna".
3. Directional influence: The radiation direction of the lightning current electromagnetic radiation energy is similar to the "omnidirectional radiation" of the spherical wave free space, corresponding to the "stereoscopic" radiant energy of a cable area and direction, but only in the total radiation power. A small amount, and the intensity of the radiated power is also inversely proportional to the square of the distance. In other words, the directionality of lightning electromagnetic radiation and cable reception basically has the characteristics of "omnidirectional low gain".
4. Polarization effects: The electromagnetic waves emitted by the radiation source have a certain “polarization direction†(the direction of change of the electric field strength vector E), and the receiving cable also has a problem of the direction of polarization received. For example, the discharge current of the lightning rod body is mainly "vertical", and its radiation field polarization direction (E) is also a "vertically polarized wave". Only vertical poles and ground lines can effectively receive this "vertical polarization". "Waves", while horizontal cables, basically do not receive such "vertically polarized wave" energy. This is the effect of polarization; both the cable reception and the direction of polarization of the lightning radiation are random factors, actually "polarization". Attenuation is very large, and the energy actually received is also a tiny amount.
Taking into account the above-mentioned various limiting factors, we can draw the following concept: The energy of lightning electromagnetic radiation to free space is only a fraction of the total energy of the lightning discharge, due to insurmountable factors such as frequency spectrum, radiation efficiency, directionality, and polarization. Impact, the actual lightning energy received by the cable and the total energy of the lightning is more "super tiny"; the so-called tens of kilovolts and tens of thousands of amps of data on the cable is against the laws of science.
"Thunder and lightning electromagnetic pulse", "lightning shock wave", "lightning wave", "surge voltage", "surge current", "lightning overvoltage", "lightning overcurrent" and many other description vocabulary are also some whimsical languages. Is not a classic technical language.
In accordance with the calculation formula given in a “Professional Lightning Protection†article, the author calculated a lightning surge “overvoltage, overcurrent†and “total energy†of a cable in a office building for a discharge lightning rod, which actually exceeded the lightning rod discharge. The total energy is also many times larger; the "professional formula" given is actually not related to any of the above-mentioned limitations, and is typical of the "followed by amateurs."
A lot of life practice tells us that during thunderstorms watching TV at home can see occasional flashes of lightning interference, and can also hear thunder and cough sounds from telephones and radios. The so-called cases of lightning-burning of household appliances, burning of security equipment, and melting of video cables have caused most of the cases where lightning strikes have caused grid faults and caused grid power surges to be destroyed.
In the early years, the author had seen several narrow-beam radars with hundreds of kilowatts and megawatts of power levels. When the wave speed came, strong electromagnetic fields could turn on indoor fluorescent lights or brighten high-pressure xenon lamps but never saw them on the head. The thunder and lightning electromagnetic field turns on the fluorescent light and the street light. If the lightning sensor really has dozens or hundreds of kilovolts, thousands of amperes, and several tens of kiloamps of amperes, the fluorescent lamps will not only light, but they will be destroyed in batches.
The conclusion is that the actual "thunderstorm sensing" energy received on the cable is not as large as the "tens of kilovolts" and "tens of thousands of amps" described by "professional lightning protection." It is only one of the total energy of lightning. The tiny amount, in essence, is not a unipolar charge, but an alternating induced electromotive force, which can be equivalent to the middle of the cable and has nothing to do with the earth. The "surge protectors" used in power grid systems actually require "tens of kilovolts" and "tens of kiloamps" of protective performance. This is used to cope with power grid surges and requires grounding to discharge surge currents. , Lightning induction and the grid "surge" compared to far less than a panic. The use of a grounded "surge protector" as a "lightning arrester" for a lightning-sensitive lightning system can only be a "modern technology joke".
2-6 Inductive Lightning Protection Design
The previous analysis of lightning induction has made it clear that the lightning induced electromotive force of the cable has nothing to do with the earth. It is a kind of high-frequency alternating voltage and its position is equivalent to the middle of the cable. According to this feature, the basic practice of lightning protection is to set a “voltage limiting protection circuit†for the input and output ports of the equipment, instead of “discharging the lightning current to the ground†and engage in “equipotential clamping†farce.
1) Video transmission line protection circuit principle Equivalent circuit: Lightning induced electromotive force Uo is formed on the shielding layer of the video cable and can be equivalent in the middle of the cable. It forms a loop through the matching resistor and the core wire at both ends.
