Personal summary of lightning protection work

Summary of five years of lightning protection experience in the customs system

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Summary of five years of lightning protection experience in the customs system Sun Haiyuan In 1997, the Shantou Customs Building suffered serious damage due to a lightning strike. Customs leaders decided to carry out comprehensive lightning protection renovation, thus kicking off the customs system lightning protection project. It has been five years since then. Now, most of the more than 30 department-level customs offices across the country have undergone lightning protection renovations for the headquarters building and some branch offices. The actual results over the past five years show that after the transformation of the system's lightning protection project, the probability of equipment being struck by lightning has been greatly reduced. Many customs have basically no lightning strikes. This is gratifying for everyone. Five years of hard work has not been in vain. During the development of customs lightning protection work, leaders of the General Administration of Customs and leaders of various customs authorities attached great importance to and provided strong support. The General Administration allocates special funds for lightning protection every year, organizes and compiles the "Customs System Lightning Protection Technical Guidelines", and frequently holds experience exchange meetings within the system. Leaders of various customs offices have personally participated in the entire process of lightning protection projects from bidding to project completion, and have assigned dedicated personnel to be responsible for the lightning protection work of their respective customs offices. This ensures that the lightning protection work of the customs system is guaranteed in terms of management, technology and funding. Since the company won the bidding for the Shantou Customs lightning protection project, it also completed many customs lightning protection projects. Therefore, I went from Shanghai Customs in the east to Lhasa Customs in the west, and from Dalian Customs in the north to Haikou Customs in the south. I have visited most of the more than 30 department-level customs offices across the country and had extensive exchanges with engineers from the technical departments of the customs. Through exchanges, we improved our understanding and increased our friendship. Later, we all became friends. I feel that now they have a strong understanding of lightning protection. Already have quite a deep understanding of it. Looking back now, their dedication and knowledge-seeking spirit are worth learning and learning from. After several years of work accumulation, I have a deeper understanding of lightning protection in the customs system. Now I will write it down and discuss it with my friends. The system lightning protection (or comprehensive lightning protection) project should include two parts: direct lightning strike and induced lightning strike. We are mainly talking about the protection of communication equipment in the customs system. Is it irrelevant to the protection of direct lightning strike? In fact, it is very relevant. Let’s not talk about the losses caused by lightning strikes that damage buildings. Without a direct lightning strike protection system, according to IEC1312 estimates, almost all lightning currents flow through conductor-type lines (power lines, signal lines) entering and exiting the building. etc.), such lightning current is not considered an induced lightning strike. Therefore, direct lightning strike protection is a prerequisite for induced lightning strike protection. At Shantou Customs, Gongbei Customs, Kunming Customs, etc., after detecting that the direct lightning protection system does not meet the requirements, we first improve it and then implement inductive lightning protection. Direct lightning strike protection already has the national standard GB50057-94 "Design Code for Lightning Protection of Buildings", so it is very clear to do it. Moreover, most of the direct lightning strike protection systems of the customs system have been completed in the civil engineering of the building. Therefore, the customs The lightning protection of the system is mainly induction lightning protection. We divide customs induction lightning protection into three categories: customs building (main customs and branch customs), inspection site, and weighbridge. The following are the solutions we have adopted at customs offices in various places. The results are still very good. We have now sorted them out for review and discussion with everyone. 1. Customs Building The customs building here refers to the office buildings of the general customs and branch customs (offices). Some customs office buildings are divided into several buildings. Now we use the building where the main computer room is located as an example to design a complete induction lightning strike protection plan. The schematic diagram is shown in Figure 1. 1. Power line protection 1). General power distribution: A. Buildings with self-contained transformers or buildings without transformers but low-voltage incoming cables with steel pipes are buried underground into the building (the length of the buried ground is not less than 20 meters) and installed at the output end of the main power distribution A three-phase power supply SPD with a nominal discharge current ≥20KA (8/20?s) and a residual voltage ≤700V. B. If the low-voltage line directly enters the building overhead, a three-phase power SPD with a nominal discharge current ≥100KA (8/20?s) and a residual voltage ≤700V should be installed at the main power distribution output end, or a nominal A combination of a three-phase power supply SPD with a discharge current ≥ 25KA (10/350?s) and a three-phase power supply SPD with a nominal discharge current ≥ 20KA (8/20?s) and residual voltage ≤ 700V. Explanation: (1) What is the maximum lightning current on distribution lines? There is no national or international standard that clearly gives a fixed value. Most of the current standards provide some estimated values. The above mentioned and the following contents are all estimated with reference to IEC1312, BG50057-94 and some ministerial standards. Five years of practical application have proven that such a configuration is very reasonable. (2) What should be the residual voltage of the power supply SPD to be qualified? In other words, there is no clear conclusion yet about the overvoltage withstand capability of electrical equipment. The current SPD design generally has two basis, one is: The insulation voltage of the equipment basically has standards. The levels are: 1500V, 2500V, 4000V, 6000V. That is to say, the residual voltage of the SPD is lower than 1500V to meet the insulation voltage requirements of the equipment. Therefore, many SPDs will now have residual voltage. The voltage value is designed within 1500V.

