Professional Analysis: Underground Smoke Sensor Alarm Thresholds and Standards

Underground fires in coal mines are among the most destructive hazards with the shortest response window. Unlike gas or dust explosions, fires are difficult to detect in early stages, but once they develop, they can quickly damage ventilation systems and trigger secondary accidents.

Smoke is one of the earliest, most stable, and most identifiable physical indicators of a mine fire. Therefore, the alarm thresholds of underground smoke sensors are a key technical component of mine safety monitoring systems.

Nexisense, with long-term experience in mine safety industrial sensors, has accumulated extensive practical knowledge in sensor application, threshold setting, and system integration. This article systematically analyzes underground smoke sensor alarm thresholds based on standards, principles, and engineering applications.

Standards Basis for Underground Smoke Sensor Alarm Thresholds

Underground smoke sensors are not ordinary fire protection devices. Their alarm parameters must meet both coal mine safety regulations and the adaptability to complex mine conditions. The main technical references include:

• Coal Mine Safety Regulations

• Related AQ series standards for coal mine safety monitoring systems

• MT/T industry technical specifications for mining sensors

• Enterprise-level engineering application and validation standards

Typical Alarm Threshold Ranges for Different Sensor Technologies

Common underground smoke sensors include ionization, photoelectric, and laser scattering types, each with distinct sensitivity and response speed:

• Ionization Smoke Sensor: Alarm threshold 0.1–0.3 dB/m, response time ≤ 30 s

• Photoelectric Smoke Sensor: Alarm threshold 0.05–0.2 dB/m, response time ≤ 15 s

• Laser Scattering Smoke Sensor: Alarm threshold 0.02–0.1 dB/m, response time ~10 s

Higher sensitivity aids early fire detection but requires stronger dust and mist interference handling.

Alarm Threshold Logic for Different Underground Scenarios

Mine environments are complex, and uniform thresholds cannot balance safety and false alarm rates. Thresholds should be set according to smoke generation mechanisms and background interference.

Belt Conveyor Galleries

• Sources: belt friction, drum overheating (high-risk fire origins)

• Recommended threshold: 0.08–0.12 dB/m

• Principles: detect early friction ignition, avoid frequent dust false alarms

Electromechanical and Transformer Rooms

• Sources: dense electrical equipment, cable overheating, short circuits

  

• Recommended threshold: 0.05–0.08 dB/m

• Principles: detect insulation aging/electrical faults early, correlate with temperature/current monitoring

Mining Workfaces

• Sources: high dust levels, complex background interference

• Recommended threshold: 0.1–0.15 dB/m

• Principles: slightly higher threshold, focus on trend changes rather than instantaneous values

Alarm Trigger Mechanism of Underground Smoke Sensors

Concentration Threshold Trigger Logic

• To avoid false alarms from transient interference, sensors typically use a combination of time + concentration criteria:

• Smoke concentration > threshold for 5 s → Level 1 alarm

• Smoke concentration > 150% of threshold for 10 s → Level 2 emergency alarm

• This design balances sensitivity and stability

Trend-based Early Warning

• Besides fixed thresholds, trend alarms are important for early fire detection:

• If smoke concentration rises faster than 0.05 dB/m/s over 30 s, an early warning is issued even if absolute threshold is not reached

• Useful for smoldering cables and slow equipment overheating

Engineering Features of Nexisense Mining Smoke Sensors

• Dust Recognition Algorithm: distinguishes smoke from dust via particle size distribution and variation

• Humidity Compensation: reduces water mist impact on optical detection in high-humidity environments

• Multi-level Alarm Output: supports audible/visual alerts, device linkage, system upload

• Standardized Signal Interface: compatible with 4–20 mA, RS485 mining control systems

  
  

Installation Guidelines and Alarm Response

Typical Installation Locations

• Belt Conveyor: 10–15 m downwind from drive drum

• Near Electromechanical Equipment: 1.5–2 m from equipment, height 1.2–1.5 m

• Main and Return Roadways: 1 sensor every 200 m

Standard Alarm Response Process

• Locate alarm point

• Automatically or manually stop relevant equipment

• Send personnel for on-site inspection

• Manually reset after hazards are removed

• Following a standardized response prevents minor hazards from evolving into major incidents

FAQ

• Why do high dust levels trigger false alarms? Dust has optical properties similar to smoke; particle feature analysis and trend recognition can reduce false alarms.

• Is a lower threshold always safer? No. Too low thresholds may cause frequent false alarms and reduce system reliability; thresholds should match the scenario.

• Do underground smoke sensors require periodic calibration? Yes. Even stable optical or laser sensors need periodic functional checks per mine regulations.

Conclusion

The alarm threshold of underground smoke sensors is not just a numeric setting, but a balance of standards, fire mechanisms, and field conditions. Scientific threshold settings, trend-based early warning, and standardized installation enable smoke monitoring systems to provide critical early intervention, giving mines valuable proactive safety control.

As coal mine safety evolves from reactive to preventive, high-reliability smoke monitoring has become an essential capability in modern mines.