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Class I: environments containing flammable gases or vapors.

Class II: environments containing combustible dusts.

Class III: environments containing "easily ignitible 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" versus "dust-ignition" proof

Strictly speaking, an electric motor is considered "explosion-proof' (to use the NEMA standard term rather than the Code wording) only when designed for a Class I location. A motor for Class II area service is considered "dust-ignitionproof." In ratings up to at least 500 hp, motor construction is basically the same for either type of service. But that is only because of convenience in design and manufacture, not because the requirements are identical.

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, a motor must be built-and labeledas "explosion-proof." Here's the reason: for a Class I environment, the NEC defines Division 1 as ". . . a location. . in which ignitible 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 ignitible 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? (For motors, the Code seems to answer that last question in Section 500-3(c), which states: "Unless otherwise specified, normal operating conditions for motors shall be assumed to be rated full-load steady conditions." But is starting a motor really "abnormal"? And what does "otherwise specified" mean?)

However, neither the motor 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 an "explosion-proof" motor as appropriate in a "Div 2" area-and we'll return to that subject a bit later. The NEC is silent concerning the manner in which a motor is rendered suitable for a specific hazardous area, saying only that the apparatus must be "approved" for the service.

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