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Electrical engineers are familiar with instrument grounding, and in this article, Changhui Instruments will discuss the grounding methods for low-voltage distribution IT systems, TT systems, and TN systems. These three grounding methods are easy to confuse; they differ in principle, characteristics, and application areas. I hope this information will be useful to electrical professionals everywhere. Colorless paperless recorder: yunrun.com.cn/product/339.html Definition: According to the current **standard “Code for Design of Low-Voltage Power Distribution” (GB 50054-2011), there are three types of grounding systems for low-voltage power distribution systems: IT system, TT system, and TN system. ①The first letter of IT, TT, and TN indicates the relationship between the power supply terminal and ground; T denotes that the neutral point of the power supply transformer is directly grounded ； The I mark indicates that the neutral point of the power transformer is ungrounded, or grounded through a high impedance. ②The second letter of IT, TT, and TN indicates the relationship between the exposed conductive parts of the electrical apparatus and ground. The T designation means that the exposed conductive parts of the electrical apparatus are directly grounded, with this grounding point being electrically separate from the grounding point at the power supply side ； N indicates that there is a direct electrical connection between the exposed conductive parts of the electrical apparatus and the power supply ground point. A comprehensive analysis of IT, TT, and TN low-voltage power distribution systems 1. IT system The IT system is one in which the power supply neutral point is not grounded, and the exposed conductive parts of electrical equipment are directly connected to ground. IT systems can have a neutral wire, but IEC strongly recommends not using one. Because if a neutral wire is installed, in an IT system any ground fault at any point on the N wire will cause the system to cease to be an IT system. http://yunrun.com.cn/upload/201808/25/201808251627521121.png Figure 1: Wiring diagram of the IT system. Characteristics of the IT system: ① When a grounding fault occurs for the first time in the IT system, there is only capacitive current flowing to the non-faulty phase, and this value is very small. The voltage across the exposed conductive parts with respect to ground does not exceed 50V; therefore, it is not necessary to immediately cut off the faulty circuit in order to maintain continuous power supply ； ②In the event of a ground fault, the voltage with respect to ground increases by 1.73 times ； ③220V loads require a step-down transformer, or must be powered by an external power source ； ④Install an insulation monitor. Application areas: Where high power supply continuity is required, such as emergency power supplies and hospital operating rooms. ⑤The IT-powered supply system offers high reliability and good safety when the power delivery distance is not very long. It is generally used in places where power outages are not allowed, or in locations that require continuous power supply under strict conditions, such as electric steelmaking processes, operating rooms in large hospitals, and underground mines. The power supply conditions in underground mines are relatively poor, and cables are prone to getting damp. ⑥By using an IT-powered supply system, even if the power supply’s neutral point is not grounded, any leakage current from a single phase to ground remains low, and this does not disrupt the balance of the power supply voltage; therefore, it is safer than systems with a grounded power supply neutral point. However, when used over long power supply distances, the distributed capacitance of the power supply lines to the ground cannot be ignored. ⑦When a load experiences a short circuit fault or leakage that causes the equipment’s casing to become charged, the leakage current forms a path through the ground; in such cases the protection device may not activate, which is dangerous. It is only safe when the power supply distance is not too long. This power supply method is rare on construction sites. 2. TT system: The TT system is one in which the neutral point of the power supply is directly grounded, and the exposed conductive parts of electrical equipment are also directly grounded. The grounding of the power supply neutral point is generally referred to as working ground, while the grounding of the exposed conductive parts of equipment is called protective ground. In a TT system, the working ground and the protective ground must be independent of each other. Device grounding can involve each device having its own separate grounding device, or it can be the case that several devices share one common grounding device. http://yunrun.com.cn/upload/201808/25/201808251729496176.png Figure 2: Wiring diagram of the TT system. Main advantages of the TT system: ① It can suppress the overvoltage that occurs in the low-voltage power grid when there is a connection between high-voltage and low-voltage wires, or when the insulation between the high-voltage and low-voltage windings of the transformer is damaged. ②Low-voltage power grids have a certain capacity to dissipate lightning overvoltage. ③Compared to the situation where the enclosure of low-voltage electrical appliances is not grounded, in the event of an accident involving contact with the enclosure, the voltage