Abstract: This article discusses the grounding methods and applicable scope of power distribution systems, and after briefly describing the RCD principle, points out the correct method for using RCD. Electricity is a clean energy source that is readily available, easy to transmit, and convenient to use. China's power industry has developed rapidly; in 2000, the national power generation reached 1368.5 TWh, and the installed power generation capacity reached 319 GW, ranking second in the world. The level of electrification has also been greatly improved. Electricity has become an indispensable energy source for various aspects of China's construction and people's lives. The use of electricity has spread to all walks of life. For example, electricity is used in metal smelting, welding, cutting, and metal heat treatment; in electrolysis, electroplating, and electrochemical processing; and also in transportation, medical care, and agricultural irrigation. Now, electricity is increasingly being used to improve the living environment. 1 Grounding Methods For a long time, the safe operation of power and the correct use of electricity have been issues of concern, and the correct grounding and effective protection technology of power distribution systems are important aspects of the safe use of electricity. In power systems, there are two grounding methods: direct neutral grounding (also known as a high-current grounding system) and ungrounded neutral (or grounded via an arc suppression coil, also known as a low-current grounding system). In high-voltage or ultra-high-voltage power systems of 110kV and above, direct neutral grounding is generally used. This is to reduce the insulation level of high-voltage electrical equipment and prevent overvoltages after a ground fault, eliminating the asymmetry caused by a single-phase ground fault. Under this grounding method, the zero-sequence current generated by the ground fault is sufficient to make the relay protection operate sensitively, thus ensuring reliable protection. Medium-voltage distribution systems generally have an ungrounded neutral, so the system can still operate asymmetrically for two hours after a single-phase ground fault. However, with the widespread use of underground power cables and the rapid increase in urban electricity load, many places have begun to adopt neutral grounding. For low-voltage distribution systems of 380/220V, except in some special cases, the vast majority are neutral grounding systems. The purpose is to prevent the risk of electric shock to operating personnel after insulation damage. Here is an example: the neutral point of the secondary side of a low-voltage three-phase four-wire transformer is grounded, but the casing of the electrical equipment is not grounded. When the casing is energized, if someone touches the casing, the current flowing through the human body is: Iren = Where: ux——phase voltage (V) rren——human body resistance (Ω) r0——grounding device resistance (Ω) Since r0 ≤ rren ≈ 1500Ω, then Iren ≈ ≌ 0.147A, which is much greater than the safe allowable value. 2 Residual Current Device (RCD ) According to national standard GB16917.1-97 "General Requirements for Residual Current Circuit Breakers with Overcurrent Protection for Household or Similar Purposes" and other standards, residual current devices can be divided into: (1) residual current operated switches (protective devices with only residual current protection); (2) residual current operated circuit breakers (protective devices with overload, short circuit and residual current protection functions); (3) residual current relays (protective devices with only residual current alarm function). 2.1 Working Principle of the Protective Device Residual current protection is a current-operated type of residual current protection. It is suitable for power transformer neutral point grounded systems (TT and TN systems), and also for some ungrounded IT systems with large ground capacitance (not applicable to phase-to-phase electric shock). The working principle of the residual current device is as follows: Three-phase lines A, B, C and neutral line N pass through a zero-sequence current transformer. The secondary coil of the zero-sequence current transformer is connected to the intermediate circuit and the trip unit. Under normal conditions (without electric shock or leakage faults), according to Kirchhoff's Current Law, the vector sum of the currents in the three phase lines and the neutral line is equal to zero, i.e.: ++ + = 0. Therefore, the sum of the magnetic flux vectors generated by the currents in each phase line in the core of the zero-sequence current transformer is also zero, i.e.: ++ + = 0. When someone experiences electric shock or a leakage fault occurs, leakage current appears. At this time, the vector sum of the primary currents through the zero-sequence current transformer is no longer zero, i.e.: Δ++ ≠ 0. The magnetic flux in the zero-sequence current transformer changes, generating an induced electromotive force on its secondary side. This signal enters the intermediate circuit. If it reaches the set value, it energizes the excitation coil, drives the main switch, and immediately cuts off the power supply, achieving electric shock protection. 