FLAMMABILITY OF NOISE ABATEMENT 

MATERIALS USED IN CABS OF MOBILE MINING 

EQUIPMENT 

Charles D. Litton, Kenneth E. Mura, Richard A. Thomas 

National Institute for Occupational Safety and Health 

Pittsburgh Research Laboratory 

P. O. Box 18070 

Cochrans Mill Road 

Pittsburgh, PA 15236 

USA 

Harry C. Verakis 

Mine Safety and Health Administration 

Approval and Certification Center 

Triadelphia, WV 26059 

USA 

ABSTRACT 

Experiments were undertaken to evaluate the flammability of typical noise abatement 

materials used to line the cab interior of mobile mining equipment. The experiments were 

conducted using three different experimental configurations: 

1.	 A ventilated tunnel test. 

2.	 A full-size cab test where the interior surfaces were lined with the sample material and 

then ignited near the floor using a methane-air burner. 

3.	 Standard ASTM E-162 tests using a radiant panel test apparatus. 

This paper describes the tests and presents the results obtained for 15 to 20 different materials 

of various chemical compositions and thickness. With one exception, materials that did not 

exhibit flame spread in the tunnel test also performed well in the full-scale cab test. All materials 

that had passed only a small-scale flammability test, such as SAE J369, failed dramatically in all 

three of the above test configurations. With one exception, all materials that performed well in 

the full-scale cab test had flame spread indices < 25, based upon the ASTM E-162 results. The 

lone exception had an ASTM E-162 flame spread index of 68. With regard to samples that failed 

the full-scale cab test, the carbon monoxide levels within the cab reached lethal concentrations in 

the range of 1.8% to 3.8% within 2 minutes from ignition of the sample—dramatically illustrating 

the severe fire environment that can result within the cab space. 

INTRODUCTION 

Data collected during the period 1990-99, indicate that 339 fires involving mobile mining 

equipment occurred at U. S. mines, resulting in 159 injuries and 2 fatalities1

. In some of these 

instances, fire spread to the cabs of these vehicles, igniting the cab lining materials, producing 

excessive levels of smoke and toxic gases, and compromising the operator’s ability to safely 

egress the vehicle2

. With respect to noise abatement, the Mine Safety and Health Administration 
 

 

(MSHA) has recently issued new regulations requiring the use of engineering controls when 

necessary to reduce operator exposure to excessive noise levels that can impair hearing3

. One 

such engineering control is the use of noise-reducing materials to line the interior surfaces of 

cabs. Selection of these materials is based primarily upon the material’s ability to reduce noise, 

without the requirement for the selection of these materials based upon their fire resistance. Yet, 

typical cabs are relatively small in volume and should fires occur within these small spaces, the 

buildup of smoke and toxic gases occurs rapidly. This consequence seriously reduces the 

operator’s ability to perform emergency maneuvers and safely exit the cab. Some of these 

materials have undergone no flammability testing, while a significant fraction of these types of 

materials have been evaluated for their flammability using only small-scale tests, such as the 

Society of Automotive Engineers test, SAE J3694

. The SAE J369 test samples measure 0.051 m 

wide by 0.356 m long and the ignition source is a Bunsen-type burner. A similar test is the Motor 

Vehicle Safety Code, Standard 302 (MVSS 302)5. A smaller percentage of these materials have 

undergone more extensive flammability tests, most notably the ASTM E-84 Tunnel Test6

, to 

arrive at flame spread indices for these materials. A series of experiments were undertaken to 

evaluate the adequacy of these different tests with respect to the full-size cab fire tests and to 

determine the potential hazard from combustion that can result should these materials ignite and 

burn within the cab space. 

Twenty-one (21) different noise-reducing materials were obtained for testing. These materials 

encompass the range of materials typically used, not only in terms of their chemical composition, 

but also in terms of their flammability and fire resistance ratings. There were materials with no 

flammability rating, to those with only small-scale flammability ratings to those with more 

extensive, larger-scale flammability ratings. 

EXPERIMENTAL 

Three different types of tests were used to evaluate the sample materials. The first test 

used a laboratory-scale tunnel, with a square cross-section of 0.21 m2

, and ventilated at a constant 

air velocity of 1.0 m/s. Details of this experimental configuration can be found in Reference 7. 

