Page 53 - Electrical Installation in Hazardous Area

32 Electrical installations in hazardous areas

The latent heat of vaporization; information again necessary in case calcu-
lation is required.
The heat capacity; information is necessary in case calculation is required.
The sub-group; unnecessary for the purposes of area classification but,
as explained in later chapters, is important in the selection of electrical
equipment for installation. All electrical equipment for surface industry is
Group 11 equipment but some is given a sub-group which relates to its
possible use. These sub-groups are described in detail later but are IIA, IIB
and IIC. Gases and vapours are associated with these sub-groups to permit
selection of appropriate equipment. Dusts are not so specifically related
to sub-groups but, as these sub-groups specdy the maximum energy which
apparatus may release in an arc or spark, they can be used to identify appa-
ratus suitable for specific dust risks. The sub-grouping system is referred
to in Chapter 1 and in later chapters.
The surface temperature class; a classification system related to the
maximum temperature achieved by electrical equipment and gases and
vapours are associated with these temperature classes on the basis of their
ignition temperatures. The temperature classification system is intended
for use with gases and vapours but, although dusts are not so specifically
associated with these classes, they spec@ the maximum temperature which
surfaces may reach and, as such, can be used to identdy apparatus suitable
for particular dust risks. This system is referred to in Chapter 1 and later
chapters.
Figure 2.2 is a typical blank form used to collect the necessary information
on the above parameters.

2.6.2 Information on fuels (dusts)

For dusts, the requirements are different. It is advisable to exclude dust
from equipment which might spark, but both layers of dust which form
on apparatus and dust clouds may be ignited by temperature. While it is
possible for dusts to be ignited by sparks or arcs this is, because of the
dust exclusion from the interior of apparatus, an uncommon problem and,
when it occurs, it is dealt with as described in later chapters. The following
information is thus important.
The dust cloud ignition temperature; the minimum temperature at atmo-
spheric pressure at which an ideal cloud of dust can be ignited.
The dust layer ignition temperature; the minimum ignition temperature of
a 5 mm layer of dust at atmospheric pressure.
The temperature class; again an equipment matter, but dusts are associated
with this classification as described earlier to permit selection of appropriate
equipment.
The conductivity of dust; a dust with a resistivity of less than 1@Qm
is considered as a conducting dust, as opposed to other dusts which are
considered as non-conducting. This has an influence on the choice of electri-
cal equipment and its installation.