of the enclosure with respect to ground can be reduced, thereby minimizing the risk of electric shock to people. ④Since the grounding current is relatively large during a single-phase ground fault, it enables the protective device (residual current circuit breaker) to operate reliably and promptly isolate the fault. The main disadvantages of the TT system: ① When low-voltage and high-voltage lines are struck by lightning, step-up and step-down overvoltages may occur in the distribution transformers. ②The protective effect of grounding the enclosures of low-voltage electrical appliances is inferior to that of IT systems. ③When the metal casing of electrical equipment becomes charged (either due to a phase wire touching the casing or due to insulation damage that causes leakage), grounding protection can **reduce the risk of electric shock. However, the low-voltage circuit breaker (automatic switch) may not trip, resulting in the voltage of the enclosure of the leaking device relative to ground being higher than the safe voltage, which constitutes a dangerous voltage. ④When the leakage current is low, a fuse may not blow even in the presence of one, so a leakage protector is also needed for protection; as a result, it is difficult to promote the TT system. ⑤The TT system’s grounding apparatus consumes a large amount of steel, and it is difficult to recycle, requiring much labor and materials. Applications of the TT system: ① In the TT system, since the grounding device is located near the equipment, the likelihood of the PE wire breaking is low, and such breaks are easy to detect. ②When the TT system equipment is operating normally, its enclosure is not charged; in the event of a fault, the high voltage on the enclosure will not be transmitted throughout the entire system via the PE wire. Therefore, the TT system is suitable for powering voltage-sensitive data processing equipment and precision electronic devices (such as automated instruments). It offers advantages in hazardous environments where there is a risk of explosion and fire. ③The TT system can significantly reduce the fault voltage on equipment with leakage currents, but it generally cannot reduce it to a safe level. Therefore, when using a TT system, a leakage protection device or an overcurrent protection device must be installed, with the former being preferred. ⑤The TT system is mainly used for low-voltage customers, that is, small customers who do not have distribution transformers and draw low-voltage power from an external source. 3. TN system: The TN system is one in which the power supply neutral point is directly grounded, and the exposed conductive parts of the equipment are directly electrically connected to the power supply neutral point. In a TN system, all exposed conductive parts of electrical equipment are connected to the protective conductor and to the ground point of the power supply, which is usually the neutral point of the power distribution system. The power system of the TN system has a single point of direct grounding, and the exposed conductive parts of electrical equipment are connected to this point through a protective conductor. A TN system is typically a three-phase power grid system with a grounded neutral point. Its characteristic is that the exposed conductive parts of the electrical equipment are directly connected to the system ground point; in the event of a short circuit due to contact with the casing, the short-circuit current forms a closed circuit through the metal wires. A metallic single-phase short circuit is formed, thereby generating a sufficiently large short-circuit current that allows the protective device to operate reliably and disconnect the fault. If the working neutral wire N is grounded repeatedly, in the event of a short circuit to the casing, part of the current may flow to the repeated grounding points, which can prevent the protective device from operating reliably or cause it to fail to operate, thereby allowing the fault to worsen. In the TN system, that is, the three-phase five-wire system, the N wire and the PE wire are laid separately and are insulated from each other; it is the PE wire, rather than the N wire, that is connected to the casing of electrical equipment. Therefore, what matters most to us is the potential of the PE wire, rather than that of the N wire; hence, the repeated grounding in the middle is not a repeated grounding of the N wire. If the PE and N wires are grounded together, since they are connected at the multiple grounding point, there is no longer any distinction between the PE and N wires in the wiring between this point and the working grounding point of the distribution transformer. The neutral current that was originally carried by the N wire is now shared by both the N and PE wires; additionally, a portion of the current is diverted through the multiple grounding point. Since it can be assumed that there is no PE wire in front of the repeated grounding point, and only a PEN wire composed of the original PE wire and N wire connected in parallel remains, the advantages of the original TN-S system will be lost; therefore, the PE wire and N wire cannot be grounded together. In the TN system, it is divided into three types: TN-S system, TN-C system, and TN-C-S system, depending on whether its protective neutral wire is separated from the working neutral wire. ◆ TN-C system: http://yunrun.com.cn/upload/201808/25/201808251718185948.png Figure 3: Wiring diagram of the TN-C