Explosion-proof electromotors are special apparatuses used in the petrochemical industry, in which risk of explosion is high.  The explosion-proof electromotors include, for example, a flameproof enclosure type “Ex d” and an increased safety type “Ex e.”
The flameproof enclosure type “Ex d” has a structure where, in case of any explosion inside the enclosure, the enclosure withstands the pressure of the explosion, and no flame inside the enclosure is transmitted to an external explosive atmosphere through any joint, opening, etc.  of the enclosure. 
The increased safety type “Ex e” has a structure that prevents sparks, arcs or high-temperature zones from occurring in electric apparatuses to thereby increase mechanical and electrical safety. 
This flameproof enclosure type electromotor is characterized in that its thickness is greater than those of the other typical products in order to secure structural rigidity of the enclosure that is capable of withstanding the explosion pressure. 
For this reason, a terminal box of the flameproof enclosure type electromotor is heavier and more expensive, and is restricted by structural modification such as a change in the number of lead wires of a cable because of stricter explosion-proof certification testing required by the standards, compared to that of the increased safety type electromotor.