0 Introduction With the development of science and technology, the application of advanced electronic equipment is becoming increasingly widespread: electronic medical diagnostic systems, communication systems, industrial automation integrated control systems, computer networks, etc. The operating voltage of these increasingly powerful and sensitive electronic devices is constantly decreasing, thus greatly increasing the possibility of damage from transient overvoltages—especially transient overvoltages caused by lightning. [b]1 Transient Overvoltage and Lightning[/b] 1.1 Transient Overvoltage Transient overvoltage refers to the spike voltage generated within microseconds to nanoseconds, as shown in Figure 1: [img=274,147]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jxdl/2000-3/23-1.jpg[/img] This spike voltage is different from the so-called overvoltage in general power supplies, because general power supply overvoltages may last for several seconds or more, and the overvoltage amplitude is relatively small. However, the amplitude of this spike voltage can sometimes be very high, and it can occur in both power supply systems and signal systems. Transient overvoltage phenomena are related to both the natural world and the operation of electrical systems. Natural phenomena such as lightning, auroras, corona discharge, static electricity, radiation, and ionization can all lead to transient overvoltages. Among various types of transient phenomena, lightning and switching impulses are the most common causes of low-voltage system accidents. When transient overvoltages enter low-voltage electronic systems, they can cause malfunctions or damage to electronic circuits. It is estimated that on average, half of the malfunctions of electronic equipment are caused by transient overvoltages, resulting in incalculable losses. Therefore, in today's electronic age, lightning and transient overvoltages have become a major public nuisance, and the losses they cause can be divided into four levels: Each impulse damages electronic equipment components, shortening their service life; multiple impulses lead to equipment damage, and replacement and maintenance require manpower and resources; and equipment failures cause business interruptions, resulting in various losses. For example, after a GSM base station in a telecommunications company is struck by lightning, mobile call revenue within the coverage area of ​​that base station decreases; the sudden interruption of services causes incalculable indirect losses to reputation, such as users being unable to make calls and complaints about the service quality of the telecommunications department (inability to guarantee smooth communication). 1.2 Transient overvoltage caused by switching: When current flows on a conductor, a magnetic field is generated to store energy. The larger the current and the longer the conductor, the more energy is stored. Therefore, when power transmission lines are interrupted or heavy loads are switched, transient voltages as high as 3500V can be measured on the line. 1.3 Transient overvoltage caused by lightning: A lightning strike generally consists of three parts of the impact current: the first part rises from 0 to 100kA within 10μs; the second part reaches 2kA within 5ms after the first part begins, with a total charge exceeding 20C; and the third part reaches 200C within two seconds. Therefore, the key characteristics of lightning strikes are high power, low energy, high current, and rapid current changes. According to the definition of IEC1312-1 (02.95), a single lightning strike for analysis consists of the following lightning strikes: * A first lightning strike of positive or negative polarity * A subsequent lightning strike of negative polarity (lightning strikes after the first) * A long-duration lightning strike of positive or negative polarity [img=280,167]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jxdl/2000-3/23-2.jpg[/img] The mathematical expression of the current waveform of the first lightning strike 10/350μs (see Figure 2) and the subsequent lightning strike 0.25/100μs is i＝(I/h)｛(t/τ1)10/［1＋(t/τ)10］｝exp（－t/τ2）………………（1） (1) Where: I———peak current; A h———correction coefficient for peak current; t———time; S τ1 — Wavefront time constant; τ2 — Wavetail time constant. We can classify lightning strikes into direct lightning strikes and induced lightning strikes based on the different forms in which lightning impulses penetrate equipment. When equipment or lines are directly struck by lightning, all the lightning current may pass through these devices or lines and be conducted back to the ground; this is called a direct lightning strike. Because the equipment or lines struck by lightning may bear the impact of a huge lightning current, the damage caused by a direct lightning strike is extremely severe. Induced lightning strikes are divided into electrostatic induction and electromagnetic induction. When lightning forms, unipolar charges (such as negative charges) concentrate at the bottom of the thundercloud. Due to the effect of the electric field on the ground, the ground below the thundercloud induces a corresponding positive charge, thus forming an electrostatic induction electric field. When lightning discharges, the electric field between the thundercloud and the ground disappears, but the lightning creates a strong electromagnetic field in the surrounding space. Due to electromagnetic induction, an induced overvoltage is generated on the protected object. The amplitude of the overvoltage is directly