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DOE-HDBK-1092-98
equipment and its associated wiring shall be installed so they are positively separated from the
nonintrinsically safe circuits. Induced voltages could defeat the concept of intrinsically safe circuits.
5.2.5 ENCLOSURES
In Class I Division 1 and 2 locations, conventional relays, contactors, and switches that have arcing
contacts shall be enclosed in explosion-proof housings, except for those few cases where general-
purpose enclosures are permitted by the NEC. By definition, enclosures for these locations must
prevent the ignition of an explosive gas or vapor that may surround it. In other words, an explosion
inside the enclosure shall not start a larger explosion outside. Adequate strength is one requirement
for such an enclosure. For an explosion-proof enclosure, a safety factor of 4 is used. That is, the
enclosure shall withstand a hydrostatic pressure test of four times the maximum pressure from an
explosion within it.
In addition to being strong, the enclosure shall be flame-tight. This term does not imply that the
enclosure is hermetically sealed but rather that the joints cool the hot gases resulting from an internal
explosion so that by the time they reach the outside hazardous atmosphere, they are too cool to affect
ignition. The strains and stresses caused by internal explosive pressures are illustrated in Figure
5-3 (dotted lines indicate the shape that a rectangular enclosure strives to attain under these
conditions). Openings in an enclosure strive to maintain the shape of the enclosure. Openings in an
explosion-proof enclosure can be threaded-joint type (Figure 5-4) or flat-joint type (Figure 5-5).
In Class II locations, the enclosure shall keep dust out of the interior and operate at a safe surface
temperature. Because there will be no internal explosions, the enclosure may have thinner wall
sections. The construction of these enclosures is known as dust-ignition-proof.
NEC Article 500
Figure 5-3. The right mixture of air and gases in an enclosure can cause an explosion that creates internal
pressures that can rupture the enclosure if not released properly.
5-15


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