Samples measuring 0.23 m wide and typical lengths between 1.22 and 1.83 m were placed along 

the centerline of the tunnel secured to a metal rack with one edge of the sample located 

approximately 0.12 m from the upstream entrance to the tunnel. A methane burner with 12 

nozzles was then positioned so that the methane-air flames impinged on both the top and bottom 

surfaces of this upstream sample edge. The approximate heat output of the burner was 25 kW. 

The time allowed for flames to impinge upon the sample varied from 30 s to a maximum of 5 

minutes, where the lower times correspond to experiments where ignition and flame spread 

occurred rapidly while the longer times correspond to experiments where little, if any, flame 

spread was observed. Each experiment was videotaped and the gases CO, CO2, and O2 measured 

continuously by flowing the gases from a sampling point located in the center of the tunnel 

exhaust section to three separate gas analyzers. For this test, samples for which flame spread did 

not advance beyond the vicinity of the ignition area were judged to pass. Samples, where flames 

spread along the surface beyond the local ignition area, were judged to have failed the test. All of 

the sample materials were tested in this experimental configuration. A photograph of the tunnel 

with the gas burner igniter is shown in Figure 1. 
Figure 1. Photograph of the ventilated tunnel with the gas burner igniting a sample material. 

Figure 2. Photograph of the full-size cab used in the second type of experiments. 
The second type of experiment used a full-size cab, typical of older cabs used on mobile mining 

equipment (Figure 2). The volume of the cab used in the experiments is approximately 2.5 m3

. 

In these experiments, sample materials were mounted on the walls and ceiling of the rear of the 

cab and, in some tests, on the floor of the cab. The samples were ignited at the base of the rear 

wall using the same gas burner used in the laboratory-scale tunnel tests. Gas temperatures were 

measured near the ceiling at three different locations along the front-to-rear centerline—0.25 m 

from front and rear of the cab, and in the cab center--and at three locations along the front-to-rear 

centerline 0.25 m from the cab ceiling—0.025 m from front and rear of the cab and in the cab 

center. A gas sampling probe was positioned to continuously flow gas samples from a point in 

the cab center 0.25 m from the ceiling to gas analyzers for measurement of CO, CO2, and O2. All 

experiments were videotaped using both a standard camera and an infrared thermal imaging 

camera. In the experiments, a sample material was judged to have passed this full-size cab test if 

visual examinations of the sample subsequent to the experiment did not indicate any damage (or 

flame spread) beyond the localized ignition area. Ten of the sample materials were tested in this 

experimental configuration. 

The third type of experiment performed on the samples was the standard ASTM E-162 Test using 

a radiant heat energy source. This test is described in detail elsewhere8

. Briefly, the test consists 

of mounting a standard sample measuring 0.152 m wide 0.457 m long at a 30o angle with respect 

to a vertically-positioned radiant panel measuring 0.305 m x 0.457 m operating at a blackbody 

equivalent surface temperature of 670 oC. The topmost edge of the sample is positioned so that it 

is approximately 0.0445 m from the surface of the radiant panel. A small pilot flame with a flame 

length of 0.051 to 0.076 m is positioned so that the flame impinges on the topmost edge of the 

sample. An exhaust stack containing eight thermocouples is mounted above the sample and the 

gas velocity at the top of the stack maintained at a value of approximately 1.3 m/s during the test. 

Once the test has begun, the pilot flame ignites combustible vapors at the topmost edge of the 

sample and the flame front begins to move downward along the sample surface. The times to 

reach successive distances of 0.0762 m are recorded until the flame has reached the distance of 

0.381 m or, in the event flames do not reach this distance, until 15 minutes have elapsed. The 

rate of progression of the flame front along the sample and the maximum gas temperature 

measured in the stack are then used to calculate a number, called the flame spread index. For this 

experiment at least 3 samples of each material were tested and the average of the three or more 

separately measured flame spread indices quoted as the flame spread index for that material. In 

addition, a sample was judged to pass the test if the resultant flame spread index had a value of 25 

or less. Sixteen of the sample materials were subjected to this standard test. 