The main point of the design of the video line protection circuit is to ensure that the voltage drop of the lightning induced electromotive force Uo at the two ends of the 75 ohm load is “limited†to below the maximum safe voltage of the circuit.
1 The influence of transmission attenuation and frequency distortion in the normal state should be within the allowable range;
2 When the lightning pulse arrives, the switch discharge element K rapidly turns on the “limit voltageâ€, and the filter network Z can absorb the lightning induced voltage to the maximum, ensuring that no excessive voltage appears on the 75 ohm load and plays a role in equipment safety protection;
3 Different video coaxial transmission methods, such as baseband transmission and RF transmission, different input and output signal amplitudes of different devices, and different signal bandwidths, the specific design requirements for the protection circuit are different; the most ideal lightning protection circuit protection circuit should be combined The "specific protection circuit" of the actual circuit design of the specific equipment is embedded together in the equipment circuit. Universal external line protector can also refer to different transmission methods to design different types of line protectors;
4 long-distance transmission cable, the line impedance of the inner and outer conductors is large, there is a significant attenuation of the lightning-induced electromotive force Uo, can make the Uo discharge constant increase, can make Uo in the line impedance and filter network buck increase, discharge Reduced tube current helps to ensure minimum lightning induced voltage on 75 ohm loads;
5 All front-end cameras are insulated from the ground. The "Lightning Protection Circuit" does not need to connect to earth. The host computer is used to safely discharge the static charge of the system by single point earth connection. The static ground potential of the whole system floats with the "ground potential" of the "host grounding point". ;
6 Engineering practice shows that the embedded “protection circuit†that combines product-specific circuit design is very effective. The analysis of the actual application product damage rate shows that the damage rate at the highest stage of products without protection circuit is sometimes as high as more than 10%, of which more than 95% are burned out of the ground potential loop. In addition, the quality problems of components and misoperation problems are also damaged. The product has added a protective circuit and the damage rate has dropped to below 2-3%. After analysis with the on-site technicians, there is no sufficient reason to indicate that the lightning is damaged, because the vast majority are not damaged by lightning. What is even more gratifying is that the intrusion damage rate of ground potential has also dropped significantly. This shows that there is also a strong ability to suppress ground potential intrusion. The simulated experimental data shows that there is enough safety suppression capability for the ground loop voltage up to 250VAC, but this does not mean that the system is allowed to have a ground loop;
7 video line protection circuit can be designed in the camera output circuit, designed in the DVR, matrix, Optical, video transmission equipment and other input circuits;
8 Lightning protection circuits are not connected to the earth and can effectively eliminate potential safety hazards of the ground potential loop.
3) Inductive lightning protection of the power system The basic design points of the security system of the security system are:
Confirming that the lightning protection and power supply system of Party A's buildings have passed the acceptance check, Party A's electricity use is safe.
The DC power source and chassis used in the security system equipment must be confirmed to be electrically isolated from the “zero and ground†wires of the power grid. The grid zero and ground wire design cannot be introduced into the security system.
Ensure that the system is "single point grounded". All cameras must be isolated from the ground. Grounded surge protectors or grounded surge protectors must not be used.
In the environment of the workshop motor group or the application environment of high-power variable-frequency motors (such as water supply, centralized air conditioning, etc.), the system power supply should use clean power (the power needs filtering).
　 Security power supply line lightning protection grounding lightning protection device will cause security risks to the security system can not be used, then the security power supply system to prevent induction thunder how to do it?
Single-point grounding effectively eliminates the "surge" intrusion of the grid ground potential. By adopting the "design points" described above, lightning-induced EMF damage can be minimized. The input of the security power supply belongs to the high voltage and low frequency components. The conventional transient lightning induced electromotive force belongs to the high frequency short pulse. The power supply filter can be used as the input protection and suppression. For the sake of more security, the output end can use the voltage limiting component protection circuit.
2-7 About Room Thunder Protection
The security room is usually located in the building of Party A. The lightning protection and power supply system has been used to provide qualified lightning protection. The security of 220VAC power belongs to safe use of electricity. According to the above “safety power supply basic design pointsâ€, it can be done.
