Page 592 - Dust Explosions in the Process Industries

Electrical Apparatuses forAreas Containing Combustible Dusts 559

In principle, heated layers of some organic dusts might decompose and develop com-
bustible gases that do not burn directly but mix with the air over time by convection and
molecular diffusion and eventually form a premixed explosive gas atmosphere inside the
enclosure. This may not be a very likely scenario in general, but in special cases where
such a chain of events cannot be excluded, the electrical components inside the enclo-
sure may have to satisfy the requirements for areas containingexplosive mixtures of com-
bustible gases and vapors.

8.3.2.2
Combustible, Electrically Conductive Dusts

In general, electrically conductive dusts are metal dusts, and hence layers of such dusts
also normally are comparatively good heat conductors. Overheating heat-producing
components due to thermal insulation by the dust layer therefore seems less likely with
metal dusts than with organic dusts.Also, layer ignition temperatures as measured in lab-
oratory tests (see Chapter '7) are generally higher for metal dusts than organic dusts.
Hence, short-circuitingof electrical components,printed circuits, and the like, on which
the dust settles, constitutes the main hazard presented by electrically conducting dusts.
This is not a genuine fire or explosion risk and should, strictly speaking, be treated sep-
arately in other contexts, such as together with problems arising from corrosive and
abrasive dusts (see Section 8.4.1 on protection by enclosures).

8.3.3
THERMAL HAZARDS ASSOCIATED WITH ACCUMULATION
OF DUST LAYERS ON EXTERNAL SURFACES OF ELECTRICAL
APPARATUS ENCLOSURES

As long as the minimum ignition temperature of the dust (for layer or cloud, depending
OB circumstances)is higher than the maximum temperature of the external enclosuresur-
face, the dust will not be ignited by the surface. This is definitely true if the enclosure
surface is entirely free of dust deposits, that is, if the hazard is limited to direct ignition
of an explosive dust cloud by the clean hot surface. It is also true if just a small part of
the enclosuresurfaceis covered by a dust layer, so that this does not significantlyhamper
the heat transfer from the interior of the enclosure, via the surface, and into the sur-
roundings. For these cases,the maximum permissible enclosuresurface temperatures can
be specified on the basis of relatively simple standard laboratory tests for minimum
ignition temperatures (see Section 8.4.2).
However, the situation is different if a substantialpart of the enclosure surface is cov-
ered by a thick dust layer. In this case, the surface temperature of the enclosure may
increase to a value significantly higher than that attained in the absence of dust. This is
because the surface temperatureof a temperature-classedenclosure is seldom a constant
inherent property of the enclosure but a result of the temperature distribution along the
heat transfer path between the heat generating component(s) inside the enclosure and
the atmosphere surrounding the enclosure. The electrical power generated inside the