Connection Methods of Three-Phase Electric Meters

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There are mainly two connection methods for three-phase electric meters: direct connection and connection via transformers.

**I. Direct Connection Method**

1. **Connection Principle**
- This method is applicable to three-phase circuits with small currents. Directly connect the three live wires (Phase A, Phase B, and Phase C) of the three-phase power supply and one neutral wire to the corresponding terminals of the electric meter. The voltage coils and current coils inside the electric meter are directly connected to the voltage and current in the circuit, so that the power consumption of the three-phase circuit can be measured.

2. **Specific Connection Steps**
- Firstly, identify the connection terminals of the electric meter. Generally speaking, a three-phase electric meter has seven terminals, numbered 1, 2, 3, 4, 5, 6, and 7 from left to right. Among them, terminals 1, 4, and 7 are the incoming wire terminals (live wires) of Phase A, Phase B, and Phase C respectively; terminals 3, 6, and 9 are the outgoing wire terminals (also live wires) of Phase A, Phase B, and Phase C respectively; terminals 10 and 11 are the incoming and outgoing wire terminals of the neutral wire.
- Then, connect the live wires of Phase A, Phase B, and Phase C of the three-phase power supply to terminals 1, 4, and 7 of the electric meter respectively. Next, connect the live wires of Phase A, Phase B, and Phase C coming out of the electric meter to the load equipment from terminals 3, 6, and 9 respectively. Finally, connect the neutral wire to terminal 10 of the electric meter and then connect it out from terminal 11 to the neutral wire terminal of the load.
- For example, in the power supply line of a small three-phase motor, if the power of the motor is small and within the current range suitable for direct connection of the electric meter, the electric meter can be connected to the circuit according to the above steps, so as to accurately measure the power consumption of the motor.

**II. Connection Method via Transformers**

1. **Connection Principle**
- When the current in the three-phase circuit is relatively large, the directly connected electric meter may not be able to withstand such a large current. At this time, a current transformer is needed to reduce the current before connecting it to the electric meter. A voltage transformer is used to convert the high voltage into a voltage suitable for measurement by the electric meter. The function of the transformers is to convert the high voltage and large current into low voltage and small current according to a certain proportion, so that the electric meter can measure safely and accurately.

2. **Specific Connection Steps**
- For the connection of the current transformer:
- The current transformer has a primary winding and a secondary winding. The primary winding is connected in series to the live wires of Phase A, Phase B, and Phase C of the three-phase power supply, and the current flows in from the P1 terminal of the transformer and flows out from the P2 terminal. The secondary winding has two terminals, usually marked as S1 and S2. Connect the S1 terminal of the current transformer of Phase A to the corresponding current input terminal of the electric meter (such as terminal 1), and connect the S2 terminal to the common current terminal of the electric meter (such as terminal 3); connect the S1 terminal of the current transformer of Phase B to another current input terminal of the electric meter (such as terminal 4), and connect the S2 terminal to the common current terminal (such as terminal 6); connect the S1 terminal of the current transformer of Phase C to the corresponding current input terminal of the electric meter (such as terminal 7), and connect the S2 terminal to the common current terminal (such as terminal 9).
- For the connection of the voltage transformer:
- The primary winding of the voltage transformer is connected in parallel to the line voltage of the three-phase power supply, and the output terminals of the secondary winding are respectively connected to the voltage input terminals of the electric meter. For example, one end (usually marked as a) of the output terminal of the secondary winding of the voltage transformer of Phase A is connected to the voltage input terminal of Phase A of the electric meter (such as voltage terminal 1), and the other end (usually marked as x) is connected to the common voltage terminal of the electric meter (such as terminal 10); the output terminals of the secondary windings of the voltage transformers of Phase B and Phase C are also connected to the corresponding voltage terminals of the electric meter in a similar way.
- Taking the large three-phase distribution cabinet in a factory as an example, since the load current and voltage are relatively high, in order to accurately measure the electric energy, it is necessary to convert the signals through current transformers and voltage transformers and then connect them to the three-phase electric meter.

When connecting a three-phase electric meter, it is necessary to ensure that the wiring is correct and strictly follow the wiring diagram of the electric meter and relevant electrical safety regulations to avoid damage to the electric meter or safety accidents caused by incorrect wiring.
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