The lightning protection of the security system is a systematic project. The "complete" design of the overall lightning protection of the entire system is eliminated, and a partial "lightning protection of the engine room" and the "equipotential connection" of the engine room cannot solve the lightning protection of the system. If the front camera pole is designed for lightning protection, or if the system uses a large number of grounding lightning protection devices, then even a good “airfield lightning protection†can hardly guarantee the safety of the system equipment.
System single point grounding—The engine room host chassis is securely connected to the earth. It is only used to discharge the static charge of the system. The engine room anti-inductive lightning protection also considers line protection enough.
2-8 Lightning Protection Concept of Security Engineering
The starting point and ultimate goal of the lightning protection design of the security system must be to ensure the safety of the system equipment and operation. This should be clearly defined in relevant industry standards. Anti-direct lightning protection should be protected by an independent lightning rod; anti-induced lightning protection is a good line protection. Pole lightning rods, grounding surge protectors, grounding surge protectors, if used for security system lightning protection is equal to planting a time bomb.
Establish the concept of "wide area information system electromagnetic compatibility design": "system single point grounding" is the first element of the lightning protection, anti-interference and system safety design of security systems and the basic principles of security design.
To do security and lightning protection should understand some of the lightning stroke probability knowledge, so as to reduce the blindness of lightning protection design and investment, in order to not be misled, in order to design a truly safe and effective lightning protection project. For example, a 3 or 5 meter high independent house or outdoor camera pole, the probability of lightning strike in the north is far less than 0.01 times / year, that is "a hundred years no chance" a lightning strike, it is necessary to do direct lightning protection? Someone told you to do it. Every outdoor pole needs to be done. Call you to buy a professional stainless steel pole lightning rod and make a professional grounding net (cage); a few hundred yuan outdoor 3, 5 meters high pole, according to the pole The design of lightning rods requires investment of up to several thousand to 1,200,000 yuan. Obviously this is a commercial purpose, allowing people to become "fat cows" and the result is a "self-destruction" system. Since the Three Gorges Project took into consideration the safety of life and property in several provinces, it was not designed for a hundred years. Is it necessary for a security pole to do so?
Conclusion:
At present, our security and lightning protection is still in an "awkward situation": do not understand the weak current system technology to do lightning protection, and do not understand the lightning protection technology of weak current system; the lightning protection project of the security system consists of security engineering companies that do not understand the lightning protection technology. Assume that while the lightning protection design of the security system and related standards are compiled and supplied by "professional lightning protection" companies that do not understand the weak current system technology, the role of the security engineering business is the "user" of lightning protection.
"Supplier" compiles relevant "standards" for "users", and stipulates how the lightning protection system should "should" and "have to" design, and what products "should" and "must" use; this situation has been confirmed by years of practice. This lightning protection design and the application of lightning protection products create a safety hazard for the monitoring system itself. What is more worth pondering is that in the industry-related standards and norms, there is no guarantee of the "system-wide security" of the basic terms, a single part of the development of "protection technology conditions", and the lack of a principle analysis, there will be no "safety "Hidden design" is responsible for accountability and responsible for the relevant provisions of the rectification; this is tantamount to giving the "security design" a green light and providing "protection umbrella." They can create a variety of pretexts for “safety hazard design†and shirk responsibility.
After all, lightning protection is a practice science, practice is the sole criterion for testing truth, and practice is the highest authority. Let history evaluate those hidden dangers and their "brilliant projects"! As a long-term and safe person, security personnel must take a good path to their own scientific development.
Lumanman's repairs are far away, and I will search for it.
The wide-area information system (WAIS) refers to a weak system that spans a wide area and has direct electrical connections between system devices and cannot achieve equipotential bonding between systems. The security system belongs to the "wide area information system".
Wide-area electromagnetic environment - Influencing the external electromagnetic environment of the wide-area information system, from the air to the underground, lightning in the air, various electromagnetic waves in the free space, the coupling of charged cables on the ground, and various types of underground potential Affects the operating quality and system safety of weak systems.
Wide area information system electromagnetic compatibility (EMC): It is assumed that the quality, safety, electromagnetic compatibility (EMC) and other problems of the various equipment (products) used in the system have been solved and are all qualified. Only the system security under the influence of the external electromagnetic environment is considered. Operational design issues. It is different from the commonly used equipment, products, and circuit board-level electromagnetic compatibility issues. Some people tried to explain the “wide-area electromagnetic compatibility of security systems†using the concepts of equipment and PCB electromagnetic compatibility.
