Rapid Detection and Suppression of Mining 

"Equipment Cab Fires 

Maria I. De Ros Charles D. Litton, Pittsburgh Research Laboratory, 

National Institute for Occupational Safety and Health, P.O. Box 18070, 

Pittsburgh, PA 15236, USA 

Abstract. The National Institute for Occupational Safety and Health (NIOSH/PRL) 

conducted a series of large-scale experiments to evaluate the effectiveness of optical 

flame detectors, photoelectric smoke detectors, and combined ionization and photoelectric smoke detectors for rapidly detecting mining equipment cab fires. The detector alarm times were then used to trigger the disc~arge of a fire inerting system inside 

the cab to suppress cab material fires. This paper discusses the types of fire detectors 

tested, the experiments that Were conducted, and the results that were obtained. Con-" 

elusions are that rapid detection of equipment cab fires can be achieved to trigger the 

discharge of a fire inerting system inside the cab to protect the operator in the cab. 

Keywords: fires, equipment, detection, suppression, mining, inerting system 

,. Introcluction 

1.1. Background 

Analyses of mining equipment fires for the 1990-1999 period showed that 171 of 

the 340 equipment fires were due to ruptured lines resulting in the spraying of 

hydraulic fluid or fuel onto the engine hot surfaces [1-3]. In at least 97 cases, these 

fires raged out of control resulting in loss of equipment and presenting significant 

risks to the cab bperatoL On 16 occasions, flammable vapors and mists penetrated 

the cab and ignited, forcing the operator to exit under hazardous conditions during 

the critical time needed to perform safe parking of the vehicle and engine shutoff. 

Findings also showed that on 10 occasions, fires originated in the cab due to electrical malfunctions that ignited combustible materials inside the cab. 

In this study, optical flame detectors, photoelectric smoke detectors, and combined ionization and photoelectric smoke detectors were evaluated for their effectiveness in rapidly detecting cab fires. The detector alarm times were then used to 

trigger the discharge of a fire inerting system inside the cab to suppress cab fires. 

Of note is that during the inertization of the cab volume for the suppression of 
cab fires, the oxygen concentrations are reduced to levels that do not support

combustion, yet are maintained sufficiently high to support life (O2, �12%) during the critical time needed for the operator to perform emergency tasks (safe

parking of the vehicle and engine shutoff), and exit the cab.

Of note is that the NIOSH/PRL experiments, for the rapid detection and suppression of cab fires, are being conducted for the first time within the mining

industry and other industries as part of extensive studies for the safety of equipment operators during fires.

The experiments were conducted in a large-scale mining equipment cab (1.8 m

long · 1.5 m high · 0.9 m wide) (Figures 1–4), using various sized fire sources located within the cab, at various distances from the detector (Figures 1–4).

1.1.1. Cab Fire Inerting System. The cab fire inerting system [4] is a commercially

available system consisting of an inert gas mixture concentration (Inergen, 45%

mixture concentration: N2, 50%; Ar, 42%; and CO2, 8%), designed (1.45 m3 of

gas) for a cab volume of 2.43 m3

. The gas mixture is contained in a canister bolted to the cab rear wall and is discharged inside the cab through a muffled nozzle

system at a discharge rate that maintains a breathable cab atmosphere [5, 6].

1.1.2. Fire Detectors. Four commercially available fire detectors were evaluated in

this study for the specific intend to test their effectiveness in detecting incipient

flaming and/or smoking fires in the cab for the early discharge of a fire inerting

Figure 1. Schematic of the cab with an optical flame detector (OFD

#1) exposed to fire sources.
Figure 2. Schematic of the cab with an optical flame detector (OFD

#2) exposed to a fire source.

Figure 3. Schematic of the cab with a smoke detector (I/PSD)

exposed to fire sources.
Figure 4. Prelit fuel tray fires (32 kW: 250 ml diesel and 250 ml

gasoline contained in a 23 cm dia fuel tray).

system inside the cab. Two of the detectors were optical flame detectors, a third

was a photoelectric smoke detector, and the fourth was a combined ionization

and photoelectric smoke detector. The number of detectors was confined to four

for experimental economic reasons. This study is not a survey and testing of commercially available fire detectors but a guide to the mining safety industry in the

use of cab fire detectors.

The two optical flame sensors detect electromagnetic radiation produced by a

flame. Both of these sensors respond to radiation produced in both the ultraviolet and infrared regions of the spectrum (UV and IR): ultraviolet sensors

respond to energy emitted between 0.18 and 0.4 lm; while infrared sensors

respond to energy between 0.75 and 20 lm [7–10]. The two sensors differ primarily by their respective field of view: one sensor, denoted as OFD (1, with a

180� field of view, was designed for wide-area coverage while the second, denoted as OFD (2, with a 45� narrow field of view, was designed as a line-of-sight

detector for short distances (<50 cm). Both of these sensors have rapid response

times in milliseconds, and may also be used as explosion detectors. For these

experiments, the detectors response times were recorded in seconds due to software–hardware constraints.

