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 generalpurpose 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.

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.

Figure 5-4. Threaded joints can be used as an escape path to cool the hot gases as they pass through the threads to the outside of the enclosure.