The design of safe operation of the security system belongs to the electromagnetic compatibility (EMC) design problem of the wide area information system, including system anti-jamming design, anti-static design, direct lightning protection and inductive lightning design; the goal is to ensure the system's own safety and ensure the system operation quality.
The lightning protection design of the cameras “pole lightning rod†and “ground lightning protection device†analyzed in the previous analysis constitutes an artificial safety hazard, which is due to the lack of understanding of the earth electromagnetic environment and the lack of basic characteristics of the wide-area work of the security system. Awareness, this is the root cause of system design hidden dangers. Concern and study of system electromagnetic compatibility design is a major issue related to the safety of weak systems. It should be mentioned on the agenda of security projects.
The basic principle of 2-2 security system security design is "single point earthing"
1. Basic concept of "single point earthing" Single point earthing means that the host of the system is grounded at one point, and the remote camera and all equipment must be insulated from the earth; specifically, "single point earthing" means that there is a direct electrical connection. The "system" should make the electrical connection "centralization point" equipment (system host or subsystem host) do a ground. For example, optical cable transmission system: the front-end multi-channel optical transmitter is the host of the subsystem, and the casing “does†connect to the ground. The cameras connected to the optical transmitter cable should be insulated from the earth. This is the “direct electrical connection†system. "Single-point grounding" does not replace the back-end system host ground because there is an optical cable in the middle that isolates the electrical connection.
2. The "single point earthing" project requires the host to "single point earthing". All equipment at the far end of the system is suspended to the ground. The electrostatic charge generated by the system is discharged through the host earthing point, and the static potential is maintained with the earth to ensure the safety of operation. . The (ground) potential of the system after single-point earthing refers to the “ground potential†of the system with respect to the zero potential of the earth, ie, the potential of the system earthing point; the “Professional Lightning Protection†in the Security Forum once received the cable. Lightning induced electromotive force is described by "overvoltage" and "high potential", claiming that "cables at both ends can be clamped to equipotential at both ends with a grounded lightning protector at both ends." The high-frequency analysis shows that for the alternating induced electromotive force on the cable, even if the grounding resistance of the lightning arrester is 0 and the ground potential at both ends is equal, the “clamping voltage†of the lightning arrester at both ends is always “clamping voltage†Equal in size, opposite in polarity, where is there any potential equalization? In addition, the grounding "discharge circuit" includes the AC and DC impedance sum of the cable and the grounding wire as well as the grounding resistance. The so-called "effective discharge of the lightning current" is merely an illusion. Lightning induced electromotive force has nothing to do with the earth. There is no problem of discharge of current to the earth; "single point earthing" is used to discharge the static charge of the system, and there is no high requirement for grounding resistance, and no special grounding net is required; "single point "Grounding" and the traditional lightning rod grounding, power grid grounding, anti-surge protection device ground leakage current is essentially different from the requirements of the difference, using ordinary wire to connect the building steel, water pipes can be connected.
3. Analysis of the rationality of "single point earthing" 1 "Single point earthing" cuts off all ground loops and effectively blocks the path of "lightning ground potential" and "grid ground potential" invading the weak current system. This is lightning protection. , Anti-surge, anti-interference is the most effective basic technical means; multi-point grounding introduces ground potential interference and grid surge, the introduction of lightning counter-attack voltage, burning of security equipment and lightning protection equipment case has been more and more security engineering damage Case confirmed.
2 The "single point earthing" of the security system is not only not inconsistent with the anti-induction mine, but also the basic concept that the lightning protection design of the security system should follow and the basic conditions of the lightning protection design; because the anti-radar does not need, it cannot be connected through any part of the system. Earth discharge; Induction thunder only needs to pass the protection circuit to suppress the voltage value when the lightning induced electromotive force reaches the device port, ensuring that it is lower than the “highest safety voltage†of the device, and the protection circuit used does not need to connect to earth.
3 The system "single point earthing" makes the entire system float with the earth point equipotential. Artificially created multi-point grounding and attempted to achieve "equipotential bonding", this is theoretically and practically impossible for the "wide area information system."
4 Adhere to the "single point grounding" safety design principle, which can avoid being misled by "grounding and lightning protection", and can avoid the investment in the complex grounding system.
