Page 600 - Dust Explosions in the Process Industries

Electrical Apparatuses for Areas Containing Combustible Dusts 567

and 8.2), dust fires are not regarded as a hazard in their own right. Therefore, areas con-
taining layers of combustibledust that are unlikely to become dispersedinto dust clouds
are not regarded as hazardous areas.

8.4.6
ENCAPSULATION BY MOLDING

As part of the effort to “harmonize” dust standards with gas standards, IEC (2002f)
produced a new standard for encapsulation of electrical equipmentfor combustibledust
atmospheres by molding. In this type of protection, electrical parts that can ignite an
explosive atmosphere are molded into a compound material in such a way that the
atmosphere cannot make contact with these parts. The compounds can be thermoset-
ting, thermoplastic, epoxy resins, elastomers, and the like, with or without fillers.
It is difficult to see that this comprehensiveEx“pD” standard for dusts, which is to a
large extent an edited copy of the correspondinggas standardIEC (2002g), is very help-
ful. As discussed in Section 8.3, the issue of preventing the formation of an explosive
atmosphere inside enclosures is not relevant for dusts. Arelevant specificissue with dusts
would rather be to make sure that molded components embeddedin dust deposits do not
give rise to self-heating or self-ignitionof the dust layer or deposit. The basic European
enclosure standard for dusts (CENELEC, 1998a),discussed in Section 8.4.4, contains a
paragraph on “materialsused for cementing,”which may be expanded to cover additional
aspects of encapsulation by molding that may be relevant for dusts. If necessary, a ref-
erence could be made to gas standards (e.g., IEC, 2002g).

8.4.7
WHYTHE CONCEPT OF FLAMEPROOF ENCLOSURES US NOT
RELEVANT FOR COMBUSTIBLE DUSTS

8.4.7.1
The Original Flameproof Concept for Gases and Vapors

This section is included mainly to support the discussion of the “Atex POOa” Directive
in Section 8.1.3. The basic concept of “flameproof” enclosure design is to confine pos-
sible gas or vapor explosions inside enclosures containing potential ignition sources to
the volume of the enclosure only. It is then assumed that combustible gas that appears
on the outside of an enclosure may give rise to formation of an explosive gas mixture
inside the enclosure by entering it through narrow holes and gaps, such as during
“thermal breathing.” Confinement of the explosion to the interior of the enclosure is
achieved by fulfillment of three basic requirements. First, the enclosure must have suf-
ficient strength to withstand the maximumpossible internal explosionpressure. Second,
any gaps or holes in the enclosure wall must be narrow and long enough to prevent hot
combustiongases produced by the internal explosion,which are expelledfrom these open-
ings, from igniting any explosive gas mixture outside the enclosure. Finally, the exter-
nal enclosure wall temperature must be lower than the minimum ignition temperature
of the explosive mixture of the gas or vapor in question.