Abstract: This paper discusses the necessity of third-level surge protection for power supplies and proposes some suggestions for improving third-level (including Class D) surge protection. Keywords: power supply surge protection, third-level lightning protection, surge protection, electrostatic discharge protection. In the 21st-century information age, lightning disasters affect almost every industry. Advanced electronic equipment, especially those with large-scale integrated circuits as core components, such as communication, computer networks, measurement, monitoring, and electrical appliances, are widely used in power, aviation, defense, communications, broadcasting, finance, transportation, petrochemicals, medical care, and other areas of modern life. Because these electronic devices generally have weak tolerance to transient overvoltages and overcurrents, accidents caused by lightning and other forms of surges penetrating into the internal components of electrical equipment, resulting in interference and damage, are on the rise year by year. Historical statistics show that over 70% of lightning-damaged electrical equipment incidents originate from power supply lines. Therefore, power system lightning protection is a crucial aspect of lightning protection for electronic equipment and its systems. Implementing multi-level lightning protection for the power supply lines of buildings and their electronic information systems is an important component of comprehensive lightning protection, a consensus within the lightning protection industry. The first and second levels of lightning protection for power systems are given great attention by designers, contractors, developers, and project acceptance personnel, from lightning protection design and equipment configuration to installation and inspection, and generally meet relevant national standards. Many articles have already discussed the first and second levels of lightning protection for power systems, so they will not be repeated here. The following is a brief analysis and discussion of the third level of lightning protection (surge protection) for power systems. There is a question regarding the necessity and importance of the third level of lightning protection for power systems. The author learned from designers at architectural design institutes that many building designs do not include the third level of lightning protection for power systems, only designing lightning protection for the main power distribution and building distribution (i.e., the first and second levels of lightning protection). The reason for this is that the configuration of electrical equipment is often variable during the design phase, and the electrical equipment requiring protection, except for the machine room, is relatively dispersed and numerous. In addition, the complexity of the project and the increased project costs can, to some extent, burden the construction and development parties. Therefore, it is often "neglected" or downplayed in the implementation of lightning protection projects. Thus, the third line of defense in the lightning protection system becomes problematic. Here's an example of lightning damage to electrical equipment: A radar station at an airport, located 200 meters above sea level in a high-risk thunderstorm area, had lightning rods and a grounding system installed during lightning protection engineering. First and second-level surge protectors were also installed in the main power distribution room and the equipment room. In the summer of 1994, lightning struck an overhead three-phase power supply line near the foot of the mountain, severely damaging the power distribution box in the pump house at the foot of the mountain. The radar station's power equipment (including 9 UPS rectifier diodes, 5 high-power diodes, and 13 500V/15A fuses) and 9 control circuit boards of the radar were also damaged, rendering the radar inoperable and causing economic losses of approximately 2 million yuan. This accident occurred on an overhead power line; here we will only discuss lightning protection for the power supply. The first and second-level surge protectors installed in the main power distribution room and equipment room were not damaged, yet the power equipment was severely damaged. Some suspect a problem with the surge protectors' quality, but the author believes this is not entirely the case. The performance and reliability of the installed power surge protectors are unclear, but it's certain they failed to prevent the intrusion of a strong lightning strike. Upon closer analysis, we can't entirely blame the two power surge protectors. With such a powerful lightning strike, causing severe damage to two power systems, even resulting in smoke and fire, could two power surge protectors withstand it? If a third-stage surge protector had been installed, creating a more robust defense, the damage to the radar station might have been different, or at least less severe. How effective is third-stage power surge protection? Power surge protectors, whether first-stage, second-stage, or third-stage, are all connected in parallel to the power line. They all shunt the lightning current entering the power line, diverting it to the grounding electrode for release, thus gradually reducing the overvoltage in the circuit. The third-stage surge protector further reduces the residual voltage of the second-stage protector and also alleviates the shunt burden on the first and second-stage protectors. In the lightning strike situation described above for the radar station, the powerful lightning current would be difficult to discharge solely through the first and second stages of the power supply. The first-stage surge protector (SPD) has a slow response, and the second-stage SPD has limited shunting capacity. Often, before it can release its current, some of the lightning current has already crossed the second stage, resulting in excessively high residual voltage at the second stage output, and the lightning's energy needs further release. This clarifies the causal relationship of the aforementioned lightning strike accident at the radar station. The combined shunting effect of one or more third-stage surge protectors can ensure more complete lightning current release, further reducing residual voltage and meeting the safety requirements of electrical equipment. Here is another example of a lightning strike: In May 2005, a lightning strike occurred at a toy factory in Dongguan City, Guangdong Province. The lightning strike damaged the surge protector installed at the power port of the factory's main computer server, emitting thick black smoke. The surge protector's circuit board was severely damaged, and the critical surge protector component, the varistor, exploded. Fortunately, the computer server was not damaged; after removing the surge protector and connecting the power supply, the computer server functioned normally. In late 