Page 165 - Electrical Installation in Hazardous Area

Area classification in areas which are not freely ventilated 131

hipped roof (dutch-barn construction) which is more common in industrial
loading and unloading areas. This latter may be with vents or without vents,
although, in most cases, vents are present particularly in cases where the
flammable gas or vapour is lighter than air.
Where no walls are present the effect of a flat roof is dependent upon
the proximity of the roof to the edge of the hazardous area which would
be created if the release were in free air (see Fig. 5.6). If the roof is more
than 5 m beyond the extent of hazardous area predicted by Fig. 3.4 and 3.5
(in Chapter 3) the roof is unlikely to have any effect but in other cases the
hazardous area is expected to reach the roof. Where a hipped (dutch-barn)
type roof is present without walls and if the top of the hazardous area
defined in the absence of the roof is within 5m of the roof, the entire area
contained by the roof will become a hazardous area of the same severity,

lea+kad
Roof

Source of
release

Fig. 5.6 Effect of flat roof within 5 m of free ventilation limit of explosive atmosphere