Use of area classification system helps in
Identifying extent of areas where explosive atmosphere may occur under normal and expected conditions.
Selecting electrical apparatus / instruments for such area.
Locating other equipment like heaters, boilers etc
2.1 CLASSES:
In this system, areas are classified and grouped according to the flammable / combustible materials, which may be present.
Class – I Locations are those in which flammable gases or vapours are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures.
Class – II Locations are those that are hazardous because of the presence of combustible dust.
Class – III Locations deal with combustible fibres. 2.2 GROUPS:
Groups exist to permit classification of locations depending on the properties of flammable vapours, liquids or gases and to permit testing and approval of equipment for such locations.
Class I hazardous locations are divided into groups in such a way that each group contains materials of similar explosion characteristics.
GROUP – A: Atmosphere containing acetylene
GROUP – B: Atmosphere containing hydrogen, or gases or vapours of equivalent hazard such as manufactured gas.
GROUP – C: Atmosphere containing ethyl ether vapours, ethylene or cyclo propane.
GROUP – D: Atmosphere containing gasoline, hexane, naphtha, benzene, butane, propane, alcohol, acetone, benzol, lacquer solvent vapour or natural gas. Class II locations are divided into three groups identifying the specific materials involved.
GROUP – E: Atmosphere containing metal dusts, including aluminium, magnesium and their commercial alloys and other combustible dusts whose particle size, abrasiveness and conductivity present similar hazards in the use of electrical equipment.
GROUP – F: Atmosphere containing combustible carbonaceous dusts, including carbon black, charcoal, coal, or coke dusts that have more than 8% total entrapped volatiles or dusts that have been sensitized by other materials so that they present an explosion hazard
GROUP – G: Atmosphere containing combustible dusts not included in Groups E or F, including flour, grain, wood, plastic and chemicals.
2.3 DIVISIONS:
Having decided that a location should be classified and having designated the gas or vapour as group A, B, C or D, the next step is to designate the location as per Division.
DIVISION 1 - Class I, Division 1 hazardous locations are defined (as per NEC code Article 500) as follows:
1. Those locations in which ignitable concentrations of flammable gases or vapours can exist under normal operating conditions. 2. Those locations in which ignitable concentrations of such gases
or vapours may exist frequently because of repair or maintenance operations or because of leakage.
3. Those locations in which break down or faulty operations of equipment or processes might release ignitable concentrations of flammable gases or vapours and might also cause simultaneous failure of electric equipment.
DIVISION 2 – Class I Division 2 Locations are as follows:
1. Those locations in which volatile flammable liquids or flammable gases are handled, processed or used, but in which the liquids, vapours and gases will normally be confined within closed containers or closed systems from which they can escape only in case of accidental rupture or break down of such containers or system or in case abnormal operation of equipment.
2. Those locations in which ignitable concentrations of gases or vapours are normally prevented by positive mechanical ventilation
and which might become hazardous through failure or abnormal operation of the ventilating equipment.
3. Those locations which are adjacent to a class I, Division 1 locations and to which the ignitable concentrations of gases or vapours might occasionally be communicated, unless communication is prevented by adequate positive ventilation from the source of clean air and effective safe guards against ventilation failure are provided.
2.4 ZONES:
Zone 0 - As a location in which ignitable concentrations of gases or vapours are present either (1) continuously or (2) for long period of time.
Zone 1 – Zone 1 locations are defined as follows:
1. Those locations in which ignitable concentrations of flammable gases or vapours are likely to exist under normal operating conditions.
2. Those locations in which ignitable concentrations may exist frequently because of repair or maintenance operation or because of leakage.
3. Those locations in which equipment is operated or processes carried on, of such nature that equipment break down or faulty operations could result in release of ignitable concentrations and also cause simultaneously failure of electrical equipment in a mode to cause the electrical
4. Those locations adjacent to a Zone 0 location from which ignitable concentrations could be communicated, unless communication is prevented by adequate positive pressure ventilation from a source of clean air and effective safe guards against ventilation failure are provided.
Zone 2 - Those locations in which ignitable concentrations of gases or vapours are not likely to occur in normal operations and if they do occur , they will exist only for short period of time.
Zone 20 - A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently.
Zone 21 - A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally.
Zone 22 - A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but if it does occur, will persist for a short period only.