5 "Single point earthing" is the "touchstone and watershed" for testing and judging the security design and hidden danger design of the security system.
2-3 Questions about "Lightning protection of buildings and their power supply system" in security engineering
First of all, please consider a question: The security engineering company designed the security project for Party A. Party A was already working and using electricity in the building. Their buildings and power supply system passed the safety inspection and they all had a daily The detection and maintenance, how do you need to do when the security project to consider and design their buildings lightning protection, power distribution surge protection, engage in what three levels of protection? Even if the building and the power supply system of Party A are protected against lightning and unqualified surges, it is not a matter for the security engineering company to govern; the building is protected against direct lightning strikes and multi-stage lightning protection of the power supply is included in the relevant management documents of the security and lightning protection project. In the propaganda description of “Professional Lightning Protectionâ€, it occupies the main space, and the lightning protection design part of the real security system highlights two major potential safety hazards of “lightning rod lightning protection†and “lightning multi-point groundingâ€. It is a question worthy of our consideration.
Lightning protection of buildings and power supply systems is a basic condition that should be possessed before the construction of security systems. It should not belong to the jurisdiction of the security system design, or it would be superfluous. The vast majority of security engineering companies do not have the corresponding qualifications.
2-4 security system direct lightning protection design
First, all equipment of the security system should work within the effective protection scope of existing buildings' lightning protection and lightning protection systems and other independent lightning rods, and the transmission cables should be buried and routed as far as possible.
Second, outdoor isolated pole camera, if it is necessary to prevent direct lightning (prove the probability of lightning strike), should set up independent lightning rod protection pole camera, vertical pole and independent lightning rod distance should be greater than 4,5 meters (counterattack pole Distance), the top of the camera pole must not be installed lightning rod, the following will not be ground network, metal pole but also do a good job with the camera's high-level, all-weather insulation, in order to deal with the lightning rod when the "step voltage" through the pole Counter-attack cameras; the use of wood or cement insulation poles is more conducive to insulation, the camera bracket is best to use engineering plastic brackets to increase the level of insulation.
Third, there are people who propose to make the "camera poles a lightning rod and insulate the camera from the poles." This is also undesirable. Because the lightning strike voltage generated by the lightning arrester strikes the air at a distance of more than 30 to 40 centimeters, the rainy days can exceed a distance of more than 1 meter, and conventional insulation cannot be achieved.
A responsible manufacturer should take the initiative to correct its own "security hidden lightning protection project."
2-5 About Lightning Sensing
The description of lightning induction, "professional lightning protection" uses "thunder and lightning electromagnetic pulse", "lightning impulse wave", "lightning wave", "surge voltage", "surge current", "lightning overvoltage", "lightning over The words "current" and many other words have been described; but it is never clear what the "lightning sensor" on the cable is.
1) False Lightning Sensing Data The so-called "leakage voltage" of several tens of kilovolts on the monitoring system cable and the argument that the surge arrester "needs several thousand tens of kiloamperes of discharge capacity" should be derived from "professionalism." "Lightning protection" artificially created multi-point grounding, the introduction of ground potential and grid surge data, which is a false data with the lightning electromagnetic induction wind horse cattle unmatched. "Surge" and "earth surge protector" are originally terms and concepts of the power grid system. Now "professional lightning protection" confuses it with lightning induction. Moreover, it simply puts the "grounded surge protection" of the power system. The device is used directly as a "lightning arrester" for weak systems.
There is also a type of lightning-sensing calculation formula given by “professional lightning protectionâ€. The calculated data is also a “lightning shock wave†of several tens of kilovolts. This is the data that is put together in the absence of a basic understanding of the ground potential, and brings together the data. The lightning-sensing "hat" is to search for "reasonable arguments" for the sales of "earth lightning protectors" and "grounded surge protectors." Reflects these people have no basic understanding of the nature of lightning induction.
