Modern buildings contain various grounding systems, such as lightning protection grounding, electrical safety grounding, AC power working grounding, and communication and computer control system grounding. Among them, the grounding of computer control systems is the most complex. In order to prevent the effects of stray capacitance coupling, static electricity, and inrush current, and to ensure the stable and reliable operation of the computer control system and the safety of operators, it is essential to establish a good grounding system. In computer system grounding, there are as many as six types of grounding for each function. These groundings are interconnected and mutually exclusive. Therefore, whether they are handled correctly during the design will directly affect the normal operation of the system and the construction progress. To avoid this situation, the following two aspects should be noted: (1) Whether the grounding point is correct. A correct grounding method can prevent crosstalk between different parts of the control system; (2) Whether the grounding is firm. Try to keep each grounding point at zero impedance to reduce the voltage drop on the grounding line. 1. Types and Functions of Grounding in Computer Control Systems According to the "Design Code for Electronic Computer Rooms" (GB50174-93), "There are generally four types of grounding in electronic computer rooms: AC working grounding, safety protection grounding, DC working grounding, and lightning protection grounding. These four types of grounding should share a common grounding device, and their grounding resistance should be determined according to the minimum value among them." The types of computer grounding systems can be divided into two categories according to their different grounding functions: working grounding and protective grounding. 1.1 Working Grounding: (1) DC Working Grounding: ① Floating grounding; ② Direct grounding: series grounding, parallel grounding, network grounding; ③ Digital grounding and analog grounding. (2) AC Working Grounding. 1.2 Protective Grounding: (1) Safety Protection Grounding: ① Shielding grounding; ② Static grounding. (2) Lightning Protection Grounding. The following analysis addresses the various grounding methods mentioned above: 1.2.1 DC Working Ground Here, DC ground refers to the ground wire of the DC power supply in the computer control system, which is the logic reference ground of the computer itself, i.e., the equipotential ground of the digital circuits in the computer control system; the zero potential of the input signal of the sample-and-hold amplifier 1 and the A/D converter (called analog ground); and the ground of the sensor (called signal ground). The purpose is to ensure that the control system circuits have a unified reference potential during operation, preventing signal errors caused by zero potential fluctuations. Having a unified reference potential also prevents interference from external electromagnetic fields. This reference potential point (surface) is a reference surface for the entire system, but it is not necessarily the zero potential of the earth; it may be charged itself. This equipotential point is also called "working ground." This "working ground" is only used to describe a system with a zero-volt potential connected to a reference point, line, or plane. Due to zero-current grounding faults and the short-term occurrence of short-circuit currents, this zero-volt potential reference point, line, or plane may have a very high potential relative to the distant earth, but it is still a zero-volt potential for the relevant system. (1) Floating ground: When the "working ground" is not directly grounded, it is called floating ground. When the insulation resistance between the "working ground" and the earth is greater than 1 mΩ, the DC working ground can be considered as floating. Its advantage is that it can avoid conductive electromagnetic interference between circuits, which generates ground loops and electrical noise. Applicable to: stray distributed capacitance coupling paths can be ignored and low frequency situations; (2) Direct ground: When the "working ground" is connected to the earth through a low impedance path, it is called direct ground. When the floating ground does not meet the above conditions, other grounding forms must be adopted. The DC direct grounding method is usually adopted. In principle, the grounding resistance of the DC ground should be as small as possible. Based on the current domestic and foreign computer production, it is generally less than 1 mΩ. There are three main methods for this: ① Series grounding: Connect the DC working ground of each device in the computer control system to the "DC ground" bus in series. ② Parallel grounding: Connect the DC ground of each device in the computer control system to the "DC ground" bus in parallel. ③ Network grounding: Use a copper busbar of a certain area to form a copper busbar network under the raised floor. To ensure insulation, a 2-3 mm thick insulating rubber or other insulating material should be placed under the copper busbar. The DC working ground of each device is connected to the network, and the network is connected to the grounding body. When the network side length is equal to the wavelength of 30 MHz and 0.5 m, there is almost no potential difference between any two points on the grounding network for all frequencies from DC to 30 MHz. The adoption of the above three grounding methods should be determined comprehensively based on the actual situation such as the area of ​​the computer room, the working nature of the computer, the situation of the interference source, and the installation requirements of the computer manufacturer; (3) Digital ground and analog ground: In the computer control system, both digital ground and analog ground are DC working grounds, but they must be grounded separately inside the system. Even if there are two grounds on the same chip, they must be grounded separately and then connected at only one point. Otherwise, the digital circuit will return to the digital circuit through the ground wire of the analog circuit, which will affect the analog signal. 