Specifying motors for hazardous locations
By Edward Cowern -- Plant Engineering, 8/1/1999
Failure to select the proper motor for use in a hazardous location can have serious consequences -- lost production, extensive property damage, and even loss of life. Selecting the proper motor requires an understanding of Underwriters Laboratories (ULÝ) and National Electrical Code (NECÝ) class, group, and division designations and T code letters.
In some plant engineering departments, there is only a vague understanding of the selection criteria for hazardous location motors. The specifier may pass the buck to another party in the hope that someone -- perhaps the motor manufacturer -- will fill in the missing specification data. In other cases, the same type of motor previously used in the plant is specified, with the hope that this approach will handle any situation. But this approach can greatly increase the cost of the project, while in some cases resulting in a motor that is inadequate for the application.
Hazardous locations are operating environments in which explosive or ignitable vapors or dust are present, or are likely to become present. Special motors are required to ensure that any internal fault in the motor will not ignite the vapor or dust. Requirements for electrical installations in hazardous locations are covered in Articles 500, 501, 502, 503, 510, 511, 513, 514, 515, and 516 of the NEC.
An alternative approach to hazardous location requirements is allowed by the relatively new article 505. This article departs from traditional hazardous location requirements and brings the NEC closer to the somewhat less stringent European code requirements by permitting classification of areas into three separate zones: 0, 1, and 2.
Article 505 is typical of how the NEC evolves. It sets forth some new principles, and then a period of time will pass before equipment suppliers have products available to match the new requirements. We can also expect "inertia of habit" to restrain the rate of change to the zone system. Perhaps the zone system will be used first by multinational companies where engineers are more familiar with it and the necessary hardware. At present, Article 505 has much more effect on wiring practices and components than on motors, so it will not be included in this discussion.
The term explosionproof is often erroneously thought to apply to any hazardous-location motor. Explosionproof motors (Fig. 1), however, are only those approved for Class I locations -- that is, where potentially explosive gases, liquids, or vapors are present. A Class I unit is constructed to contain an explosion within itself without rupturing. After the initial pressure buildup on ignition, the hot gas is forced to cool by passing through long, tight passageways (flame paths) before escaping from the motor. The temperature of gas escaping from the motor will then be below the minimum ignition temperature (MIT) of the gases or vapors in the atmosphere surrounding the motor.
Class designations
Every motor approved for hazardous locations carries a UL nameplate that indicates the motor is approved for that duty (Fig.2). This label identifies the motor as having been designed for operation in Class I or Class II locations. The Class identifies the physical characteristics of the hazardous materials present at the location where the motor will be used. Some motors may be approved for both Class I and II locations.
Class I covers gases, vapors, or liquids that are explosive or otherwise pose a threat as ignitable mixtures. A familiar example of a Class I material is gasoline. It is explosive as a vapor and ignitable as a liquid. Some of the most common Class I substances are listed in Table 1.
Class II covers dusts -- specifically, dust in amounts sufficient to create explosive mixtures, and dusts that are electrically conductive. A prime example of a hazardous dust is wheat flour. As a compact mass, flour burns or smolders; but when it is finely distributed in air, it is highly explosive. Also included in Class II are electrically conductive metallic and nonmetallic dusts, such as powdered aluminum and magnesium, and pulverized coal. Aluminum and magnesium dusts can burn violently even when not suspended in air; but when airborne, they are explosive. Some common Class II substances are listed in Table 2.
Class III locations do not normally require hazardous-location motors. Specifying a hazardous-location motor for Class III locations is a common error. Section 503-6 of the NEC permits a totally enclosed fan-cooled or nonventilated motor to be used in Class III locations. A totally enclosed motor can be purchased at lower cost than a motor approved for hazardous locations. NEC Section 503-6 also allows the use of an open drip-proof motor in Class III locations, if the inspection authority is satisfied that proper housekeeping will be maintained.
Class III locations are those where easily ignitable fibers and "flyings" are likely to be present. Such substances are commonly encountered in the textile, woodworking, and plastics industries. Class III materials are not normally airborne, because they are fairly heavy and settle rapidly. They are, however, quite flammable, and, therefore, create a potentially hazardous condition near electrical equipment. Common Class III substances are listed in Table 3.
