The National Electric Code (NEC) written by The National Fire Protection Association (NFPA) states that all lighting used in a “hazardous location” needs to be explosion proof.
A hazardous location is defined as any location where there are high enough concentrations of combustible particles in the air to cause a fire or explosion. These combustible materials include, but are not limited to, gasses, liquids, dust, and some fibers. Whenever any of these hazards are present, workers must use specially-designed equipment to reduce the chance of igniting any of these flammable materials.
Two types of fixtures are certified for use in hazardous locations, explosion-proof lights and intrinsically safe fixtures. Explosion-proof light fixtures work by ensuring any ignition of the flammable airborne particulates in the fixture housing remains contained inside the fixture. Keeping this ignition from extending to the environment outside of the fixture prevents what could potentially be a large explosion. Intrinsically safe fixtures are incredibly low powered and are not capable of producing sufficient spark or heat to ignite the flammable particulates.
In a standard light fixture, the bulb, contacts, wiring, and any switches are all directly exposed to the local atmosphere. The spark from a loose contact or the movement of the switch, and even the heat of the bulb can be enough to ignite a flammable atmosphere. In an explosion proof light fixture, any explosions that may occur are contained inside the fixture.
Some people assume that these fixtures work by having an airtight seal around the bulb and any switches so as to not allow the flammable particulates to become exposed to heat or a spark. This is not correct. More accurately, the fixture allows for a small amount of air to circulate within the fixture, and should the flammable particulates ignite, the fixture is shaped in such a way that it will completely contain the flame within the fixture housing.
The type of lights needed in a hazardous environment is determined by the type of potentially flammable materials that will be in the air in the location where the fixtures are to be used. According to OSHA, there are 3 classes, or types of hazardous locations, locations, listed.
Class I Flammable gasses or vapor are present
Class II Combustible dust
Class III Fibers or other airborne flammable materials
In addition to the three classes, each class is broken down into two divisions. Division 1 is considered normal plant conditions and Division 2 is for abnormal plant conditions. In order to create the safest possible environment, it is very important to use the correct light for the correct location and division. Using the wrong light could be a disastrous mistake.