Page 608 - Dust Explosions in the Process Industries

ElectricalApparatuses for Areas Containing Combustible Dusts 575

The term source ofrelease, which is indeed appropriate for gases, is not useful in the
context of dusts and should be replaced by a more relevant term, such as area ofdust
cloud generation.

8.6.3
PROTECTION BY ENCLOSURES

8.6.3.1
IP Enclosures

At least four reasons for using dust tight or dust protected enclosureshave been put for-
ward. To avoid ingress of
0 Combustible dust that can form an explosive dust cloud inside the enclosure.
0 Combustible dust that can form a combustible dust layer inside the enclosure.
0 Electricallyconductive dust that can cause electricalshort-circuitinginside the enclosure.
0 Abrasive or corrosive dust that can damage delicate mechanical and electrical com-
ponents inside the enclosure.

Only ingress of dust that can form explosive dust clouds is within the “Atex” domain
for dusts as presently defined by the existing European directives. However, ingress of
dust that can form explosive dust clouds can be effectively prevented by IP 5X or 6X
enclosures.
Safe design of electrical equipment for areas containingcombustibledusts should, with
very few exceptions,be based on two simple concepts: The first is use of enclosuresthat
prevent ingress of dust to the required extent; the second is adoption of measures that
keep the temperatureof any surfacein contact with dust clouds or layers sufficientlylow
to effectively prevent ignition. This sound philosophy is expressed clearly in current
European standards. For example, in CENELEC (1998a, 1998b), “The ignition pro-
tection is based on the limitation of the maximum surface temperature of the enclosure,
and on the restriction of dust ingress into the enclosureby the use of ‘dust tight’ or ‘dust
protected’ enclosures.”

8.6.3.2
Pressurized Enclosures

The new “pressurization”standard for dusts (IEC,2001b) is self-contradictorybecause it
rests on a fundamentally wrong assumption.This assumptionis that combustibledust that
enters the interior of an electrical apparatusor componentenclosurethough unintentional
tiny holes and gaps in the enclosure wall can accumulate as an explosive dust cloud inside
the enclosure.In reality, only dust layers can be generatedby this kind of dust ingress.Hence,
for the new standard to have some relevance, terms like explosive dust clouds, explosive
dust air mixtures, combustible dust atmospheres, and explosive concentrations of com-
bustible dust mixed with air must be replaced by layers of combustible dust throughoutthe
standard.