In a three-phase power system, the three live wires are referred to as phase lines. The ends of these three wires are connected to different loads, while the tails of the three-phase lines are joined together, forming a neutral line. This neutral line is also known as the "zero line" because, under balanced conditions, no current flows through it. Additionally, the neutral line is either directly or indirectly connected to the earth, which keeps its voltage close to zero.
Typically, in a three-phase circuit, the live lines are color-coded: red, yellow, and blue. The neutral line is usually black. In a single-phase lighting circuit, the live wire is often yellow, the neutral is blue, and the ground wire is yellow-green. However, in some regions, the live wire may be red, the neutral black, and the ground wire yellow-green.
China currently uses a three-phase four-wire power supply system, which separates industrial and residential electricity. Industrial power includes three live lines, with a voltage of approximately 380V between them, and each phase is 120 degrees out of phase. For residential use, there is a live wire (fire line), a neutral wire, and a ground wire. The voltage between the live wire and the neutral wire is typically around 220V.
Electrical appliances require both a live and a neutral wire to create a complete circuit, allowing current to flow. The neutral wire is generally considered to be at zero potential, meaning it carries no charge. The ground wire, on the other hand, is not powered regardless of whether the switch is on or off. Its primary function is to provide a safe path for excess current or high voltage to be directed into the earth, preventing electric shocks and reducing the risk of electrical accidents.
The ground wire connects the equipment or appliance casing to the earth, serving as an essential safety measure. If the ground wire is missing or not properly installed, it can lead to dangerous situations. In everyday use, the ground wire is connected to the earth, while the neutral wire is also connected to the earth. Therefore, theoretically, there should be no voltage difference between the neutral and ground wires.
However, under certain conditions, a 220V voltage can appear between the neutral and ground lines:
1. **External damage**: If the wiring is damaged due to external forces, such as construction or accidents, this can cause a voltage difference.
2. **Poor installation quality**: If the neutral wire is poorly connected or has contact issues, it can become disconnected, leading to a voltage buildup.
3. **Unreliable grounding**: If the ground wire is not properly installed or has high resistance, it may act like a live wire when a fault occurs.
4. **Neutral wire disconnection**: When the neutral line is broken, the voltage between the neutral and ground can rise significantly.
5. **Reversed wiring**: If the neutral and live wires are accidentally swapped, this can result in unexpected voltages.
Understanding these scenarios helps in identifying and addressing potential electrical hazards, ensuring safer and more reliable power systems.
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