Can the neutral wires of two transformers be shared?

Whether the neutral wires of two transformers can be shared depends on specific conditions and application scenarios.

 The following is a detailed analysis:

 I. Basic Principles

The main functions of the neutral wire (neutral line, N wire) are to **balance the three-phase current** and **provide a

 single-phase circuit**. Its potential is theoretically zero (achieved through the neutral point grounding). The core issue of

 sharing the neutral wire between two transformers lies in whether the potentials of the neutral points are consistent, and

 whether it meets the electrical safety specifications.

 II. Situations Where the Neutral Wire Can Be Shared

1. Transformers in Parallel Operation

   - **Conditions**: The two transformers need to meet the **parallel operation conditions** (the same voltage level, phase

 sequence, short-circuit impedance, and similar capacities).

   - **Wiring**: The neutral wires can be connected together, and the neutral points are grounded jointly. At this time, the 

neutral wire current is shared by the two transformers, and the system is stable.

   - **Application Scenarios**: Transformers that are backup to each other in the factory power distribution system, and the

 load switching is realized through the tie switch.

2. The Same Grounding System and Consistent Neutral Point Potentials

   - **Conditions**:

     - The **neutral point grounding resistances of the two transformers are the same** (generally ≤ 4Ω), and they are connected 

to the same grounding grid.

     - The neutral wire is only used as a current circuit and does not involve the potential difference of different systems.

   - **Example**: Multiple transformers in the same factory area adopt the **TN-S system**, the neutral wire (N wire) and the

 protective earth wire (PE wire) are strictly separated, and the neutral points are grounded together.

 III. Situations Where Sharing the Neutral Wire is Prohibited

1. Independent Transformers Not in Parallel Operation

   - **Risks**: The potentials of the neutral points of the two transformers may be different, resulting in **circulating current

 in the neutral wire** or **neutral point shift**, causing the following problems:

     - The voltage of the single-phase load is abnormal (higher or lower than 220V), damaging the equipment.

     - The neutral wire is live, increasing the risk of electric shock.

     - In case of a fault (such as the failure of the neutral point grounding of a certain transformer), the voltage is conducted

 to another system through the neutral wire.

YouTube：https://youtube.com/shorts/XYRiZyqxiDE 

 2. Mixing of Different Grounding Systems

   - If one transformer is a **TN system** (the neutral point is directly grounded), and the other is an **IT system** 

(the neutral point is not grounded), connecting the neutral wires will lead to the inability to identify the grounding fault and

 the failure of the protection device.

3. Severe Load Imbalance or Large Capacity Differences

   - When the load differences between the two transformers are large, the current in the neutral wire may exceed the design 

capacity, resulting in overheating of the wire or burning of the contacts.

 IV. Safety Precautions

1. **Grounding Specifications**:

   - The neutral point of each transformer must be **independently grounded** (the grounding resistance meets the standard),

 and it is prohibited to ground the neutral wires in series (as described in Summary 3).

   - When sharing the neutral wire, it is necessary to ensure that the impedance of the grounding grid is uniform to avoid

 potential differences.

2. **Protection Devices**:

   - If the neutral wires are connected, **zero-sequence protection** needs to be configured to monitor the unbalanced current.

   - Avoid using the residual current device (RCD), otherwise it may malfunction due to the current in the neutral wire.

3. **Suggestions for Application Scenarios**:

   - **Industrial Scenarios**: If a backup power source is required, give priority to using the **bus tie switch**, and the neutral

 wire is switched synchronously with the phase wire to avoid long-term sharing.

   - **Civil Scenarios**: It is strictly prohibited to mix the neutral wires of different transformers, and ensure that the single-phase

 load is powered by the same transformer.

V. Summary

- **Can be Shared**: Transformers in parallel operation, with matched parameters and in the same grounding system.

- **Prohibited from Being Shared**: Independent transformers, transformers with unmatched parameters, and transformers with

 different grounding systems.

**Final Suggestion**: When it comes to the transformation of the power system, professional personnel are required to conduct 

**short-circuit current calculations** and **relay protection designs** according to the transformer models, load characteristics, 

grounding methods, etc., to ensure compliance with standards such as the Code for Design of Low-Voltage Power Distribution

 (GB 50054).