Page 597 - Dust Explosions in the Process Industries

564 Dust Explosions in the Process Industries

8.4.3
OTHER REQUIREMENTS TO IP ENCLOSURES

International standards (see Section 8.4.4), such as CENELEC (1998a, 1998b),require
that enclosures satisfy some further requirements in addition to the ability to prevent
ingress of dust and maintain a surface temperature below a prescribed upper limit:
Thermal endurance. Enclosures made of plastic materials must be able to withstand
certain specified thermal load tests, both in the range of low and high temperatures.
This also applies to any plastic materials used for cementing.
Mechanical strength. Enclosuresmust be able to withstand relevant tests for mechan-
ical strength.
Grounding and bonding of metallic parts. All metal parts, which by becoming elec-
trostatically charged can give rise to electrostatic discharges that can ignite clouds or
layers of the dust in question, must be properly grounded and bonded.
Antistatic properties. To avoid propagating brush discharges (see Section 1.1.4.6 in
Chapter 1) enclosures made of plastic materials must satisfy certain requirements to
the maximum permissible insulation resistance to ground, the maximum permissible
breakdown voltage across the thickness of the plastic wall, or the minimum permis-
sible thickness of external plastic insulation on metal.
Protection against incendiary metal particle sparks. Exposed parts of enclosures
must not contain metals able to generate impact sparks that can ignite clouds or layers
of the actual dust (see Section 1.1.4.5 in Chapter 1, Section 5.4 in Chapter 5, and
Section 7.12 in Chapter 7).

8.4.4
INTERNATIONAL STANDARDS FOR DESIGN OF ENCLOSURES
FOR ELECTRICALAPPARATUSES FOR AREAS CONTAINING
COMBUSTIBLE DUSTS: AN OVERVIEW

The IEC (1992) suggested that safe design of equipment for areas containing combustible
dusts be based essentially on two basic principles: isolation of potential ignition sources
by means of enclosures that keep the dust out to the required extent (IP 5X or 6X) and
prevention of ignition of the actual dust (layer or cloud) by the enclosure surface. This
philosophy has been adopted in current European standards, as expressed in the two
standardsCENELEC (1998a, 1998b):“The ignitionprotectionis 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 ‘dusttight’ or ‘dustprotected’enclosures.”In addi-
tion, the points mentioned in Section 8.4.3 are accountedfor. There seems to be no valid
reason for departing from this simple, sound philosophy as the basis in international
standards work also in the future. The philosophy also seems to be largely in agreement
with prevailing standards and philosophy of standardizationin the United States.
The IEC (1999a, 1999b) seriously considered adopting the two CENELEC (1998a,
1998b)standards.However, as discussed in Sections 8.1.1 and 8.1.3, it eventuallydecided
to depart from this approach. Instead, it developed a series of new standards with the