Page 193 - Electrical Installation in Hazardous Area

Area classification practice for dusts 157

upon its dispersion and settling of the dust to form a layer. The layers of
dust formed also create the following hazardous areas.
An uncontrolled dust layer (i.e., one not contained in depth by house-
keeping) will produce a Zone 20. Such layers are normally limited to the
inside of vessels, etc., and because they are so thick (in excess of 5mm)
may have properties which are different to those of controlled layers. Such
layers would not be acceptable outside containment such as process vessels
in view of Section 31 of the Factories Acts'.
A controlled dust layer (i.e., one which is contained in depth to less
than 5 mm by housekeeping or other means) formed by a continuous grade
source of release will only give a Zone 21 if disturbed often, and only
a Zone 22 if rarely disturbed. The problems are principally the clouds
produced by the layers when disturbed as the electrical equipment is
designed to operate in this depth of dust layer.
Where the layer of dust can be controlled to less than 0.5mm thickness
with confidence it is unlikely that there will be enough dust to create an
explosive atmosphere and, therefore no hazardous area is presumed to exist
as a result of its presence, as when disturbed it is not capable of the produc-
tion of an ignitable dust cloud.

Zone 21

A primary grade source of release will give rise to a Zone 21 if it produces
a cloud. The extent of that Zone 21 will depend upon the dispersion of the
dust cloud or the settling of the dust to form a layer. As before, the layer
will be considered to produce the following Zones.
Where an uncontrolled dust layer (one not controlled to less than 5mm
by housekeeping, etc.) is formed the result is a Zone 20 in the area covered
by the layer.
Where a controlled dust layer whose thickness is controlled to less than
5mm is formed, but is regularly disturbed, then the result is a Zone 21 in
the area around the layer. Where the layer is not regularly disturbed then
the hazardous area reduces to Zone 22.
Where a dust layer is controlled to less than 0.5mm no hazardous area
is presumed to exist for reasons already described.

Zone 22
Where the source of release is of secondary grade then any cloud produced
will give a Zone 22. As before, in the case of layers produced by this cloud
the following is the case.
Where an uncontrolled dust layer is formed then, once again, a Zone 20
will exist in the region of the layer.
Where the dust layer is controlled by housekeeping or other means to
less than 5mm, a Zone 21 will exist where it is disturbed regularly, and a
Zone 22 will be formed where it is only disturbed rarely.