I. Basic Principles of Lightning Protection Lightning and other strong interference can cause serious damage to communication systems and the resulting consequences, making lightning protection essential. Lightning consists of high-energy low-frequency components and highly penetrating high-frequency components. It primarily damages equipment in two ways: one is through direct conduction of lightning through metal pipelines or ground wires; the other is through surges generated by lightning electromagnetic pulses inducing surges in metal pipelines or ground wires via various coupling methods. The vast majority of lightning damage is caused by this induction. For electronic information equipment, the main hazard comes from the coupled energy of lightning electromagnetic pulses, which generate transient surges through the following three channels: Metal pipeline channels, such as water pipes, power lines, antenna feeders, signal lines, and aviation obstruction light leads; ground wire channels, ground potential backflash; and spatial channels, the radiated energy of electromagnetic fields. Among these, surges in metal pipeline channels and ground potential backflash in ground wire channels are the main causes of damage to electronic information systems. The most common form of damage is lightning damage caused by power lines, therefore, it needs to be a key focus of prevention. Because lightning can penetrate electronic information systems indiscriminately, lightning protection is a complex systemic project. The core of lightning protection is discharge and equalization. 1. Discharge involves releasing the energy of lightning and its electromagnetic pulse through the ground, adhering to the principle of hierarchy—that is, discharging as much excess energy as possible and as far as possible into the ground before it enters the communication system; hierarchy means weakening lightning energy according to the established lightning protection zones. Lightning protection zones, also known as electromagnetic compatibility zones, divide the environment into several areas based on the varying intensities of lightning and its electromagnetic pulses felt by people, objects, and information systems: LPZOA zone: All objects in this zone may be directly struck by lightning, therefore all objects may conduct away all lightning current; the electromagnetic field in this zone is not attenuated. LPZOB zone: Objects in this zone are unlikely to be directly struck by lightning, but the electromagnetic field in this zone is not attenuated. LPZ1 zone: Objects in this zone are unlikely to be directly struck by lightning; the current flowing to each conductor is further reduced compared to LPZOB zone; the electromagnetic field attenuation and effectiveness depend on the overall shielding measures. If further reduction of the guided current and electromagnetic field is required in subsequent lightning protection zones (such as LPZ2 zone), subsequent lightning protection zones should be introduced. The selection of these zones should be based on the environmental requirements of the system to be protected and the specific conditions of the lightning protection zones. The higher the protection zone number, the lower the expected interference energy and voltage. In modern lightning protection technology, the establishment of lightning protection zones is of great significance, as it guides the implementation of shielding, grounding, and equipotential bonding techniques. 2. Equipotential bonding aims to maintain a potential difference in all parts of the system that is sufficient to cause damage. This means that the potential of all metallic conductors in the system's environment and the system itself remains essentially equal during transient phenomena. This is essentially based on equipotential bonding. A potential compensation system consists of a reliable grounding system, metal conductors for equipotential bonding, and equipotential connectors (lightning arresters). During the extremely short time of a transient phenomenon, this potential compensation system can quickly establish an equipotential between all conductive components in the protected system's area, including active conductors. This complete potential compensation system can create an equipotential zone in a very short time, which may have a potential difference of tens of kilovolts relative to distant areas. Importantly, within the area where the protected system is located, there is no significant potential difference between all conductive components. 3. The lightning protection system consists of three parts, each with its own important function and no two parts are interchangeable. External protection, consisting of lightning arresters, down conductors, and grounding electrodes, can directly conduct most of the lightning energy into the ground for discharge. Transition protection, consisting of reasonable shielding, grounding, and wiring, can reduce or block induction introduced through various intrusion channels. Internal protection, consisting of equipotential bonding and overvoltage protection, can balance the system potential and limit the overvoltage amplitude. II. Function and Technical Parameters of Surge Arresters Surge arresters, also known as equipotential connectors, overvoltage protectors, surge suppressors, surge absorbers, and lightning protection devices, are used for power line protection. Given the current characteristics of lightning damage, lightning protection based on surge arrester solutions is the simplest and most economical lightning protection solution, especially in lightning protection renovation. The main function of a surge protector is to maintain or limit the potential across its terminals to a certain range during transient events, transferring excess energy from active conductors to the ground for discharge. It is a crucial component in achieving equipotential bonding. Some key technical parameters of surge protectors include: rated operating voltage, rated operating current, and the current-carrying capacity of specially approved series-parallel power surge protectors. Current-carrying capacity, the surge protector's ability to transfer lightning current, is measured in kiloamperes and is related to the waveguide type. Functionally, surge protectors can be