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Area Classifications

Wednesday, October 21, 2015

When electrical apparatus for signalling and lighting was introduced in coal mines it was accompanied by flammable gas and dust explosions that were electrically initiated. From there technical standards began to be developed to recognise the features of electrical apparatus that would prevent these electrically initiated explosions. Presently several methods of protections are utilised.  

These methods include apparatus designed to prevent the entry of flammable dust or gas into the interior, apparatus that is strong enough to contain and cool any combustion gases produced internally or electrical devices designed to prevent a spark strong enough to ignite a specific hazardous gas.

The probability of a hazard existing in flammable concentrations will vary in different locations. A location that has a close proximity to an open source of hazard will have a higher likelihood of a flammable atmosphere. Whereas the probability of a flammable atmosphere being present outside a flanged pipe containing flammable liquid is much lower because it would only occur if the flange leaks. Instead of working with an endless assortment of scenarios, distinct zones have been defined.

This practice of dividing the potentially dangerous explosive atmospheres into zones takes the different dangers from explosive atmospheres into account and allows protection measures to be applied that reflect the situation from a safety and economic viewpoint.

Classification of Hazardous Places

Gases/Vapours

Zone 0: A place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapour or mist is present continuously or for long periods or frequently

Zone 1: A place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapour or mist is likely to occur in normal operation occasionally

Zone 2: A place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapour or mist is not likely to occur in normal operation but, if it does occur, will persist for a short period only

Dusts

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: An area in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur, occasionally in normal operation

Zone 22: An area 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

In places of work the potentially explosive atmospheres are normally classified at most as Zone 1 and 2 and/or 21 and 22. Zone 0 and 20 are restricted to very small inaccessible areas in work places or are usually restricted to the inside of technical equipment.