Page 24 - Electrical Installation in Hazardous Area

Introduction 3

Fuel (Flammable
gas, vapour, Oxygen (Air)
mist or dust)

Ignition source (Operation or
maloperation of electrical equipment)
Fig. 1.1 The explosion triangle

the basis for technology since the turn of the twentieth century when the
problem was first identified in the mining industry. While in other areas of
risk the approach is often based much more heavily on statistical analysis
than is the case here, the approach in respect of explosive atmospheres is
well established and accepted, having been in use since the early 1900s. The
presence of many subjective areas which make statistical analysis difficult
have also limited the statistical approach although there have been many
attempts to apply such an approach. Thus current and foreseeable future
technology is based upon that currently used, and there is no indication
of a radical change to readdress the technology on a statistical basis as is
done, for example, in the nuclear industry.
A typical attempt to analyse the statistical level of security achieved in
relation to gas, vapour and mist releases is that in a paper by W.A. Hicks
and K.J. Brown at the 1971 Institution of Electrical Engineers Conference'
which identified the risk of ignition as between and Many
others however have produced different figures as the assumptions made
in respect of the subjective elements of the technology vary.
The technology is currently based upon the identification of the risk of an
explosive atmosphere being present in a particular place coupled with the
identification of the likelihood of electrical equipment within the explosive
atmosphere malfunctioning in a way which would cause it to become a
source of ignition coincident with the presence of that explosive atmosphere.
The objectives are not just to identify these coincidences but to utilize the
information so obtained to influence the design of particular process plants
and similar operational situations in a way so as to minimize the risk of
creation of an explosive atmosphere, and hence the risk of an explosion
due to electrical installations. To this end, the generality of the approach is
to seek out situations where an explosive atmosphere is normally present
of necessity due to the process involved, situations where the likelihood of
its presence is high and situations where the likelihood is of its presence is