Page 164 - Electrical Installation in Hazardous Area

130 Electrical installations in hazardous areas

E = Extent of hazardous area with adjustments for wall proximity,
C = Distance source of hazard/wall end
a=E-C

Fig. 5.4 Typical two-wall effect (b)

a = Taken from Table 3.2 on the basis of maximum release with
a minimum of 0.5 m

Fig. 5.5 Typical three-wall effect

all cases this approach will be valid for both primary and secondary sources
of release and, in general, the retention of explosive atmosphere will not
be sufficient to increase the severity of classification (i.e., Zone 1 from a
secondary source of release).

5.3 Roofs without walls or associated with one, two or
three walls

5.3.1 Roofs without walls

There are two scenarios which must be considered here. The first is the
flat roof, which is so typical of petrol station canopies and the second, the