The other two fire detectors that were evaluated were smoke detectors. One of

the smoke sensors was a photoelectric smoke detector, denoted as PSD, which

detects light scattered by smoke particles within a small chamber [11]. Photoelectric smoke detectors are generally more sensitive to the larger smoke particles produced from smoldering fires than to the much smaller particles produced from

flaming fires. The other smoke sensor was a combined ionization and photoelectric smoke detector, denoted as I/PSD. This detector utilizes a small ionization

chamber to generate an alarm to the smaller particles produced from flaming fires
[12] while the photoelectric component generates an alarm when larger smoke particles from smoldering fires are produced.

2. Experiments

A total of 47 experiments were conducted with the two optical flame detectors,

the photoelectric smoke detector, and the combined ionization and photoelectric

smoke detector, positioned within the equipment cab. For these experiments, various sized fire sources were used (�32, �0.5, �0.05 kW). In addition, a fourth fire

source, a small candle, was also used with an estimated heat release rate of

0.005 kW. The fire sources were located at various cab locations and distances

from the detector (Figures 1–4). Detector response times as a function of fire size,

location and distance were recorded.

For the fire sources 32, 0.5, and 0.05 kW, a 50/50 mixture of No. 2 diesel fuel

and gasoline were used with surface areas of 0.041, 0.002, and 0.00049 m2

, corresponding to diameters of 0.23, 0.05, and 0.025 m, respectively.

The fire heat release rate (kW), Qf, was calculated from the standard formula

[13]:

00 Q Þ; f 1⁄4 ðAsÞðHcÞðmf

where As is the surface area of the liquid fuel surface (m2); Hc is the average heat

of combustion of the fuel mixture (approximately 40 kJ/g) and mf¢¢ is the mass

flux of fuel from the surface given by the expression; mf¢¢ = 55(1 ) e

2.1d

); with d

equal to the fuel surface diameter, in m.

Combining these expressions yields heat release rates of 32, 0.5, and 0.05 kW

for the three fuel surface areas defined above. The calculated heat release rate for

the 32 kW fire sized source is reported in the following:

ð0:041 fuel tray surface areaÞm2

ð40 kJ/gÞð19:5 g/m2 � sÞ 1⁄4 32 kW:

2.1. Optical Flame Detector with a 180� Field of View

Twenty-one experiments were conducted with the OFD (1, positioned at the upper

left corner of the rear cab wall (0.23 m distance from ceiling and 0.26 m distance

from wall; detector dimensions, 25 cm dia · 12 cm high) (Figure 1). For these

experiments, three sized fire sources were used: 0.5 kW (5 ml of diesel and 5 ml of

gasoline contained in a 5 cm dia tray), 0.05 kW (2.5 ml of diesel and 2.5 ml of

gasoline contained in a 2.5 cm dia tray), and 0.005 kW (candle), located at various cab locations and distances from the detector (Figure 1). The cab floor locations, shown in Figure 1, were: (1) center of front wall (under the control panel,

at 20 cm distance from the front wall; distance from the detector, 1.65 m); (2) center of rear wall (distance, 1.14 m); (3, 4) center of front and rear left side walls
(distances, 1. 4 and 1.14 m, respectively); and, (5, 6) center of front and rear right

side walls (1.5 m and 1.22 m, respectively). In addition, a flame source (7) was

placed at the center of the control panel (distance, 1.6 m).

2.2. Optical Flame Detector with a 45� Field of View

Four experiments were conducted with the OFD (2, positioned at the center of

the cab ceiling (detector dimensions, 7.5 cm dia · 5 cm high) using three sized fire

sources (0.5, 0.05, and 0.005 kW) (Figure 2). For these experiments, the fuel sources (1) and (2) were located directly under the detector, at 50 cm (0.5 kW fire

source) and 30 cm (0.5, 0.05, 0.005 kW fire sources) distances from the detector

(Figure 2).

2.3. Combined Ionization and Photoelectric Smoke Detector

Eighteen experiments were conducted with the I/PSD, positioned at the center of

cab ceiling (detector dimensions, 7.5 cm dia · 5 cm high), using four sized fire

sources (32, 0.5, 0.05, and 0.005 kW), located at various cab locations and distances from the detector (Figure 3). The 32 kW fire source consisted of 250 ml of

diesel and 250 ml of gasoline, contained in a fuel tray (23 cm dia) (Figure 4)

equipped with electric matches for remote ignition at cab door closing.

