ISA-TR12.24.01 is concerned with the classification of hazardous areas where flammable gas or vapor risks may arise, in order to permit the proper selection and installation of apparatus for use in such hazardous areas (see Notes 1 and 4).

It is intended to be applied where there may be a risk of ignition due to the presence of flammable gas or vapor, mixed with air under normal atmospheric conditions (see Note 2), but it does not apply to:

a) mines susceptible to firedamp;

b) the processing and manufacture of explosives;

c) areas where a risk may arise due to the presence of ignitable dusts or fibers;

d) catastrophic failures which are beyond the concept of abnormality dealt with in this recommended practice (see Note 3);

e) rooms used for medical purposes;

f) areas where the presence of flammable mist may give rise to an unpredictable risk and which require special consideration (see Note 5).

This recommended practice does not take into account the effects of consequential damage.

Definitions and explanations of terms are given together with the main principles and procedures relating to hazardous area classification.

For detailed recommendations regarding the extent of the hazardous areas in specific industries or applications, reference may be made to the codes, standards, or recommended practices relating to those industries or applications.

NOTE 1 — For the purpose of this standard recommended practice, an area is a three-dimensional region or space.

NOTE 2 — Atmospheric conditions include variations above and below reference levels of 101.3 kPa (1.013 mbar) and 20 °C (293 K), provided that the variations have a negligible effect on the explosion properties of the flammable materials.

NOTE 3 — Catastrophic failure in this context is applied, for example, to the rupture of a process vessel or pipeline, and such other events that are not predictable.

NOTE 4 — In any facility , irrespective of size, there may be numerous sources of ignition apart from those associated with electrical apparatus. Appropriate precautions will be necessary to ensure safety in this context. This recommended practice may be used with judgment for other ignition sources.

NOTE 5 — Mists may form or be present at the same time as flammable vapors. This may affect the way flammable material disperses and the extent of any hazardous areas. The strict application of area classification for gases and vapors may not be appropriate because the flammability characteristics of mists are not always predictable. Whilst it can be difficult to decide upon the type and extent of zones, the criteria applicable to gases and vapors will, in most cases, give a safe result. However, special consideration should always be given to the danger of ignition of flammable mists.

Purpose

The purpose of this recommended practice is to provide guidelines for classifying hazardous (classified) locations for the selection and installation of electrical equipment. Basic definitions given in the 1995 edition of IEC 79-10, Electrical apparatus for explosive gas atmospheres, Classification of hazardous areas, have been followed in developing this recommended practice. This publication is only a guide and requires the application of sound engineering judgment.

Electrical installations in areas where flammable liquids or gases are produced, processed, stored or otherwise handled can be suitably designed if the locations of potential sources of release and accumulation are clearly defined. Once a location has been classified, requirements for electrical equipment and associated wiring should be determined from applicable publications. Applicable publications may include NFPA No. 70 (NEC) or API RP 14F. Reference Section 1.2 for other possible applicable publications.