1.2.2 AC Working Grounding AC working grounding, also known as power grounding, is the grounding of AC power supply and AC high-current circuits in the control system. Its nature is the same as that of the working ground in the power system. In fact, it is the ground of the AC power transformer in the low-voltage power system. It belongs to the ground of the high-current system. The purpose of doing this is only one: safety. This ground has two functions: (1) It connects all the equipment together through low-impedance conductors and connects the system to the grounding device. Thus, all the equipment has the same potential, and this potential is the ground potential. In this way, even under fault conditions, there is no unsafe potential difference between any two exposed non-energized metals or between non-energized metals and the ground; (2) Equipment grounding provides a safe low-impedance return path for fault current, which can allow overcurrent devices to quickly clear the fault and minimize damage. At the same time, it ensures that the interference signal of the circuit leaks into the ground and does not interfere with sensitive signal circuits or measurement circuits. For DC ground, it is noise ground. my country's general electrical regulations stipulate that the grounding resistance of AC working ground should be less than 4Ω. 1.2.3 Protective grounding The function of protective grounding is to prevent operators from being electrocuted due to insulation damage or degradation of electronic equipment and to ensure the safety of the equipment. (1) Safety protective grounding: Abbreviated as safety ground, it refers to the metal casing of power equipment that may be carrying dangerous phase voltage due to the damage of the insulation of the incoming power supply. In order to prevent such dangerous voltage from endangering personal and equipment safety, the non-energized metal casing (such as cabinet casing, component casing, etc.) during normal operation is grounded. Advantages: The casing grounding can make high-frequency interference voltage form a low impedance path, preventing it from interfering with electronic circuits. According to the different characteristics of power supply wiring methods in my country, there are five forms of safety grounding: TN-C system, TN-S system, TN-C-S system, IT system, and TT system. At present, the safety grounding of computers at home and abroad mostly adopts the second and fifth methods. The safety protective grounding resistance should be less than 4Ω; (2) Static grounding: It is a type of grounding designed to eliminate the static charge generated during the operation of the computing system. Since the humidity in the computer room is generally not less than 50%, and most computer rooms use anti-static raised floors, the possibility of static electricity generation is small. Therefore, this grounding is generally not required. Its grounding resistance should generally be less than 10Ω; (3) Shielding grounding: It is generally used in communication systems. It connects the equipment shield and the ground with a low-resistance wire to form an electrical path, providing a path for high-frequency interference signals. Its functions are twofold: (1) to prevent the information processed by the computer from being stolen; (2) to prevent external electromagnetic fields from interfering with the normal operation of the computer system. Its grounding resistance should generally be less than 2Ω. Shielding grounding is already included in the safety protection grounding of the computer casing, and is generally not set separately in the design. 2 Grounding methods and functions of computer grounding system The grounding system here refers to the relationship between the AC working grounding, DC working grounding, safety protection grounding, lightning protection grounding and the earth of the computer control system. The grounding methods of the grounding system can be divided into two categories: independent grounding and common grounding. They are discussed below: 2.1 Independent grounding: Each type of grounding is connected to an independent grounding device, and some groundings are connected to the independent grounding device through the grounding bus. Features: The computer's direct ground, power ground, and building's lightning protection ground are all independently connected to the earth, which looks perfect on the surface. Disadvantages: In actual engineering, the investment is high and the construction is difficult. This grounding system is only used when it is absolutely necessary under specific conditions. (1) When the DC working ground is floating, the relationship between the grounds is: When the DC ground of the computer control system is floating, except for the lightning protection ground, the other grounds can be connected to the same busbar and then connected to the grounding device; in order to prevent backflash, the distance from the lightning protection grounding device is required to be greater than 5m. (2) When the DC ground is directly grounded, the relationship between the grounds is: ① The DC ground and the lightning protection ground are independent, and the safety ground and the AC ground share a grounding device. Disadvantages: It is difficult to achieve the requirement of electrical independence between these three grounding devices during construction; ② The DC ground, AC ground and safety ground share a grounding device, and the lightning protection ground is a separate grounding device. Advantages: Convenient construction and