In a three-phase power system, the three live wires are referred to as phase lines. The ends of these three lines are connected to different loads, while the tails of the three-phase power are joined together. This common point is called the neutral line, often also referred to as the "zero line." The term "zero line" comes from the fact that, during balanced three-phase conditions, no current flows through the neutral line. Additionally, it 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 single-phase lighting circuits, yellow often represents the live wire, blue is the neutral, and yellow-green indicates the ground wire. However, in some regions, red may be used for the live wire, black for the neutral, and yellow-green for the ground.
China currently uses a three-phase four-wire power supply system, which includes both industrial and residential electricity. Industrial power consists of three live wires with a voltage of approximately 380V between them, with each phase separated by 120 degrees. Residential electricity, on the other hand, includes a live wire (fire line), a neutral wire, and a ground wire, with a standard voltage of about 220V between the live and neutral lines.
For an electrical appliance to function properly, it requires a complete circuit formed by the live and neutral lines. Under normal conditions, the neutral line carries no voltage, making it safe to touch. However, the ground wire is always connected to the earth, regardless of whether the switch is on or off. Its main purpose is to provide a safe path for excess current in case of faults or electric leakage, preventing potential hazards to people and equipment.
The ground wire is designed to connect the metal casing of electrical appliances to the earth, offering protection against electric shocks. If the ground wire is not properly installed or fails, it can lead to dangerous situations. In everyday use, the ground wire is connected to the earth and the outer casing of appliances, while the neutral wire is also grounded. 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 factors, such as construction or accidents, it can cause a fault between the neutral and ground lines.
2. **Poor installation**: Inadequate construction practices can result in poor contact, leading to overheating and disconnection of the neutral line. This can cause the live wire to pass through the appliance, creating a voltage between the neutral and ground.
3. **Unreliable grounding**: If the ground wire is improperly installed or has high resistance, a short circuit between the live and ground wires can create a dangerous voltage on the ground wire.
4. **Neutral line disconnection**: When the neutral line is broken, the voltage between the neutral and ground lines can rise significantly, especially when loads like bulbs are connected.
5. **Reversed neutral and live lines**: If the neutral and live lines are accidentally swapped, it can cause unexpected voltages and unsafe conditions.
Understanding these scenarios helps in identifying and preventing electrical hazards, ensuring safer operation of electrical systems.
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