Page 70 - Electrical Installation in Hazardous Area

46 Electrical installations in hazardous areas

and the resultant necessity to exclude it and select only equipment which
has been shown to be suitable for the specific environment.

3.1.4 General consideration of release

Taking account of the above it is necessary to consider all points from which
flammable material may be released and to identify areas contaminated by
those releases appropriately. Historically this has been done by a general-
ized method of area classification but since 1970 the more exact source of
hazard method of area classification which is considerably more objective
has come to the fore and the examples given in this chapter are based on
this latter method.

3.2 Generalized method of area classification

The generalized method of area classification is the method which has been
historically used. The knowledge of the performance of plant gained over
many years has now, however permitted a more precise method, known
as the source of hazard method. That does not mean that the generalized
method may not be used but it has become less popular as its correct
application often gives more extensive hazardous areas than those which
actually exist. This increases the costs of operation of a given plant and
sometimes leads to conflict due to the perceived extent of areas of high
hazard (Zones 0 and 1) by regulatory bodies and others as being exces-
sively large which, as one of the principal objectives of area classification
is to influence plant design, leads to excessive expenditure. Occasionally if
not carefully executed it can also lead to certain hazardous areas not being
adequately identified which leads to unnecessary danger.
The generalized method of area classification does not require each source
of hazard to be uniquely identified but that a judgment be made on the basis
of the presence of flammable materials in a particular area, together with
a general identification of types of process equipment which are known to
leak. This requires a set of basic yardsticks against which such judgments
may be made.

3.2.1 Generalized zonal classification specification

Zone 0
Zone 0 is an area (Zone) which includes the interiors of closed or venti-
lated vessels containing flammable liquids or vapours and air. Clearly this
only applies to the vapour spaces of closed or ventilated tanks containing
flammable liquids but as these tanks usually have a varying quantity of
liquid within them, the Zone 0 extends to the lowest liquid level possible.