low investment; widely used in current engineering practice. However, the requirement that the distance between the two grounding devices be greater than 5m must be met. ③ The computer's DC ground and safety ground share one grounding device, other power equipment shares one grounding device with the computer equipment's AC ground, and lightning protection ground is a separate grounding device. Advantages: It can avoid interference from non-computer system power equipment to the computer's AC signals. The distance between the three independent grounding devices should be greater than 5m. 2.2 Shared Grounding: From an electrical safety perspective, the most economical and practical measure is to use equipotential bonding, and the overall equipotential bonding is actually shared grounding. That is, all grounding devices of a building are connected to a single grounding device. Otherwise, when the lightning protection device is struck by lightning, its potential rises (up to millions of volts), causing electric shock or fire, and the probability of damage to equipment is much higher than with shared grounding. In addition, when a grounding fault occurs in the housing of a phase-line equipment in a 220/380V system, and the protection switch fails to trip the fault point in time, a high dangerous voltage will inevitably exist between the exposed conductive parts of the grounded electrical device and other independently grounded metal bodies. When constructing a shared grounding system, the following should be noted: various grounding systems share a grounding device when using single-point or multi-point grounding, but they cannot be connected arbitrarily. The correct connection method is: each equipment or cabinet has its own grounding busbar connected to its respective grounding busbar, and then these busbars are connected to the grounding device. Before this, the grounding branch lines of the equipment and the grounding busbar should be strictly insulated. There are three types of shared grounding connection forms. (1) Single-point grounding: a point in the circuit is defined as the zero potential reference point, and all zero potential grounding points in the computer control system are directly connected to it. When there are multiple cabinets in the system, the grounding of each cabinet and its electronic circuits can be separated from other cabinets, and the grounding of each cabinet is only connected to the grounding reference point of the entire system. Advantages: Eliminates common impedance coupling and low-frequency grounding loops, and works well at frequencies less than 1 MHz; (2) Multi-point grounding: Each grounding is connected to the zero-volt potential grounding reference plane with the most complementary connection line, so that the coupling resistance and standing wave effect of the grounding wire are minimized. This reference plane can be the shell of a device. When multiple devices are used, the grounding network (wire) is laid throughout the entire system. Advantages: It can be applied to high-frequency circuits with frequencies greater than (equal to) 10 MHz. The grounding connection is easier to achieve than single-point grounding during construction, and the standing wave effect at high frequencies can be minimized; (3) Hybrid grounding: A grounding method in which multiple points and single points coexist. Features: This grounding system has a relatively simple structure and is easier to construct, but the system is not safe enough, so this grounding system is rarely used at home and abroad. 3 Analysis based on examples In actual work, not all computer stations have the above-mentioned grounding. Generally, it should be considered comprehensively according to the working nature of the computer and the actual situation. A certain computer room has a grounding method where the system uses a common grounding. The DC working ground uses multi-point grounding. To ensure a unified potential reference plane for this computer's DC system, the DC grounds of these three devices should be connected to the same grounding device. This design uses a grounding network as the system's grounding device. The three devices are connected to the floating network with the shortest possible connection. ① is a zero-volt potential grounding reference plane with multiple grounding points. When the side length of this grounding grid is equal to 0.5m of the 30MHz wavelength, there is almost no potential difference between any two points on the grounding grid for all frequencies from DC to 30MHz. When the individual dimensions of the raised floor are 0.6 × 0.6 (m), the side length of the grounding grid should preferably be 0.6m. Points ② to ⑤: The network and the equipment's AC working ground and safety protection ground are connected together to the distribution box terminal block, and then led from the terminal block to the system grounding device. This method of ensuring all devices have the same grounding potential is very safe and prevents interference problems caused by improper grounding. 4. Conclusion Currently, there is a confusing issue regarding computer grounding in China: how to correctly select the grounding electrode to connect to the computer's grounding system. A common mistake made by many computer manufacturers is to require the installation of a separate, independent grounding electrode from the power system (including lightning protection). This practice is unsafe and is not permitted under the electrical regulations of some developed countries because it not only fails to eliminate noise but may also become a source of computer noise. Numerous engineering practices have shown that independent grounding systems for power supply, logic, security protection, and lightning protection are far more likely to be damaged by lightning strikes than shared grounding systems. Editor: He Shiping