However, this design can only solve the insulation voltage. If the voltage between the phase line and the neutral line is 1500V, the equipment may have problems because the voltage directly enters the rectifier and subsequent circuits. The 1500V voltage may damage these circuits, which is not bad. This depends on the overvoltage withstand capability of the equipment itself. To ensure safety, an SPD must have phase-to-neutral protection. (3) There is a high-voltage lightning protection device on the high-voltage side of the transformer. Therefore, the low-voltage side can be regarded as only induced lightning. If low-voltage lines enter directly overhead, direct lightning strikes on the lines must be considered. 2) Install a three-phase power supply SPD with a nominal discharge current ≥20KA (8/20?s) and a residual voltage ≤700V at the output end of the main power distribution on each floor of the building. Note: There are two main purposes for installing floor SPD: one is to serve as a double protection for distribution lines together with the SPD of the building's main power; the other is to protect the overvoltage induced on the building lines and various switch overvoltages. 3) Install a three-phase power supply SPD with a nominal discharge current ≥20KA (8/20?s) and a residual voltage ≤700V at the output end of the main power distribution in each main computer room and customs declaration hall (including customs brokers) in the building. . Explanation: (1) Some people may ask if this is what is commonly called level three lightning protection? no. The main difference between the two is: the design idea of ??multi-stage SPD is mainly based on the above-mentioned insulation voltage levels. Generally, the first-stage residual voltage is designed to be 4000V, the second-stage residual voltage is designed to be 2500V, and the third-stage is designed to be 1000V. The residual voltage of the SPD used in the current solution is 700V, which means that if the main power distribution SPD or the floor SPD fails, the residual voltage on the power line is still 700V, and the equipment is safe. If there is a problem with one of the above three levels, the equipment may be at risk of lightning strikes. The actual results also prove that the current solution is highly safe. (2) Should SPD be installed on every floor? Is it okay to install it in compartments or only on floors where important equipment is installed? Of course this is also possible, but the effect will be less effective. 2. Signal line protection The main communication lines external to the customs building, within the building and between buildings are: optical fiber, DDN, X.25, intermediate frame relay, dial-up backup, trunk line, ISDN, microwave feeder, satellite feeder, wireless intercom feeder , cable TV feeders, network lines (Category 5 lines, etc.), monitoring lines, internal telephones, etc. Corresponding protective measures should be taken for these lines according to specific conditions. See Figure 1. 1) Optical fiber: Optical fiber does not require special lightning protection equipment, as long as the reinforced steel wire of the optical fiber is grounded. 2), DDN, Signal SPD with voltage ≤100V. 3). Dial backup: Install signal SPD with nominal discharge current ≥5KA (8/20?s)/line, 150V≤residual voltage≤250V. 4). Trunk line: Install signal SPD with nominal discharge current ≥5KA (8/20?s)/line and 50V≤residual voltage≤200V. 5), ISDN: Install signal SPD with nominal discharge current ≥5KA (8/20?s)/line, 30V≤residual voltage≤100V. 6) Microwave feeder: The microwave frequency commonly used by customs is up to 2.4GHZ and the impedance is 50?. Therefore, the installation frequency and impedance meet this requirement. The nominal discharge current is ≥10KA (8/20?s)/line, and the residual voltage is ≤ 100V feeder SPD. 7) Satellite feeder: There is a 1GHZ intermediate frequency signal on the satellite feeder, and most of the impedance is 75?. Therefore, the installation frequency and impedance meet this requirement. The nominal discharge current is ≥10KA (8/20?s)/line, and the remaining Feeder SPD with voltage ≤100V. 8) Wireless intercom: The customs wireless intercom generally operates at several hundred MHZ and has an impedance of 50?. Therefore, the installation frequency and impedance meet this requirement. The nominal discharge current is ≥10KA (8/20?s)/line, and the residual Feeder SPD with voltage ≤100V. 9) Cable TV: my country’s TV signal frequency is within 1GHZ, and the impedance is 75?. Therefore, the installation frequency and impedance meet this requirement. The nominal discharge current is ≥10KA (8/20?s)/line, and the residual voltage is ≤100V. feeder SPD. 10) Network cable: The network cable of the customs system is mainly Category 5 cable or Category 5e cable. The generally installed transmission rate is 100Mbps, the nominal discharge current is ≥3KA (8/20?s)/line, and the residual voltage is ≤20V dedicated signal. SPD. 11) Monitoring line: Generally, it is a CCTV closed-circuit monitoring line, with an installation operating frequency of about 20MHZ, a nominal discharge current ≥3KA (8/20?s)/line, and a residual voltage ≤20V dedicated signal SPD. 12) Internal telephone: There are many internal telephones, and the nominal discharge current is ≥10KA (8/20?s)/line, and the residual voltage is ≤250V special signal SPD. Note: (1) The signal lines entering and exiting the building must be equipped with SPD. If the signal line in the building is unshielded and the line length is between 50m and 100m, then one end of the signal line should be equipped with SPD. If the signal line is longer than 100m, both ends of the signal line should be equipped with SPD. For customs, such lines are mainly network cables. The nominal discharge current of the SPD on the signal line in the building is ≥500A (8/20?s)/line. (2) The residual voltage of the signal SPD should be greater than the maximum value of the normal operating voltage on the line, and at the same time less than the maximum withstand voltage value of the equipment connected to the signal line.

3. Electromagnetic field Everyone attaches great importance to the power lines and signal lines mentioned above, and they know how to deal with them. However, sometimes after doing both of the above, the equipment is still damaged by lightning strikes. The main reason is the space caused by lightning strikes. For the customs office building, the main sources of electromagnetic fields in the building are: after the building itself is struck by lightning, the lightning electromagnetic field generated in the building by the lightning current on the down conductor, lightning strikes occur near the building, and the lightning electromagnetic field is attenuated by the building After subtracting the residual electromagnetic field in the building and the electromagnetic field generated in the building after lightning is introduced into various wires (including ground wires). The main protective measures against lightning electromagnetic fields are as follows: See Figure 3. 1) Shielding: We recommend that the lines in the building be shielded in three ways: the first is the power cord, the second is the signal line entering from the outside (such as telephone lines, etc.), and the third is the network cable in the building. These three types of wires should be shielded in metal trunkings. At least the vertical parts should be shielded separately. For horizontal layout on the second floor, these three types of wires should also be separated or shielded separately. This is actually a complementary interference between the three lines, which does not interfere with the equipment in the building and is not interfered by the electromagnetic field in the building. 2) Place the equipment correctly: The equipment should be placed in the center of the room, where the lightning electromagnetic field is the weakest. It should not be too close to the window, not too close to the building pillars, not too close to the main ground wire and outside lines without SPD installed. Generally, All should be above 1 meter. 3) The computer room with centralized equipment should be located below the top four floors, preferably around the second floor. The computer room should be shielded, the floor should be anti-static, walls and ceilings should be shielded, and equipment should be placed in special cabinets. Note: (1) Generally speaking, a magnetic field of 0.3GS can seriously interfere with the operation of the equipment, and a magnetic field of 2.4GS can cause permanent damage to the equipment. In fact, this phenomenon is very common in our daily life. When you are on a wired phone call, your mobile phone rings. The noise you hear on the wired phone call is caused by electromagnetic interference. (2) There are formulas to calculate the lightning electromagnetic field in the