The cab floor locations were: (1) center of front wall (under the control panel,

at 20 cm distance from front wall; distance from the detector, 1.47 m); (1a) center

front floor (distance, 1.37 m); (2) center of rear wall (distance, 1.58 m); and, (3, 4)

center of front and rear side left walls (distances for each location, 1.5 m). In

addition, one of the fire sources (5) was placed at the center of the control panel

(distance, 0.84 m) (Figure 3). Additional experiments (two experiments) were conducted with the I/PSD exposed to smoldering fire sources (consisting of 10 pieces

of electrical cord, 2.5 cm long, contained in a 13 cm dia tray), located at the center of the front and rear cab floor (distances, 1. 37 m and 1.4 m, respectively).

2.4. Photoelectric Smoke Detector

Two experiments were conducted with the PSD, positioned at the center of the

cab ceiling (detector dimensions, 7.5 cm dia · 5 cm high), using smoldering fire

sources similar to the sources used for the I/PSD, located at the center of the

front and rear cab floor (distances from the detector, 1.37 and 1.4 m, respectively).

2.5. Cab Fire Inerting System Experiments

Two experiments were conducted with the cab fire inerting system for the suppression of cab fires. For these experiments, the discharge of the fire inerting system

inside the cab was manually triggered 5 s after the 32 kW fuel tray was lit

(Figure 5). The trays (1) and (2), equipped with electrical matches for remote ignition at cab door closing, were located at the center of the front and rear cab floor

(distances from the detector, 1.37 and 1.4 m, respectively) (Figure 5).
Figure 5. Schematic of the cab with a cab fire inerting system.

3. Results and Discussion

3.1. Detector Response Times

Tables 1–3 and Figure 6 show the results for all the experiments. Figure 6 shows

the response times (2 s) yielded by the optical flame detector with a 180� field of

view (OFD (1), exposed to a 0.5 kW fire source located on the cab floor at the

center of the front wall (distance from the detector, 1.65 m). The detector yielded

similar response times for the 0.05 kW and 0.005 kW fire sources, located at all

cab locations and distances (maximum distance, 1.65 m) (Table 1). Of note is that

for these experiments, the detectors response times were recorded in ‘‘seconds’’

rather than in ‘‘milliseconds’’ due to software–hardware constraints.

For the OFD (2, similar response times of 2 s were obtained for all fire sources,

located directly under the detector at 30 cm distance (Table 1). The detector failed

to detect the 0.5 kW fire source (the only source tested) at a distance of 50 cm

(Table 1).

For the combined ionization and photoelectric smoke detector (I/PSD),

response times of 5 s were obtained for the 32 kW fire source, located at the center of the front cab floor (Table 2), at a distance from the detector of 1.37 m. For

this type of detector, under these experimental conditions, the rapid response

times are due to the fast rise of smoke particles within the flame by means of the

buoyancy effect [14–17]. For the smaller fire sources (0.5 and 0.05 kW), the I/PSD
Table 1

