What are the grounding characteristics of power supply systems?

The type of grounding system to use depends crucially on the application

The basic power supply systems used for power supply in construction projects include three-phase three-wire system, three-phase four-wire system, etc. The International Electrotechnical Commission (IEC) has established unified regulations for this, referred to as TT systems, TN systems, and IT systems. The TN systems are further divided into TN-C, TN-S, and TN-C-S systems. TT power supply system: It refers to a protection system in which the metal casing of electrical equipment is directly connected to ground; it is known as a protective grounding system, or 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 loaded 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 coming into contact with the casing or due to insulation failure that causes leakage), ground protection helps to **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 low, a fuse may not blow even in the presence of one, 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. 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 casing of the equipment becomes charged, the neutral grounding protection system can convert 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; thus it 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. TN-C power supply system: It uses the working neutral wire as both the neutral wire and the protective wire; it can be referred to as the protective neutral wire, and is denoted by NPE. The characteristics of this power supply system are as follows. 1) Due to the unbalanced three-phase load, there is an unbalanced current in the working neutral wire, resulting in a voltage with respect to ground; therefore, the metal enclosures of electrical equipment connected to the protective wire also have a certain voltage. 2) If the working neutral wire is broken, the enclosure of the equipment protected by earth bonding becomes charged. 3) If the phase wire of the power supply comes into contact with the ground, the potential of the equipment’s enclosure rises, causing a dangerous potential to spread along the neutral wire. 4) When a leakage protector 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 leakage circuit breaker cannot be closed ； Furthermore, the working neutral wire must not be disconnected under any circumstances. Therefore, in practice, the working neutral wire can only allow repeated grounding on the upper side of the residual current device. 5) The TN-C power supply system is only suitable for situations where the three-phase loads are roughly balanced. TN-S power supply system: It is a power supply system that strictly separates the working neutral wire N from the dedicated protection wire PE; it is known as the TN-S power supply system. The characteristics of the TN-S power supply system are as follows: 1) During normal operation of the system, no current flows in the dedicated protection wire, while only unbalanced current flows in the working neutral wire. There is no voltage between the PE wire and ground; therefore, the neutral connection for the metal casing of electrical equipment is made through 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 be connected to 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. Before starting construction work on a building, the requirements of “three connections and one leveling” are met (electrical connection, water supply connection, road access, and leveled ground – a TN-S power supply system must be used). TN-C-S power supply system: In temporary power supply for construction projects, if the initial part of the system uses a TN-C configuration, but construction regulations require that a TN-S power supply system be used at the construction site, then it is possible to separate the PE wire from the main distribution box at the later part of the system. This system is called a TN-C-S power supply system. The characteristics of the TN-C-S system are as follows. 1) The working neutral wire N is connected to the dedicated protection wire PE; when the unbalanced current in this circuit is high, the neutral grounding protection of electrical equipment is affected by the potential of the neutral wire. 2) PE wires must under no circumstances be connected to the leakage protectors, as the activation of the leakage protector at the end of the circuit can cause the preceding leakage protectors to trip, resulting in a widespread power outage. 3) Except at 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 shall not be installed on the PE wire, nor shall a large conductor be used as a substitute for the PE wire. The TN-C-S power supply system is a temporary adaptation of the TN-C system. When a three-phase power transformer has proper working grounding and the three-phase loads are relatively balanced, the TN-C-S system is still a viable option for electrical use in construction projects. However, in cases of unbalanced three-phase loads or when there are dedicated power transformers at construction sites, a TN-S power supply system must be used. IT power supply system: I indicates that there is no working ground on the power supply side, or the grounding is through a high impedance. Every two letters, T, indicates that the electrical equipment on the load side is provided with grounding protection. The IT power 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 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. By 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 line to the ground cannot be ignored. When a load experiences a short circuit fault or leakage that causes the equipment enclosure to become charged, the leakage current forms a path through the ground, and the protection device may not activate. It is only safe when the power supply distance is not too long. This power supply method is rare on construction sites.

Great post; the detailed explanation is worth saving.