Page 50 - Electrical Installation in Hazardous Area

Area classification 29

Zone Y
Areas not classified as Zone Z in which accumulations or layers of combus-
tible dust may be present under abnormal conditions and give rise to
ignitable mixtures of dust and air (explosive atmosphere) are designated
as Zone Y.
These definitions, which are similar to those appearing in BS 6467, Part Z3
and other documents, are on the face of it very different to those for gases,
not least because only two Zones exist as a result of the exclusion from the
scope of BS 6467, Part 23 of the interior of dust handling equipment. Taking
this fact into account, however, the differences are not as great as would
initially seem the case and, allowing for the differences in performance of
released dust from that of released gas, vapour or mist, there appears to be
an easily identified relationship between Zone 1 and Zone Z and likewise
between Zone 2 and Zone Y.
These definitions in respect of dusts will be replaced in the near future
by three new definitions in IEC 1241-34 which is an international document
and thus will bring the definitions for dust hazardous areas up to the same
status as those for gases, vapours and mists.

Zone 20

Zone 20 is a Zone in which combustible dust, as a cloud, is present contin-
uously or frequently, during normal operation, in sufficient quantity to be
capable of producing an explosible concentration of combustible dust in
mixture with air (explosive atmosphere), and/or where layers of dust of
uncontrollable and excessive thickness can be formed. An example of this
Zone is the inside of processing equipment.

Zone 21

Zone 21 is a Zone not classified as Zone 20 in which combustible dust, as a
cloud, is likely to occur during normal operation in sufficient quantities to
be capable of producing an explosible concentration of combustible dust in
mixture with air (explosive atmosphere). Examples of these areas are given
as those immediately surrounding powder filling or discharging sites.

Zone 22
Zone 22 is a Zone not classified as Zone 21 in which a combustible dust,
as a cloud, can occur infrequently, and persist only for a short period, or
in which accumulations or layers of combustible dust can give rise to an
explosive concentration of combustible dust in mixtu~ with air (explosive
atmosphere). Examples of this are given typically as areas of mills where