Page 67 - Electrical Installation in Hazardous Area

3

Area classification practice for

gases, vapours and mists in freely
ventilated situations

Introduction

In Chapter 2 the mechanizms associated with area classification, together
with the constitution of teams to carry it out, and the detail of information
necessary to produce an area classification were examined. This chapter will
now look at the actual production of an area classification for a particular
works or plant. This will be based upon the information produced as a
result of the execution of the exercises envisaged in Chapter 2 and available
knowledge of the performance of flammable gases, vapours and mists when
released into an atmosphere or other area where sufficient oxygen exists to
support combustion.
A tremendous amount of research work has been carried out in a rela-
tively coordinated way on the production of methods of protection of
electrical equipment intended for use in explosive atmospheres. The same
cannot, unfortunately, be said of work carried out in respect of area clas-
sification which has been less intensive and more fragmented. This is
recognized in BS 5345, Part 2' which attempted to describe the detail of area
classification but, while describing excellently all of the facets of area classi-
fication and the procedures necessary for its execution, the code' was forced
to direct its readers to other industrial and similar for the detail
of extents of hazardous areas. This situation was made necessary because
of the diversity of view of those involved in area classification which has
lead, and continues to lead, to differences in the treatment of hazardous area
definition in different industries and sometimes in different companies.
The reassuring thing in all of this is that adoption of almost any published
industry code will produce an acceptably secure area classification if its
application is by personnel knowledgeable in the business of area classifi-
cation, operating without undue commercial pressures and with access to
full information of the operations carried out at the site being classified and
the flammable materials involved. It must be remembered, however, that
each industry or company code is based upon a particular approach to area
classification and these approaches may vary between codes. Extreme care
is therefore necessary if a particular plant is classified using information
from more than one code. For this reason, the objective of this chapter is
to examine the background philosophy of area classification and provide