Area classification is a method of analyzing and classifying the environment where explosive gas atmospheres may occur so as to facilitate the proper selection and installation of equipment to be used safely in that environment. The classification also takes into account the ignition characteristics of the gas or vapor such as ignition energy (gas group) and ignition temperature (temperature class).
The purpose of classifying hazardous areas is to avoid ignition of those releases which invariably occur from time to time in the operation of facilities handling flammable liquids and vapors, thereby minimizing the probability of coincident exposure of a flammable atmosphere and an electrical source or another sources of ignition such as mechanical sources or radio frequency electromagnetic radiation for example.
It is not the aim of hazardous area classification to guard against the ignition of major releases of flammable materials under catastrophic failure, e.g. rupture of a pressure vessel. The occurrence of such releases must be kept within acceptable limits by the correct design, construction, maintenance and operation of the facilities in conjunction with an adequate safety management system.
The classification of the hazardous areas according to the frequency and duration of the existence of a flammable gas/air mixture provides a means to perform the correct selection of equipment to be installed in areas according to their degree of hazardous area classification. Its zoning restrictions shall also be applied when using any temporary or mobile electrical equipment in the area. It is used also to ensure that the sources of ignition are segregated from sources of flammable gas.
The HAC commonly use the reference from NFPA 497/499 diagram, however this diagram only can be used for the chimical listed under table 4.4.2 of the NFPA 497, otherwise the vapor release calculation methods as guiden by IEC 60079-10 shall be apply for most of chemical mixture since there are not listed at NFPA 497. This become a common mistake that can cause the HAC study can not validated and the consequences, this will lead to the false judgement.