Page 171 - Electrical Installation in Hazardous Area

Area classification in areas which are not freely ventilated 137

V = Vents (louvres or otherwise)
a = Extent of hazardous area (Table 3.2) with a minimum of 0.5m
Hazardous area Source of release
Not acceptable - Continuous grade
Zone 1 - Primary grade
Zone 2 - Secondary grade

Fig. 5.11 Hazardous area is a room with openings (with or without louvres)

Where a building (as described above) contains primary grade sources of
release, unless local forced ventilation is fitted in a way which removes the
release separately, the entire interior of the building will be Zone 1, and
a Zone 1 will exist at all ventilation openings based upon the maximum
number of primary grade sources of release which are considered as
releasing at the same time, based upon Tables 3.1 and 3.2, with a minimum
of 0.5 m (not normally acceptable for personnel entry).
It is not considered as acceptable to have any continuous grade sources
of release releasing into such a building without additional precautions.

Buildings provided with forced ventilation

General forced ventilation is the most common way in which the building
may be scoured with air. The objective is normally to ensure that the
building is swept with air while at the same time, ensuring that the
forced ventilation provided does not create hazardous areas around the
building. To do this it is necessary to first determine the maximum release
of flammable material from any one secondary grade source of release (it
is not considered as necessary to consider more than one secondary grade
source of release releasing at any given time) and, if greater, the maximum
release from any possible combination of primary grade sources of release