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Abstract: This paper addresses the two different types of protective grounding methods currently used in China’s public distribution networks, and outlines how power supply companies should guide customers in making the right choice when using these methods, as well as the issues that need to be taken into account during their actual application. ?? Keywords: protective grounding ； Grounding protection ； Neutral grounding protection ； Distribution network ； Residual current device Grounding protection and neutral connection protection are collectively referred to as protective grounding; it is an important technical measure taken to prevent electric shock incidents and ensure the proper operation of electrical equipment. The differences between these two types of protection are mainly reflected in three aspects: first, the protection principles are different. The basic principle of ground protection is to limit the leakage current from a faulty device to the ground, ensuring that it remains within a safe range; once this value exceeds a set threshold, the protector will automatically cut off the power supply ； The principle of neutral grounding protection is to use the neutral grounding circuit so that, in the event of insulation failure and the equipment coming into contact with its casing, resulting in a single-phase metallic short circuit, the short-circuit current triggers the protective devices on the circuit to operate promptly. Second, their scope of application is different. Based on relevant factors such as load distribution, load density, and load characteristics, the \"Technical Regulations for Low-Voltage Power Supply in Rural Areas\" define the scope of application for the operating systems of these two types of power grids. The TT system is typically used in rural public low-voltage power grids; it belongs to the type of grounding protection within protective grounding ； The TN system (which can be further divided into TN-C, TN-C-S, and TN-S types) is mainly suitable for urban public low-voltage power grids as well as the dedicated low-voltage power grids of industrial and mining enterprises. This system belongs to the zero-grounding protection type among protective grounding methods. The current low-voltage public distribution networks in China generally use the TT or TN-C systems, employing a mixed single-phase and three-phase power supply approach. That is, a three-phase four-wire 380/220V power distribution system that supplies power to both lighting loads and motor loads. Third, the line structures are different. A grounding protection system consists only of phase wires and a neutral wire. Three-phase power loads may not require a neutral wire; it is sufficient to ensure that the equipment is properly grounded. In such a system, apart from the grounding of the power supply’s neutral point, no other grounding connections should be made ； The neutral grounding protection system requires that the presence of a protective neutral wire be ensured under all circumstances; if necessary, this protective neutral wire can be installed separately from the neutral grounding wire. Meanwhile, the protective neutral wire in the system must have multiple points for repeated grounding. 1. Current status of the use of protective grounding at the client side in rural low-voltage public distribution networks As we know, most household appliances in China use three-core power cords with three-prong plugs, or four-core power cords with four-prong plugs. For three-phase four-wire electrical equipment, an additional yellow-green protective grounding wire is connected to the appliance’s casing to ensure protective grounding while the appliance is in operation. Since the existing public power distribution networks do not employ a unified and dedicated grounding (or neutral) wire to accommodate this. At the same time, not every customer possesses the professional technical knowledge in this area; furthermore, there are constraints and influences posed by factors such as the objective living conditions in urban areas, the non-standard design and construction of housing electrical distribution systems, and inadequate safety awareness and management by power supply authorities. For ordinary electricity customers, it is not an easy task to properly and effectively implement protective grounding. Therefore, most customers tend to discard the protective ground wire required by the product design after purchasing it. Even with certain measures taken and the use of protective grounding wires, it is often difficult to meet the technical standards specified in the regulations; there are numerous safety hazards, and this can even create many potential causes for accidents. As a result, the protective grounding that was originally required in products to ensure safe use of electrical appliances by customers has become nothing more than a gimmick and an unnecessary burden. Through on-site visits and household surveys, the author found that among the 500 randomly selected households (or organizations), not a single one was able to choose to use protective grounding correctly ； After purchasing electrical appliances, 95.6% of households discard the ground wire ； Customers who opted for protective grounding but used it incorrectly accounted for 4.4% of the total number of households ； Customers who are unaware of what type of power distribution system their local grid uses account for 98% of all households ； Less than 1% of customers are able to correctly distinguish between grounding protection and neutral connection protection. 