proportional to the amplitude of the lightning current and inversely proportional to the distance from the lightning strike point. When the electromagnetic field of lightning reaches 0.07 Gauss and 2.4 Gauss, it can cause microelectronic devices to malfunction and be damaged. This is equivalent to the magnetic field at a distance of 2.8 km and 80 m from a lightning rod when a 100 kA lightning current flows through it. Generally, the distance between electronic equipment and lightning rods inside buildings is much smaller than this distance. This also means that dangerous overvoltages may occur within a 2.8 km radius of the lightning strike center. The following describes several coupling intrusion methods of lightning strikes into low-voltage systems and illustrates them with examples: 1.3.1 Resistive coupling overvoltage, see Figure 3 (such as coupling caused by shielding layer resistance or grounding resistance) [img=604,293]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jxdl/2000-3/23-3.jpg[/img] When lightning strikes point a, the discharge current and voltage drop generated by the lightning current flowing into the ground create a potential difference E between points a and b. Because the cable shielding at end b of building b was not grounded, a potential difference ΔV will occur between the cable shielding and the grounding system of building b. A portion of ΔV is applied as a common-state voltage ΔV1 between the equivalent load Zb and the grounding system of building b. In July 1997, the control tower of Nanchang Xiangtang Airport was struck by lightning, damaging part of the computer network, while the automatic telegraph communication interface within the tower remained normal. The airport communication station was far from the control tower; apart from the damaged interface board connected to the automatic telegraph communication cable of the control tower, most other communication electronic equipment within the communication station was normal. The communication cables between the communication station and the control tower were all buried underground. After careful analysis of the cause of the accident, it was found that when the communication cable from the control tower entered the distribution frame within the communication station, the cable shielding was not grounded, leading to damage to the corresponding communication interface board. This is a typical example of lightning strikes damaging equipment through resistive coupling overvoltage via the shielding. We can also derive the backflashover overvoltage when both ends of the cable shielding are grounded during a lightning strike, as shown in Figure 4. [img=560,290]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jxdl/2000-3/24-4.jpg[/img] Backflash overvoltage refers to the rise in grounding potential of equipment during a lightning strike, causing a high voltage between the grounded casing and the conductive parts of the equipment that may damage it. Figure 4 shows the backflash overvoltage on the equipment load Zb at building b when lightning strikes at point a: Vf＝i2R2－i2（R1＋Zb）－dφ／dt＝i2R2－i1（R1＋Zb）－L1di1／dt…………………………………（2） (2) Where: i1———the impulse current of the cable core; i2———the impulse current of the cable shield; φ———the magnetic flux of the cable core and cable shield circuit; R1———the resistance of the cable core; R2———the resistance of the cable shield; Zb———the impedance of the equipment load at building b; L1———the inductance of the cable core. The backflash overvoltage on the equipment load Zb at building b when the shields at both ends of the cable are open can be obtained by setting i1 and i2 to zero respectively. 1.3.2 Capacitive Coupling Overvoltage Due to the effect of the electrostatic field during the lightning formation process, charge accumulates on all conductive objects in the electric field. After a lightning strike, the electrostatic field disappears, and the redistribution of charges creates currents inside objects and across their impedance, resulting in a voltage drop and overvoltage. 1.3.3 Inductively Coupled Overvoltage When a lightning strike current flows through a conductor, the magnetic field generated around the conductor induces an overvoltage in adjacent transmission lines. In April 1996, lightning struck an 84-meter-high microwave tower about 16 meters north of the Jiangxi Provincial Power Dispatch Bureau building. There was a cable trench on the north side of the building. The equipment connected by shielded signal cables was undamaged, except for an RS232 communication line that ran from the computer room on the eighth floor through this cable trench to the terminal in the dispatch room on the tenth floor. The RS232 interfaces at both ends of the signal line (the host computer on the eighth floor and the terminal on the tenth floor) were damaged. Analysis of the accident suggests that when lightning struck the microwave tower, the lightning current discharged to the ground along the almost vertical tower material (angle steel), generating a horizontal magnetic field. This magnetic field induced the signal lines inside the building that were parallel to the tower material, causing damage to the RS232 interface, as shown in Figure 5. Similarly, when lightning strikes the lightning rod of a building, the lightning current discharged along the lightning conductor may induce the transmission lines inside the building. [img=301,418]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jxdl/2000-3/25-5.jpg[/img] [b]2 Comprehensive Protection Against Lightning and Transient Overvoltage[/b] 2.1 Lightning Protection Zones The IEC standard divides the space requiring protection into different lightning protection zones: Zone Z0A—Objects within this zone are likely to be directly struck by lightning; Zone Z0B—Objects within this zone are unlikely to be directly struck by lightning, but the electromagnetic field within the zone is not attenuated; Zone Z1—Materials within this zone are unlikely to be directly struck by lightning, and the electromagnetic field within the zone may be attenuated, the degree of attenuation depending on the shielding measures. In practical applications, we generally simply divide lightning protection into external lightning protection and internal lightning protection. 