Enclosures

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.

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