2. Correctly understand and master the differences and application scopes of the two protection methods of protective grounding. Practice has shown that the use of protective grounding is an effective safety protection measure in China’s low-voltage power networks at present. Since protective grounding is further divided into grounding protection and neutral grounding protection, and the objective environments in which these two different protection methods are used vary, improper selection not only affects the protection performance available to customers but also impacts the reliability of power supply in the electrical grid. So, as electricity consumers in a public distribution network, how can one choose and use protective grounding in a proper and reasonable manner? The specific protection measures that electricity customers should adopt depend first and foremost on the type of distribution system in their power supply system. If the customer’s public power distribution network is a TT system, the customer should adopt grounding protection uniformly ； If the customer’s public power distribution network is a TN-C system, zero-grounding protection should be adopted uniformly. The TT system and the TN-C system are two systems with their own distinct characteristics. Although both can provide customers with a mixed single-phase and three-phase power supply of 220/380V, they cannot replace each other; moreover, their requirements regarding protective measures are entirely different. This is because, within the same power distribution system, if both types of protection are present simultaneously, in the event that a device equipped with grounding protection experiences a phase-to-case fault, the voltage of the neutral wire with respect to ground will rise to half or more of the phase voltage. As a result, all devices that rely on neutral grounding (since their metal casings are directly connected to the neutral wire) will acquire the same high potential, causing the metal parts such as the device casings to have a high voltage with respect to ground, thereby posing a risk to the safety of those using the equipment. Therefore, the same protection method can only be used in the same power distribution system; the two protection methods cannot be used together. Secondly, customers must understand what protective grounding is, and be able to correctly distinguish between grounding and neutral grounding protection. Protective grounding refers to the grounding system installed to prevent electrical hazards to human safety, as household appliances and electrical equipment may become charged due to insulation damage, causing their metal casings to carry electricity. Connecting the metal enclosure directly to the grounding electrode using a protective ground wire (PEE) is called grounding protection, as shown in Figure 1 ； When the metal enclosure is connected to the protective wire (PE) and the protective neutral wire (PEN), it is referred to as zero protection, as shown in Figure 2. 4. How to properly select and use grounding protection and neutral connection protection By standardizing the design and construction standards for the power distribution systems within customers’ buildings, and by adopting a local three-phase five-wire system or single-phase three-wire system in place of the three-phase four-wire system or single-phase two-wire system used in TT or TN-C systems for the indoor power distribution in newly built or renovated customer buildings, effective protective grounding can be achieved at the customer’s end. The so-called \"local three-phase five-wire system or single-phase three-wire system\" means that after low-voltage lines are connected to the customer’s premises, the customer must change from the traditional wiring methods used previously. On top of the existing three-phase four-wire and single-phase two-wire wiring systems, an additional protection wire is added to each electrical outlet that requires grounding protection, connecting to its grounding terminal. For ease of maintenance and management, the junction of the indoor outlet and the outdoor inlet of this protection wire should be located on the power distribution panel where the power is supplied, after which the connection method for the protection wire shall be determined based on the customer’s power distribution system. 4.1 Requirements for the installation of the grounding protection wire (PEE) in TT systems When the customer’s power distribution system is of the TT type, this system requires that the customer implement grounding protection measures. Therefore, in order to meet the requirements regarding the grounding resistance value for grounding protection, customers must install artificial grounding devices outdoors in accordance with the provisions of the \"Technical Regulations for Low-Voltage Power Supply in Rural Areas\". The grounding resistance must satisfy the following formula: Re ≤ Ulom/Iop. Where: Re is the grounding resistance (Ω); Ulom refers to the voltage limit (V), and under normal conditions it can be taken as 50 V for the effective value of alternating current; Iop is the operating current of the residual current (leakage) protector at the next higher level (A). For ordinary customers, using angle steel with dimensions of 40×40×4×2500 millimeters and driving it vertically into the ground at a depth of 0.6 meters will suffice to meet the requirements regarding the grounding resistance value. Then, a round steel bar with a diameter of ≥φ8 is welded and extended 0.6 meters above the ground, after which a wire of the same material and type as the power supply phase wire used for connection is used to link it to the protective wire (PEE) of the distribution panel. 