Technical and Specific Definition of “Explosion-Proof"
There are several accepted methods for making electrical equipment safe for use in NEC Division or Zone 1 environments:
I. The safest generally is using intrinsically-safe voltages. The idea here is that the voltage is so low that it could not cause a spark capable of setting off an explosion or of igniting the hazardous material. The key disadvantage of this technique is that it prohibits the use of sufficient electricity to operate small motors.
II. Oil immersion: the potentially arcing (sparking) parts are submerged in oil so that the hazardous materials can not come close enough in sufficient concentration to ignite.
III. Sand immersion: the same idea as oil immersion except that the chamber is filled with a type of sand.
IV. Pressurization: the potentially arcing parts are surrounded by a safe gas (usually air or nitrogen) which is pumped into the chamber where they are located in sufficient quantity and at sufficient pressure to prevent the entry of dangerous concentrations of the hazardous material.
V. Explosion proofing: The potentially arcing parts are encapsulated in a special housing which is designed to prevent explosions in two ways: 1. It prevents the entry of the hazardous material in potentially hazardous concentrations. 2. If hazardous materials do manage to enter the encapsulation chamber, the chamber is strong enough contain any resulting explosion or fire and prevent its spread outside the chamber to cause a secondary explosion or fire.