divided into those that protect against direct lightning strikes and those that protect against induced lightning strikes. Surge protectors that protect against direct lightning strikes are typically used for line protection where direct lightning strikes are possible, such as protection at the boundary between LPZOA and LPZ1 zones. Their current-carrying capacity is measured and represented using a 10/35μs current waveform. Surge protectors that protect against induced lightning strikes are typically used for line protection where direct lightning strikes are unlikely, such as protection at the boundary between LPZOB and LPX1 zones, or LPZ1 zones. The response time, measured by an 8/20μs current waveform, represents the time required for a surge protector to control transient phenomena; this time is related to the waveform characteristics. Residual voltage, the voltage limiting capability of a surge protector against transient phenomena, is related to the lightning current amplitude and waveform characteristics. III. Selection of Surge Protectors To achieve ideal protection results with surge protectors, it is crucial to "install appropriate surge protectors in suitable locations." Therefore, the selection of surge protectors is extremely important. 1. The distribution of lightning current among various facilities entering a building is as follows: approximately 50% of the lightning current is discharged to the ground through external surge protection devices, and another 50% is distributed within the metallic materials of the entire system. This evaluation model is used to estimate the current-carrying capacity of surge protectors and the specifications of the metallic conductors used for equipotential bonding at the boundaries of LPAOA, LPZOB, and LPZ1 zones. The lightning current at this location is a 10/35μs current waveform. 1. Regarding the distribution of lightning current among various metallic materials: The amplitude of the lightning current in each part depends on the impedance and reactance of each distribution channel. The distribution channel refers to the metallic material that may be allocated lightning current, such as power lines, signal lines, water pipes, metal structures, and other grounding. Generally, it can be roughly estimated based on their respective grounding resistance values. In cases where it is uncertain, it can be assumed that the resistances are equal, i.e., the current is evenly distributed among the metallic pipes. 2. When power lines are introduced overhead and may be directly struck by lightning, the lightning current entering the protection zone inside the building depends on the impedance and reactance of the external lead-in line, the surge protector discharge branch, and the user-side line. If the impedances at both ends are the same, the power line is allocated half of the direct lightning current. In this case, a surge protector with direct lightning protection function must be used. 3. The subsequent evaluation mode is used to evaluate the lightning current distribution at the boundary of protection zones after LPZ1. Since the insulation impedance on the user side is much greater than the impedance of the surge protector discharge branch and the external lead-in line, the lightning current entering the subsequent lightning protection zone will be reduced, and no special estimation is required numerically. Generally, the current-carrying capacity of surge protectors used in subsequent lightning protection zones is required to be below 20kA (8/20μs), and surge protectors with high current-carrying capacity are not necessary. The selection of surge protectors for subsequent lightning protection zones should consider energy distribution and voltage coordination between different levels. When many factors are difficult to determine, series-parallel surge protectors are a good choice. The series-parallel design is a concept proposed based on the characteristics of many applications and protection level distinctions in modern lightning protection (as opposed to traditional parallel surge protectors). Its essence is an effective combination of multi-stage dischargers and filter technology through energy coordination and voltage distribution. Series-parallel lightning protection has the following characteristics: wide application. It can be used not only in conventional applications but also in locations where the protection zone is difficult to distinguish. The voltage division and delay effect of induced decoupling devices under transient overvoltages helps to achieve energy coordination. It slows down the rise rate of transient interference to achieve low residual voltage, long lifespan, and extremely fast response time. 4. The selection of other surge protector parameters depends on the level of the lightning protection zone where each protected object is located. Its operating voltage is based on the rated voltage of all components installed in the lead circuit. For series-parallel surge protectors, their rated current must also be considered. 5. Other factors affecting lightning current distribution in electric arc lines: A decrease in transformer terminal grounding resistance will increase the distributed current in the electric arc lines. An increase in the length of the power supply cable will reduce the distributed current in the power lines and achieve a balanced current distribution among the major conductors. Excessively short cable length and excessively low neutral line impedance will cause current imbalance, resulting in differential mode interference. Connecting multiple users in parallel with the power supply cable will reduce the effective impedance, leading to an increase in the distributed current. In a networked power supply state, temporary lightning current mainly flows into the power lines, which is why most lightning losses occur at the power lines. IV. Installation of Surge Protectors 1. Power lines should implement multi-level protection. Multi-level protection uses each lightning protection zone as a hierarchy to gradually reduce lightning energy (energy distribution), ensuring that the limiting voltages at each level are coordinated, ultimately limiting the overvoltage value within the equipment insulation strength (voltage coordination). Multi-level protection becomes necessary