2004, to improve its lightning protection system, the factory installed power supply and signal line lightning protection equipment, including four-level power supply lightning protection (the main power distribution, sub-distribution power, and main electronic system were each equipped with level one, two, and three surge protectors, respectively; ten computer power ports were specially equipped with 2.5kA surge protectors for fine protection!) and lightning protection for computer, telephone, and monitoring signal lines. In this lightning strike, apart from the aforementioned power surge protector (level four SPD) on the computer server being damaged, the other surge protectors and electrical equipment were undamaged. It is conceivable that the level four SPD played a crucial role in saving the factory's computer server, and it is also a result of the comprehensive protection provided by the multi-level power supply lightning protection. This is because lightning (induced) entering the power line cannot have a definite path; it flows randomly, towards areas with low grounding resistance. In this case, before the lightning current entered the computer server, the installed power surge protector effectively diverted and limited the voltage, protecting the computer server. The damage to the surge protector due to excessive lightning current is a normal phenomenon. This lightning strike incident fully demonstrates the importance of multi-level surge protection for power supplies and its protective role for electrical equipment systems. It also reminds us that the third-level surge protection device (SPD) is indispensable, and critical equipment should also be protected by a fourth-level surge protection device (SPD) to ensure the safety of electrical equipment. The three-level surge protection mentioned above is designed for lightning intrusion into the main power line. In actual power equipment systems, some terminals are still some distance from the distribution cabinet (second-level surge protection), and induced lightning can still occur in this section of the line. It is particularly worth mentioning that in recent years, there have been more and more high-rise buildings. The taller the building, the greater the probability of being struck by lightning. When lightning strikes the lightning rod or lightning protection strip of a high-rise building, the lightning current flowing down the down conductor (usually the steel reinforcement of the building) and being released to the ground will generate an electromagnetic field, inducing lightning in the surrounding metal, including the power lines inside the building. The main power distribution (first-level surge protection) of high-rise buildings is usually located at the bottom of the building. Thus, the protection of electrical equipment in high-rise buildings from induced lightning mainly relies on the third-level surge protection device (SPD). In addition to lightning protection, surge protectors (SPDs) also provide surge protection and static electricity protection. Surges occur widely in daily life, including when power is switched on and off, plugged in and unplugged, starting and stopping elevators, electrical switches, motors, drilling, welding, electrical equipment damage, and short circuits. Compared to lightning, although surge pulse voltage is lower, its pulse width, duration, and intensity are still considerable. Surges can interfere with and damage electrical appliances, and even cause fires. Moreover, surges occur within the power system itself; they can occur on main power lines, branch lines, generators, power distribution equipment, UPS, AC/DC power supplies, and even at the terminals of electrical equipment. Once they occur, they directly enter the electrical equipment, causing damage. Surges happen frequently and are difficult to prevent! Therefore, surges pose a significant threat to electrical equipment and should not be taken lightly! Static electricity occurs when a thundercloud or other charged object approaches a metal conductor. The conductor then induces a charge of opposite polarity to the object. If this static charge is not released promptly, its voltage can accumulate to hundreds or even thousands of volts. Once static electricity forms on a power line, it threatens the safety of electrical appliances. Surges, static electricity, and lightning share similar characteristics, and their protection can be achieved through surge protectors (SPDs). It is more appropriate to refer to these power protectors collectively as power surge protection devices (SPDs). Based on the characteristics of lightning, surges, and static electricity, their location within the power system, and the degree of threat they pose to electrical appliances, it can be seen that the third-level power SPD offers a wider protection range and is more effective at absorbing various surges. Its function and effect are no less than that of the first and second-level power SPDs. Therefore, when implementing lightning protection for the power system of buildings and electrical equipment systems, only by simultaneously implementing three levels of surge protection can the damage caused by lightning, surges, and static electricity to electrical appliances be effectively prevented, thereby ensuring the safety of electrical equipment. The practice of lightning protection engineering has fully demonstrated this point. Regardless of the project size, as long as the first and second-level power supply lightning protection are done well, and the third-level power supply lightning protection is also adequately addressed, the lightning protection project can achieve good results. It is particularly important to emphasize that the third-level power supply SPD protection should not be neglected; otherwise, all previous efforts will be wasted, leading to disastrous consequences! Especially in high-rise buildings, electrical appliances with long power lines spanning multiple buildings, exposed electrical terminals, unattended base stations, computer rooms, monitoring, surveillance, and regulatory centers, the third-level power supply SPD protection should be emphasized and strengthened. In computer rooms, monitoring centers, and other locations with a large number and concentrated equipment, it is recommended to treat their distribution boxes with second-level power supply SPD protection and install third-level power supply SPDs at the terminals of important equipment. For critical equipment in particularly important electronic information systems, such as servers in computer networks, it is also necessary to connect a lower-level power supply SPD in series at the power port of the equipment for more refined protection. Any missing or inadequate third-level power supply SPDs in building and electrical equipment systems must be implemented and improved. Applications of Class D (Level 3) SPDs in Other Areas: The aforementioned