Frequently Asked Questions

Frequently Asked Questions about EX Equipment

Explosion Proof (EX)

MLEx - EX - explosion proof label

"Explosion proof" products are capable of containing an explosion. The term "explosion proof" does not indicate that the product is capable of withstanding an internal explosion without allowing flames or hot gases to escape to trigger an explosion in the surrounding atmosphere.

The "explosion proof" term is assigned to those products which are certified by the national rating agencies such as Factory Mutual or Underwriters Laboratories after meeting their specifications and passing their tests. Unless certified by one of these agencies, the product does not meet the "explosion proof" requirements of the National Electrical Code.

Intrinsically Safe

"Intrinsically safe" products receive their classification because their electrical power usage is below the level of power required to set off an explosion within a given hazardous area. In addition, "intrinsically safe" products are incapable of storing large amounts of energy which might spark an explosion when discharged.

Hazardous Area

Both national rating agencies, as well as the American National Standards Institute adhere to the same NFPA definitions of what contributes to a hazardous area. These areas are defined as Class I (combustible gas and liquids), Class II (combustible dust), and Class III (combustible fibers). Class I is subdivided into groups A (acetylene), B (Hydrogen and butadiene), C (diethyl ether, ethylene, isoprene and UDMH), and D (acetone, gasoline, lacquer solvent, styrene, propane and natural gas). Class II is divided into Groups E (metal dust), F (carbon black, coal, and coke), and G (flour, starch and grain dusts).

All classes include two divisions. Division I covers electrical equipment directly exposed in an explosion atmosphere of the material of a specific group. Division II covers electrical equipment in an explosive atmosphere only when accident or fallout occurs, or in a properly vented direct exposure.

Qualification for a rating automatically qualifies the equipment for a lower class and group. For example: Class I equipment can be used in Class II and Class III applications with no restrictions.

An "explosion proof" rating is given only to a single piece of equipment for a specific class, division, and group. A piece of equipment, a system, or parts of a system can receive an "intrinsically safe" rating for a class, group and division. The rating agencies usually test the equipment as a system, and all parts of the system receive the highest class and group reached by the system regardless of any previous "explosion proof" rating. The entire system also receives a collective rating, which will generally be that of the lowest rated piece within the system. If the system is not modified by the end user, it retains the rating. The end user must install the equipment as supplied, and installation procedures are not specified by the National Electrical Code.

What the Explosion Proof Label Really Means

MLEx - EX - explosion proof label

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).

"Explosion" Proof versus "Dust Ignition" Proof

Strictly speaking, equipment is considered "explosion proof" (to use the NEMA standard term rather than the Code wording) only when designed for a Class I location. Equipment for Class II area service is considered "dust ignition proof." Because Class III applications are found almost entirely in the textile industry, which is a limited market, we will confine our attention here to the far more widely encountered Classes I and II. Each is divided by Code Article 500 into two main subdivisions: Division 1 and Division 2 (or "Div 2" in the commonly used chemical industry shorthand term).

Here is where many misunderstandings have arisen. In a Division 1 location, equipment must be built-and labeled as "explosion proof." Here's the reason: for a Class I environment, the NEC defines Division 1 as ". . . a location. . in which ignitable concentrations of flammable gases or vapors can exist under normal operating conditions; or . . . in which . . . [such concentrations]. . may exist frequently because of repair or maintenance operations or because of leakage; or . . . in which breakdown or faulty operation of equipment or processes might release ignitable concentrations . . . and might also cause simultaneous failure of electric equipment."

That's quite a mouthful. Obviously, it's subject to a lot of interpretation. How often is "frequently"? What are "normal" operating conditions?

However, neither the supplier nor the equipment user makes the choice of whether or not the area is properly considered Division 1. That's up to the AHJ-"the Authority Having Jurisdiction" over Code enforcement. The AHJ may be a local or state Fire Marshal, an electrical or building inspector, or an insurance agent. Other industry standards and publications offer guidelines for "area classification," a subject that need not concern us here.

Once the decision is made, the Code simply says that "explosion proof apparatus," as defined earlier, is a "protection technique" applicable to "Class I, Division 1 and 2 locations." That implies "explosion proof" equipment as appropriate in a "Div 2" area. The NEC is silent concerning the manner in which equipment is rendered suitable for a specific hazardous area, saying only that the apparatus must be "approved" for the service.