Page 46 - Electrical Installation in Hazardous Area

Area classification 25

atmosphere with a degree of regularity and which requires consideration of
the introduction of equipment that may become an ignition source. It does
not seek to address the possibility of explosions which occur in catastrophic
failure conditions which, by and large, are not easily predictable. It must
be remembered, however, that a procedure to deal with catastrophe is
necessary even though that procedure does not form part of the area
classification process and the requirements which that process engenders.
There are basically three situations which can predictably occur in any
operation of in which flammable materials are involved. First, a situation
in which an explosive atmosphere is present for long periods or always as
a result of operational necessity. Typical of such situations are the interiors
of process vessels or equipment processing gases, vapours mists or dusts
where air is also present, the interiors of stock tanks where liquids, gases
or vapours are present with air, silos and mills where dust and air are both
present, etc.
Second, a situation in which explosive atmospheres occur regularly in
normal operation of the plant or process. Typical of such situations are
those surrounding rotating seals on machines which are the subject of wear,
manual loading points on such things as vessels where gases, vapours,
mists and dusts are loaded, powder bag loading points, paint spraying
facilities, etc.
Third, a situation in which explosive atmospheres occur rarely and
normally result from failure of equipment or procedures. Typical of such
situations are the failure of a gasketted joint in a process pipe releasing a
gas, vapour, mist or liquid, the overflowing of a vessel due to failure of
process control functions releasing a liquid, powder handling rooms where
dust deposits can form and then be agitated into clouds, etc.

2.3 General approach to area classification

Modem area classification is based, unlike its more historical predecessor
described in BS/CP 1003l, upon identification of individual sources of
hazard and their result. BS/CP 1003 was based upon a general appraisal of
the areas at risk and identification of the hazard from this general appraisal.
This led to variation in application by different organizations which is
some cases led to over classification where entire plants were classified as
having high level of risk and, more worryingly, underclassification where
inadequately protected electrical equipment was installed. Older, already
classified, plants need therefore to be reasessed in accordance with modern
thinking if they were originally classified in this generalized way.
Identification of sources of hazard is formalized in respect of gases,
vapours and mists in BS 5345, Part 22 and although no such formal
identification exists for dusts in BS 6467 Part Z3 the document likely to
succeed it (EC 1241-34) contains a similar form of identification. Therefore
sources of release can be categorized, be they gas, vapour, mist or dust.