THEORETICAL 

In general, data from the first and second experimental configurations are most easily 

analyzed in terms of the heat release rates and total heat outputs calculated from the measured 

oxygen concentrations. The flame spread index data from the standard ASTM E-162 experiments 

represent components from both flame spread and heat release rate. Heat release rate, QTOT, 

expressed in kilowatts (kW), can be calculated for the tunnel experiments and the full-size cab 

experiments using the O2 data from the general expression: 

QTOT = QO2 = 13.1 DO2 (1) 

Where DO2 is the rate of depletion of O2, g/s; 
 

and the factor of 13.1 kJ/g of O2 consumed in equation (1) an average value for polymeric 

materials. 

For the ventilated tunnel, DO2 is calculated from the expression 

DO2 = 1.43 x 10-3 V0A0 (∆O2) (2), 

where the ∆ gas concentration is the change in gas concentration, in ppm, from its initial value; 

V0 is the air velocity through the tunnel, 1.0 m/s; and 

A0 is the tunnel cross-sectional area, 0.21 m2

. 

For the full-size cab tests, the depletion rate of O2, may be calculated from the data of gas 

concentrations vs time to yield the average heat release rates. In addition, for these tests, the 

maximum O2 depletion may be used to calculate the energy output from the combustion of the 

samples using the expression 

E (kJ) = 13.1ρO2VC (%∆O2/100) (3) 

where	 ρO2 is the density of O2, 1428 g/m3

, 

VC is the cab volume, 2.5 m3

, and 

%∆O2 is the average per cent O2 consumed. 

For the standard E-162 test, a flame spread index, IS, is calculated from the expression 

IS = FSQ (4) 

where FS = 1 + 1/(t3 – t0) + 1/(t6 – t3) + 1/(t9 – t6) + 1/(t12 – t9) + 1/(t15 – t12), and the times t3 

through t15 are the arrival times (in minutes) at the respective distances of 0.076, 0.152, 

0.228, 0.304, and 0.38 m along the length of the sample. The quantity FS is a measure of 

the flame spread rate along the sample surface. 

Q is given by Q = CT/β, with C (a constant) having a value of 5.7, β (a calibration 

constant for the apparatus) having an average value of 30 oC/kW, and T being the 

maximum stack temperature difference (oC) between the temperature-time curve for the 

sample and that for a similar curve using an inert sample. Q is, then, a measure of the 

heat output from the sample. 

Previous tests of mine conveyor belting9 have indicated that the propensity for flame spread 

scales with the ratio of heat release rate to forced convective air velocity, QTOT/V0. For these 

experiments, a heat parameter, HP, is defined that is the energy output per unit volume, kJ/m3

. 

For the ventilated tunnel test, HP is obtained by dividing the calculated heat release rate, QTOT, by 

the volumetric air-flow rate through the tunnel, V0A0. For the cab experiments, HP is the energy 

output divided by the cab volume. In the results that follow, the heat parameters calculated for 

the ventilated tunnel and full-size cab experiments are reported for comparison. 

RESULTS AND DISCUSSION 

In the data that follows, the tunnel and cab heat parameters (HP) are the net values 

obtained after subtracting the contribution from the methane-air burner used to ignite the samples. 

For the tunnel tests, the measured HP of the burner was 125 kJ/m3

, while for the cab experiments 

the measured HP was 650 kJ/m3

, corresponding to a burner duration of five minutes and an 

oxygen depletion of 3.47%. For the materials tested in the tunnel, the HP ranged from a low 
value of 115 to a maximum value of 1165. For these tests, materials with HP’s less than 225 did 

not support flame spread. For the full-size cab tests, the HP ranged from a low value of 115 to a 

maximum value of 2750. For the E-162 tests, values of the flame spread index ranged from a low 

of 13 to a maximum of 772. It is worth noting that the sample material with the maximum flame 

spread index had a relatively low HP for the tunnel test (210), but a correspondingly high HP for 

the cab experiment (2050). Only three materials had flame spread indices less than 25 and one 

other material had an average flame spread index of 68, based on two series of tests, one yielding 

an IS of 95 and the second yielding a value of 40. The data from the experiments are summarized 

in Table 1. 

