Classification of Hazardous Areas

Scope of IEC 60079
The classification of hazardous areas where flammable gas or vapour risks may arise is a primary concern in the proper selection and installation of apparatus for use in such hazardous areas (see notes 1 and 4). IEC 60079-10 is intended to be applied where there may be a risk of ignition due to the presence of flammable gas or vapour, mixed with air under normal atmospheric conditions (see note 2).

Exclusions to IEC 60079

1. mines susceptible to firedamp;
2. the processing and manufacture of explosives;
3. areas where a risk may arise due to the presence of ignitable dusts or fibres;
4. catastrophic failures which are beyond the concept of abnormality dealt with in this standard (see note 3);
5. rooms used for medical purposes;
6. areas where the presence of flammable mist may give rise to an unpredictable risk and which require special consideration (see note 5). This standard 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 relating to those industries or applications.

Notes

1. For the purpose of this standard, an area is a three-dimensional region or space.
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.
3. Catastrophic failure in this context is applied, for example, to the rupture of a process vessel or pipeline, and such events that are not predictable.
4. In any process plant, 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 standard may be used with judgement for other ignition sources.
5. Mists may form or be present at the same time as flammable vapours. This may affect the way flammable material disperses and the extent of any hazardous areas. The strict application of area classification for gases and vapours 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 vapours will, in most cases, give a safe result. However, special consideration should always be given to the danger of ignition of flammable mists.

Definitions and terms
explosive gas atmosphere
a mixture with air, under atmospheric conditions, of a flammable material in the form of gas or vapour in which, after ignition, combustion spreads throughout the unconsumed mixture.

NOTE Although a mixture which has a concentration above the upper explosive limit (UEL) is not an explosive gas atmosphere, it can readily become so and, in certain cases for area classification purposes, it is advisable to consider it as an explosive gas atmosphere.

hazardous area
an area in which an explosive gas atmosphere is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of apparatus

non-hazardous area
an area in which an explosive gas atmosphere is not expected to be present in quantities such as to require special precautions for the construction, installation and use of apparatus

Zones
Hazardous areas are classified into zones based upon the frequency of the occurrence and duration of an explosive gas atmosphere, as follows:

zone 0
an area in which an explosive gas atmosphere is present continuously or for long periods

zone 1
an area in which an explosive gas atmosphere is likely to occur in normal operation

zone 2
an area in which an explosive gas atmosphere is not likely to occur in normal operation and, if it does occur, is likely to do so only infrequently and will exist for a short period only

NOTE Indications of the frequency of the occurrence and duration may be taken from codes relating to specific industries or applications.

source of release
a point or location from which a flammable gas, vapour, or liquid may be released into the atmosphere such that an explosive gas atmosphere could be formed [IEV 426-03-06, modified]

Grades of release
There are three basic grades of release, as listed below in order of decreasing likelihood of the explosive gas atmosphere being present:

1. continuous grade;
   a release which is continuous or is expected to occur for long periods
2. primary grade;
   a release which can be expected to occur periodically or occasionally during normal operation
3. secondary grade.
   a release which is not expected to occur in normal operation and if it does occur, is likely to do so only infrequently and for short periods

A source of release may give rise to any one of these grades of release, or to a combination of more than one.

release rate
the quantity of flammable gas or vapour emitted per unit time from the source of release

normal operation
the situation when the equipment is operating within its design parameters

NOTES

1. Minor releases of flammable material may be part of normal operation. For example, releases from seals which rely on wetting by the fluid which is being pumped are considered to be minor releases.
2. Failures (such as the breakdown of pump seals, flange gaskets or spillages caused by accidents) which involve urgent repair or shut-down are not considered to be part of normal operation.

ventilation
movement of air and its replacement with fresh air due to the effects of wind, temperature gradients, or artificial means (for example fans or extractors)

Explosive limits

lower explosive limit (LEL)
the concentration of flammable gas or vapour in air, below which the gas atmosphere is not explosive

upper explosive limit (UEL)
the concentration of flammable gas or vapour in air, above which the gas atmosphere is not explosive

NOTE For the purpose of this standard, the terms "explosive" and "flammable" should be considered synonymous.

relative density of a gas or a vapour
the density of a gas or a vapour relative to the density of air at the same pressure and at the same temperature (air is equal to 1,0)

flammable material
a material which is flammable of itself, or is capable of producing a flammable gas, vapour or mist

flammable liquid
a liquid capable of producing a flammable vapour under any foreseeable operating conditions

flammable gas or vapour
gas or vapour which, when mixed with air in certain proportions, will form an explosive gas atmosphere

flammable mist
droplets of flammable liquid, dispersed in air so as to form an explosive atmosphere

flashpoint
the lowest liquid temperature at which, under certain standardized conditions, a liquid gives off vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture

boiling point
the temperature of a liquid boiling at an ambient pressure of 101,3 kPa (1 013 mbar)

NOTE For liquid mixtures, the initial boiling point should be used. Initial boiling point is used for liquid mixtures to indicate the lowest value of the boiling point for the range of liquids present, as determined in a standard laboratory distillation without fractionation.

vapour pressure
the pressure exerted when a solid or liquid is in equilibrium with its own vapour. It is a function of the substance and of the temperature

ignition temperature of an explosive gas atmosphere
the lowest temperature of a heated surface at which, under specified conditions, the ignition of a flammable substance in the form of a gas or vapour mixture with air will occur

NOTE IEC 60079-4 and IEC 60079-4A standardize a method for the determination of this temperature.