Page 103 - Electrical Installation in Hazardous Area

Area classification for gases, vapours and mists 79

c

a
3

Fig. 3.14 Open drains. a = 1 m

will introduce. In other cases no Zone 0 exists. It would not be consid-
ered as acceptable to have a flammable material exhausting into an open
drain continuously. If the release into the drain is abnormal, a Zone 0 will
still exist within the drain at any point where, because of the geometry of
the drain, liquid collects and will become static and persist until it has all
evaporated.
Zone 1 occurs where the drain acts as a long narrow pool and the area
around it will contain an explosive atmosphere from time to time, due to
wind action. A Zone 1 will exist around the drain (as shown in Fig. 3.14)
where release into the drain is in either normal or abnormal operation. This
is nominally chosen as 1 m which, although to a degree arbitrary, is likely
to be larger than any practical distance achieved. Collection of flammable
liquids at low spots in the drain are unlikely to affect the extent of the
Zone 1 described here. The combination of normal release into the drain
coincident with drain blockage could produce an external pool which will