How to distinguish: TN-C system, TN-S system, TN-C-S system, TT system

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How to distinguish: TN-C system, TN-S system, TN-C-S system, TT system. The basic power supply systems used in building electrical installations include three-phase three-wire system and three-phase four-wire system, etc., but the meanings of these terms are not entirely precise. The International Electrotechnical Commission (IEC) has established unified regulations for this, referred to as the TT system, TN system, and IT system. The TN systems are further divided into TN-C, TN-S, and TN-C-S systems. What follows is a brief introduction to various power supply systems. (1) Basic methods of power supply for electrical installations: According to the various protection methods and terminology defined by IEC, low-voltage distribution systems are classified into three categories based on their grounding methods, namely TT, TN, and IT systems, which are described below. (1) TT power supply system: The TT system refers to a protection system in which the metal casing of electrical equipment is connected directly to the ground; it is known as a protective grounding system, or simply the TT system. The first symbol, T, indicates that the neutral point of the power system is directly grounded ； The second symbol, T, indicates that the exposed metal conductive parts of the load equipment, which are not in contact with live parts, are directly connected to the ground, regardless of how the system is grounded. All groundings of the load in the TT system are referred to as protective grounding. The characteristics of this power supply system are as follows. 1) When 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 hazardous voltage. 2) When the leakage current is relatively small, even a fuse may not blow, so a leakage protector is also needed for protection; therefore, the TT system is difficult to implement on a wide scale. 3) The grounding systems in TT systems consume a large amount of steel, and they are difficult to recycle, requiring significant labor and materials. Some construction companies currently use the TT system; when they borrow power from this system for temporary use, a dedicated protective wire should be used to reduce the amount of steel required for grounding devices. The newly added dedicated protective wire, the PE wire, is separated from the working neutral wire, the N wire. Its characteristic is that: ① The common grounding wire has no electrical connection with the working neutral wire ； ②During normal operation, current can flow in the working neutral wire, while no current flows in the dedicated protection wire ； The TT system is suitable for areas where grounding protection is scattered. (2) TN power supply system: This type of power supply system is a protection system in which the metal casing of electrical equipment is connected to the working neutral wire; it is known as the neutral connection protection system, and is denoted by TN. Its features are as follows. 1) Once the enclosure of the equipment becomes charged, the neutral grounding protection system can transform the leakage current into a short-circuit current. This current is very large, being 5.3 times that of the TT system; in effect, it represents a single-phase to ground short-circuit fault. The fuse in the circuit breaker will blow, and the trip mechanism of the low-voltage circuit breaker will immediately activate, causing the faulty equipment to lose power, thereby ensuring safety. 2) The TN system saves materials and labor time, and is widely used in China and many other countries; it clearly has more advantages over the TT system. In TN power supply systems, they are classified into TN-C and TN-S types, depending on whether the protective neutral wire is separated from the working neutral wire. (3) TN-C power supply system: In this system, the working neutral wire is also used as the protective wire for grounding; it can be referred to as a protective neutral wire, and is denoted by NPE. (4) TN-S power supply system: This is a system in which the working neutral wire N and the dedicated protective wire PE are strictly separated from each other. It is called a TN-S power supply system, and the characteristics of this system are as follows. 1) When the system is operating normally, there is no current in the dedicated protection wire; only an unbalanced current exists in the working neutral wire. There is no voltage between the PE wire and ground, so the zero protection for the metal casing of electrical equipment is connected to the dedicated protection wire PE, ensuring safety and reliability. 2) The working neutral wire is used only in single-phase lighting load circuits. 3) The dedicated protection wire PE must not be disconnected, nor must it feed into the leakage circuit breaker. 4) Leakage protectors are used on the main power lines; the working neutral wire shall not have multiple grounding points, while the PE wire may have multiple grounding points, but it does not pass through a leakage protector. Therefore, leakage protectors can also be installed on the main power supply lines of the TN-S system. 5) The TN-S power supply system is safe and reliable, and is suitable for low-voltage power supply systems in industrial and civil buildings. The \"three connections and one leveling\" prior to construction work in building projects (electrical connection, water supply connection, road access, and leveled ground – a TN-S power supply system must be used. Last edited by btlhrdb on 2009-2-25 09:20)