A brief discussion on the grounding of low-voltage explosion-proof distribution devices and its solutions

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A Brief Discussion on the Grounding of Low-Voltage Explosion-Proof Distribution Devices and Its Solutions. A Brief Discussion on the Grounding of Low-Voltage Explosion-Proof Distribution Devices and Its Solutions, analyzed by Explosion-Proof Distribution Boxes/Co., Ltd.; the details are as follows: 1. Grounding is an important safety measure to prevent electrical equipment from leaking current. \"Ground\" generally refers to the earth. But electrically, it has a deeper meaning. Grounding is the establishment of an electrical conductive path between the components of one system and another system (or with a certain reference point). In electrical and electronic systems, the term “ground” usually has two meanings: one is “earth,” and the other is “system reference ground.” Grounding refers to connecting a certain part of an electrical device to the ground through a grounding device. A brief overview of the grounding of low-voltage distribution systems and its solutions ； Connecting to ground refers to linking the conductive parts of electrical equipment that are not charged under normal conditions (such as metal casings) to the ground wire of the power supply grid. Since the earth contains conductive substances such as water found in nature, it is also capable of conducting electricity. When a charged conductor comes into contact with the ground, a hemispherical \"ground field\" is formed with the point of contact as its center. At this point, the grounding current flows into the ground through the conductor from the grounding point, and spreads in a hemispherical pattern in all directions. Grounding and neutral connection are important safety measures to prevent electric shock in the event of a leakage current in electrical equipment. The ground is usually taken as the zero potential point of the system. The ideal ground must be an ideal conductor with zero potential and zero impedance; there should be no voltage difference between any points on it, and it can serve as a reference point for all voltages in the system. There are two purposes for grounding: one is for safety, known as protective grounding, and the other is to provide a stable zero potential reference point for signal voltages or system voltages, known as signal ground or system ground. Protective grounding refers to establishing a good metallic connection between the metal casing of electrical equipment, which is not charged under normal conditions, and a dedicated grounding device. Its function is to limit the voltage with respect to ground to a specified safe range when the metal casing of the device becomes accidentally charged, thereby eliminating or reducing the risk of electric shock. Protective grounding is most commonly used for electrical equipment in low-voltage ungrounded distribution networks. The function of protective grounding is that when the metal casing of an electrical device becomes charged, if a person touches this casing, since the resistance of the human body is much higher than that of the grounding conductor, most of the current flows into the ground through the grounding conductor, while only a small amount of current passes through the human body. At this time, by properly controlling the ground resistance, electric shock accidents can be reduced. However, in TT power supply systems, the voltage on the equipment enclosure of devices using this protection method remains quite dangerous to humans. Therefore, this protection method is only applicable to construction sites with TT power supply systems, where the resistance of the protective grounding is required to be no more than 4Ω. Protective grounding is a widely used safety measure, mainly applicable to three-phase three-wire power systems with a neutral point that is not directly grounded. In a ground-insulated power system, unless otherwise specified, all metal parts that may develop dangerous voltages due to insulation failure or other reasons shall be grounded. Protective zero connection involves linking the non-live conductive parts of electrical equipment during normal operation to the ground neutral wire of the power grid, in order to prevent electric shock accidents. Protective neutral grounding is typically used in three-phase four-wire power systems with a directly grounded neutral point and a voltage of 380/220V; it must also be used in combination with other safety measures such as fuses and circuit breakers to achieve its purpose. When the metal casing of an electrical device becomes charged, the short-circuit current forms a closed circuit through the neutral wire, resulting in a single-phase short-circuit fault. Since the impedance of the neutral wire is very low, the short-circuit current is extremely high, usually several times or even dozens of times the rated current. Such a severe single-phase short-circuit causes the protective devices to operate quickly and accurately, thereby cutting off the power supply in question and ensuring personal safety. Its power supply system is a zero-connected protection system, namely the TN system. Whether the protective neutral is separated from the working neutral allows the TN power supply system to be divided into three types: TN-C, TN-S, and TNC-S. Whether protective grounding or protective neutralizing is used, it is not allowed in the same system to ground some equipment while neutralizing another. In the same system, if some devices are grounded while others are connected to the neutral wire, then when a grounded device experiences a shell fault, the potential of the