Page 175 - Electrical Installation in Hazardous Area

Area classification in areas which are not freely ventilated 141

(Note

Continubus grade
source of release
Fig. 5.14 Effects of point source ventilation. Notes: (1) Interior of building and ex-
terior around openings are Zone 2 to take account of ventilation failure. (2)
Immediate area around continuous (or primary) grade source of release
is Zone 0 (Zone 1). (3) Where source is a point source and ventilation
velocity local to it is 2m/s Zone 0 (Zone 1) is restricted Table 3.2 with a
minimum of 0.5 m

grade sources of release. The effects of tlus ventilation may also be consid-
ered when deciding on the additional level of general ventilation necessary
for such a location. These situations are described in Fig. 5.14.
Where a continuous grade source of release is present in an area which
is not freely ventilated, local extract ventilation (e.g., for an oil/water
separator in a building) may be provided locally to the source of release to
remove any gas or vapour which is normally released. Unless the source of
release has additional modes of release it is, in such cases, reduced, as far
as the area in question is concerned, to a secondary grade source of release
on the basis that it will release into the area only when the local ventilation
fails. This does suppose that the ventilation is quickly repaired if it does fail
and here repair within the hour will be necessary. If the source can behave
in other modes then these must be separately considered. Where the source
of release can be described as a point source then, provided that the airflow
around the source as a result of the ventilation is at least 0.5 m/s, the Zone 0
will be less than 0.5m.
Where a primary grade source of release is protected by local ventilation,
the situation is similar to that existing for a continuous grade source of