Page 169 - Electrical Installation in Hazardous Area

Area classification in areas which are not freely ventilated 135

Roof

Ground
a = Taken from Table 3.2 on the basis of maximum release
(minimum figure 0.5 m)

Fig. 5.10 Effect of roof on three-wall system

Any door which enters the room will be considered as a source of release
equivalent to the quantity of vapour released within the room and Table 3.2
should be used to determine the extent of the hazardous area outside
the room, around the door or other opening. Where liquids above their
flashpoints but below their boiling points are contained within the room,
drainage should be provided to ensure that a significant pool cannot form,
and the external hazardous area should be based on Table 3.8 with a mini-
mum of 0.5 m in cases where no mist is assumed. This latter is a safety factor
to take account of unknowns. Where the room contains a liquid contained
above its boiling point then the area outside the room (which is a hazardous
area) should be based on Table 3.10. These areas outside the room will, be
Zone 2 in the case of secondary grade sources of release (the only sources
of release considered as acceptable in these circumstances) even though the
retention problem is likely to lead to the classification of the room as Zone 1.

5.4.1 The application of additional general ventilation

Where a room contains sources of release then it is almost certain that
additional ventilation will be necessary. A room may be provided with
additional ventilation in one of three ways.