neutral wire will rise, causing the shells of all devices connected to the neutral wire to acquire a dangerous voltage. High voltage can penetrate into the low voltage side when the insulation between the high-voltage and low-voltage sides of a transformer is damaged, or when a broken high-voltage conductor comes into contact with the low-voltage wiring. To prevent and reduce this risk, in low-voltage distribution systems with an ungrounded neutral point, in addition to implementing reliable protective grounding, protective measures must also be in place to prevent high voltage from leaking into the low-voltage system. To this end, in a low-voltage power grid with an ungrounded neutral point, the neutral point or one of the phases should be connected to the ground through a breakdown fuse. Under normal conditions, the low-voltage system remains ungrounded. When high voltage penetrates into the low-voltage area, the fuse is blown; the fault current flows into the ground through the grounding system, which in turn triggers the protective devices in the high-voltage system to cut off the power supply, thereby providing protection. To ensure the safety of the protective zero system, one or more points on the neutral wire are connected repeatedly to the ground through a grounding device; this is known as repeated grounding. Its purpose is to reduce the voltage of leaky devices with respect to the ground. Leakage protectors can be used not only to safeguard human safety but also to monitor the insulation condition of low-voltage systems or equipment ； The wiring downstream of the leakage protector installation point should have good insulation from ground ； RCDs do not provide protection against two-phase electric shock, but they reduce the risk in case of a broken neutral wire. Furthermore, it also reduces the imbalance of the three-phase load, which leads to a decrease in the voltage of the neutral wire with respect to ground; this thereby reduces the risk and shortens the duration of short-circuit faults. Since the repeated earth connections and the working earth connection form a parallel circuit, the short-circuit current increases, thereby accelerating the operating time of the circuit breaker ； The lightning protection performance of overhead lines has been improved. The lightning current can be diverted, thereby limiting the overvoltage generated by lightning ； Repeated grounding refers to the reconnection of one or more points on the neutral wire to the ground through grounding devices, which can improve the safety of the circuit. 2. The grounding types, requirements, and application scopes of low-voltage systems – although fairly comprehensive and effective protection schemes have already been provided in the past design codes and standards for low-voltage distribution systems to ensure the safety of equipment and personnel – However, the protective measures to ensure the data security of information networks seem to be “relatively inadequate”. Given the above circumstances, when addressing grounding design issues and various solutions in information networks, it is not advisable to strictly adhere to existing design specifications. For the grounding system of low-voltage distribution systems in the information network era, it is necessary not only to ensure the safe and reliable operation of power supply lines, electrical equipment, and people. Moreover, it is even more necessary to protect the security of the data processed, stored, and transmitted by information network devices. For this reason, when designing a grounding system, we need to consider not only the grounding resistance of various \"grounding wires\". Great attention also needs to be paid to grounding impedance ； “The actual wiring method of the ground wire ； Factors such as whether the users’ daily management and maintenance of network devices are \"reasonable\". Practice has shown that a properly designed grounding system and correct wiring practices are among the prerequisites for ensuring that network devices operate efficiently and with high bandwidth. The International Electrotechnical Commission (IEC) specifies that the protective grounding types for low-voltage distribution systems are divided into three systems: TN, IT, and T. The grounding methods for power distribution systems in our country follow the standards set by IEC. These methods are still classified into three categories – TN, TT, and IT – based on the combination of grounding for the power distribution systems and electrical equipment. The characteristics of these systems are represented by symbols, with the first letter indicating the relationship between the power source and ground. “T” indicates a firm ground connection at a certain point ； ““I” indicates that all live parts are insulated from ground or grounded at a certain point through impedance. The second letter indicates the relationship between the electrical equipment’s enclosure and ground. ““T” indicates a solid grounding of the enclosure, independent of the power supply grounding, while “N” indicates that the enclosure is firmly connected to the system ground point. The subsequent letters indicate the combination of the neutral line and the protective line in the power grid. ““C” indicates that the neutral line and the protective wire are combined; “S” indicates that the neutral line and the protective wire are separate. TN system: The power supply neutral point is directly grounded and a neutral wire (N wire) is provided; it is a three-phase four-wire system. All exposed conductive parts of electrical equipment in this system shall be connected to the common protective wire (PE wire), which is connected to the neutral wire at the