Two Wiring Methods of Three-phase Electric Meters
The wiring methods of three-phase electric meters are mainly divided into two types: **direct connection method** and **connection method via transformers**. The following is the specific description:
I. Direct Connection Method (Without Transformers)
Applicable Scenarios
- **Small load current** (usually ≤ 100A), and the rated current of the electric meter matches the load.
- **Low-voltage three-phase four-wire system** (such as residential buildings, small factories).
Wiring Steps
1. **Confirm the terminals of the electric meter**:
- A three-phase electric meter usually has 8 terminals (3 phase line inlets, 3 phase line outlets, 1 neutral line inlet, and 1 neutral line outlet).
- Some electric meters are labeled as: L1, L2, L3 (phase lines), and N (neutral line).
2. **Wiring sequence**:
- **Inlet side**: Connect the phase lines (L1, L2, L3) and the neutral line (N) of the three-phase power supply to the corresponding terminals of the electric meter.
- **Outlet side**: Connect the phase lines and the neutral line of the load to the outlet terminals of the electric meter.
3. **Precautions**:
- **Consistent phase sequence**: Ensure that the phase sequence of the power supply (L1→L2→L3) is consistent with the labeling of the electric meter terminals to avoid reverse connection.
- **The neutral line should not be disconnected**: The neutral line needs to be reliably connected. Disconnection may cause the electric meter to burn out.
- **Tighten the terminals**: Prevent overheating caused by poor contact.
II. Connection Method via Transformers (With Transformers)
Applicable Scenarios
- **Large load current** (usually > 100A), and it is necessary to convert it into a small current through a current transformer (CT).
- **High-voltage systems** or scenarios where isolated measurement is required.
Wiring Steps
1. **Wiring of the current transformer**:
- **Primary side**: Put the transformer over the phase lines (L1, L2, L3), and pay attention to the direction of the transformer (the P1 end faces the power supply side).
- **Secondary side**: Connect the secondary windings (S1, S2) of the transformer to the corresponding terminals of the electric meter respectively (such as 1→S1, 3→S2).
- **Short circuit and grounding**: The S2 terminal needs to be reliably grounded to avoid high-voltage hazards.
2. **Wiring of the voltage circuit**:
- **Direct connection**: If the system voltage is 220V/380V, the phase lines and the neutral line can be directly connected to the voltage terminals of the electric meter.
- **Via a potential transformer (PT)**: In a high-voltage system, it is necessary to step down through the PT and then connect it to the electric meter.
3. **Connection of the electric meter terminals**:
- Connect the current and voltage circuits according to the instruction manual of the electric meter. Usually:
- Current: L1 is connected to CT S1, L2 is connected to CT S1, and L3 is connected to CT S1.
- Neutral line: N is connected to the neutral line of the power supply.
4. **Precautions**:
- **Matching of the transformer transformation ratio**: The reading of the electric meter needs to be multiplied by the transformation ratio of the transformer (for example, for a 100/5A transformer, the actual current = the reading of the electric meter × 20).
- **Avoid open circuit**: The secondary side of the transformer is strictly prohibited from being open-circuited, otherwise, a high voltage will be generated.
- **Correct polarity**: Ensure that the polarities of the CT and PT are consistent, otherwise, the electric meter will run in reverse.
III. Comparison of the Two Methods
| Items | Direct Connection Method | Connection Method via Transformers |
| Applicable Current | Small current (≤ 100A) | Large current (> 100A) |
| Wiring Complexity | Simple | More complex (requiring transformers and secondary circuits) |
| Cost | Low (without transformers) | High (requiring transformers and accessories) |
| Safety | Direct contact with high voltage, higher risk | Isolated measurement, high safety |
IV. Safety and Specifications
1. **Power-off operation**: Disconnect the power supply before wiring to avoid electric shock or short circuit.
2. **Specification standards**: Comply with national standards (such as GB/T 17215) and the requirements of the local power department.
3. **Regular calibration**: Calibrate the accuracy of the electric meter after newly installing or replacing the transformer.
If you need to further understand the wiring diagram of a specific model of the electric meter or the selection method of the transformer, you can provide the model of the electric meter and system parameters to assist in the analysis!