Group designations
Within Class I and Class II, group designations are assigned to various combustible substances on the basis of their behavior after ignition. Group designations A through G are arranged in descending order according to the stringency of motor design requirements; Group A requirements would require the longest flame paths and tightest fits. Groups A through D fall within Class I, and Groups E, F, and G fall within Class II. Class III materials are not broken down by group.
Gasoline and acetylene provide an illustration of the group concept. Both are Class I substances. Acetylene is designated as a Group A substance while gasoline falls within Group D. MIT of automotive gasoline is 280 C (536 F), slightly below the 305-C (581-F) MIT of acetylene. An acetylene explosion, however, is more intense than a gasoline explosion, so acetylene is grouped well above gasoline.
It is a common misconception that Class I transcends Class II and that a Class I motor will automatically satisfy any Class II requirement. But, a Class I motor is designed primarily to confine the effects of an internal motor explosion. Design is based on the assumption that, over a period of time, normal heating and cooling will cause the motor to breathe the surrounding atmosphere, and the atmosphere within the motor will, eventually, become the same as that of the operating environment. A subsequent internal fault can, therefore, cause an explosion within the motor.
A Class II motor, however, is designed to maintain the motor's surface temperature at a level such that Class II materials in the motor operating environment will not be heated to their MIT. If the operating environment contains both Class I and Class II substances, a dual-rated Class I/Class II motor must be specified.
Another common misconception is that because the classes and groups exist -- then there should be suitable products (motors or other equipment) to operate in the defined environment. As it turns out, Classes and groups are used for all types of equipment including enclosures, light fixtures, heating elements, and operator devices. But just because there is a definition doesn't mean that a matching product is available. In the case of motors, this is especially true for Class I Groups A and B. Apparently the market for motors to operate in these environments is so limited, and the designs so difficult, that most manufacturers do not make them.
Still another misconception is that explosionproof motors are inherently weatherproof. These motors are not gasketed or sealed and, because of the exacting requirements for explosion containment, they have no weep holes.
The most common hazardous location motors are made for Class I and Group D and Class II Groups F and G. Several manufacturers can build motors for Groups C and E, but they are normally made on a special order basis.
Divisions
Hazardous locations are further broken down into Division 1 and Division 2. The distinctions are defined in detail in Article 500 of the NEC. Simply stated, a Division 1 location is one in which ignitable substances are likely to be present continuously or intermittently in the course of normal operations. In a Division 2 location, ignitable materials are handled or stored in a manner that allows the combustible substance to escape in the event of spill, accident, or equipment failure.
For a complete list of Class I materials refer to NFPA 325, "Guide to Fire Hazard Properties of Flammable Liquids, Gases and Volatile Solids."
Division distinctions are concerned primarily with installation procedures required by the NEC. Class I and Class II motors for hazardous locations have no division designation on the UL label. All Class I and Class II motors are designed to meet Division 1 requirements and are suitable for installation in both Division 1 and Division 2 locations.
Hazardous-location motor T codes
All hazardous-location motors manufactured after February 1975 carry a T-code designation (Table 4). The T code identifies the maximum absolute motor surface temperature that will be developed under all conditions of operation, including overload up to and including motor burnout. The T- code designation of the motor must be correlated with the minimum ignition temperature (MIT) of the substances in the motor's operating environment.
The presence of acetone or gasoline, for example, will affect motor selection. Acetone and gasoline are both Class I Group D materials. Acetone has an MIT of 465 C (869 F); Table 4 indicates that a motor with a T1 rating (450-C maximum surface temperature) would be acceptable for operation in an acetone environment.
Gasoline, however, has an MIT of 280 C (536 F). For operation in an environment containing gasoline, no less than a T2A motor, designed to develop a surface temperature no greater than 280 C, should be specified (Table 4). Although T codes and ignition temperatures are conservatively assigned and are based on worst case testing procedures, an extra margin of safety should be provided by specifying a T2B or higher T-rated motor, designed to develop a maximum surface temperature of 260 C (500 F).
Meeting some of the lower temperature T-code requirements necessitates the use of automatic thermal overload devices (on fractional horsepower motors) or normally closed (NC) winding thermostats in larger (integral horsepower) motors.
Winding thermostats are control devices with relatively low current capacity. They have to be connected to the motor's magnetic starter to cause it to interrupt power to the motor when the internal temperature gets too high. Failure to make the required "control circuit" connection will negate the motor nameplate T-code rating.