building. The following are two typical calculations. A. A 10-story building is equipped with 10 down conductors. For example, when the current at the lightning strike point on the roof is 10KA (10/350?s), the safety distance on the ninth floor (top two floors) (2.4 GS), is 1.67 meters, which means that the equipment must be at least 1.67 meters away from the down conductor to be safe. Therefore, after considering various factors in some literature, it is proposed that the equipment should be more than 1 meter away from the down conductor. B. It is also the 10-story building above. The steel shielding grid of the building is 2m*2m. A 100KA lightning strike occurred 100m away from the building. The safety distance in the LPZ1 zone inside the building is 2m. In other words, the equipment in the building should be at least 2m away from the exterior wall to be safe. Therefore, some literature, after considering various factors, proposes that the equipment should be at least 1 meter away from the exterior wall, especially the window. It is dangerous for someone to place their equipment on a windowsill. (3) If the building lines are not shielded and the lines are long, SPD must be installed to be safe. 4. Grounding There are two main problems in grounding. One is the size of the grounding resistance, and the other is the grounding method. These two issues have been debated for more than two years and are now basically consistent. 1) Ground resistance size: According to GB50174-93 "Electronic Computer Room Design Code", the ground resistance of the customs computer room can be set as ≤ 4?. In fact, the lightning protection ground resistance in GB50057-94 only requires ≤ 10?, but some equipment The working grounding requirement is ≤1Ω. Therefore, some customs stipulate that the grounding resistance requirement is ≤1Ω. 2) Grounding method: The customs adopts a special grounding method, that is, connecting all ground networks, and each floor is connected to the foundation of the building nearby. If it is an old building, the main ground wire should be above 16mm2. 3) Equipotential connection: The equipment room where the equipment is concentrated should be equipped with a grounding busbar, and all equipment shells, SPDs, and metal parts that are not charged during normal operation should be connected to the grounding busbar nearby. Note: (1) Of course, the smaller the ground resistance of the ground network, the better. However, the geological conditions in some places are very poor. It is very difficult to reduce the ground resistance to less than 1°, and the cost will be very high. Therefore, the national standard sets the lightning protection ground at ≤10?. (2) Domestic and foreign academic circles have debated for many years whether dedicated grounding or independent grounding. With the advancement of science and technology, a common understanding has been basically reached and the dedicated grounding method is adopted. 2. Inspection site The customs inspection site here refers to the on-site office space of the customs located at docks, airports and other places. There is usually a small computer room here. Since it is an office site, it is important to ensure the normal working of the equipment. It is also the focus of lightning protection. The schematic diagram is shown in Figure 2. Inspection sites are generally several offices in one building. Some inspection sites are divided into two or more office locations (some share the same building with other units). We usually consider the following. 1. Power line protection 1). General power distribution: A. A building with its own transformer or a building without a transformer but with low-voltage incoming cables covered with steel pipes buried underground into the building (the length of the buried ground is not less than 20 meters) at the output end of the main power distribution Install a three-phase power supply SPD with a nominal discharge current ≥20KA (8/20?s) and a residual voltage ≤700V. B. If low-voltage lines directly enter the building overhead, a three-phase power supply SPD with a nominal discharge current ≥100KA (8/20?s) and a residual voltage ≤700V should be installed at the main power distribution output end, or a nominal A combination of a three-phase power supply SPD with a discharge current ≥ 25KA (10/350?s) and a three-phase power supply SPD with a nominal discharge current ≥ 20KA (8/20?s) and residual voltage ≤ 700V.

