Explosion Proof Motors Classification Essay

Selecting a motor for a location where potentially explosive gases or vapors are present as a normal part of a manufacturing process can be a daunting task.

Hazardous locations are operating environments in which explosive or ignitable vapors or dust are present, or are likely to become present. In such environments, you need special motors to ensure that any internal fault in the motor will not ignite the vapor or dust. The National Electrical Code (NEC) covers hazardous locations in Chapter 5.

"Explosionproof" doesn't necessarily apply to a motor, just because it operates in a hazardous location. Explosionproof motors are only those approved for Class I locations. Those are the locations where potentially explosive gases or vapors are present as a normal part of a manufacturing process, instead of an unusual condition. If the hazardous material could result from only a ruptured container or some other unusual condition, then you have a Division 2 location instead.

A Class I motor tries to contain an explosion within itself without rupturing. After the initial pressure buildup on ignition, the hot gas must cool by passing through long, tight passageways (flame paths) before escaping from the motor. The temperature of gas escaping from the motor will be less than the minimum ignition temperature (MIT) of the gases or vapors in the atmosphere surrounding the motor.

Meaning of motor class designations. Every motor approved for hazardous locations carries an Underwriters' Laboratories' label that identifies it as suitable for operation in Class I or Class II locations. Some motors may carry approval for both Class I and Class II locations. The Class identifies the physical characteristics of the hazardous materials present where the motor will operate.

Class I covers gases, vapors, or liquids that are explosive or pose a threat as ignitable mixtures. A familiar example of a Class I material is gasoline. It's explosive as a vapor and ignitable as a liquid.

Class II covers dusts. Specifically, it covers electrically conductive dusts and dust in amounts sufficient to create explosive mixtures. A prime example of a hazardous dust is wheat flour. As a compact mass, flour burns or smolders. When finely distributed in air, it's highly explosive. Also included in Class II are electrically conductive metallic dust such as aluminum and nonmetallic dusts, such as pulverized coal. Aluminum and magnesium dusts can burn violently even when not suspended in air. When airborne, they're explosive.

Class III covers ignitable fibers or materials producing combustible flyings. These locations don't normally require hazardous-location motors. Specifying a hazardous-location motor for Class III locations is a common error. Class III materials are not normally airborne, because they are fairly heavy and settle rapidly. They are, however, quite flammable. Thus, they create a potentially hazardous condition when near electrical equipment. Some common Class III substances include baled waste kapok, cocoa fiber, cotton, excelsior, hemp, jute, rayon, sawdust, and Spanish moss.

The NEC allows you to use a totally enclosed fan-cooled or nonventilated motor in Class III locations. This gives you a cost-savings in purchase price. The NEC also allows you to use an open drip-proof motor in Class III locations, if you can prove to the AHJ you will employ proper housekeeping.

Meaning of group designations. Within Class I and Class II only, various combustible substances fall into group designations, based on their behavior after ignition. Group designations A through D are in descending order according to the stringency of motor design requirements for explosive vapors and gases. Group A motors have the longest flame paths and tightest fits. Actually, no Group A or B motors are in production. One justification for Sec. 500-5(a)(2) Ex. 1 and 2 is it allows you a way to take a normally Group B classification for butadiene and certain ethers and use Group C or even Group D equipment. You'll need these exceptions if you're specifying motors for these locations. Groups A through D fall within Class 1, and Groups E, F, and G fall within Class II.

Gasoline and acetylene provide an illustration of the group concept. Both are Class I substances. Acetylene is a Group A substance. Gasoline falls within Group D. MIT of gasoline is 280øC (536øF), slightly below the 350øC (581øF) MIT of acetylene. An acetylene explosion is more intense than a gasoline explosion, so acetylene is in a group well above gasoline. The 1999 NEC uses the maximum experimental safe gap (MESG) values and the minimum igniting current (MIC) ratios to establish group classifications. If you look at Sec. 500-5(a), you can see what the actual ranges are for each group classification.

Don't fall into the trap of thinking Class I transcends Class II. It's not true that a Class I motor will automatically satisfy any Class II requirement. A Class I motor's design focuses on confining the effects of an internal motor explosion. This design rests on the assumption that, over time, normal heating and cooling will cause the motor to breathe the surrounding atmosphere. This means the atmosphere within the motor will eventually become the same as that of the operating environment. A subsequent internal fault, therefore, can cause an explosion.

The design of a Class II motor focuses on maintaining the motor's surface temperature at a level such that Class II materials in the motor's operating environment will not heat up to their MIT. If the operating environment contains both Class I and Class II substances, you must specify a dual-rated Class I/Class II motor.

Meaning of division. Hazardous locations break down even further, into Division 1 and Division 2. Simply stated, a Division 1 location is one in which ignitable substances are likely to be present continuously or intermittently during normal operations. In a Division 2 location, operators handle and store ignitable materials in a manner that allows the combustible substance to escape in the event of spill or equipment failure.

Division distinctions focus primarily on the NEC's required installation procedures. Class I and Class II motors for hazardous locations have no Division designation on the UL label. All Class I and Class II motors meet Division I requirements and are thus suitable for installation in both Division 1 and Division 2 locations.

New hazardous-location motor T codes. All motors manufactured after February 1975 carry a T code designation. The T code identifies the maximum absolute motor surface temperature that will develop under all conditions of operation, including motor overload and even burnout.

The T code designation of the motor must correlate with the 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 1, Group D materials. Acetone has an MIT of 465øC (869øF). Table 3 shows us that a motor with a T1 rating (450øC maximum surface temperature) would be acceptable for operation in an acetone environment.

Gasoline, however, has an MIT of 280øC (536øF). For operation in an environment containing gasoline, the minimum motor you can specify is a T2A motor. This motor's design limits surface temperature 280øC. Although the T codes and ignition temperatures are conservative, you should provide an extra margin of safety by specifying a T2B motor. This motor's design limits surface temperature to 260øC (500øF).

Additional sources of information. Two other publications of the National Fire Protection Association (NFPA) will help you select the right motor. NFPA publication 497 provides a comprehensive listing for hazardous gas applications, while NFPA publication 499 provides a comprehensive listing for hazardous dust applications.

The field service representative of your insurance underwriter can also provide advice. This person has access to a large base of other motor specifiers and users, and may be able to help you do more with less. If you're uncertain that your motor will be safe in the environment in which it must operate, always consult your insurance company and the motor manufacturer for assistance. Never guess - lives are at stake.

TAGS: Design

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