What the Explosion Proof Label Really Means
Who decides these things and what are their credentials?
Labels on foods, spelling the nutritional ingredients; labels on pesticides, spray cans, and fertilizers warning of hazardous contents or threats to the environment-we're all growing ever more accustomed to product labeling assurances of just what we are (or are not) getting.
For electric equipment including aerial lifts, forklifts and other material handling products, the marking we're used to is the "FM” or “UL” label, certifying that Factory Mutual or Underwriters Laboratories has found such equipment safe for use in a particular environment. But why FM or UL? And just what does the certification mean?
For most electrically powered equipment, the only safety labeling issue concerns the risk of fire or explosion originating with the equipment and spreading to its surroundings. The nature of those surroundings is not defined by FM or UL, but by the National Fire Protection Association (NFPA) in Standard No. 70, the National Electrical Code. The language of the Code needs to be clearly understood.
First, in Article 100, the Code offers this general definition of "explosion proof apparatus": "Apparatus enclosed in a case that is capable of withstanding an explosion of a specific gas or vapor that may occur within it and of preventing the ignition of a specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within, and that operates at such an external temperature that a surrounding flammable atmosphere will not be ignited thereby."
Note that the Code definition does not require the apparatus to be designed so that an internal explosion cannot occur. The only requirement is that any such explosion be confined to the apparatus interior.
Second, Article 500 of the Code defines specific environments as follows:
- Class I: environments containing flammable gases or vapors
- Class II: environments containing combustible dusts
- Class III: environments containing "easily ignitable fibers or flyings"
Within Classes I and II are several "groups" of materials having similar characteristics. For example, under Class I are Groups A (acetylene); B (predominantly hydrogen); C (ethyl ether or ethylene); and D (gasoline, acetone, ammonia, butane, methane, natural gas, etc.). Under Class II we find Groups E (combustible metal dusts); F (carbonaceous materials such as carbon black or coal); and G (other dusts such as flour, wood, and plastic).
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