 

Sample Designation IS (E-162) Tunnel HP Cab HP 

TFMFR 772 210 2050 

TFM 13 115 115 

PNR 527 720 2430 

PVN 347 675 2610 

UPSS1 (1st Set) 95 135 850 

UPSS1 (2nd Set) 40 “ “ 

UPSS2 20 125 230 

TFMT 273 675 -----

PPFKA 23 125 650 

PMFUF 163 390 1000 

PHUF 581 1165 -----

SC4 331 600 -----

SC1 144 330 -----

SC2 124 275 -----

SC3 361 490 -----

AAWM 536 710 2380 

AAWMFM 228 640 2750 

Table 1. Summary of the flame spread indices, IS, and the heat parameters for both tunnel and 

cab experiments. Only those sample materials tested using the E-162 radiant panel test apparatus 

are shown. The additional five materials were subjected to the tunnel test only, with average HP 

values in the range of 600 to 1000. 

Correlations were obtained between the cab HP and the tunnel HP as shown in Figure 3. 

Although there is scatter in the data, a least-squares regression of cab HP vs tunnel HP (the solid 

line of Figure 3) yielded a correlation factor, r2

, of 0.75. 
HP (Cab) 

5000 

4000 

3000 

2000 

1000 

0 

0 200 400 600 800 1000 1200 1400 

HP (Tunnel) 

Figure 3. Cab HP vs the tunnel HP for the 10 sample materials tested.

Additionally, the measured flame spread indices were plotted as a function of the tunnel HP and 

are shown in Figure 4. With the exception of one point (sample TFMFR, IS = 772), there is a 

good correlation between the different experiments as shown by the least-squares regression solid 

line with an r2

 of 0.84. 

0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

Flame Spread Index 

0 200 400 600 800 1000 1200 1400 

HP (Tunnel) 

Figure 4. A plot of the measured E-162 flame spread index vs the tunnel HP for the sixteen 

sample materials tested. 

It is worth noting that all of the sample materials with IS < 25 had corresponding tunnel HP’s less 

than 200 and that all sample materials with 25 < IS < 200 had corresponding tunnel HP’s in the 

range of 200 to 400, except for the one sample previously noted. With regard to test data 

obtained by the standard ASTM E-84 tunnel test6

, seven of the sample materials had flame spread 

indices less than 25. Four of these sample materials were found to be inadequate in either the 

tunnel or cab experiments, or both, which is an indication that the use of this standard test would 

not be appropriate for the measure of fire resistance for materials used in the cabs of mining 

equipment. 
 

The propensity to spread flame, once a material is ignited, is an important consideration in the 

selection of noise-reducing materials. However, the small volume typical of most cabs increases 

the potential fire hazard due to the rapid accumulation of lethal gases, such as CO, within this 

volume, coupled by a corresponding rapid depletion of O2. The magnitude of this hazard is 

apparent in Table 2, where the maximum levels of CO and minimum levels of O2 measured 

during the full-size cab tests are tabulated. These table values have been corrected for the effects 

of the gas burner ignition source, and represent levels indicative of the sample material 

combustion only. Six of the 10 materials produced CO in excess of 1.5%, while five of the 10 

materials depleted O2 levels to well below 15%, resulting in lethal atmospheres inside the cab. 

Sample Designation Cab HP Maximum CO (%) Minimum O2 (%) 

TFMFR 2050 1.791 10.00 

TFM 115 0.037 20.34 

PNR 2430 2.561 7.96 

PVN 2610 3.811 7.00 

UPSS1 850 0.261 16.41 

UPSS2 230 0.004 19.73 

PPFKA 650 0.247 17.48 

PMFUF 1000 2.211 15.61 

AAWM 2380 3.691 8.23 

AAWMFM 2750 2.011 6.26 

Table 2. The maximum levels of CO and minimum levels of O2 measured during the full-size 

cab experiments, corrected for the contribution by the gas burner ignition source. 

In addition to the magnitude of these levels, it is also important that the rapidity with which these 

atmospheres are created also be quantified. This aspect of the problem is demonstrated in Figures 

5 and 6, where the best/worse case for CO accumulation and O2 depletion, respectively, are 

shown as a function of time. %CO

3.50 

3.00 

2.50 

2.00 

1.50 

1.00 

0.50 

0.00 

 Worst Case 

Best Case 

0 50 100 150 200 250 300 350 

Time (seconds) 

Figure 5. The measured CO concentrations for the best and worst sample materials as a function 

of time from ignition of the gas burner. 

