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Monday, April 6, 2009

Hazardous Area Classification

Hazardous area fall into two categories; hazards due to flammable gases (vapors or mists), and hazards due to combustible dusts (fibers or particles).
Gaseous Hazards
Explosive gas atmospheres are classified into zones based on the frequency and duration of their occurrence as below:

Zone 0: an area in which an explosive gas atmosphere is present continuously, for long periods, or is present frequently

Zone 1: an area in which an explosive gas atmosphere gas is likely to occur in normal operation occasionally

Zone 2: an area in which an explosive gas atmosphere is not likely to occur in normal operation, and if it does occur it will exist for a short period only.

Electrical apparatus for potentially explosive atmospheres is divided into the following groups:

Group I: mines susceptible to fire damp (methane)

Group II: other industries
High surface temperatures can cause ignition of flammable gases or vapors therefore the surface temperature of equipment in hazardous area must not exceed the ignition temperature of these gases or vapors.

Group I: Electrical equipment may not have a surface temperature that exceeds 150oC where coal dust can form a layer, and 450oC for internal surface where the above risk is avoided by sealing against ingress or dust.

Group II: electrical equipment may not have a surface temperature that exceed its specified temperature class, as listed in the table below:

Temperature class of electrical equipment

Maximum temperature of electrical equipment

Ignition temperature Of gases or vapor

T1

≤ 450 oC

450 oC

T2

≤ 300 oC

300 oC

T3

≤ 200 oC

200 oC

T4

≤ 135 oC

135 oC

T5

≤ 100 oC

100 oC

T6

85 oC

85 oC




Electrical apparatus of Group II may be subdivided according to the nature of the potentially explosive atmosphere for which it is intended.

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