2.2 External Lightning Protection System The external lightning protection system consists of lightning rods (or lightning protection strips, lightning protection networks), down conductors, and grounding systems. The primary function of a lightning rod is to protect buildings from lightning strikes by providing a fast and reliable path for the lightning current to be discharged into the ground. If lightning strikes an object without a reliable path to the ground, the high voltage effect, high thermal effect, mechanical effect, electrostatic induction, electromagnetic induction, and backflashover of the lightning current can all generate tremendous destructive force. 2.2.1 Research on the protection range of lightning rods (or lightning protection strips, lightning protection networks) shows that even if the tip of a lightning strike is tens of meters above the ground, its direction of travel may still be unaffected by the characteristics of the ground and underground. The specific height depends on the intensity of the lightning. In particular, the trajectory of low-intensity discharges to the ground is unstable and may approach the building at an angle. This indicates that the traditional "protection angle" of lightning rods is unreliable. Therefore, GB50057-94 "Code for Design of Lightning Protection of Buildings" no longer uses the concept of "protection angle" for lightning rods. Instead, based on the IEC1024-1 approach, when designing lightning arresters, the rolling sphere method and lightning protection grid can be used alone or in any combination to calculate the protection range of lightning rods (or lightning protection strips, lightning protection networks). The rolling sphere radius is specified on page 104 of the standard. In practice, we comprehensively adopted the "ball shear method" to calculate the protection range of lightning rods in the lightning protection renovation of the Guiye 623 line. The Guiye 623 line has a voltage level of 6kV, a total length of 5km, and runs through paddy fields and hills. This line frequently suffers severe lightning damage: porcelain insulators are blown off, lines are broken, transformers are damaged, and lightning enters the low-voltage system. Our chosen solution is to install V-shaped lightning rods on each pole (the lightning rod base has obtained a national patent), and the rod length and angle have been carefully calculated. As can be seen from different angles in Figures 6 and 7, the V-shaped lightning rods can effectively protect the transmission line from direct lightning strikes. When thunderclouds lower their height and approach the line from the side at an angle, the V-shaped lightning rods can also effectively protect the transmission line from direct lightning strikes. Since the end of 1995, when the Guiye 623 line adopted V-shaped lightning rods for lightning protection renovation, and since the renovation was completed and put into operation in early 1996, it has achieved satisfactory lightning protection results. [img=267,226]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jxdl/2000-3/25-6.jpg[/img][img=289,204]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jxdl/2000-3/25-7.jpg[/img] For microwave stations with tower heights greater than 60m or located on high mountains, this paper proposes a lightning protection method for microwave (communication) antennas to prevent direct lightning strikes—consisting of a lightning protection device on the tower top and horizontal or independent lightning rods around the tower. Their specific dimensions and protection range should be calculated using the "spherical cutting method" based on the actual situation. The lightning protection device on the tower top and the horizontal rod can be made of steel that meets lightning interception requirements, and the cost is low, as shown in Figure 8. In summary, the author believes that in lightning protection practice, the lightning protection specifications should be strictly followed, and applied flexibly and rigorously in combination with the actual on-site conditions of the protected object. [img=457,344]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jxdl/2000-3/26-8.jpg[/img] 2.2.2 Lightning Down Conductors Conventional lightning down conductors are made of round steel, angle steel, and multi-strand copper core wires with a certain cross-sectional area. When the lightning impulse current flows through the lightning down conductor, a strong magnetic field is generated around the down conductor. Therefore, for low-voltage systems, the above types of down conductors are not ideal. Electrostatic field coaxial cables greatly overcome the shortcomings of conventional down conductors. They can change the transient response, and their characteristic impedance is only one-quarter that of copper core wires, and their impulse impedance is less than one-tenth that of the tower impedance. The capacitance between the inner and outer conductors of this type of cable is relatively large, which reduces its impedance and makes the voltage drop across the cable cross-section as small as that the insulating medium can withstand. When lightning discharge current flows to ground through the inner conductor of the coaxial down conductor, the grounded outer shielding layer acts as a shield to prevent high voltage between the down conductor and the building, as well as the induction of strong magnetic fields around the down conductor into the low-voltage system within the building. Therefore, electrostatic coaxial cables can safely guide lightning current from communication towers or high-rise buildings to the ground. 