4.00 
0.00 

5.00 

10.00 

15.00 

20.00 

25.00 

%O2

 Worst Case 

Best Case 

0 50 100 150 200 250 300 350 

Time (seconds) 

Figure 6. The measured O2 concentrations for the best and worst sample materials as a function 

of time from ignition of the gas burner. 

 

Figures 5 and 6 are typical, in terms of the time to produce a life-threatening atmosphere within 

the cab volume, of those experiments resulting in high levels of CO and low O2 concentrations. 

In general, about 20 seconds elapse before the sample material ignites and within less than 10 to 

20 seconds from sample ignition, CO levels exceed 1%. This rapid development of CO is due not 

only to the small cab volume but also to the lack of O2 to support the combustion so that there is a 

rapid transition to fuel-rich conditions resulting in significant increases in the production of CO. 

CONCLUSIONS 

The results of this study indicate that the majority of noise-reducing materials used to line 

the interior surfaces of cabs of mobile mining equipment are prone to rapid flame spread. Also, 

the combustion of these materials within the small volume of the cab rapidly results in the 

formation of life-threatening atmospheres due to the production of CO and depletion of O2. 

For the laboratory-scale tunnel experiments, a heat parameter value of less than 200 appears to be 

an indicator of good fire resistance. However, one material with a tunnel HP of 210 performed 

poorly in the full-size cab test and also in the standard E-162 radiant panel test. 

Those materials with E-162 flame spread indices of 25 or less performed well in both the tunnel 

experiments and the full-size cab experiments. One sample material with an average flame 

spread index of 68 also performed well in both the tunnel and cab experiments. 

As mentioned previously, seven of the sample materials had flame spread indices less than 25, 

based upon the standard ASTM E-84 tunnel test6

. Four of these sample materials were found to 

be inadequate in either the tunnel or cab experiments, or both. These results indicate that the use 

of this standard test would not be appropriate for the measure of fire resistance for materials used 

in the cabs of mining equipment. 
 

The full-size cab test data show, in general, that good fire resistance of noise abatement materials 

for use in cabs of mining equipment can be achieved by a flame spread index of 50 or less. As a 

means of improving fire safety, the Mine Safety and Health Administration (MSHA) presently 

recommends a flame spread index of 25 or less for noise abatement materials used in cabs or 

compartments of underground equipment and 50 or less for cabs or compartments in surface 

equipment. The lower recommended flame spread index as applied to underground equipment is 

based on the more confined environment for escape. 

REFERENCES 

1.	 DeRosa, Maria I., Analyses of Mobile Equipment Fires for All U. S. Surface and 

Underground Coal and Non-Coal Mining Categories, From 1990 Through 1999, 7th 

Symposium, IAFSS, Worcester, MA, June 16-21, 2002. 

2.	 Mine Safety and Health Administration, U. S. Department of Labor, Accident 

Investigation Report, Fatal Fire Accident, Danley Quarry, Vulcan Materials 

Company, Nashville, Davidson County, TN, July 26, 1993. 

3.	 Title 30, Code of Federal Regulations, PART 62, Section 62.130, July 1, 2002. 

4.	 Surface Vehicle Standard, SAE J369, Society of Automotive Engineers, Inc., 400 

Commonwealth Drive, Warrendale, PA 15096, Jan., 1994. 

5.	 Title 49, Code of Federal Regulations, PART 571, Sec. 571.302, pp. 683-686, 

October, 1999. 

6.	 ASTM Fire Test Standards, 2nd Edition, ASTM, 1916 Race St., Philadelphia, PA 

19103, 1988, pp.9-26. 

7.	 Lazzara, C. P. and Perzak, F. J., Conveyor Belt Flammability Tests. Comparison of 

Large-Scale Gallery and Laboratory-Scale Tunnel Results, 23rd International 

Conference of Safety in Mines Research Institutes, Washington, DC, 1989, pp. 138­

150. 

8.	 ASTM Fire Test Standards, 2nd Edition, ASTM, 1916 Race St., Philadelphia, PA 

19103, 1988, pp.98-109. 

9.	 Perzak, Frank J. et al, Hazards of Conveyor Belt Fires, U. S. Bureau of Mines Report 

of Investigations 9570, 1995, 33 pp.