Optical Flame Detector Response Times for Various Sized Fire Sources,

Cab Locations, and Distances from the Detector

Fire size Response time Distance from sensor Cab location
 

OFD #1: positioned in the cab, at the upper left corner of rear wall
 

Cab Floor

0.5 kW 2 s 1.65 and 1.14 m Center front and rear walls

2 s 1.4 and 1.14 m Center front and rear left side walls

2 s 1.5 and 1.22 m Center front and rear right side walls

2 s 1.6 m Center control panel

0.05 kW 2 s 1.65 and 1.14 m Center front and rear walls

2 s 1.4 and 1.14 m Center front and rear left side walls

2 s 1.5 and 1.22 m Center front and rear right side walls

2 s 1.6 Center control panel

0.005 kW (candle) 2 s 1.65 and 1.14 m Center front and rear walls

2 s 1.4 and 1.14 m Center front and rear left side walls

2 s 1.5 and 1.22 m Center front and rear right side walls

2 s 1.6 m Center control panel

OFD #2: positioned in the cab, at the center of cab ceiling

Directly under the detector

0.5 kW No response 50 cm Vertical distance from detector

2 s 30 cm Vertical distance from detector

0.05 kW 2 s 30 cm Vertical distance from detector

0.005 kW (candle) 2 s 30 cm Vertical distance from detector

Table 2

Smoke Detector Response Times for Various Sized Fire Sources, Cab

Locations, and Distances from the Detector

Fire size Response time Distance from sensor Cab location

I/PSD; positioned in the cab, at the center of cab ceiling

Cab floor

32 kW 5 s 1.37 m Center front floor

0.5 kW 10 s 1.47 and 1.58 m Center front and rear walls

10 s 1.5 and 1.5 m Center front and rear left side walls

10 s 0.84 m Center control panel

0.05 kW 10 s 1.47 and 1.58 m Center front and rear walls

10 s 1.5 and 1.5 m Center front and rear left side walls

10 s 0.84 m Center control panel

0.005 kW (candle) No responsea 1.47 and 1.58 m Center front and rear walls

No responsea 1.5 and 1.5 m Center front and rear left side walls

No responsea 0.84 m Center control panel

Smoldering fire sourceb 60 s 1.37 and 1.4 m Center front and rear floor

PSD; positioned in the cab, at the center of cab ceiling

Cab floor

Smoldering fire sourcec 60 s 1.37 and 1.4 m Center front and rear floor

a Due to lack of smoke evolution. b Consisting of 10 pieces of electrical cord, contained in a 13 cm dia tray; each piece, 2.5 cm long.

Same as described for the combined ionization and photoelectric detector experiments. c
Table 3

Activation of a Fire Inerting System in the Cab, at Detector Fire

Response time (5 s) for the Suppression of Fires Inside the Cab

Fire size Fire suppression time Distance from sensor Cab location: cab floor
 

Prelit fuel traysa (32 kW) 20th s

b 1.37 and 1.4 m Center front and rear floor

a 250 ml of diesel and 250 ml of gasoline, contained in a 23 cm dia tray. b After the start of the discharge of the gas mixture (O2, �14%) (complete gas mixture discharge, 120th s).

Figure 6. OFD #1 response time, exposed to a 0.5 kW fire source in

an equipment cab, at all cab locations and distances from the detector.

yielded slower response times of 10 s at all cab locations and distances (maximum

distance, 1.58 m) (Table 2). The I/PSD failed to detect the candle fire source

(0.005 kW) due to lack of smoke particle evolution. The I/PSD was effective in

detecting smoldering fire sources within 60 s, located at the center of the front and

rear cab floor (distances, 1.37 and 1.4 m, respectively) (Table 2). Similar response

times were obtained with the PSD, under similar experimental conditions

(Table 2).

3.2. Activation of a Fire Inerting System in the Cab

For the suppression of fires inside the cab, the I/PSD response times of 5 s were

used to manually trigger the discharge of the fire inerting system in the cab

(Table 3). The expectations were that the discharge of the gas mixture in the cab

would be effective in suppressing the fires inside the cab by inerting the cab volume.

For these experiments, the cab fires (32 kW), located at the center of the front

and rear cab floor, were suppressed within the first 20 s after the start of discharge

of the gas mixture (O2, �14%) (Table 3). Of note is that the 32 kW fire source

was mainly used to evaluate the effectiveness of the fire inerting system in suppressing large cab fires. In real situations, these fires might have been already

detected at their smoldering or early flaming stage of combustion. However, it is

important to stress that at fire detection time (conveyed by detector fire alarm
and/or visual observations), the operator needs to rapidly perform safe parking of

the vehicle and engine shutoff, and exit the cab, accompanied or preceded by the

automatic or manual activation of the fire inerting system in the cab.

4. Conclusions

Results show that the optical flame detector with a 180� field of view (OFD (1)

was effective in detecting within 2 s fires ranging from 0.5 to 0.005 kW, located at

various cab locations and distances from the detector. Similar response times were

obtained for the optical flame detector with a 45� field of view (OFD (2), exposed

to similar sized fire sources located directly under the detector at 30 cm distance.

At 50 cm distance, the OFD (2 failed to detect the fire.

For a combined ionization and photoelectric smoke detector (I/PSD), response

times of 5 s were obtained for the 32 kW fire source. For much smaller fire sources (0.5 and 0.05 kW), the I/PSD yielded a slower response time of 10 s, at all cab

locations and distances from the detector. The I/PSD failed to detect the

0.005 kW candle fire. The I/PSD and PSD were effective in detecting small smoldering fires within 60 s.

The I/PSD response time of 5 s was then used to manually trigger the discharge

of the fire inerting system in the cab for the suppression of a 32 kW fire placed

inside the cab. The cab fires were suppressed within the first 20 s after the start of

the discharge of the gas mixture.

In conclusion, the data indicate that the OFD (1 flame detector and the I/PSD

smoke detector, installed within the operator�s cab, provide the operator with sufficiently early warning of early flaming fires (OFD (1 and I/PSD; 2 and 10 s,

respectively) and smoldering fires (I/PSD; 60 s) to activate a cab fire suppression

system, to perform safe parking of the vehicle and engine shutoff, and exit the

cab. Of note is that the I/PSD detector offers additional qualities such as manageable size and cost effectiveness.

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