Page 190 - Electrical Installation in Hazardous Area

1 54 Electrical installations in hazardous areas

The ignition energy of a dust cloud can be determined by dispersing dust
suspended in a cloud, in air which is inside a tube and then introducing
an ignition source. Some figures for this are given in Table 6.2. While this
has been done for a variety of dusts, and figures exist, the information is
only of value in respect of the use of electrical equipment which creates arcs
or sparks to which the dust cloud has access. This situation is unusual as
the normal approach is to exclude the dust from the interior of the equip-
ment where such arcs or sparks could occur. Historically, there has been
no internationally recognized test method and the available information is
from a series of tests which were, nonetheless, carried out by those expert
in the field.
At the time when the dust cloud ignition temperatures listed in Table 6.2
were produced there was no officially recognized international method
for determination of dust-layer ignition temperature. One of the common
historic methods for determination of a dust-cloud ignition temperature
was, however, to determine the minimum temperature at which a cloud of
the dust in air will ignite in a series of tests in a Godbert-Greenwald furnace
or similar apparatus. A series of tests is required because of the difficulty of
achieving the worst case, and a uniform cloud. If one addresses the figures
given in Table 6.2 it will be noted that, with the exception of sulphur, for
all of the materials included the temperature is around 400 "C or above.
An International Standard (IEC 1241 - Z4) now exists, produced by
the International Electrotechnical Commission (IEC) for the standard test
methods for these ignitions.

6.1.2 The ignition of dust layers
The ignition of dust layers is a complex phenomenon, depending upon the
particle size of the layer and the environment in which ignition is to be
achieved (e.g., whether it has been pre-heated, etc.). Again, there was no
standard test for this when the figures given in Table 6.2 were produced
and, in general, the ignition temperature was achieved by heating a 5mm
layer of dust on a hotplate and determining the temperature at which igni-
tion occurs. This is difficult as dusts tend to smoulder when ignited and
while some will continue to burn, and to progress along the layer away
from the source of ignition, some will not and will extinguish as soon as
the source of ignition is removed. Notwithstanding this difference, the igni-
tion temperature is considered to be the temperature at which smouldering
begins, even if it extinguishes on removal of the heat source. It should be
noted that the layer ignition temperature is, in most cases, lower than the
cloud ignition temperature. An International Standard now exists for the
method of measuring this parameter - IEC 1241-24.

6.1.3 Production of flammable gases and vapours by dusts

Some dusts, particularly when heated, give off gases and vapours. This
action is caused by the dust beginning to decompose and it is possible