4.2 Requirements for the installation of the protective earth wire (PE) in the TN-C system Since this system requires customers to use an earth-connected protection method, it is necessary to add a dedicated protective wire (PE) on top of the existing three-phase four-wire or single-phase two-wire system. This protective wire is derived from the protective neutral wire (PEN) in the customer’s power distribution panel, and it is connected together with the existing three-phase four-wire or single-phase two-wire system. To ensure the safe and reliable operation of the entire system, special attention must be paid during use: once the protective wire (PE) is separated from the protective neutral wire (PEN), a neutral wire N and a protective wire (PE) are formed at the client side; these two wires must not be combined back into a single PEN wire. To ensure the reliability of the repeated grounding of the neutral conductor (PEN), repeated grounding wires should be installed at both the beginning and end of the main conductors in a TN-C system, as well as at all branch T-connecting points and at the ends of the branches. Similarly, in three-phase four-wire systems, repeated grounding should also be carried out at the point where the service line enters the building, before the (PEN) wire is separated into the neutral conductor (N) and the protective conductor (PE). The cross-sectional area of the conductors used to protect the PEN, N, or PE is always selected in accordance with the standards for the cross-sectional area and type of conductors used for the phase wires. 5. Several issues to note when using protective grounding After selecting the appropriate protection method based on their own power distribution system, customers should also pay special attention to the following points: 5.1 In a TT system, all exposed conductive parts of electrical appliances used by customers must be provided with grounding protection. In a TT system, if the exposed conductive parts of electrical equipment are not grounded, any break in the insulation will result in dangerous voltages appearing on the casing. If a person comes into contact with such a casing, the current flowing through their body can reach several hundred milliamps, enough to be fatal. When grounding protection is applied to the exposed conductive parts, the presence of an RCD can cause the power supply to be disconnected, thereby ensuring personal safety. 5.2 In a TN-C system, all the exposed conductive parts of the electrical appliances used by customers must be connected to the protective neutral wire through a protective wire; it is strictly prohibited for the protective wire (PE) to become disconnected. In a TN-C system, the protective neutral wire is used to prevent electrical equipment from causing injury to people due to charged exteriors resulting from insulation failures, by connecting the exposed conductive parts of such equipment to the protective neutral wire via a protective wire. It provides protection primarily by utilizing the short-circuit current that is generated when a phase wire makes contact with the housing. This short-circuit current flows through the phase-wire to-neutral wire circuit, rather than passing through the power supply’s neutral grounding device, which triggers the overcurrent protection device to cut off the power supply and thus provide protection. Its protection efficiency is better than that of grounding protection. However, in the actual implementation process, if there is even the slightest carelessness and the protective requirements are not followed strictly according to the regulations, the risk of electric shock caused by the neutral grounding protection system remains very high. If the protective wire (PE) connecting to the customer’s electrical equipment is broken, or if the equipment does not have such a protective wire, in the event of insulation failure that causes the equipment’s casing to become charged, it will not result in a single-phase metallic short circuit; instead, the equipment’s casing will become electrically charged, posing a risk to both human safety and the equipment itself. 5.3 Proper placement of fuses In TT systems, it is not advisable to install devices on the N line that could disconnect it; when it is necessary to disconnect the N line, protective devices should be used that can disconnect both the phase line and the N line simultaneously. In a TN-C system, it is strictly prohibited to disconnect the PEN wire, and no electrical apparatus that can disconnect the PEN wire shall be installed. When it is necessary to install electrical equipment on the PEN line, the phase circuit must be disconnected accordingly. 5.4 Proper installation and use of the final residual current device Installing a residual current device is an effective technical measure to prevent damage caused by residual currents in low-voltage power grids. In low-voltage distribution networks, as the final level of protection for customers, RCDs (Residual Current Devices, also known as leakage circuit breakers) are typically used as additional protection. When choosing to install an RCD, customers must take full account of the power supply circuit, power supply method, power supply voltage, and the grounding type of the system ； It is also necessary to strictly distinguish between the neutral wire and the protective wire; the neutral wire of a three-pole four-wire or four-pole RCD should be connected to the RCD. It should be noted in particular that, regardless of the type of power distribution system used by the customer, once the neutral wire passes through an RCD, it may no longer be used as a protective wire; it cannot be grounded again or connected to any exposed conductive parts of the equipment, nor can it be connected to the RCD. After the RCD is installed, the neutral wire on the load side must not be shared with other circuits, and the insulation resistance of the protected electrical equipment and circuits during normal operation should be no less than 0.5 MΩ. For TT systems, low-voltage residual current protection generally adopts a multi-level protection approach that includes a main leakage protector (intermediate protection) and a final protection device. The final stage of protection among these is a self-protection device on the client side. For residential lighting customers, since the installation of power distribution protection devices is generally quite simple, regardless of the system they use, it is advisable to opt for multi-functional RCDs that