Page 183 - Electrical Installation in Hazardous Area

Area classification practice

for dusts

In previous chapters the area classification of explosive atmospheres
constituted of gases, vapours and mists and air have been discussed
together with the legislation which addresses the total problem of explosive
atmospheres. Among this legislation are requirements for the handling of
combustible dusts (the term combustible is synonymous with the term
flammable which is applied to gases, vapours and mists). As discussed in
Chapter 1, Regulation 31(1) and 31(2) of the Factories Acts' requires certain
actions where Combustible Dusts may be present or released.
First, it requires that processing plant which may produce dust clouds
or release dust be enclosed as far as possible to prevent the general devel-
opment of a dust cloud. This is already done in the case of such things as
cyclones where the dust is contained by the cyclone during processing, but
it also requires further action to restrict the travel of any release (in other
words it requires positive attempts to be made to contain and define the
limits of travel of released dust).
Second, it requires that, where dust can be released, accumulation of
dust in these enclosed areas be limited by housekeeping which removes
layers of dust and the like to prevent accumulation over time. It must be
remembered that on release dust does not, like gas or vapour, disperse over
time but settles as a layer and, to prevent such layers accumulating, dust
which settles should be regularly removed either automatically or manually.
Sources of ignition should be excluded where dust clouds or layers can
occur in such density or thickness to constitute a hazard.
Third, it requires exclusion or effective enclosure of any source of ignition
in an area where, even after taking all of the actions described above, dust
clouds or layers may occur and sources of ignition cannot be excluded.
The differences between the performance of gases and dusts cannot be
overstressed. In the case of gases, vapours and mists great reliance is placed
on providing the most effective means of dispersion possible, while in the
case of dusts, housekeeping and containment come to the fore. The one
thing that both have in common is the requirement placed upon industry
to minimize the risks of release of either, and to approach the protection of
the results of release only when the release cannot reasonably be prevented.
The areas where dust is present, to an extent where ignition could occur,
have historically been been classified only where dust could be released
from process as Zone Z and Zone Y. The definitions of these two Zones
are given in Chapter 2 and clearly address only problems of release in that