2) Qualitative Analysis of Lightning Electromagnetic Radiation and Electromagnetic Induction The process of thundercloud discharge is mainly the energy consumption of thermal energy, light energy, mechanical energy, and acoustic energy. The “electromagnetic radiation energy†of lightning current is only a small part of the total energy of lightning. Lightning induction refers only to the "electromagnetic radiation" energy actually received by a certain cable. There is a certain "electromagnetic coupling relationship" between "radiation" and "reception". This "electromagnetic coupling relationship" is affected and limited by many factors:
1. Spectrum effect: Only high frequency can form effective radiation, while high frequency radiation is just the higher harmonic component of lightning current, which is not the main energy spectrum range of lightning;
2. Antenna efficiency and electromagnetic radiation of lightning currents: Electromagnetic radiation is closely related to the size and wavelength (frequency) of the antenna structure. The antenna efficiency for strict design and precision machining is around 50 to 70%. Different harmonic frequencies, different lightning rod heights, and equivalent "antenna lengths" with different inter-cloud discharge currents are unlikely to have a good matching relationship with harmonic frequencies, so the actual lightning current electromagnetic radiation is of low efficiency and super Inefficient electromagnetic radiation; the same reason, "receiver antenna" also has the best frequency and size matching problem, the best match to achieve high efficiency reception, apparently monitoring cables as lightning radiation "antenna" is also inefficient, ultra-low Efficient "receiving antenna".
3. Directional influence: The radiation direction of the lightning current electromagnetic radiation energy is similar to the "omnidirectional radiation" of the spherical wave free space, corresponding to the "stereoscopic" radiant energy of a cable area and direction, but only in the total radiation power. A small amount, and the intensity of the radiated power is also inversely proportional to the square of the distance. In other words, the directionality of lightning electromagnetic radiation and cable reception basically has the characteristics of "omnidirectional low gain".
4. Polarization effects: The electromagnetic waves emitted by the radiation source have a certain “polarization direction†(the direction of change of the electric field strength vector E), and the receiving cable also has a problem of the direction of polarization received. For example, the discharge current of the lightning rod body is mainly "vertical", and its radiation field polarization direction (E) is also a "vertically polarized wave". Only vertical poles and ground lines can effectively receive this "vertical polarization". "Waves", while horizontal cables, basically do not receive such "vertically polarized wave" energy. This is the effect of polarization; both the cable reception and the direction of polarization of the lightning radiation are random factors, actually "polarization". Attenuation is very large, and the energy actually received is also a tiny amount.
Taking into account the above-mentioned various limiting factors, we can draw the following concept: The energy of lightning electromagnetic radiation to free space is only a fraction of the total energy of the lightning discharge, due to insurmountable factors such as frequency spectrum, radiation efficiency, directionality, and polarization. Impact, the actual lightning energy received by the cable and the total energy of the lightning is more "super tiny"; the so-called tens of kilovolts and tens of thousands of amps of data on the cable is against the laws of science.
"Thunder and lightning electromagnetic pulse", "lightning shock wave", "lightning wave", "surge voltage", "surge current", "lightning overvoltage", "lightning overcurrent" and many other description vocabulary are also some whimsical languages. Is not a classic technical language.
In accordance with the calculation formula given in a “Professional Lightning Protection†article, the author calculated a lightning surge “overvoltage, overcurrent†and “total energy†of a cable in a office building for a discharge lightning rod, which actually exceeded the lightning rod discharge. The total energy is also many times larger; the "professional formula" given is actually not related to any of the above-mentioned limitations, and is typical of the "followed by amateurs."
A lot of life practice tells us that during thunderstorms watching TV at home can see occasional flashes of lightning interference, and can also hear thunder and cough sounds from telephones and radios. The so-called cases of lightning-burning of household appliances, burning of security equipment, and melting of video cables have caused most of the cases where lightning strikes have caused grid faults and caused grid power surges to be destroyed.
In the early years, the author had seen several narrow-beam radars with hundreds of kilowatts and megawatts of power levels. When the wave speed came, strong electromagnetic fields could turn on indoor fluorescent lights or brighten high-pressure xenon lamps but never saw them on the head. The thunder and lightning electromagnetic field turns on the fluorescent light and the street light. If the lightning sensor really has dozens or hundreds of kilovolts, thousands of amperes, and several tens of kiloamps of amperes, the fluorescent lamps will not only light, but they will be destroyed in batches.
The conclusion is that the actual "thunderstorm sensing" energy received on the cable is not as large as the "tens of kilovolts" and "tens of thousands of amps" described by "professional lightning protection." It is only one of the total energy of lightning. The tiny amount, in essence, is not a unipolar charge, but an alternating induced electromotive force, which can be equivalent to the middle of the cable and has nothing to do with the earth. The "surge protectors" used in power grid systems actually require "tens of kilovolts" and "tens of kiloamps" of protective performance. This is used to cope with power grid surges and requires grounding to discharge surge currents. , Lightning induction and the grid "surge" compared to far less than a panic. The use of a grounded "surge protector" as a "lightning arrester" for a lightning-sensitive lightning system can only be a "modern technology joke".