2) Install a single-phase power supply SPD with a nominal discharge current ≥20KA (8/20?s) and a residual voltage ≤700V in the computer room and each office. 2. The signal line protection inspection site generally only has one DDN and one dial-up backup communication line. A signal SPD with nominal discharge current ≥5KA (8/20?s)/line and 30V ≤ residual voltage ≤ 100V is installed on the DDN line. Install a signal SPD with nominal discharge current ≥5KA (8/20?s)/line, 150V≤residual voltage≤250V on the dial-up backup line. Some customs inspection sites are close to the customs office building, and Category 5 lines communicate directly. At this time, each line should be installed with a transmission rate of 100Mbps, a nominal discharge current ≥3KA (8/20?s)/line, and a residual voltage ≤ 20V dedicated signal SPD. 3. The electromagnetic field caused by lightning strikes in the electromagnetic field inspection field mainly considers various external incoming wires such as down conductors, main ground wires, and nearby lightning strikes. The main measures are: 1) Correct wiring: strong and weak wires should be separated by more than 1 meter, and weak signal wires should preferably be covered with shielding tubes. 2) Place the equipment correctly: The equipment should be placed in the center of the room, where the lightning electromagnetic field is the weakest. It should not be too close to the window, not too close to the building pillars, not too close to the main ground wire and outside lines without SPD installed. Generally, All should be above 1 meter. 4. The grounding should be done in a special grounding method, and the grounding resistance requirement should be ≤4?. 3. The lightning protection of the weighbridge customs system is mainly the lightning protection of the equipment in the weighbridge room. According to its structural characteristics, we make the following considerations. The schematic diagram is shown in Figure 3. 1. Power line protection. The power distribution of the weighbridge room comes from the customs main power distribution. Generally speaking, the main power distribution has been protected. Therefore, a nominal power meter is installed at the output end of the main power distribution of the weighbridge room. Single-phase power supply SPD with discharge current ≥20KA (8/20?s) and residual voltage ≤700V is sufficient. 2. Signal line protection The signal lines entering and exiting the weighbridge room are mainly: data lines (optical fibers or Category 5 lines) for communication with the customs network, and sampling lines for measuring equipment. 1) Optical fiber: No need to install SPD, but direct lightning protection must be provided during layout. 2) Category 5 lines or Category 5e lines: the installation transmission rate is 100Mbps, the nominal discharge current is ≥3KA (8/20?s)/line, and the residual voltage is ≤20V. Special signal SPD. 3) Sampling lines: The sampling lines from the floor scale to the floor scale room are all covered with steel pipes and buried in the room. At least they use shielded wires and the distance between the lines is not long. Therefore, there is no need to install SPD on these lines. 3. The equipment in the electromagnetic field scale room should be placed as far away from the window as possible and 1 meter away from the wall. These measures are simple but effective. 4. When installing the floor scale, a ground grid is made, and the ground resistance is generally within ≤4?. The equipment inside the floor scale can only be used here. The customs system’s lightning protection mainly considers the above aspects. In recent years, the customs has introduced an H986 system. This system should also be the focus of lightning protection. Since there has not been good communication with the designer of H986, the design of H986 There is no perfect solution for protection yet. This is the next step. In the lightning protection work of the customs system, satisfactory results have been achieved, but there are also some regrets, mainly as follows: 1. The customs card reader has been damaged every time lightning strikes, but one has not yet been found. Effective solutions to protect it. Analysis of the main reasons: 1) The card reader is made of plastic shell and has no electromagnetic shielding function, but it is generally placed in windows and other places with strong electromagnetic fields. Therefore, it is more likely to be damaged by electromagnetic fields. 2) The card reader used by the customs is a special equipment for the customs. We do not understand it. Moreover, it is not clear which part of the circuit of the damaged card reader is damaged. Therefore, we cannot accurately find out the cause. 3) All the connections to the card reader are indoors, and in many cases there is nothing wrong with the computer connected to it, but it is just broken, which makes it difficult to analyze. 2. The photoelectric converter is also damaged in lightning strikes. This is difficult to understand because: 1) In most cases, there is no problem with the UPS, etc. This means that the lightning does not come from the power line. 2) Optical fibers do not conduct lightning. 3) The photoelectric converter and data switch are in the same cabinet, and the signal line does not leave the cabinet. At this time, the data switch connected to the photoelectric converter is not broken, only the photoelectric converter is broken. 3. In some places, the equipment may be damaged by lightning strikes, but no traces can be found on site. Lightning protection is a systematic project. Weili people will spare no effort to continuously explore in the field of lightning protection, seek more scientific and safer lightning protection solutions, and contribute to the safety of customs system equipment.