Thread Content
The IEC International Electrotechnical Commission classifies grounding methods for distribution networks into: TT systems, TN systems, and IT systems. The meaning of the alphanumeric codes representing these grounding types – TN, TT, and IT – is as follows: All three of these types use two letters to indicate the relationship between the three-phase power system and the exposed conductive parts of electrical equipment (such as equipment enclosures and bases) and the ground. The first letter indicates the ground connection of the power system; that is, T: it denotes a point that is directly grounded (usually the system’s neutral point) ； I: Indicates no grounding (all live parts are isolated from ground), or grounding through impedance (resistors, reactors) and through equivalent circuits. The second letter indicates the relationship between the exposed conductive parts of the electrical apparatus and ground: T means independent grounding from the grounding points of the power system; N means direct electrical connection to the grounding points of the power system. In the TN system, in order to indicate the relationship between the neutral wire and the protective wire, the following letter may sometimes be added after the TN code: S: indicates that the neutral wire and the protective wire are structurally separate ； C: Indicates that the neutral wire and the protective wire are combined together structurally (PEN wire). (1) TT system: The TT system is a three-phase four-wire system with the neutral point directly grounded; it is a system in which both the power supply system and the exposed conductive parts of electrical equipment are directly grounded separately. Its neutral wire is led out after being grounded at the power supply side, and serves only as the working neutral wire; the exposed conductive parts of the electrical equipment at the point of use are directly grounded on site. (2) TN system: A TN system is a power supply system in which there is one direct grounding point, and the exposed conductive parts of the load equipment are connected to this grounding point through a protective wire. Based on the arrangement of the neutral wire and the protective wire, there are three types of TN systems: (1) TN—C system. The TN—C system is a three-phase four-wire system with the neutral point directly grounded; in this system, the neutral wire and the protective wire are combined into one. This system is currently used by many high-voltage users in low-voltage power grids. Its characteristics are: the neutral point of the power transformer is grounded, and the protective earth wire (PE) shares the same wire as the working neutral wire (N). (1) It utilizes the neutral wire of a neutral-point grounded system as the return path for fault current; when the phase wire of an electrical device comes into contact with the enclosure, the fault current flows back to the neutral point through the neutral wire. Due to the high magnitude of the short-circuit current, an overcurrent protector can be used to cut off the power supply. The TN-C system generally uses zero-sequence current protection ； (2) The TN-C system is suitable for situations where the three-phase loads are roughly balanced. If the loads are unbalanced, there will be unbalanced currents in the PEN wire; in addition, harmonic currents generated by certain load devices also flow into the PEN wire. As a result, the neutral line N becomes charged, and its voltage is likely to exceed 50 V. This not only causes the equipment enclosures to become charged, posing a safety risk to people, but also makes it impossible to obtain a stable reference potential ； (3) The TN-C system should provide repeated grounding for the PEN wire; this serves to effectively reduce the voltage of the neutral wire with respect to ground when a device connected to neutral comes into contact with its casing. As can be seen from the above, the TN-C system has the following defects: (1) When the three-phase load is unbalanced, an unbalanced current appears in the neutral wire, and a voltage is generated between the neutral wire and ground. When a three-phase load is severely unbalanced, touching the neutral wire can lead to an electric shock accident. (2) The neutral wire passing through the leakage protection switch can only be used as a working neutral wire, and cannot be used as the protective neutral wire for electrical equipment; this is determined by the working principle of the leakage switch. (3) For single-phase electrical equipment equipped with a dual-pole leakage protection switch, such as those used in TN-C systems where the protective neutral wire serves to protect the metal enclosure, it is strictly prohibited to connect this wire to the working neutral wire of the circuit, nor shall it be connected to the PEN wire located before the leakage protection switch; however, misconnections can easily occur during use. (4) The connection wires of the repeated grounding device must not be connected to the working neutral wire that passes through the leakage circuit breaker. The TN-S power supply system completely separates the working neutral wire from the protective neutral wire, thereby overcoming the shortcomings of the TN-C power supply system; as a result, the TN-C system is no longer used in construction sites today. (II) TN-S system: The TN-S system is a three-phase five-wire system with the neutral point directly grounded. In this system, the neutral wire and the protective wire are separate entities. It offers high safety and reliability, and its use is being gradually promoted at present. Its characteristics include: the neutral point of the power transformer being grounded, with the protective neutral wire (PE) sharing the same path as the working neutral wire (N). (1) It utilizes the neutral wire of a neutral-point grounded system as the return path for fault current; when the phase wire of an electrical device comes into contact with the enclosure, the fault current flows back to the neutral point through the neutral wire. Due to the high magnitude of the short-circuit current, an overcurrent protector can be used to cut off the power supply. The TN-C system generally uses zero-sequence current protection ； (2) The TN-C system is suitable for situations where the three-phase loads are roughly balanced. If the loads are unbalanced, there will be unbalanced currents in the PEN wire; in addition, harmonic currents generated by certain load devices also flow into the PEN wire. As a result, the neutral line N becomes charged, and its voltage is likely to exceed 50 V. This not only causes the equipment enclosures to become charged, posing a safety risk to people, but also makes it impossible to obtain a stable reference potential ； (3) The TN-C system