NOTE 1 – The source of dust release can be from a dust containment or from a dust layer.

NOTE 2 – Sources of dust release are divided into the following grades depending on the order of decreasing severity:

a) continuous formation of a dust cloud: locations in which a dust cloud may exist continuously, or may be expected to continue for long periods or for short periods which occur frequently;

b) primary grade of release: a source can be expected to occasionally release combustible dust in normal operation;

c) secondary grade of release: a source which is not expected to release combustible dust during normal operation; if it releases, it is likely to do so only infrequently and for short periods only.