Two Core Wiring Methods for Three-Phase Electric Meters

The two core wiring methods for three-phase electric meters are direct connection type and current transformer-connected type. They are applicable to different load current scenarios, with significant differences in wiring logic and application scope, detailed as follows:

1. Direct Connection Type
2. Core Principle

The current coil of the electric meter is directly connected in series with the phase lines of the three-phase circuit, and the voltage coil is directly connected in parallel between the phase lines (for three-phase three-wire systems) or between the phase lines and the neutral line (for three-phase four-wire systems). No additional transformers are required, and electric energy metering is directly completed through the internal coils of the meter.

2. Scenari applicativi

This type is suitable for scenarios with small load currents within the rated current range of the meter, ad esempio:

Small-scale three-phase power equipment (per esempio., three-phase motors with power < 10kW);

Mixed load circuits such as three-phase lighting and office power supply;

Three-phase power metering in residential communities or small commercial shops.

3. Classification and Wiring Key Points

Three-phase three-wire direct connection: Suitable for three-phase three-wire systems (without neutral lines, per esempio., high-voltage side or pure power supply). The meter has 6 wiring terminals in total, divided into 3 groups of incoming lines (Phase A, B, C) E 3 groups of outgoing lines. The phase sequence must be strictly corresponding and reverse connection is strictly prohibited.

Three-phase four-wire direct connection: Suitable for three-phase four-wire systems (with neutral lines, capable of bearing single-phase and three-phase loads). The meter has 8 wiring terminals in total, including the incoming/outgoing lines of Phase A, B, C and the incoming/outgoing line of the neutral line. The neutral line must be reliably connected without loose contact.

4. Advantages and Disadvantages

Advantages: Simple wiring, basso costo, no transformer error, and direct and reliable metering accuracy.

Disadvantages: Only applicable to small-current loads and unable to meet the requirements of high-current industrial scenarios.

1. Current Transformer-Connected Type
2. Core Principle

When the load current exceeds the rated current of the meter (per esempio., > 100UN), the current transformer is used to convert the large current into a standard small current (usually 5A) that the meter can bear according to a fixed transformation ratio (per esempio., 100/5, 200/5). The current coil of the meter is connected to the secondary side of the transformer, and the voltage coil is still directly connected in parallel in the circuit.

2. Scenari applicativi

This type is suitable for high-current industrial power consumption scenarios, ad esempio:

High-power three-phase motors (with power > 30kW), large compressors, pumps and other equipment;

Power distribution rooms of factories and main metering circuits of production lines;

Power metering of main power distribution boxes in construction projects.

3. Wiring Key Points

The polarity of the current transformer must be correct (P1 is connected to the incoming line side, P2 to the outgoing line side; S1 on the secondary side is connected to the incoming current terminal of the meter, S2 to the outgoing current terminal of the meter). Otherwise, the meter will reverse or cause metering errors.

The secondary side of the transformer must be reliably grounded and open circuit is strictly prohibited (open circuit will generate high voltage, endangering equipment and personal safety).

Electric energy calculation needs to be multiplied by the transformer transformation ratio, per esempio., Meter reading × (100/5) = Actual power consumption.

It is divided into three-phase three-wire and three-phase four-wire transformer connection methods. The three-phase four-wire system needs an additional neutral line connection to ensure accurate metering of single-phase loads.

4.Advantages and Disadvantages

Advantages: Can adapt to any high-current load and expand the metering range of the meter.

Disadvantages: Requires additional configuration of transformers, has high wiring complexity, and the metering accuracy is affected by the transformer accuracy.

Comparison Table of Two Wiring Methods