Page 588 - Dust Explosions in the Process Industries

Electrical Apparatuses for Areas Containing Combustible Dusts 555

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Zone 21

Figure 8.1 Standardized system for marking zones 20,2 I, and 22 on hazardous area classification
maps of an industrial plant (From CENELEC, 2002).

periods when the bags are blown or shaken to release accumulated dust. Hence, the
dusty side of the filter is classified as zone 21. The clean side of the filter normally has
only negligible quantities of dust. However, filter bags may burst or other abnormal sit-
uations may arise causing explosive dust clouds to be generated om the clean side of the
filter, which is therefore classified as zone 22.