in the following situations: failure of a certain level of surge protector or failure of a certain circuit of the surge protector. The residual voltage of the surge protector does not match the equipment insulation strength, and the cable length inside the building is relatively long. 2. In almost all cases, cable protection should be divided into at least two levels, and the same level of surge protector may also contain multiple levels of protection (such as series-parallel surge protectors). To achieve effective protection, appropriate surge protectors can be installed at the interfaces of each surge protection zone. Surge protectors can be for a single electronic device or a space containing multiple electronic devices. All conductors passing through surge protection zones that are usually spatially shielded should be connected to surge protectors at the interface of the surge protection zone. In addition, the protection range of surge protectors is limited. Generally, the protection effect will deteriorate when the distance between the surge protector and the equipment line exceeds 10m. This is because there is a oscillating voltage caused by reflection on the cable between the surge protector and the equipment to be protected, and its amplitude is proportional to the line length and load impedance. 3. In multi-level protection using power surge protectors, if energy distribution is not carefully considered, more lightning energy may be introduced into the protected area. This requires that surge protectors be selected according to the aforementioned evaluation model. Generally, surge protectors have the characteristic that the larger the lightning current, the higher the residual voltage. After energy distribution, the lightning current flowing through the final surge protector is extremely small, which is beneficial for voltage limiting. Note that selecting a surge protector with a low response voltage as the final stage of protection without considering voltage coordination is dangerous. The key to achieving energy distribution and voltage coordination lies in utilizing the inductive reactance of the cable itself between the two surge protectors. The inductive reactance of the cable itself has a certain effect on impeding buried current and voltage division, allowing more lightning current to be distributed to the preceding stage for discharge. Generally, the cable length between the two surge protectors is required to be around 15m, suitable for situations where the protective ground wire is laid close to other cables or is within the same cable. The length of branch lines on the cable affects the required cable length; when there is a certain distance (>1m) between the protective ground wire and the protected cable, a cable length greater than 5m is sufficient. In situations where it is not suitable to use the cable itself as a decoupling measure, such as when the interfaces of the two surge protection zones are close or the cable length is short, specialized decoupling devices can be used, in which case there is no distance requirement. 4. Decoupling devices are an important measure for achieving energy distribution and voltage coordination. The following materials can be used as decoupling devices: cables, inductors, and resistors. Series-parallel surge protectors are a combination of surge protectors that considers energy distribution and voltage coordination, using filters as decoupling devices, and are suitable for various applications. 5. In some extreme cases, installing surge protectors may actually increase the possibility of equipment damage and must be avoided; such situations may occur when a surge protector protects several lines, but one of the protectors on one line fails or has an excessively slow response. This could cause common-mode interference to be converted into differential-mode interference, damaging the equipment. This necessitates multi-level protection and careful maintenance of the surge protectors. Installing surge protectors arbitrarily without considering lightning protection zones, energy coordination, and voltage distribution—for example, installing only one surge protector at the front end of the equipment—will result in a strong lightning current being attracted to the front end of the equipment due to the lack of upstream protection, causing the residual voltage of the surge protector to exceed the equipment's insulation strength. This requires surge protectors to be installed according to a hierarchical principle. 6. In other cases, incorrect installation will render the equipment ineffectively protected. Excessively long surge protector connection lines can cause extremely high voltages due to inductive reactance during surge protector operation, resulting in dangerous voltages applied to the equipment. This problem is even more pronounced in the application of final-stage surge protectors. The solution is to use short connection lines, or at least two separate connection lines to distribute the magnetic field strength and reduce voltage drop. Simply thickening a single connection line is ineffective. If necessary, the wiring of the protected line can be modified to bring it closer to the equipotential bonding bus (grounding point) to reduce the connection line length. Surge protector output lines, input lines, and grounding lines being laid close together or side-by-side can also have a significant impact on series-parallel surge protectors. When the output lines (protected lines), input lines (unprotected lines), and grounding lines of a series-parallel power surge protector are laid close together, transient surges are induced in the output lines. Although their intensity is lower than before, they can still be dangerous. The solution is to separate or perpendicularly lay the input and grounding lines from the output lines, minimizing the length of parallel laying and increasing the distance between them. The surge protector's grounding wire is not connected to the protective ground of the protected equipment, meaning it uses a separate surge protector grounding. This will create a dangerous voltage between the protected wire and the equipment's protective ground during transients. The solution is to connect the surge protector's grounding wire to the equipment's protective ground.