2.2.3 External lightning protection grounding of the grounding system is mainly for quickly and safely conducting lightning current to the ground in the event of a lightning strike. The concept of lightning protection grounding for low-voltage systems is not limited to external lightning protection grounding; it also encompasses grounding concepts such as DC working ground, AC working ground, and protective ground. Modern grounding systems should be as low-impedance as possible to provide protection against lightning strikes and transient overvoltages, as well as suppress noise from communication electronic systems. Grounding methods should be designed according to the single-point principle, that is, a combined grounding method where DC working ground, AC working ground, protective grounding, and lightning protection grounding share a single grounding device. This is currently recommended by various international and domestic lightning protection standards. Grounding technology itself is not a precise discipline and should not be applied rigidly in practical applications. In areas with high ground resistivity, such as high-altitude microwave stations and television stations, equipotential bonding is more economical and effective than simply reducing the system's grounding resistance. Jiangxi 704 Television Station is located on a 1400-meter-high mountaintop in Huangyangjie, Jinggang Mountain, with vegetation only about 0.4 meters thick and rock below, making the terrain rugged and prone to lightning strikes. In the lightning protection renovation project for this station, after reducing the absolute grounding resistance to a certain level, we considered that simply extending the grounding electrode and increasing the grounding grid area to further reduce the grounding resistance would not be effective in resisting lightning strikes. Furthermore, the television station's limited grounding budget did not allow for increased engineering work. Therefore, we adopted strict equipotential bonding measures, which achieved excellent results in actual operation. The so-called combined grounding method is not absolute. Firstly, buildings and their internal systems should generally share a common ground, but some equipment manufacturers emphasize that their equipment's operating ground must be independently grounded. There are probably two reasons for this: one is concern about the unreliability of other grounding systems in the building; the other is concern about leakage current and power frequency interference signals entering the system. Secondly, specialized transmitting equipment, such as airport beacon transmitters, whose operating ground serves as auxiliary transmitting electrodes, must be independently grounded. Furthermore, in areas with dispersed deployments and strong lightning, such as the meteorological equipment near the runway at Changbei Airport, independent lightning rods can be installed for lightning protection. The independent lightning protection grounding network and the equipment's operating grounding network must maintain a certain distance. Independent grounding networks within the same system can be connected using grounding network connectors (such as the GAP-480D from OBO, Germany). Normally, the two grounding networks are isolated; during a lightning strike, when the potential difference between them exceeds 1000V, the grounding network connectors automatically conduct to balance the potential of the two grounding networks. 2.3 Internal Lightning Protection System An internal lightning protection system includes shielding, equipotential bonding, amplitude limiting, routing of metallic conductors, and grounding measures. These measures are indispensable in a complete internal lightning protection system, providing protection against coupling mechanisms that generate transient overvoltages from various aspects. Due to space limitations, the following discussion focuses on the main limiting device for modern low-voltage power supplies—the transient overvoltage protector (also known as a surge suppressor, or simply a power surge protector): 3.3.1 Power Surge Protector The main function of a power surge protector is to quickly and instantly conduct a lightning surge to the grounding electrode when it arrives, and to reduce the residual voltage on the line after the surge protector to a level sufficient to protect the equipment. The performance of the power surge protector determines its function. Current carrying capacity, clamping voltage, and response time are the three main performance indicators of a power surge protector. Understanding the meaning of these indicators requires some in-depth knowledge. The main working principle of a surge protector is to protect