The term “Flameproof” as used in the United Kingdom, India and many other Commonwealth countries is synonymous with the term “Explosionproof” as used in the U.S.A., or “explosion proof type-d protection” used in Germany and other continental countries. By definition as used in the “Specification for flameproof enclosures of electrical apparatus (Second Revision) - IS:2148-81, is, “An enclosure for electrical machinery or apparatus that will withstand, when the covers or other access doors are properly secured, an internal explosion of the flammable gas or vapour which may enter or which may originate inside the enclosure, without suffering damage and without communicating the internal flammation (or explosion) to the external flammable gas or vapour in which it is designed to be used, through any joints or other structural openings in the enclosure.” The question is often asked as to how an equipment or enclosure designed to prevent any flame from within to the outside could possible have entry within the enclosure of the volatile or vapour surrounding it? This entry is due to the fact that all apparatus are breathing types, i.e. when the electrical enclosure is operating, it emits heat, which invariably expands the air inside, expelling the same through joints. When the electrical is switched off, the air inside contracts, permitting the external atmosphere containing the volatile gas or vapour to enter the enclosure. A sequence of these operations eventually reach a stage where the concentration of the atmosphere within a enclosure becomes ignitable explodes due to sparking or hot spot. When this occurs, the flames and hot gases within are prevented from transmission to the outside due to the width of the joints and the corresponding gaps provided. Thus such enclosures are described as “flameproof” or, “explosionproof.”

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