NEC Standard Explosion-proof Enclosure Classifications

Explosion-Proof Enclosure: 

An enclosure which can withstand the pressures resulting from an internal explosion  of specified gases, and contain such an explosion sufficiently to prevent the ignition of the explosive atmosphere surrounding the enclosure (for Class I, II and III).

Oil Immersion: 

The electrical apparatus are arranged such that the arcing contacts, connections, etc., are immersed in oil. Arcing is confined under the oil such that it will not ignite an explosive mixture of specified gases above the oil or in the atmosphere surrounding the enclosure (for Class I, II and III).

Purged & Pressurized Enclosures: 

The accumulation of ignitable gases or vapors in the enclosure is prevented by  maintaining, inside the enclosure, sufficient flow of clean air or inert gas (for Class I, II and III).

Intrinsically Safe Equipment: 

Wiring that is incapable of releasing sufficient electrical or thermal energy, under normal and abnormal conditions, to cause ignition of a specific hazardous atmospheric mixture in its most easily ignitable concentration (for Class I, II and III).

Dust Ignition-Proof: 

The enclosure prevents the entrance of dust, and external surfaces shall not reach temperatures  capable of igniting or discoloring dust on the enclosure or igniting dust-air mixtures in the surrounding atmospheres (for Class II).