Components of the Three-Wire System

 Power Positive (V+)
Provides the DC power required for sensor operation (typically 12–24 V DC). Example: 24 V DC input to drive the sensor circuitry.
Power Negative/Common Ground (GND)
Serves as the return path for the power supply and may also act as the signal reference ground.
Ensures a complete current loop and provides a reference potential for the signal.
Signal Output Line (SIGNAL)
Transmits an analog signal proportional to gas concentration (e.g., 4–20 mA, 0–5 V, 0–10 V, or digital signals).
 Delivers detected gas concentration data to controllers or alarm systems via this line.

Working Principle Separate Power and Signal Paths: The three-wire system isolates the power supply (V+ and GND) from the signal output (SIGNAL), avoiding interference issues seen in two-wire systems where power and signal share lines. Noise Resistance: Independent signal lines reduce voltage drop and electromagnetic interference during long-distance transmission, improving measurement accuracy. Versatility: Suitable for scenarios requiring stable power and isolated signal output, such as industrial environments monitoring toxic/flammable gases.

Advantages of the Three-Wire System
Stability: Separated signal and power lines minimize noise interference.
Compatibility: Supports multiple output types (analog/digital).
Safety: Meets explosion-proof requirements (e.g., Ex certification) in hazardous areas through isolated design.
Simplified Wiring: More cost-effective than four-wire systems for medium-distance installations.

Typical Applications

• Fixed gas monitoring in industrial facilities (e.g., petrochemical plants, gas stations).

• Long-distance transmission (over 50 meters) with high signal accuracy requirements.

• Integration with PLCs or DCS systems using standard industrial signal inputs.

The three-wire system enables fixed gas detectors to deliver reliable gas concentration monitoring in complex industrial environments, balancing cost, performance, and installation complexity.