Page 594 - Dust Explosions in the Process Industries

E/ectrica/Apparatuses for Areas Containing Combustible Dusts 56 I

According to IEC (2001c), the degrees of protection offered by a given enclosure are
specifiedby two digits, the first referring to ingress of solid objects,the second to ingress
of water. For solid objects six levels of protection are defined, ranging from objects
larger than 50 mm (digit 1) to dusts (digits 5 and 6). For water, the correspondingrange
is from gentle dripping (digit 1) to continuous complete immersion (digit 8). The code
also specifies the test methods by which enclosures can be checked for compliancewith
the requirements of the various degrees of protection.
It is important to note that the IEC (2001~)IP code, does not cover protection against
ingress of explosive gases. As discussed in Section 8.3.1, satisfactoryprotection against
the ingress of gas is very difficult to achieve using normal enclosure technology, such
as flanges with gaskets, because gas molecules migrate though even very tiny open-
ings, in particular if there is a pressure drop across the opening. For this reason, it has
been necessary to introduce a number of additional technologies to prevent either
ingress of explosive gases into enclosures (oil-filled, pressurized, and molded enclo-
sures), ignition of explosive gas that has entered the enclosure (enclosures filled with
glass beads or sand, elimination of potential ignition sources inside enclosure), or
transmission of an explosion inside the enclosure to a possible external explosive
atmosphere (“flameproof” enclosures).
However, in the case of dusts, none of these additional measures are required to pre-
vent formation of explosive dust clouds inside enclosures. Common enclosure technol-
ogy, such as flanges with gaskets, provides the protection required.
The IEC IP code specifies two levels of prevention of ingress of dust into enclosures,
“dust protected” (digit 5) and “dusttight” (digit 6), which are defined as follows:
0 Dust protected. A limited quantity of dust is allowed to penetrate into the enclosure
under certain conditions (IP 5X).
0 Dust tight. No dust is allowed to penetrate into the enclosure (IP 6X).

IEC (2001~)also specifies the tests to which enclosuresof categoriesIP 5X and IP 6X
must be subjected. The test apparatus used should incorporate the same basic features
as illustrated in Figure 8.4.
The enclosure to be tested is placed inside a test chamber where a very dense cloud
of fine talcum powder is maintained continuously during the test period, either by a
powder circulationpump, as illustrated in Figure 8.4, or by some other means. Depending
on the practical circumstances in the industry in which the enclosure to be tested is to
operate, tests can be conducted either with a slight negative pressure inside the enclo-
sure to be tested, as illustrated in Figure 8.4, or with no pressure difference across the
enclosure wall.
CENELEC (1998b) specifies the IP requirements for combustible dusts as follows:

0 Zones 20 and 21:IP 6X
0 Zone 22: IP GX for electrically conductive dusts and IP 5X for electrically noncon-
ductive dusts.
CENELEC (1998b) also specifiesmarking codes to be used to identify the degree of
protection offered by a given enclosure.
Greiner (2001) illustrated the use of IP enclosures to prevent ingress of dust into var-
ious electrical apparatuses, such as electrical motors and terminal boxes. With adequate
design of the flange/gasket arrangement, IP 6X could be obtained quite readily.