Review of Hazardous Area Classification

HAC is a method of analyzing and classifying the environment where explosive gas atmospheres may occur to allow the proper selection of electrical apparatus to be installed in that environment. Area classification may be carried out by direct analogy with typical installations described in established codes, or by more quantitative methods that require a more detailed knowledge of the plant. The starting point is to identify sources of release of flammable gas or vapour. These may arise from constant activities; from time to time in normal operation; or as the result of some unplanned event. In addition, inside process equipment may be a hazardous area, if both gas/vapour and air are present, though there is no actual release.

A hazardous area extent and classification study involves due consideration and documentation of the following:

The flammable materials that may be present.

The physical properties and characteristics of each of the flammable materials.
The source of potential releases and how they can form explosive atmospheres;
Prevailing operating temperatures and pressures;
Presence, degree and availability of ventilation (forced and natural);
Dispersion of released vapours to below flammable limits;
The probability of each release scenario.

Gas and Vapors

Class I locations are those in which flammable vapors and gases may be present.


Class II locations are those in which combustible dust may be found.

Fibers and Flyings

Class III locations are those which are hazardous because of the presence of easily ignitable fibers or flyings.

Components for an Explosion to Occur


This needs to be present in a relatively high quantity to produce an explosive mixture (e.g. gas, vapours, mists and dusts).


Oxygen is required in high quantities and in combination with the flammable substance to produce an explosive atmosphere.


A spark or high heat must also be present.