Page 170 - Electrical Installation in Hazardous Area

136 Electrical installations in hazardous areas

Openings (normally louvered) may be fitted to create more airflow
in the building

Where openings are provided they do increase ventilation, but in no way
as significantly as artificial ventilation unless they satisfy the criteria set out
earlier in this chapter. Such openings are not in many cases practicable
unless louvred, however, as they remove much of the protection from
the environment which is often the reason why a room is necessary.
The effect of such additional ventilation is to reduce the retention of
explosive atmosphere in the building to a level where the source of
hazard/classification relationship (i.e., secondary grade source of release
gives Zone 2) is maintained. Because of the limited effect which they
have, however, they do not allow parts of the building to be delineated
as particular risks (e.g., parts Zone 1 and parts Zone 2) and the entire
building will adopt the classification produced by the most onerous source
of release contained within it. They must also be strategically placed to
take maximum account of the internal situation. In buildings where internal
activities produce temperature gradients of in excess of 3°C between the
interior of the building and the outside air, then significant chimney effects
can occur and movement of explosive atmospheres within a building will
be affected to some extent by their density relative to air. The results of
these considerations generally result in louvered openings being placed at
both the top and the bottom of the building.
In addition, care must be taken to ensure that the effect of any air entering
through louvered openings is general, rather than limited, to specific areas
as the airflow is not sufficient to ensure that releases do not contaminate
all of the building. The layout of equipment within the building is also
important as it could produce blindspots. In most cases considerations such
as this tend to create a scenario where louvres are necessary all around the
building, as shown in Fig. 5.11, at both high and low level which is not
always ideal. The result will still usually lead to the entire interior of the
building being classified as Zone 1 if a primary grade source of release is
present which, because of toxic and asphyxiant considerations, may often
mean that access to such buildings is severely restricted.
To sum up, buildings which merely have openings to allow enhanced
natural ventilation without meeting the criteria described in Section 5.1.1
normally require ventilation openings as shown in Fig. 5.11 and, in the
main, will only be suitable for secondary grade sources of release. Even in
such cases, layout inside the building is of critical importance.
Where buildings ventilated naturally by openings not complying with the
criteria for unrestricted ventilation, provided the ventilation openings have
been provided with sufficient thought to internal airflow, secondary grade
sources of release within the building will produce a Zone 2 classification
and external Zones 2 at the openings, based upon the maximum single
secondary grade release and using Table 3.2 as a basis, with a minimum
of 0.5 m.