electrical equipment from lightning strikes by clamping the instantaneous lightning surge to a safe voltage. This involves the magnitude of the incoming lightning. The international standard test index is generally a 10kA, 8/20us (Figure 2) lightning surge. In practical applications, the lightning current surge can reach tens to hundreds of kA. In this case, the performance of the surge protector needs to be evaluated from two aspects. On the one hand, it's crucial to consider whether the surge protector's maximum withstand current meets the requirements. If the surge protector's maximum withstand current is less than the actual lightning current entering the line, the surge protector itself will be damaged. In our application at Fuzhou 708 TV station (on the mountaintop), we found that while the surge protector with a smaller withstand current successfully protected the equipment, it was itself damaged by the lightning current. Later, we replaced it with German OBO V25 (primary) and V20 (secondary) surge protectors. After undergoing several large lightning strikes, both the protected equipment and the surge protector itself remained unharmed. This is because the V25's maximum withstand current can reach 100kA. On the other hand, the clamping voltage capability of internationally advanced surge protectors has a conversion ratio of 1:3 to the lightning surge current. Although the residual voltage of the surge protector may meet safety requirements under a standard test current of 10kA and an 8/20us surge, the actual surge current often exceeds this test specification. For example, when the lightning current reaches 20kA or 50kA, the residual voltage on the output side of the surge protector may exceed the equipment's safety protection voltage. The concept of graded protection is undoubtedly the best solution here. For example, if the incoming surge is 50kA and 8/20us, it is certain that all similar surge protectors (assuming the maximum withstand current of the surge protector is greater than 50kA) will have a residual voltage of over 1500V (the input voltage module of communication equipment can generally withstand instantaneous surge voltages of up to 1000V), which will lead to damage to the protected equipment. At this time, a second-stage surge protector should be installed to clamp and convert the residual voltage of the first stage back to a safe voltage range. According to statistics, 80% of lightning damage to electronic equipment is caused by lightning surges entering the power supply section, so the selection of power surge protectors is extremely important. Currently, there are dozens of 380V low-voltage power surge protectors (including imported and domestic ones) on the domestic market. However, the widespread application of professional and high-end low-voltage surge protectors in China has only occurred in recent years. Since 1993, we have selected high-performance surge protectors from Liebert (USA), Redeton (UK), OBO (Germany), and the Sino-US joint venture Aieruo for units that have suffered severe lightning strikes, based on their specific conditions. After several years of testing, all have achieved excellent results. This also provided us with firsthand information on the application of these surge protectors. Surge protector standards are specified in international standards such as ANSI/IEEE, UL, IEC, CCITT, FCC, CSA, DIN, and NEC. Authoritative domestic testing institutions for low-voltage surge protectors include: the former Ministry of Posts and Telecommunications Telecommunications Bureau's Communication Protection Technology Maintenance and Support Center; the former Ministry of Posts and Telecommunications Communication Product Protection Performance Supervision and Testing Center; the Shenyang Lightning Protection Test Station of China Railway Signal & Communication Corporation; the Electronic Industry Safety and Electromagnetic Compatibility Testing Center; and the Electric Power Research Institute of the Ministry of Electric Power. The test reports provided by these institutions, especially those of the Ministry of Posts and Telecommunications, offer reliable and trustworthy evidence for the main performance indicators of power surge protectors. As long as the power system is strictly regulated and equipped with high-performance surge protectors, the damage caused by transient overvoltages (due to lightning, power grid equipment switching, etc.) from the power supply to the low-voltage system can be reduced to near zero. [b]4 Prospects[/b] For modern low-voltage systems, lightning protection and transient overvoltage protection have become extremely important. The implementation of comprehensive lightning protection engineering can provide comprehensive protection for the safety of low-voltage systems, as proven by the author's extensive experience in lightning protection engineering. [b]References:[/b] ［1］ [US] G. Sharrick, translated by Hou Jinghan. Grounding Engineering. People's Posts and Telecommunications Press, 1988. ［2］ Zeng Yonglin. Grounding Technology. Water Resources and Electric Power Press, 1979. [3] Protection against lightning electromagnetic pulses IEC-1312-1 (1995.2), IEC-1312-2 (1994.11), IEC-1312-2 (1996.10) [4] Domestic lightning protection standards GB50057-94 Code for Design of Lightning Protection of Buildings YD5068-98 DL548-94 Code for Lightning Protection and Grounding Design of Mobile Communication Base Stations GB50174-93 Code for Design of Computer Rooms GB50200-94 Technical Specifications for Cable Television System Engineering