Differences between the Neutral Point and the Neutral Wire
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In an electrical power system, the neutral point and the neutral wire are two related yet distinct concepts. The following are their main differences:
### Definition - **Neutral Point**: In a three-phase alternating current power system, the common connection point where the ends (or starting points) of the three-phase power windings are connected together is called the neutral point. It is an equipotential point of the three-phase power source, and its potential is zero or close to zero relative to the earth under normal circumstances.
- **Neutral Wire**: The neutral wire is a conductor led out from the neutral point. It is connected to the earth and is mainly used in three-phase four-wire or three-phase five-wire power supply systems to provide a circuit for single-phase electrical equipment, enabling the current to flow back to the power source and ensuring the normal operation of the circuit.
### Function - **Neutral Point**: During the normal operation of the power system, the neutral point mainly plays a role in balancing the three-phase voltages. Since it is impossible for the three-phase loads to be completely symmetrical, the existence of the neutral point can keep the three-phase voltages relatively stable and avoid serious imbalance of the phase voltages caused by asymmetrical loads. In addition, the neutral point also provides a reference point for the grounding protection.
- **Neutral Wire**: The main function of the neutral wire is to provide a circuit for single-phase loads, allowing the current to flow out from the power source, flow back to the power source after passing through the loads, and form a complete circuit. Meanwhile, the neutral wire also plays a role in stabilizing the phase voltages to some extent. Especially when the three-phase loads are unbalanced, the current passing through the neutral wire can adjust the voltages of each phase to keep them within a reasonable range.
### Grounding Method
- **Neutral Point**: The neutral point can be directly grounded, or it can be ungrounded or grounded through equipment such as arc suppression coils. The system with the neutral point directly grounded is called a large-current grounding system. In such a system, when a single-phase earth fault occurs, the earth fault current is large, which will make the protection device act quickly to cut off the faulty line. While the system with the neutral point ungrounded or grounded through an arc suppression coil is called a small-current grounding system. When a single-phase earth fault occurs, the earth fault current is small and the system can continue to operate for a period of time.
- **Neutral Wire**: The neutral wire must be reliably grounded in the power supply system, usually connected to the earth at the neutral point of the power source. This can ensure that the potential of the neutral wire is close to the earth potential during normal operation, providing a safe circuit for single-phase equipment. Meanwhile, it is also beneficial for making the leakage current flow into the earth through the grounding wire when leakage and other faults occur, triggering the protection device to act and ensuring the safety of people and equipment.
### Potential Characteristics
- **Neutral Point**: During normal operation, the potential of the neutral point is theoretically zero, but in practice, due to factors such as unbalanced three-phase loads, there may be a certain displacement voltage generated, causing its potential to deviate slightly from the zero potential. However, within the allowable range, this potential shift will not affect the normal operation of the system.
- **Neutral Wire**: The potential of the neutral wire is basically the same as the earth potential under normal circumstances, which is zero potential. However, in some special cases, such as the neutral wire being broken or having poor contact, the potential of the neutral wire may increase, resulting in abnormal phase voltages of electrical equipment and even posing a threat to the safety of people and equipment.
### Current Situation - **Neutral Point**: When the three-phase loads are symmetrical, the current at the neutral point is zero. But when the three-phase loads are unbalanced, there will be current passing through the neutral point, and this current is called the unbalanced current. The magnitude of the unbalanced current is related to the degree of imbalance of the three-phase loads.
- **Neutral Wire**: In a three-phase four-wire system, under normal circumstances, there is current passing through the neutral wire, and its magnitude is equal to the vector sum of the unbalanced currents of the three-phase loads. When the three-phase loads are balanced, the current of the neutral wire is zero.
