Page 168 - Electrical Installation in Hazardous Area

1 34 Electrical installations in hazardous areas

E = Extent of hazardous area taking account of walls if roof is not present
C = Distance from source to end wall
D = Distance from source to furthest corner of roof
F=E-D
a=E-C

Fig. 5.9 Effect of roof over two walls

5.4 Rooms above ground

As previously stated, an above-ground room with no attempts made to
give ventilation will have a very long retention of any release of flammable
material. A method of calculating this is given in Chapter 4 based upon a
proposed new British Standard3. It is difficult, however, to be sure that all
pockets of a room will be swept clear within this specified time by crack
ventilation, and retention must be assumed to be capable of removing the
source of hazard/classification relationship. It is not considered possible
to countenance a room with no specifically included ventilation require-
ments being used for any purpose which means that it will contain sources
of release more onerous than secondary grade sources of release. In such
circumstances the interior area classification will, because of the paucity
of ventilation, give an internal classification of Zone 1, which will clearly
introduce significant difficulty in the use of the room.