Page 48 - Electrical Installation in Hazardous Area

Area classification 27

leak once in, say, 3 years according to records in a particular plant and there
would be a desire to identify it as a secondary grade source of release from
that information. The generality is, however, that it may just be accidental in
the particular set of circumstances surrounding that case at that particular
time and if a different period of 3 years was chosen, releases would be seen
to be much more frequent. Therefore, the general case which defines it as a
primary grade of release needs to be used in all circumstances, unless a very
high level of confidence exists the circumstances of the particular situation
are such as to jus* a differing approach in that case. Such a situation is
expected to be very rare indeed and so the general case, as identified in this
book, should be used in all but very few circumstances. Should sufficient
confidence exist, however, an alternative approach is clearly acceptable but
a full justification must be produced in all such cases.

2.5 Hazardous zonal classification

Having identified the types of release which we expect it is then possible to
identify the risk of the presence of an explosive atmosphere in a given area.
This is determined by the likelihood of release, the nature of the release and
the ventilation in the area of release. Clearly, with the exception of releases
inside vessels, the persistence of explosive atmospheres is determined to
a significant extent by such things as the level of ventilation at the loca-
tion of the release and the characteristics of dispersion of the flammable
material. It is necessary to define the risk of the presence of an explosive
atmosphere in some way which is related to the source of release and these
other parameters.

2.5.1 Gases, vapours and mists

The methods of defining a risk of the presence of an explosive atmosphere
for gases, vapours and mists are highly developed and such areas are
divided into three levels.

Zone 0

Zone 0 is a zone in which an explosive gas, vapour, mist/air mixture (explo-
sive atmosphere) is continuously present or present for long periods.

Zone 1

Zone 1 is a zone in which an explosive gas, vapour, mist/air mixture (explo-
sive atmosphere) is likely to occur in normal operation.