grounding point. Our country refers to this system as protective zero connection. The power side of the TN system has a direct grounding point, and an N wire is led out; it is a three-phase four-wire system. In the system, the enclosures of electrical equipment are directly connected to this point through a protective wire, a practice commonly known as protective grounding. Depending on the different combinations of the neutral wire and the protective wire in the system, it is further divided into the following three forms. TN—C system: In this system, the neutral wire and the protective wire are combined into one, which is known as the TN—C system. Due to low investment and the saving of conductive materials, it was widely used in our country in the past. When the three-phase load is unbalanced or there are only single-phase electrical devices, normal load current flows through the PEN wire; sometimes third harmonic currents also flow through it. The voltage drop generated by these currents on the PEN wire results in the electrical equipment’s casing becoming charged, thereby creating a voltage with respect to ground. Under normal operation, this voltage ranges from a few volts to several dozen volts, which is below the safe voltage of 50V. However, when there is a break in the PEN wire or a short circuit relative to ground, the potential of the PEN wire increases, resulting in a voltage relative to ground that exceeds the safe level and thereby increasing the risk of electric shock. At the same time, the PEN wires within the same system are connected to each other; fault voltages can travel along these PEN wires to other areas that have not suffered any faults, which may lead to new electrical failures. Additionally, since the entire system uses PEN wires for equipment grounding, electrical isolation cannot be achieved, and this fails to ensure the safety of personnel carrying out electrical maintenance. As a result, this approach is hardly used internationally. TN—S system: In this system, the neutral wire and the protective wire are separate, which is what is referred to as the TN—S system. The advantage of this system is that the PE wire does not carry load current under normal conditions; it becomes charged only in the event of a ground fault. As a result, it does not cause electromagnetic interference to other devices connected to the grounded PE wire. Therefore, this system is suitable for use in data processing and precision testing equipment. A break in the N wire does not affect the safety of devices connected to the PE wire in terms of protection against indirect electric shock. Such systems are commonly used in environments with poor conditions, where high requirements are placed on safety and reliability, as well as in situations where devices are sensitive to electromagnetic interference. However, such a system cannot address issues such as the propagation of fault voltages to the ground and the rise in neutral point potential caused by relative ground short circuits. TN—C—S system: In this system, the neutral wire and the protective wire are first combined, and then separated again, resulting in the TN—C—S system. After PEN is divided into PE and N wires, it cannot be combined or interchanged with the PE wire again; otherwise, it becomes a TN—C system. This system combines the features of both the TN-C system and the TN-S system; it has a simple power supply circuit structure while still ensuring a certain level of safety. It is often used in locations at the end of distribution systems where the environmental conditions are poor, or in places with equipment for data processing. Since the PE wire carries a certain voltage from the PEN wire at the upstream end, the equipment’s casing becomes charged, and contact with it can result in electric shock. IT systems: The power supply neutral point is either ungrounded or grounded through an impedance; generally, no neutral wire (N wire) is provided, and it belongs to a three-phase three-wire system. The exposed conductive parts of the equipment in this system are directly grounded separately through their respective PE wires. In the system, the enclosures of electrical equipment are in direct electrical connection with the ground, a practice commonly known as protective grounding. This grounding point is not connected to the grounding point at the power supply terminal. In this system, since the casings of all devices are directly grounded through their respective PE wires, and there is no electromagnetic connection between these PE wires, it is also suitable for powering data processing devices and precision testing equipment. This prevents dangerous fault voltages from spreading along the PE wires to other areas that have not suffered any faults. In the TN system, since the PE (PEN) conductors are connected together, it is difficult to determine the cause and location of ground faults. Therefore, the TT system has been designated by power supply authorities as the grounding system to be used for supplying electricity from urban public low-voltage grids to consumers. However, this grounding protection system does not guarantee safety in all situations; when the insulation of equipment in the system is damaged or a relative ground short circuit occurs, the casing of the equipment can become charged. If a person comes into contact with a live enclosure, since the contact resistance of the human body is much higher than that of the protective grounding resistance, the single-phase short-circuit current flows into the ground through the grounding system; as a result, the current flowing through the human body is relatively small, thereby reducing the risk of electric shock.