offer leakage protection, short-circuit protection, overload protection, and overvoltage protection. Electrical equipment equipped with an RCD and electrical equipment without an RCD on the same circuit cannot share the same grounding electrode. The RCD wiring method for the TT system is shown in Figure 1. For TN-C systems, since multiple-stage protection for residual current protection is not permitted, only a final RCD can be installed at the power consumer’s receiving end. The RCD wiring method is shown in Figure 2. For ordinary residential customers, due to limitations in living conditions, they can only use the wiring method shown in the part of Figure 2 that is not marked with a “*” ； For institutional customers, it is recommended to use the wiring method indicated by the “*” in Figure 2. This method involves treating the client’s installation as a partial TT system, that is, connecting the exposed metal parts of the electrical equipment protected by the RCD to a dedicated grounding conductor using PEE wires. Since this PEE wire is not connected to any PE wires outside the local TT system, any hazardous fault voltages that arise outside that local TT system will not cause a potential difference to be induced in this PEE wire. As a result, its protection sensitivity is much higher than that of the wiring methods that do not feature the “*” symbol; however, the specialized grounding device required for it cannot be installed in ordinary households. To prevent customers from shutting down the RCD on their own, it is recommended that power supply companies install the RCD at the beginning of the power supply line in the customer’s distribution panel when installing the panel, and place the customer’s knife switch fuses after the RCD, thereby improving the efficiency of the RCD’s operation. 5.5 Standardizing indoor wiring Standardize the indoor wiring and installation procedures for clients, and carry out electrical installations in strict accordance with the requirements of the \"Technical Regulations for Low-Voltage Power in Rural Areas\". The wiring method for electrical appliances in the same location should be consistent; for example, the switches in the distribution panel should have their wires leading towards the panel, and for three-phase four-wire systems, the wires should be arranged from left to right as N, A, B, C ； The single-phase arrangement consists of a neutral wire and a phase wire. The switches of all electrical equipment should control the phase wire. Special attention must be paid to the wiring requirements for the socket: it must be a single-phase 2-hole socket. When installed horizontally, the live wire should be connected to the right terminal of the socket, while the neutral wire should be connected to the left terminal; when installed vertically, the live wire should be connected to the upper terminal of the socket, and the neutral wire to the lower terminal ； Single-phase 3-hole socket: in the TT system, the terminal at the upper hole facing the socket is connected to the ground wire, while in the TN-C system it is connected to the protective neutral wire; the terminal at the right hole is connected to the phase wire, and the terminal at the left hole is connected to the neutral wire ； It is a three-phase 4-hole socket; in the TT system, the terminal at the top of the socket is used for connecting the ground wire, while in the TN-C system it is used for connecting the protective neutral wire. The phase wires are connected to phases A, B, and C respectively via the terminals in the left holes. When sockets of different voltages are installed in the same location, they should be clearly distinguishable from each other, and plugs cannot be inserted into one another. 5.6 Eliminate illegal electricity use Customers must strictly abide by the \"Rural Safe Electricity Use Regulations\" when using electricity, and avoid any illegal practices related to electricity use. First, it is necessary to follow the instructions provided with the electrical appliances carefully. For electrical devices that require protective grounding, the appropriate method of protective grounding must be chosen based on the power system in use. Secondly, it is necessary to regularly test the operational reliability of RCDs. Those that do not function properly should be reported to the power supply department promptly for replacement or repair. When an RCD fails to operate as intended after triggering, the cause of the fault must be investigated immediately; power can only be restored once the faulty device has been fixed. It is strictly prohibited to disable the RCDs and force power to be supplied. Thirdly, it is necessary to select the appropriate size for fuses and circuit breakers based on one’s own power consumption. It is strictly prohibited to use copper or aluminum wires as substitutes for fuses. This is especially true for electricity consumers who rely on zero-grounding protection; if fuses and circuit breakers are not selected properly, in the event of a leakage fault in electrical equipment, the short-circuit current will not be sufficient to cause the fuse to blow in time, thereby preventing the power supply from being disconnected and hindering the effective operation of the zero-grounding protection system. This is because the system utilizes the single-phase metallic short circuit that occurs when the insulation of the equipment is damaged and comes into contact with the casing; the resulting high short-circuit current causes the overcurrent protection device to activate promptly, thereby cutting off the power supply to the equipment with a leakage fault. If the fuse selected has a blowing current value that is greater than the short-circuit current value, the fuse will not blow in time and thus will fail to cut off the power supply. Fourth, one should not think that installing an RCD solves all problems; even the slightest sense of luck can become a hazard to safe electricity use