U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health TRAPED

MENT OF HEALTH & HUMAN SERVICES . USA

MULTIPLE FIRE SENSORS FOR MINE FIRE DETECTION 

AND NUISANCE DISCRIMINATION

 Objective

To determine the effectiveness of multiple types of fire sensors for

early and reliable mine fire detection.

Background

The National Institute for Occupational Safety and Health

(NIOSH) has evaluated the effectiveness of multiple types of fire

sensors for early mine fire detection and nuisance signal

discrimination. Reliable early fire detection in underground mines

can be hindered by false fire alarms produced by carbon monoxide

(CO) and smoke particulate emissions from diesel equipment,

welding, and metal-burning activities. Another false fire alarm

source is the cross-interference of hydrogen (H2) produced by

battery-charging operations with a CO sensor's chemical cell.

Workers can learn to ignore repetitious "false" alarms associated with

these emissions. This jeopardizes their safety in the event of a mine

fire.

Approach

An experimental mine fire detection program was begun at the

NIOSH Pittsburgh Research Laboratory (PRL) to determine the

effectiveness of combinations of multiple types of fire sensors for

early mine fire detection and nuisance signal discrimination. Optical

and ionization smoke sensors, CO sensors, and metal oxide

semiconductor (MOS) sensors were used to collectively differentiate

fire products-of-combustion (POC) signatures from nuisance

emissions produced by diesel equipment, metal flame cutting, and the

charging of battery-operated mine equipment. As part of this

research program, coal, conveyor belt, electrical cable insulation, and

diesel fuel mine fire detection experiments were done in PRL's Safety

Research Coal Mine (SRCM) (see figures 1 and 2). An in-mine

evaluation of the sensors in an operating coal mine complemented the

research. Fire sensor alarm values for the smoke and MOS sensors

were defined as a 10-standard-deviation change in the sensor's signal

from its ambient value. The CO sensor alarm value was defined to

be a 5-ppm increase above ambient. Systematic interpretation of the

results was accomplished with the application of a neural network

analysis program.

Results

Experimental results showed the clear advantage of smoke sensors

over CO sensors for early mine fire detection. It was shown that

smoke sensors could detect smoldering belt combustion smoke

particulates long before alarm levels of 5-ppm CO were detected. In

the presence of diesel emissions, an MOS sensor, which is sensitive

to nitric oxides (NOx), responded to smoldering coal and belt POC

with a signal output indicative of a decrease in the sensor element

resistance in contrast with an increase in the sensor element

resistance associated with the presence of NOx from diesels. It was

shown in an operating coal mine that the MOS sensor can detect

nuisance emissions associated with diesel equipment and acetylene

torch cutting. Without the MOS sensor signal, the CO and ionization

smoke sensor would identify these nuisance emissions as a fire. At

a battery-charging station, several hundred ppm of H2 produced a

false indicated CO alarm value because of chemical cross

interference in the CO sensor's chemical cell. Without the smoke

sensor signal, the battery-charging activity would be considered a

fire. A neural network was developed that was able to predict for the

in-mine SRCM fire detection experiments the occurrence of a coal or

conveyor belt fire in the presence of steady-state diesel emissions

based on CO, MOS, and optical smoke sensor responses.

Recommendations

Multiple types of fire sensors, such as smoke, CO, and MOS

sensors, should be implemented whenever possible to reduce the risk

of nuisance fire alarms and provide early mine fire warning

capability based on the expected fire and nuisance emissions sources.

A rule-based system derived from the expected sensor responses

could be implemented into a mine atmospheric warning system. To

enhance miner safety, a smoke sensor should be used in a conveyor

belt entry for early fire detection and at a battery-charging station to

Technology News

Milestones in Mining Safety and Health Technology

SAFER HEALTHIER PEOPLE

No. 498 September 2002
Figure 1.—Sensor station in the Safety Research Coal Mine.

Figure 2.—Smoldering coal fire test in the Safety Research Coal Mine.

eliminate false chemical cell CO alarms. With the operation of diesel

equip-ment in underground mines, a NOx-sensitive MOS sensor can

be used to discriminate diesel emissions and provide a positive

indication of a fire. A combination of a CO sensor, smoke sensor,

and MOS NOx-sensitive sensor is one possible sensor combination

for early mine fire detection and nuisance signal discrimination.

For More Information

For additional information, contact John C. Edwards, Ph.D.,

NIOSH, Pittsburgh Research Laboratory, Cochrans Mill Rd., P.O.

Box 18070, Pittsburgh, PA 15236-0070; phone: (412) 386-6760;

fax: (412) 386-6595; e-mail: JCEdwards@cdc.gov

To receive additional information about occupational safety

and health problems, call 1-800-35-NIOSH (1-800-356-4674),

or visit the NIOSH Web site at www.cdc.gov/niosh

Mention of any company name or product does not constitute

endorsement by the National Institute for Occupational Safety

and Health.