Page 191 - Electrical Installation in Hazardous Area

Area classification practice for dusts 155

that the gases and vapours given off may be flammable. Initially, to
determine the situation, the dust in question is heated in a tube with a flame
applied above it. If any flammable gases or vapours are emitted during the
decomposition of the dust then an ignition will occur. If an ignition is
obtained then the released gases and vapours need to be identified and, if
the dust can reach the temperature necessary to release them in the practical
handling and process situation, then precautions to prevent their ignition
have to be taken.

6.1.4 Other important dust properties

While the objective of the construction of electrical equipment for use in
dust situations is to exclude the dust from the interior, insofar as is neces-
sary to prevent an ignition, this may not mean total exclusion. The dust
may, however, be conducting and, in such cases, must be totally excluded
from the apparatus to prevent malfunction.

6.2 Area classification for dust releases

The method of classifying a hazardous area where dust presence is the
problem is little different from that of classification in the case of gases or
vapours. It is first necessary to identify the possibility of the presence of a
dust cloud or layer from process information or likely release possibilities.
These will then be graded as sources of release in the same way as for gases
and vapours, after which the type and extents of hazardous areas may be
identified.

6.2. I Sources of dust release
Sources of Release of dusts are graded exactly as in the case of gas or vapour
release and are classified as follows:

Continuous grade source of release

A continuous Grade Source of Release is one which releases a dust cloud
for a large part of the time and, is in definition, synonymous with its gas or
vapour counterpart. Typical of such areas are those inside process equip-
ment such as hoppers, silos, etc.

Primary grade source of release

Primary Grade Sources of Release in the case of dusts are those areas
where uncontrolled layers or clouds of dust are not normally present but