2-6 Inductive Lightning Protection Design
The previous analysis of lightning induction has made it clear that the lightning induced electromotive force of the cable has nothing to do with the earth. It is a kind of high-frequency alternating voltage and its position is equivalent to the middle of the cable. According to this feature, the basic practice of lightning protection is to set a “voltage limiting protection circuit†for the input and output ports of the equipment, instead of “discharging the lightning current to the ground†and engage in “equipotential clamping†farce.
1) Video transmission line protection circuit principle Equivalent circuit: Lightning induced electromotive force Uo is formed on the shielding layer of the video cable and can be equivalent in the middle of the cable. It forms a loop through the matching resistor and the core wire at both ends.
The main point of the design of the video line protection circuit is to ensure that the voltage drop of the lightning induced electromotive force Uo at the two ends of the 75 ohm load is “limited†to below the maximum safe voltage of the circuit.
1 The influence of transmission attenuation and frequency distortion in the normal state should be within the allowable range;
2 When the lightning pulse arrives, the switch discharge element K rapidly turns on the “limit voltageâ€, and the filter network Z can absorb the lightning induced voltage to the maximum, ensuring that no excessive voltage appears on the 75 ohm load and plays a role in equipment safety protection;
3 Different video coaxial transmission methods, such as baseband transmission and RF transmission, different input and output signal amplitudes of different devices, and different signal bandwidths, the specific design requirements for the protection circuit are different; the most ideal lightning protection circuit protection circuit should be combined The "specific protection circuit" of the actual circuit design of the specific equipment is embedded together in the equipment circuit. Universal external line protector can also refer to different transmission methods to design different types of line protectors;
4 long-distance transmission cable, the line impedance of the inner and outer conductors is large, there is a significant attenuation of the lightning-induced electromotive force Uo, can make the Uo discharge constant increase, can make Uo in the line impedance and filter network buck increase, discharge Reduced tube current helps to ensure minimum lightning induced voltage on 75 ohm loads;
5 All front-end cameras are insulated from the ground. The "Lightning Protection Circuit" does not need to connect to earth. The host computer is used to safely discharge the static charge of the system by single point earth connection. The static ground potential of the whole system floats with the "ground potential" of the "host grounding point". ;
6 Engineering practice shows that the embedded “protection circuit†that combines product-specific circuit design is very effective. The analysis of the actual application product damage rate shows that the damage rate at the highest stage of products without protection circuit is sometimes as high as more than 10%, of which more than 95% are burned out of the ground potential loop. In addition, the quality problems of components and misoperation problems are also damaged. The product has added a protective circuit and the damage rate has dropped to below 2-3%. After analysis with the on-site technicians, there is no sufficient reason to indicate that the lightning is damaged, because the vast majority are not damaged by lightning. What is even more gratifying is that the intrusion damage rate of ground potential has also dropped significantly. This shows that there is also a strong ability to suppress ground potential intrusion. The simulated experimental data shows that there is enough safety suppression capability for the ground loop voltage up to 250VAC, but this does not mean that the system is allowed to have a ground loop;
7 video line protection circuit can be designed in the camera output circuit, designed in the DVR, matrix, Optical, video transmission equipment and other input circuits;
8 Lightning protection circuits are not connected to the earth and can effectively eliminate potential safety hazards of the ground potential loop.
3) Inductive lightning protection of the power system The basic design points of the security system of the security system are:
Confirming that the lightning protection and power supply system of Party A's buildings have passed the acceptance check, Party A's electricity use is safe.
The DC power source and chassis used in the security system equipment must be confirmed to be electrically isolated from the “zero and ground†wires of the power grid. The grid zero and ground wire design cannot be introduced into the security system.
Ensure that the system is "single point grounded". All cameras must be isolated from the ground. Grounded surge protectors or grounded surge protectors must not be used.
In the environment of the workshop motor group or the application environment of high-power variable-frequency motors (such as water supply, centralized air conditioning, etc.), the system power supply should use clean power (the power needs filtering).
