Here are key data that you need to properly select, install, and maintain hazardous-area motors.

Hazardous areas motors are those motors designed with specific characteristics so that they'll operate safely in a definitive hazardous environment. In some instances, a weather-protected, or a dripproof machine may be used depending upon the degree and type of hazard in the location.

Before you can properly select a motor for use in a hazardous area, it's important for you to remember that motors designed for use in one type of hazardous location usually cannot be used in another type of hazardous location. Therefore, it's essential that you have a clear and thorough understanding of hazardous-area classifications, characteristics, and requirements before attempting to select the right motor for use in a particular hazardous location.

Motors for use in hazardous (classified) areas must meet stringent Underwriters Laboratories (UL) and NEC requirements, and are designed in accordance with National Electrical Manufacturers Association (NEMA) standards. Motors for use in hazardous areas are listed in the UL "Redbook," Hazardous Location Equipment Directory.

Also, hazardous area motors are required to have special features that minimize the possibility of the motor causing ignition of the specific explosive atmosphere surrounding it.

Motors for Class I, Groups C and D locations are often referred to as Explosionproof motors. The term "explosionproof apparatus" is defined in Article 100 of the NEC as follows:

Apparatus enclosed in a case that is capable of withstanding an explosion of a specified 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.

To date, UL does not list motors for use in Groups A and B.

Motors for use in Class II locations are designated as dust-ignition proof.

UL lists motors built only by one manufacturer for Class III applications; however, totally enclosed, nonventilated motors and the so-called lint-free or self-cleaning textile squirrel-cage motors are commonly used in these locations.

All hazardous-area motors must be marked to show Class, Group, and operating temperature or temperature range referenced to a 40 [degrees] C ambient. Temperature identification numbers marked on the motor must be in accordance with Table 500-3(d) of the NEC. (See accompanying table.)

You also should be aware that hazardous-area motors manufactured after October 24, 1997 must be of "energy-efficient" design as required by the Energy Policy Act of 1992 (EPACT).

Class I motors

Motors for Class I locations are often termed "explosionproof" motors and are made of a heavy metal enclosure without ventilating openings. Such motors must be built to provide three performance attributes:

* Strength;

* Joints that will not permit flame or hot gases to escape; and

* Cool operation to prevent ignition of the outside atmosphere.

Motors built expressly for use in these hazardous areas are designed assuming that the explosive atmosphere that surrounds the motor also is present within the motor. You should understand that explosionproof motors are not designed to keep gases and vapors out. Instead, the motor enclosure is designed to be strong enough to withstand the forces created by an explosion of the gases or vapors within the motor without rupturing. At the same time, the enclosure must be capable of preventing the escape from it of hot gases that might cause ignition and explosion in the area.

The fits between various enclosure parts are made tight enough and with extra-long or extra-large flange mating surfaces so that any flame of an internal explosion is cooled sufficiently as it passes through mating parts. This performance characteristic prevents ignition of the outside area atmosphere. These fits are often referred to as explosionproof joints or flame paths. Sometimes, the fits are threaded, such as where a terminal box cover attaches to a conduit. In instances where threaded joints are used, at least five threads must be fully engaged.

Flat joints are also used. These work in a manner similar to the threaded joint. The joints must be wide enough and the clearance small enough so that flames will be quenched and not propagate from the interior of the motor to the atmosphere surrounding it.

In addition, explosionproof motors must operate at a temperature low enough so that the atmosphere around it will not be ignited. The operating temperature of the motor must be marked on its housing. The intent of the temperature-marking requirement is explained in Sec. 500-2 of the NEC. The surface temperature of the equipment must not exceed the ignition of the specific gas or vapor that will be found in the classified area. This means that the ignition temperature of the flammables found within the area in which the motor is to be installed must be known. This information can be obtained from NFPA 497M or NFPA 325M. These documents list the various hazardous materials alphabetically in tables that include their ignition temperatures in [degrees] C and [degrees] F.

Class II motors

In Class II locations, which are hazardous because of the presence of combustible dust, UL standards call for a different type of construction. Class II motors are designed to preclude dust and to operate at specified limited temperatures.

Article 502 of the NEC defines the term "dust-ignitionproof" as "enclosed in a manner that will exclude dusts and... will not permit arcs, sparks, or heat otherwise generated or liberated inside of the enclosure to cause ignition of exterior accumulations or atmospheric suspensions of a specified dust on or in the vicinity of the enclosure."

Assemblies that generate heat, such as motors, are tested with a dust blanket to simulate the operation of the motor in a Class II location. The maximum surface temperature under actual operating conditions must not exceed 165 [degrees] C for equipment not subject to overload. For equipment that may be overloaded, such as motors and power transformers, the maximum surface temperature allowed is 120 [degrees] C.

Closs III motors

Motors suitable for Class III locations must function at full rating without developing surface temperatures high enough to cause excessive dehydration or gradual carbonization of accumulated fibers or flyings. These motors, or other devices, have the same surface temperature limitations as Class II equipment, and construction is similar.

As required by the NEC, these motors must be supplied with telescoping or closed-fitting mating parts, or have other effective means to prevent the escape of sparks or burning material. They also must be constructed so that no openings occur through which sparks or burning material might escape, or through which adjacent combustible material might be ignited.


Hazardous areas are locations where some flammable material is or may be present in an easily ignitable state. These are classified in the NEC by type of flammable material (Class I, II, or III), and by degree of hazard (Div. 1 or 2). In addition, for purposes of testing and approval of equipment, various air mixtures are classified as Group A, B, C, D, E, F, or G.

Article 500 of the NEC defines the three classes as follows:

* Class I: Those locations in which flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures.

* Class II: Those locations in which combustible dusts are present in the air in quantities sufficient to produce explosive or ignitable mixtures.

* Class III: Those locations in which easily ignitable fibers or flyings are present; however, such fibers or flyings are not likely to be in suspension in air in quantities sufficient to produce ignitable mixtures.

Generally speaking, Div. 1 Iocations are more hazardous because of intensity, frequency, etc. of the dangerous material; Div. 2 locations essentially are less hazardous. Additional details on Classes, Groups, temperatures, etc. are provided in Articles 500, 501, 502, and 503 of the NEC.