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Chapter 1
SUMMARY AND RECOMMENDATIONS
A. Background
The Occupational Safety and Health A~ministrat:ion (OSHA)
requested that the Committee on Evaluation of Industrial
Hazards of the National Research Council's (NRC) National
Materials Advisory Board (NMAB) classify a submitted list
of several hundred gases, vapors, and dusts in accordance
with the groups presented in Article 500 of the National
Electrical Code (NEC). In work previously conducted for
the U.S. Coast Guard by the NRC Committee on Hazardous Mate-
rials,{ a method to classify flammable gases and vapors had
been established and this method was used by the NMAB com-
mittee to prepare a matrix of combustion-relevant properties
and classifications of gases, vapors, and selected solids.2
It appeared that no system for classifying the identi-
fied combustible dusts was available, but a review by the
committee of the few dusts classified in Class II, Groups
E, F. and G of NEC Article 500 indicated that the layer
ignition temperature and the electrical conductivity of the
dusts were the crucial factors. There were, however, no
well-established test methods to determine either of these
properties for dusts. Accordingly, the committee established
its Pane] on Dust Test Equipment to recommended the needed
methods.
Layer ignition temperature and electrical resistivity
were deemed to be the crucial properties for the following
reasons:
I. It was assumed that the equipment used in the areas
exposed to the dust would be dust-ignition-proof since NEC
Section 502-l requires that the equipment be dust-tight.
Accordingly, electrically energized parts would not be
exposed to the combustible dust, and the minimum ignition
energy of the dust, an important consideration for intrin-
sica1ly safe equipment and for protection against ignition
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by discharge of static electricity, would not have to be
considered in the classification scheme.
~ if.
2. A review of published data3~8 indicated that, with
some exceptions (approximately 50 out of 1500), the temper-
ature of a heated surface required to cause ignition was
always lower for a layer of dust on the surface than it was
for a dust cloud in a heated chamber. In those cases in
which the cloud ignition temperature was lower than the
layer ignition temperature, it was apparent that either:
There was a change in state of the dust from
a solid to a gas (a typical example would be
asphalt), or
The material in the cloud ignition temperature
test was somewhat different from that in the
layer ignition temperature test because of oxi-
dation of each particle (an example would be
atomized aluminum), or
The difference in temperature was within the
test tolerance and, as such, is probably not
significant.
The panel reasoned that dust cloud ignition temperatures
need not be considered in the classification scheme because
of the small number of dusts having cloud ignition tempera-
tures that might be lower than layer ignition temperatures.
See Appendix C for a list of dusts which have been tested
and found, under test conditions, to have cloud ignition
temperatures lower than the layer ignition temperatures).
3. Although particle size and shape are known to affect
the critical properties of dust, the availability of a rela-
tively simple and specific test method to determine these
properties would permit the specific dust available in the
area under consideration to be evaluated, should there be
any question as to the group classification.
4. The thermal insulating properties of a dust will
have some ef feet on the surface temperature of the electri-
cal equipment on which it collects, particularly equipment
that generates cons iderable heat such as lighting f ixtures
and motors. However, this effect was judged to be relative-
ly minor for most combustible dusts.
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5. The abrasiveness of the dust was'not judged to be
,a problem if the dust is kept out of moving parts such as
motor bearings.
6. The previous history of a dust, particularly metal
dusts, was judged to be a critical' factor with relation to
electrical resistivity. The panel felt that for such highly
conductive dusts, the electrical resistivity of the solid
from which the dust is formed generally should be consid-
ered the electrical resistivity of the dust itself.*
7. Although moisture content is known to have an
effect on the ignition properties of dust, the major effect
appears to be to accelerate spontaneous ignition when the
dust is in deep layers. The effect of moisture content on
the ignition properties was judged to be of only minor sig-
nificance with respect to ignition by the heated surface
of electrical equipment because the heat of the surface
would drive off the moisture quick~y.9
B. Objectives
The panel set forth the following objectives:
1. Evaluate methods presently used to test for the
layer ignition temperature and electrical resistivity
of combustible dusts. ' '
2. Suggest any modification of these methods needed
to account for particle size and shape and the need to
characterize and define dust.
3. Recommend the test methods to be used to classify
the identified combustible dusts according to the groups
given in NEC Article 500.
4. Describe testing needed to validate these recom-
mended test methods.
*In special situations, the method recommended in this re-
port can be used if the sample's tested are representative
of those actually present in the workplace. That samples
are representative is, of course, subject to agreement by
the responsible code enforcing authority.
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C. Recommendations
The panel reviewed the methods presently used to deter-
mine the layer ignition temperature and the electrical
resistivity of dusts. The panel recommended:
I. A modified version of the proposed International
Electrotechnical Commission hot plate method to determine
the layer ignition temperature, and
2. A modified version of the method presented in the
Instrument Society of America's Area Classification in
Hazardous Dust Locations, ISA-SI2.10-1973, to determine
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the electrical resistivity.
The recommended methods are described in detail along
with the panel's rationale in Chapters 2 and 3 of this
report.
The panel initially considered methods of testing for
thermaT/oxidative stability as well as layer ignition tem-
perature and electrical resistivity. A review of the liter-
ature disclosed several different techniques for the deter-
:mination of long-term stability, including thermogravi-
metric analysis, an active oxygen method, a modified ASTM
oxygen bomb method, differential scanning calorimetry, and
pressure differential scanning calorimetry. It appeared,
however, that no single standardized analysis technique was
suitable for all types of materials and that no such tech-
nique could be established. The panel, therefore, decided
that the method recommended for determining the ignition
temperature of a dust layer would provide sufficient infor-
mation on short-term therma1/oxidative stability and that
no technique should be recommended at this time for deter-
mining long-term thermal/oxidative stability.
Representative terms from entire chapter:
layer ignition
