Page 72 - Electrical Installation in Hazardous Area

40 Electrical installations in hazardous areas

The Generalized method if properly used must, however, be accepted as
valid as it over, rather than under, classifies. Regrettably, there is ample
scope for incorrect use and, in addition, the areas designated as Zone 1
and Zone 2 often become larger than would be acceptable if the source of
hazard method were used. This can cause considerable loss of credibility
when identified by third parties and the generalized method is now seldom
used except for very congested parts of plant and, even here, it is only used
as an adjunct to the source of hazard method.

3.3 The source of hazard method of area classification

The source of hazard method of area classification was introduced
progressively from around 1970 onwards. The generalized method
described in Section 3.2 had been used extensively before that time and
there was growing concern in industry that the hazardous areas being
described by that method were too large to represent the true picture. This
was particularly true in the case of Zone 1 which covered sigruficant areas
and in doing so led to questions being asked in respect of free access of plant
personnel to this zone (the frequency of release of flammables in Zone 1
is significant and as a significant number of flammables can be toxic or
asphyxiant, exposure of personnel may not be considered acceptable). This
and the cost implication led to many pressures being applied to restrict
the size of these hazardous areas. The use of the generalized method was
found not to lend itself to doing this without introducing significant risk and
attempts to achieve the required reduction led to a bewildering variation
of conclusions which threatened the confidence which was placed in area
classification.
These problems led to a much closer examination of the technology of
area classification which, in turn, led to the development of the source of
hazard method. This method is very different to the generalized method in
that each source of release and the mode of release at that point, have to
be identified. Release quantities and dispersal criteria have to be identified
and from this information the extent of a hazardous area emanating from a
particular release determined. The method of determination of this extent is
partly by the use of mathematical approaches similar to those explained in
Chapter 4, although these are to a degree inexact, as witnessed by the failure
of those involved to agree a single mathematical approach, and partly by
experimental evidence and observation. Although there is still some contro-
versy regarding these it is noticeable that most of the approaches published
give relatively similar results and smaller hazardous areas than was the
case with the generalized method. In addition, there is no evidence in over
25 years of use of the source of hazard method that the results achieved
have increased the risk of explosion where the technique has been properly
applied.
While the source of hazard method identifies each source of release and
the hazardous area created by it, there is still the problem of multiple