Page 176 - Electrical Installation in Hazardous Area

142 Electrical installations in hazardous areas

release except that, because the release is not continuous, the time taken to
repair the ventilation may be longer and a period of within one shift (i.e.,
eight hours) is considered appropriate.
Where a secondary grade source of release is protected by local ventila-
tion, only the area local to the source of release is considered as Zone 2, as
far as that release is concerned. Failure of the ventilation coincident with
the actual release of flammable material is considered as not happening
within the scope of area classification, provided the ventilation is repaired
expeditiously (again, within one shift - eight hours - is appropriate). The
Zone 2 limited to the immediate locale of the source of release if it is the
only source of release in the area. It should be stressed, once again, that
because of the velocity of release and quantity of material released from
most secondary grade sources of release this type of limitation by point
source ventilation is not easy or, in some cases, possible.

Use of dividers in buildings together with selective ventilation

Hazardous Areas within an area, such as a building, which is not freely
ventilated may also be achieved by the use of dividing walls or curtains,
with ventilation arranged so that airflow is normally into the space enclosed
by the walls or curtains. This technique is often used where manual loading
activities are involved and releases in normal operation cannot be excluded.
It is not considered that such an approach is acceptable for continuous
grade sources of release as the entire part of the area enclosed by the walls
or curtains will adopt the classification appropriate to the source of release
which would be Zone 0 in this case. The approach, could of course be used
where a continuous grade source of release has additional local ventilation
as described earlier in this chapter.
Where a primary grade source of release, inside an area which is not
freely ventilated, is further enclosed in walls or curtains in such a way
that the ventilation in the enclosed place causes a general inflow of air
through all apertures, the classification of the area within the walls or
curtains will normally be Zone 1 (if the ventilation within the enclosed area
is sufficient - see Chapter 4). However, the effect of the source of release
on the area outside the curtains will be that of a secondary grade source
of release, giving only Zone 2 in those areas. This, of course, presumes that
ventilation failure is corrected within one shift, as before.
Where a secondary grade source of release, inside an area which is not
freely ventilated, is further enclosed in walls or curtains achieving the above
objectives, the source of release need not be taken into account in determi-
nation of the zonal classification of the remainder of the area, provided that
the ventilation is expeditiously repaired after breakdown (say within one
shift - eight hours)
The walls or curtains used for the above purposes should be of gas-tight
construction, apart from openings intended to be present, such as doorways,