　 Security power supply line lightning protection grounding lightning protection device will cause security risks to the security system can not be used, then the security power supply system to prevent induction thunder how to do it?
Single-point grounding effectively eliminates the "surge" intrusion of the grid ground potential. By adopting the "design points" described above, lightning-induced EMF damage can be minimized. The input of the security power supply belongs to the high voltage and low frequency components. The conventional transient lightning induced electromotive force belongs to the high frequency short pulse. The power supply filter can be used as the input protection and suppression. For the sake of more security, the output end can use the voltage limiting component protection circuit.
2-7 About Room Thunder Protection
The security room is usually located in the building of Party A. The lightning protection and power supply system has been used to provide qualified lightning protection. The security of 220VAC power belongs to safe use of electricity. According to the above “safety power supply basic design pointsâ€, it can be done.
The lightning protection of the security system is a systematic project. The "complete" design of the overall lightning protection of the entire system is eliminated, and a partial "lightning protection of the engine room" and the "equipotential connection" of the engine room cannot solve the lightning protection of the system. If the front camera pole is designed for lightning protection, or if the system uses a large number of grounding lightning protection devices, then even a good “airfield lightning protection†can hardly guarantee the safety of the system equipment.
System single point grounding—The engine room host chassis is securely connected to the earth. It is only used to discharge the static charge of the system. The engine room anti-inductive lightning protection also considers line protection enough.
2-8 Lightning Protection Concept of Security Engineering
The starting point and ultimate goal of the lightning protection design of the security system must be to ensure the safety of the system equipment and operation. This should be clearly defined in relevant industry standards. Anti-direct lightning protection should be protected by an independent lightning rod; anti-induced lightning protection is a good line protection. Pole lightning rods, grounding surge protectors, grounding surge protectors, if used for security system lightning protection is equal to planting a time bomb.
Establish the concept of "wide area information system electromagnetic compatibility design": "system single point grounding" is the first element of the lightning protection, anti-interference and system safety design of security systems and the basic principles of security design.
To do security and lightning protection should understand some of the lightning stroke probability knowledge, so as to reduce the blindness of lightning protection design and investment, in order to not be misled, in order to design a truly safe and effective lightning protection project. For example, a 3 or 5 meter high independent house or outdoor camera pole, the probability of lightning strike in the north is far less than 0.01 times / year, that is "a hundred years no chance" a lightning strike, it is necessary to do direct lightning protection? Someone told you to do it. Every outdoor pole needs to be done. Call you to buy a professional stainless steel pole lightning rod and make a professional grounding net (cage); a few hundred yuan outdoor 3, 5 meters high pole, according to the pole The design of lightning rods requires investment of up to several thousand to 1,200,000 yuan. Obviously this is a commercial purpose, allowing people to become "fat cows" and the result is a "self-destruction" system. Since the Three Gorges Project took into consideration the safety of life and property in several provinces, it was not designed for a hundred years. Is it necessary for a security pole to do so?
Conclusion:
At present, our security and lightning protection is still in an "awkward situation": do not understand the weak current system technology to do lightning protection, and do not understand the lightning protection technology of weak current system; the lightning protection project of the security system consists of security engineering companies that do not understand the lightning protection technology. Assume that while the lightning protection design of the security system and related standards are compiled and supplied by "professional lightning protection" companies that do not understand the weak current system technology, the role of the security engineering business is the "user" of lightning protection.
"Supplier" compiles relevant "standards" for "users", and stipulates how the lightning protection system should "should" and "have to" design, and what products "should" and "must" use; this situation has been confirmed by years of practice. This lightning protection design and the application of lightning protection products create a safety hazard for the monitoring system itself. What is more worth pondering is that in the industry-related standards and norms, there is no guarantee of the "system-wide security" of the basic terms, a single part of the development of "protection technology conditions", and the lack of a principle analysis, there will be no "safety "Hidden design" is responsible for accountability and responsible for the relevant provisions of the rectification; this is tantamount to giving the "security design" a green light and providing "protection umbrella." They can create a variety of pretexts for “safety hazard design†and shirk responsibility.
After all, lightning protection is a practice science, practice is the sole criterion for testing truth, and practice is the highest authority. Let history evaluate those hidden dangers and their "brilliant projects"! As a long-term and safe person, security personnel must take a good path to their own scientific development.
Lumanman's repairs are far away, and I will search for it.
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