Page 51 - Electrical Installation in Hazardous Area

30 Electrical installations in hazardous areas

dust released from leaks can settle and give rise to dust layers which can
be agitated into a cloud by physical shocks or air turbulence.
These newer definitions show a much clearer relationship to the zonal
definitions for gases, vapours and mists while still drawing attention to the
differences which exist. Typical of these are the ability of dust to settle and
persist even in well-ventilated locations and its ability to burn as a layer
in addition to its dangers as a cloud. The general approach has historically
been to attempt, in Zones 21 and 22 (Zones Z and Y) to limit dust layers to
less than 5mm thickness and electrical equipment is designed assuming a
5 mm thickness of dust (See BS 6467, Part 23 and IEC 1241-34). The latter of
these two documents also identifies lmm as the thickness of the layer of
dust above which an explosive atmosphere is possible. The legal require-
ments in the UK in regard to housekeeping (see Chapter 1) need to take
these two figures into account and ensure that the layers allowed to accu-
mulate do not infringe these limits for a particular classification.

2.5.3 Relationship between sources of release and Zones
In the case of gases, vapours and mists where ventilation is good, for
example outdoor situations, there is a clear relationship between the grade
of release and the zonal classification which is as follows: continuous grade
of release gives rise to a Zone 0; primary grade of release gives rise to a
Zone 1; secondary grade of release gives rise to a Zone 2. This relation-
ship, however, ceases to be true as soon as any of the parameters change.
A primary grade source of release may, for instance, only give rise to a
Zone 2 if special local ventilation is applied to effectively dilute the release
to below its explosive concentration at the point of leakage. Likewise, a
secondary grade source of release may give rise to a Zone 1 if local venti-
lation is restricted.
In relation to dusts, the situation is much more tenuous as the formation
of layers is not really Zone specific and the classification depends much
more upon human intervention as housekeeping is a significant contributor
to the situation.

2.6 Collection of information
In order to carry out a formal area classification exercise it is necessary to
first collect information on both the flammable material and the methods
utilized to contain it and the local ventilation conditions need to be
identified.

2.6.1 lnformation on fuels (gases, vapours and mists)

Taking the situation in respect of gases, vapours and mists, the following
information in respect of the flammable materials is necessary as a
minimum.