2.2 Performance Parameter Description of Residual Current Device 2.2.1 Rated Leakage Operating Current (IΔn) This refers to the leakage operating current value at which the residual current device must reliably operate under specified conditions. The national standard (GB6829-86) specifies 15 sensitivity levels: 0.006, 0.01, 0.015, 0.03, 0.05, 0.075, 0.1, 0.2, 0.3, 0.5, 1, 3, 5, 10, and 20A. Sensitivity below 0.03A (30mA) is considered high sensitivity, 0.03-1A is medium sensitivity, and above 1A is low sensitivity. 2.2.2 Rated Leakage Non-Operating Current (I△n0): This is a necessary technical parameter to prevent maloperation of the leakage current device (RCD), representing the permissible three-phase unbalanced leakage current during normal grid operation. The national standard stipulates that I△n0 must not be less than half of I△n. 2.2.3 Leakage Operating Time: The operating time is from the moment the leakage current is suddenly applied until the protected main circuit is completely disconnected. To achieve safety protection against electric shock and meet the needs of graded protection, residual current devices (RCDs) are divided into three types: fast-acting, time-delaying, and inverse-time. 2.2.4 Sensitivity α Generally, the leakage signal current cannot be very large, and to ensure personal safety, China stipulates a signal current of 30mA for direct contact protection, while in other countries it can be as low as 6mA. Increasing the core cross-sectional area and the number of turns N1 can increase the excitation impedance Zm and the load impedance ZL, thus achieving high sensitivity. 3 Grounding of Low-Voltage Distribution Systems 3.1 Three Grounding Systems In China's "Code for Design of Civil Electrical Equipment" (JGJ/T16-92), low-voltage distribution systems are divided into three types: TN, TT, and IT. In this system, the first capital letter T indicates that the neutral point of the power transformer is directly grounded; I indicates that the neutral point of the power transformer is not grounded (or grounded through high impedance). The second capital letter T indicates that the casing of the electrical equipment is directly grounded, but not connected to the grounding system of the power grid; N indicates that the casing of the electrical equipment is connected to the neutral line of the system. TN system: The neutral point of the power transformer is grounded, and the exposed parts of the equipment are connected to the neutral line. TT system: The neutral point of the power transformer is grounded, and the casing of the electrical equipment does not have a dedicated protective grounding wire (PE). IT system: The neutral point of the power transformer is not grounded (or grounded through high impedance), and the casing of the electrical equipment does not have a dedicated protective grounding wire (PE). 3.2 TN system The neutral point grounding of the power transformer in the power system can be divided into three categories according to the different ways in which the exposed conductive parts of the electrical equipment are connected to the system: namely TN-C system, TN-S system, and TN-C-S system. These are introduced below. 3.2.1 TN-C system Its characteristics are: the neutral point of the power transformer is grounded, and the protective neutral (PE) and the working neutral (N) are shared. (1) It uses the neutral line (zero line) of the neutral point grounding system as the return conductor of the fault current. When the phase line of the electrical equipment touches the casing, the fault current returns to the neutral point through the zero line. Since the short circuit current is large, the power supply can be cut off by an overcurrent protection device. The TN-C system generally adopts zero-sequence current protection; (2) The TN-C system is suitable for three-phase loads that are basically balanced. If the three-phase loads are unbalanced, there will be unbalanced current in the PEN line. In addition, the harmonic current caused by some load equipment will also be injected into the PEN, so the neutral line N will be energized and is very likely to be higher than 50V. This not only makes the equipment casing energized, causing safety hazards to people, but also makes it impossible to obtain a stable reference potential; (3) The TN-C system should repeatedly ground the PEN line. Its function is to effectively reduce the voltage of the neutral line to ground when the phase of the equipment connected to the neutral line comes into contact with the casing.