system. In the TN-C system, the functions of the PE wire and the N wire are combined; a single conductor called PEN performs the functions of both. At electrical equipment, the PEN wire is connected to both the load neutral point and the exposed conductive parts of the equipment. Due to its various inherent technical drawbacks, it is now rarely used, especially in civil power distribution; the TN-C system is essentially not permitted. Features of the TN-C system: ① When the equipment enclosure is energized, the neutral protection system can convert the leakage current into a short-circuit current; in effect, this is a single-phase to ground short-circuit fault. The fuse will blow or the automatic switch will trip, thereby cutting off power to the faulty equipment, which enhances safety. ②The TN-C system is only suitable for situations where the three-phase loads are roughly balanced. If the loads are unbalanced, there will be unbalanced currents in the working neutral wire, resulting in a voltage with respect to ground; therefore, there is a certain voltage on the metal enclosures of electrical equipment connected to the protective wire. ③If the working neutral wire is broken, the enclosure of the equipment connected to the protective ground becomes live. ④If the phase wire of the power supply is grounded, the potential of the equipment’s enclosure rises, causing a hazardous potential to spread along the neutral wire. ⑤When a residual current circuit breaker is used on the main line of a TN-C system, all additional grounding connections behind the working neutral wire must be removed; otherwise, the residual current circuit breaker will not be able to close. Additionally, all such additional grounding connections must be removed, and under no circumstances shall the working neutral wire be disconnected. Therefore, in practice, the working neutral wire can only be re-grounded above the leakage circuit breaker. ◆ TN-S system yunrun.com.cn/tech/2160.html http://yunrun.com.cn/upload/201808/25/201808251706042838.png Figure 4: Wiring diagram of the TN-S system. The neutral wire in the TN-S system is the same as that in the TT system. Unlike the TT system, the exposed conductive parts of electrical equipment are connected to the power supply’s neutral point through the PE wire, sharing the same grounding electrode as the system’s neutral point, rather than being connected to a separate grounding electrode of their own. The neutral wire (N wire) and the protective wire (PE wire) are separate. The most significant feature of the TN-S system is that once the N wire and the PE wire are separated at the system’s neutral point, there must be no electrical connection between them; if this condition is violated, the TN-S system ceases to function. Features of the TN-S system: ① During normal operation of the system, there is no current in the dedicated protection wire; only an unbalanced current exists in the working neutral wire. The PE wire has no voltage with respect to ground; therefore, the neutral connection for the metal enclosures of electrical equipment is made via the dedicated protection wire PE, ensuring safety and reliability. ②The working neutral wire is used only in single-phase lighting load circuits. ③The dedicated protection wire PE must not be disconnected, nor must it feed into the leakage circuit breaker. ④Leakage protectors are used on the main power lines, so leakage protectors can also be installed on the main supply lines of the TN-S system. ⑤The TN-S power supply system is safe and reliable, and is suitable for low-voltage power supply systems in industrial and civil buildings. ◆ TN-C-S system: http://yunrun.com.cn/upload/201808/25/201808251744195994.png Figure 5: Wiring diagram of the TN-C-S system. The TN-C-S system is a combination of the TN-C system and the TN-S system; in this system, the section starting from the power source uses the TN-C configuration. Because there are no electrical devices in this section; it merely serves to transmit electrical power. At a point near the electrical load, the EN wire is separated to form separate N and PE wires. From this point on, the system is equivalent to a TN-S system. Features of the TN-C-S system: ① The TN-C-S system can reduce the voltage between the motor casing and ground, but it cannot completely eliminate this voltage. The magnitude of this voltage depends on the load imbalance and the length of the line. It is required that the current due to load imbalance not be too large, and repeated grounding should be provided on the PE line. ②Under no circumstances should the PE wire be connected to the leakage protector, as the activation of the leakage protector at the end of the circuit will cause the preceding leakage protectors to trip, resulting in a widespread power outage. ③Apart from the main distribution box where the PE wire must be connected to the N wire, the N wire and PE wire must not be connected at any other distribution boxes; switches and fuses are not allowed to be installed on the PE wire. In fact, the TN-C-S system is a modified version of the TN-C system. When the working grounding of a three-phase power transformer is in good condition and the three-phase loads are relatively balanced, the TN-C-S system performs quite well in the practice of construction power supply. However, in the case of unbalanced three-phase loads and when there is a dedicated power transformer at construction sites, a TN-S power supply system must be used.