With the development of international microelectronics technology and the rapid advancements in communication, computer, and automatic control technologies, buildings are becoming increasingly high-quality and multifunctional, forming a new architectural form—the intelligent building. Due to the numerous information systems present in intelligent buildings, the "Code for Design of Lightning Protection of Buildings" (GB50057-94) outlines requirements for installing surge protectors to ensure the safe and stable operation of these systems. Essentially, a surge protector is a material used for equipotential bonding. Its selection refers to determining the type of surge protector used for electronic equipment within different lightning protection zones, based on the severity of different lightning electromagnetic pulses and the location of the equipotential bonding point, to achieve equipotential bonding with the shared grounding body. This article will discuss the surge protector's maximum discharge current (Imax), continuous operating voltage (uc), protection voltage (up), leakage current (ip), and alarm methods. According to Article 6.4.4 of GB50057-94 "Code for Design of Lightning Protection of Buildings", "Surge protectors must be able to withstand the expected lightning current passing through them and shall meet the following two additional requirements: the maximum clamping voltage during a surge, and the ability to extinguish the power frequency follow current generated after the lightning current has passed." That is, the maximum clamping voltage of the surge protector plus the induced voltage across its terminals should be consistent with the basic insulation level of the system and the maximum allowable surge voltage of the equipment. 1. Maximum Discharge Current According to Article 6.4.6 of GB50057-94 "Code for Design of Lightning Protection of Buildings", the maximum discharge current of surge protectors installed at the boundary of LPZ0A, LPZ0B, and LPZ1 zones is calculated. According to GB50057-94 "Code for Design of Lightning Protection of Buildings", "50% of the total lightning current flows into the lightning protection device of the building, and the other 50% flows into various external conductive objects, power cables, communication cables, and other facilities entering the building." The lightning surge is decomposed through power cables, information cables, metal pipes, etc. introduced by the building. The shunt current value of the lightning current in the low-voltage power supply cable of the main distribution room is calculated. When the line is shielded, the lightning current is reduced to 30% of its original value. According to the "Design Specification for Lightning Overvoltage Protection Engineering of Communication Bureaus (Stations)" YD/T5098-2001, the charge of a 10/350μs pulse waveform is approximately 20 times the charge of an 8/20μs simulated lightning wave waveform. The specific calculation is as follows. The maximum discharge current of the primary surge protector. According to Articles 6.4.8 and 6.4.9 of GB50057-94 "Code for Design of Lightning Protection of Buildings", surge protectors installed in LPZ1 and LPZ2 zones with a nominal discharge current (rated discharge current) greater than 5ka, use PU40 400 or PU15 400 as secondary protectors. 2. Protection Voltage While selecting an appropriate residual voltage for the surge protector is important, when the surge protector is installed in a low-voltage power grid, we should consider the system's residual voltage even more. That is, while considering the protector's residual voltage, we must also consider the impact of the surge protector's installation method on the system's residual voltage. The installation method of the protector is shown in the right figure. Because the maximum average current gradient of the lightning surge in the system does not occur during the first lightning strike, but rather during subsequent strikes, according to the requirements of the "Design Specification for Lightning Overvoltage Protection Engineering of Communication Bureaus (Stations)" YD/T5098-2001, the length of the connection wire between the terminal block of the modular protector and the phase and neutral wires should be less than 0.5m, and the length of its grounding wire should be less than 1m. It is possible to select a suitable location in the low-voltage cabinet to make the total connection wire length less than 1m. Therefore, the calculation of the maximum average gradient, system residual voltage, and protector protection voltage is crucial. Selecting a protection voltage of approximately 2kV is appropriate. When power is supplied to the distribution boxes in various computer rooms, important electrical equipment, and floor distribution boxes, it has already undergone multiple delays and decoupling effects in the cables, and its wavefront time will be much greater than 10μs. The lightning current energy has also undergone multiple shunting and attenuation, and the energy will be less than 5ka. Therefore, the maximum average current gradient of each line = 5ka / 2 × 30% / 10us = 0.075ka/us. When the surge protector is installed as shown in the figure above: the maximum surge voltage of ab = u11 + ur + u12 = 1.5 × 0.075 + ur = 0.1kV + ur (assuming l1 + l2 = 1.5m). Since the equipment in the computer room, such as servers, computers, and switching matrices, are special protection equipment, their rated impulse voltage is 1.5kV. At this time, the protection voltage of the selected surge protector should be less than 1.4kV. Therefore, the protection voltage of the secondary surge protector (at 3~5ka) is less than 1.2kV, which is appropriate. 3. Maximum Continuous Operating Voltage (UC): According to Article 6.4.5 of GB50057-94 "Code for Design of Lightning Protection of Buildings", the maximum UC in a TN power supply system is greater than 1.15 × 220V = 253V. Meanwhile, Article 6.4.6 stipulates that "in locations where the power supply voltage deviation exceeds 10% or where harmonics increase the voltage amplitude, the continuous withstand voltage of the SPD should be increased according to the specific circumstances." Some distribution box manufacturers only choose 275V. The author believes that choosing 275V for the continuous operating voltage of a TN power supply system is inappropriate because the GA172-98 "Surge Protector for Computer Information Systems" product standard stipulates that the nominal conduction voltage of the surge protector should be greater than 2.2 times the system operating voltage, that is, greater than 484V in a 220V operating system. We know that the main component of a voltage-limiting SPD is the varistor. According to the table showing the relationship between continuous withstand voltage and varistor voltage (nominal conduction voltage) in the varistor classification standard, the varistor voltage is not a fixed value but a range. Relative to 484V, we can conclude that the continuous withstand voltage should be greater than 350V. Continuous withstand voltage and residual voltage are contradictory; a high continuous withstand voltage results in a longer protector lifespan but also a higher residual voltage; a low continuous withstand voltage results in a shorter protector lifespan but also a lower residual voltage. However, under a 5-10kA lightning current surge, a protector with a continuous withstand voltage of 3501V will have a residual voltage less than 100V compared to a protector with a continuous withstand voltage of 440V. This will not significantly increase the system residual voltage. Therefore, choosing a protector with a relatively high continuous withstand voltage (such as 440V) to improve its lifespan is reasonable. 4. Leakage Current According to Article 6.1.1 of GA173-98 "Surge Protectors for Computer Information Systems," the leakage current of parallel-type power surge arresters should be less than 20μA. The larger the leakage current Io, the greater the likelihood that the surge protector will accumulate energy and generate heat. Furthermore, the leakage current increases with the temperature of the varistor. Therefore, the varistor is in a vicious cycle, indicating that the greater the rate of change (increase) of the leakage current over time, the faster the surge protector accumulates energy, resulting in poorer performance and a shorter lifespan. Generally, a leakage current of less than 10μA is preferable. 5. Alarm Methods Currently, there are three types of alarm methods available: remote signaling and telemetry alarms, suitable for unattended operation; visual alarms, which achieve alarm functionality through mechanical design; and audible and visual alarms, which require inspection after thunderstorms or periodic checks and are applicable to all situations, currently the most widely used alarm method. Additionally, there are audible and visual alarms, which require an additional alarm module and should be used with caution. Because the audible and visual alarm module may be damaged first during a lightning strike, losing its alarm function, and if the product is also damaged at the same time, people will not notice because they rely on the audible and visual alarm. When a second lightning strike occurs, the lightning will take advantage of the vulnerability and damage subsequent protection equipment.