Three-Phase Four-Wire System in Electrical Engineering

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Detailed Introduction to the Three-Phase Four-Wire System in Electrical Engineering:

### Basic Principle
- **Three-Phase Power Source**: It is generated by a three-phase alternating current generator and can provide three electromotive forces that are equal in magnitude, the same in frequency, and with a phase difference of 120° from each other. These three electromotive forces are represented by Phase A, Phase B, and Phase C respectively.
- **Connection Method**: The star connection method is usually adopted. That is, the ends of the three windings of the three-phase power source are connected together to form a neutral point. A line is drawn from the neutral point, which is called the neutral wire or the zero wire and is denoted by N. Then, three lines are respectively drawn from the starting ends of the three windings. These three lines are the phase wires, also known as the live wires, and are represented by A, B, and C respectively. In this way, the three-phase four-wire system is formed.

### Voltage Relationship
- **Line Voltage**: It refers to the voltage between any two phase wires in the three-phase four-wire system. Its effective value is usually 380V. Numerically, the line voltage is √3 times the phase voltage.
- **Phase Voltage**: It refers to the voltage between any one phase wire and the neutral wire. Its effective value is generally 220V. The phase voltage is the voltage supplied for single-phase loads, such as various electrical appliances in households.

### Main Functions
- **Provide Two Kinds of Voltages**: It can simultaneously provide a line voltage of 380V and a phase voltage of 220V to meet the requirements of different types of loads. The 380V voltage is suitable for high-power three-phase loads such as three-phase motors, while the 220V voltage is applicable to low-power single-phase loads such as single-phase lighting and sockets, making the application of the power system more flexible and extensive.
- **Balance Three-Phase Loads**: When the three-phase loads are unbalanced, the neutral wire can provide a current loop to keep the potential of the neutral point of the three-phase loads stable and ensure that each phase load can work normally. Without the neutral wire, when the three-phase loads are unbalanced, it will lead to the shift of the potential of the neutral point, causing changes in the voltages of each phase load and possibly damaging electrical equipment.

### Application Scenarios
- **Industrial Field**: It is widely used in various factories, mines, metallurgy, chemical and other places to provide power sources for high-power equipment such as three-phase motors, electric welding machines, and cranes. Meanwhile, it can also meet the needs of single-phase loads such as workshop lighting and office electricity.
- **Commercial Field**: For example, in shopping centers, office buildings, hotels and other places, it is used to supply power for large three-phase equipment such as air-conditioning systems, elevators, and ventilation equipment, as well as for single-phase loads such as lighting and sockets.
- **Civil Field**: In some multi-story residential buildings, villas and other buildings, if the three-phase four-wire system is adopted for power supply, it can meet the needs of high-power electrical appliances in households, such as air conditioners and electric water heaters, which use 380V voltage, and at the same time can also provide 220V voltage for other single-phase electrical appliances.

### Wiring and Installation Requirements
- **Conductor Selection**: Select conductors with appropriate cross-sectional areas and materials according to the magnitude of the load current and the laying method of the lines. Generally speaking, the cross-sectional areas of the phase wires and the neutral wire should be selected according to the calculated current, and the cross-sectional area of the neutral wire is usually not less than half of that of the phase wire.
- **Color Identification**: For the convenience of distinction and maintenance, it is usually stipulated that the Phase A adopts a yellow conductor, the Phase B adopts a green conductor, the Phase C adopts a red conductor, and the neutral wire adopts a light blue conductor. In the actual wiring process, the wiring should be carried out strictly in accordance with the color identification to avoid wrong wiring.
- **Grounding Requirements**: The neutral wire usually needs to be grounded on the side of the power transformer to form a zero potential. On the user side, depending on specific circumstances, repeated grounding is sometimes required to improve the safety and reliability of the system. However, it should be noted that the neutral wire and the protective earth wire (PE wire) should be strictly distinguished and not mixed.