Explosion proof lighting is a special type of product designed to withstand the toughest environments, specifically hazardous locations.
Per Article 500 of the National Electrical Code (NEC), an area is considered a hazardous location when there are (or may be) ignitable concentrations of combustible substances in the air. These substances include vapors, dust, certain fibers, and flammable gasses. To be considered explosion proof, a lighting fixture must be able to contain an internal combustion by preventing it from propagating outside the unit.
Many people assume that these fixtures work via a seal around the bulb (and its other components), which prevents flammable compounds from entering the device. However, that's only half the story. Explosion proof lighting contains a unique feature called a flame path, which is a special type of joint between different parts of the enclosure. This flame path is essentially a controlled leak that allows expanding gasses from an internal explosion to cool and safely vent to the outside, without igniting any explosive atmosphere that may be outside the lamp. The class, division and group that the enclosure is rated for determines the allowable leak rate (flame path gap and length) to prevent external ignition. For example, a Group A enclosure for acetylene has a much smaller leak rate than a Group D enclosure for propane.
It is worth noting that products that are appropriate in hazardous areas are not always explosion proof. There are many other methods to mitigate explosion risk. Other techniques include intrinsic safety, encapsulation, oil immersion, pressurization, and so on, depending on each application’s specific needs. Project managers should review a product's certifications prior to bringing it to the job site to ensure the equipment is appropriate for their specific application.
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