Page 73 - Electrical Installation in Hazardous Area

Area classification for gases, vapours and mists 49

sources in the same or close locations. It is clearly not likely that all of
the sources of release will release at the same time but in multiple cases
some may, and the extent of any hazardous area so produced requires to
be identified. In the case of secondary grade sources of release the release
is so infrequent that it is unlikely that such sources need to be in any way
added, but the same cannot be true of primary grade sources of release
which release much more frequently. In such cases it is necessary to take
account of simultaneous release. Clearly in the case of many such sources
of hazard it would be unreasonable to assume that all released simulta-
neously and therefore some sort of guide is necessary. Table 3.1 gives a
method of calculating total release from multiple sources of release when
deciding upon ventilation requirements for indoor areas, but clearly it can
also be applied with some confidence to multiple sources of release in any
situation. It was first published in 1973 in the RoSPA/ICI Code2.

Table 3.1 Simultaneous release from
sources of hazard

Number of sources Number releasing
of hazard 1 simultaneously

1 1
2 2
3-5 3
6-9 4
10-13 5
14-18 6
19-23 7
24-28 8
29-33 9
34-39 10
40-45 11
46-51 12

Another difference brought about by the source of hazard approach is
the clear identification for the first time that Zone 1 as not automatically
surrounded by Zone 2. While this was also true in the generalized method
this fact tended to be obscured by the generality and it became common
belief that Zone 1 was always surrounded by Zone 2. The source of
hazard method automatically dispersed that misunderstanding as to have
a hazardous area it is first necessary to identify a source of release which
creates it. Therefore, a leak would have to be identified as possibly behaving
in two distinctly different ways to produce both a Zone 1 and a Zone 2. Thts
of course leads to another fact that was not before clearly identified and
that fact is that a single source of leakage can behave in different ways
in different circumstances. When a source of hazard is identified under the