Class D surge protection for power supplies is not yet widely implemented. It remains largely confined to certain high-end information systems (such as computer networks, communication systems, and monitoring systems), important units and departments (public security, customs, command centers, etc.), and important locations (radar stations, power plants, etc.). Many electrical devices not only lack Class D surge protection but also lack even basic surge protection. For example, computers and electrical appliances in elevators, water and gas pumping stations, communication stations, monitoring stations, parking lots, weighbridges, shopping malls, schools, hospitals, and countless household appliances are almost forgotten. This stems from a lack of awareness and a lack of safety consciousness regarding lightning and surge protection. Most buildings were not designed or implemented with lightning and surge protection for electrical appliances during construction. Some people harbor a侥幸心理 (a sense of complacency), believing they have escaped lightning strikes for many years. Thus, these numerous, scattered electrical devices become the "targets" of lightning strikes and the "objects" of surges. Data collected by the Guangdong Provincial Meteorological Bureau's Lightning Protection Testing Institute reveals the alarming scale of lightning-related accidents. Incomplete statistics indicate that lightning-related accidents cause an average annual economic loss of nearly 300 million RMB in Guangdong. The collected cases show that the majority of damage from lightning accidents involves scattered electrical equipment without lightning protection. Implementing Class 3 surge protectors (SPDs) for such a large number of electrical devices presents numerous difficulties and is impractical. However, installing Class 3 (D-class) surge protectors (the most basic lightning and surge protection) on some valuable electrical equipment remains necessary and relatively easy to implement. While Class D SPDs have lower surge protection capabilities than Class B and Class C SPDs, their current carrying capacity is sufficient to withstand induced lightning strikes and weaker lightning waves and industrial surges. They also offer rapid response and low residual voltage. Induced lightning strikes account for over 85% of lightning-related injuries. Therefore, Class D surge protectors provide good protection against induced lightning strikes and surges for indoor electrical equipment. Class D power supply SPDs are inexpensive and cost-effective, requiring no special lightning protection grounding (the power supply's working ground is sufficient!), and are easy to install; ordinary electricians can perform the installation. To prevent lightning and various forms of surges from entering electrical equipment and causing interference, damage, and accidents, all types of electrical equipment should be equipped with Class D power supply SPDs. It's never too late to mend the fence after the sheep are lost; otherwise, the loss may still occur. Don't take chances. Selection of Class D power supply SPDs: Class D power supply SPDs should use the following circuitry: Features: 1. Effectively suppresses common-mode and differential-mode surges; 2. Suitable for various power systems; 3. Significantly reduces leakage current through the MOV, extending the SPD's standby life; 4. PE and N are isolated, ensuring safety and reliability, and preventing the impact of ground potential rise on the power supply and equipment. Features: 1. Full protection modes for common mode and differential mode, low residual voltage; 2. Fast response, response time ≤25ns; 3. PE and N isolation, safe and reliable, avoiding the impact of ground potential rise on power supply and equipment; 4. Extremely short grounding wire, which is conducive to surge release. The main performance characteristics of Class D power supply SPDs include nominal discharge current, maximum surge discharge current, protection level, and response time. Standards for Class D power supply SPDs vary by region. Germany uses a nominal discharge current of 1 to 5kA (8/20μs); my country does not have a unified standard, but typically uses a nominal discharge current of 5 to 15kA (8/20μs) or a maximum discharge current of 10 to 25kA (8/20μs). The appropriate discharge current for a power supply SPD depends on the environment of the installation site (surge nature, intensity, and probability), the condition of the power line (length, shielding, indoor or outdoor installation), and the importance of the protected electrical equipment and its surge resistance and requirements. To ensure long-term stable operation of the power system, the continuous operating voltage of the surge protection device (SPD) should be low to achieve a lower level of protection. Of course, the SPD's protection mode, overheat protection for leakage current, safety fault indication, alarm indication, and the power system type to which the SPD is compatible should all be considered. Selecting a high-performance, reliable SPD that is compatible with the energy levels of the first two stages of the power supply SPD is crucial for the third stage of power surge protection and must be strictly controlled. Installation of the third-stage (Class D) power SPD: The third-stage (Class D) power SPD should be installed near the power interface of the power supply equipment. The SPD's grounding terminal should be connected to the power supply's working ground (PE), and the connection should be as short and straight as possible, generally not exceeding 0.5 meters in length. If the power supply does not have a working ground, a separate lightning protection ground should be installed. Lightning protection grounding can be achieved by chiseling small holes in the building's walls and columns to expose the reinforcing steel, welding a steel bar with bolts onto it, and then connecting it to the SPD with a copper wire with a round-hole connector. The grounding resistance should be ≤10Ω. For surge protection, in addition to installing a third-level (Class D) power SPD as mentioned above, depending on the actual situation, shielding (lines, computer rooms, equipment), equipotential bonding, and establishing a joint grounding system should also be implemented. The third-level power SPD protection should be thoroughly and meticulously implemented to build an insurmountable line of defense against surges, ensuring the safety of information systems and electrical equipment. Due to limited expertise, errors in this article are inevitable; I sincerely request corrections from experts and colleagues. References: 1. Code for Design of Lightning Protection of Buildings GB50057-94 (2000 Edition); 2. Correctly Selecting the Current Capacity of SPD Based on Lightning Factor (Liu Jike, Post and Telecommunications Design Institute, Ministry of Information Industry); 3. New Generation Multifunctional Power Safety Socket.