should provide repeated grounding for the PEN wire; this serves to effectively reduce the voltage of the neutral wire with respect to ground when a device connected to neutral comes into contact with its casing. As can be seen from the above, the TN-C system has the following defects: (1) When the three-phase load is unbalanced, an unbalanced current appears in the neutral wire, and a voltage is generated between the neutral wire and ground. When a three-phase load is severely unbalanced, touching the neutral wire can lead to an electric shock accident. (2) The neutral wire passing through the leakage protection switch can only be used as a working neutral wire, and cannot be used as the protective neutral wire for electrical equipment; this is determined by the working principle of the leakage switch. (3) For single-phase electrical equipment equipped with a dual-pole leakage protection switch, such as those used in TN-C systems where the protective neutral wire serves to protect the metal enclosure, it is strictly prohibited to connect this wire to the working neutral wire of the circuit, nor shall it be connected to the PEN wire located before the leakage protection switch; however, misconnections can easily occur during use. (4) The connection wires of the repeated grounding device must not be connected to the working neutral wire that passes through the leakage circuit breaker. The TN-S power supply system completely separates the working neutral wire from the protective neutral wire, thereby overcoming the shortcomings of the TN-C power supply system; as a result, the TN-C system is no longer used in construction sites today. (III) TN—C—S system: The TN—C—S system is a three-phase four-wire system in which the neutral wire is directly grounded; in this system, the neutral wire and the protective wire are combined in certain parts of the system. This system is currently widely used in the low-voltage power grids of high-voltage users. It consists of two grounding systems: the first is the TN-C system, and the second is the TN-S system; their boundary lies at the connection point of the N wire and the PE wire. (1) When a single-phase ground fault occurs in electrical equipment, in a TN-S system ； (2) When the N wire is disconnected, the fault is the same as that in the TN-S system ； (3) In the TN-C-S system, the PEN should be grounded repeatedly, while the N conductor should not be grounded repeatedly. The equipment enclosures connected to the PE wire are never charged during normal operation, so the TN-C-S system enhances the safety of operators and equipment. At construction sites, the TN-C-S system is generally adopted when the substation is located far from the site or when there is no dedicated transformer for construction purposes. In most TT and TN systems used in engineering design, the neutral point of the transformer is directly grounded; however, in some projects it is directly grounded within the low-voltage distribution cabinets. Both of these grounding methods are correct. It should be noted here that the direct grounding points in the power distribution system must be installed in accordance with the design requirements; when the transformer’s neutral point is designed to be grounded, it must be grounded at the transformer’s neutral point ； The design specifies that the transformer’s neutral point should not be grounded; however, when grounding is required within the low-voltage distribution cabinet, it must be done directly within that cabinet. It is a mistake to think that grounding a point in the power system necessarily means grounding it at the transformer’s neutral point. When discussing the TN-S system, the IEC stipulates that the N wire and the PE wire of the entire system are separate. The “entire system” should be understood as the load part of the power distribution system; that is, for the wires coming from the low-voltage distribution cabinets, the N and PE wires must not be connected further. In contrast, for the power supply part, the N and PE wires can be connected at one point or at multiple points. In a TN-S system, the PE wire must be laid together with the L and N wires. Although no standard specifies this, if the PE wire is laid separately, some people may not understand it. The PE wire can be laid alongside the L and N wires, or it can be laid separately; it is even possible for the L and N wires to be laid on the ground while the PE wire is laid underground, with both then coming together at the equipment. To determine whether this system belongs to the TN-S type, it is not determined by whether the PE wire is laid together with the L and N wires, but rather by whether the PE and N wires are directly connected at the power supply side. (3) IT systems: IT systems are those in which the three-phase three-wire system has a neutral point that is either ungrounded or grounded through a sufficiently large impedance (about 1000Ω), and the exposed conductive parts of electrical equipment are grounded. It is generally used in areas where power outages are not allowed, as well as in environments with harsh conditions that are prone to single-phase grounding or fires and explosions, such as coal mines, chemical plants, and textile factories. It can also be used in rural areas. In recent years, it has been increasingly employed as an emergency power source in important buildings, as well as for power and lighting systems in critical locations such as hospital operating rooms. The TN-C system is suitable for: 1. General industrial buildings and facilities where the three-phase loads are relatively balanced, the third harmonic current in the circuit is not high, and there is professional personnel for maintenance and management. 2. Not suitable for low-voltage sensitive electronic devices and explosive hazardous environments. The TN-C-S system is a widely used power distribution system suitable for industrial and residential buildings. The power supply lines use the TN-C system, and once inside the building, the PEN wire is re-grounded to separate into the PE wire and the N wire. This wiring system is simple yet ensures a certain level of safety. The TN-S system is suitable for: 1. Locations where single-phase loads are concentrated. 2. Locations equipped with precision electronic and data processing equipment. 3. Locations with requirements for fire and explosion prevention. 4. Locations with a large number of devices generating third-harmonic current. The TT system is suitable for: 1. Buildings powered by the power supply department via a low-voltage distribution system and located far from substations. 2. Precision electronic and data processing equipment with high requirements for voltage interference. 3. Locations with requirements for fire and explosion prevention.