We take a close look at the whole labeling process
LABELS ON FOODS, SPELLING OUT 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 motors, the marking we're used to is the "UL label," certifying that Underwriters Laboratories has found such a motor safe for use in a particular environment. But why UL? And just what does the certification mean?
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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 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.
How a motor is to be rendered "explosion-proof" is not dealt with in NEMA Standards MGI, either. But to safely confine an internal explosion, which could initiate with an arc associated with insulation failure, a motor must have a certain level of frame strength; the hardware holding the parts together must not fail; and the joints between parts, as well as the necessary clearance around the shaft passing through bearing assemblies, must not permit escape to the outside atmosphere of any flame or hot gas that could ignite that atmosphere.
Origin of the label
Many years ago, UL established a motor test program to verify that capability for any motor manufacturer. A satisfactory product was then entitled to be labeled for the application. The "label" is a UL nameplate attesting to the Class and Group appropriate to the atmosphere for which the design is acceptable.
Is every motor built subjected to such testing? No. Once the prototype has been approved, UL sets up a "Label Service Procedure" to monitor production of the design at specific plants and periodically sends out inspectors to see that no changes have been made. Each label used must be accounted for. Each is coded to indicate what specific factory produced the motor.
To maintain the motor's integrity in service, any repairs to the unit must follow the UL "Repair and Relisting Procedure," subscribed to by many apparatus service centers, which involves its own written procedures and surveillance. Otherwise, the label is considered void even though it remains in place on the motor.
That's because any construction changes can compromise enclosure integrity-substituting low-strength bolts; drilling holes to mount identification plates; changing a bearing seal; even tool marks or burrs on machined surfaces at assembly joints, or in seal fits.
In addition to adequate strength and "flame sealing," the temperature of any external surface of a Class I Division 1 motor exposed to the environment must not exceed 80% of the ignition temperature of the gas or vapor involved. But what temperature is that? The Code requires, in Section 500-3(d), that the motor nameplate carry an "Identification Number" expressing the maximum surface temperature within which the motor will operate under those "normal conditions." Given in Code Table 500-3(d), those codes range from T1 (meaning maximum surface temperature of 450 deg C) to T6 (denoting 85 deg C).
Groups C and D, for which Division 1 motors are most commonly labeled, include a large number of gases and vapors belonging to various chemical families. Although NFPA Standard 325M lists ignition temperatures of many such materials, more are being developed all the time. The numbers show wide variation. For example, acetone at 869 deg F; ammonia at 1204 deg ; benzene at 1040; and hexane at 437 all fall within Class I Group D. The user and the AHJ must work together to determine that the temperature identification code on the nameplate for the proposed motor is compatible with the ignition temperature of the actual atmosphere to be encountered.
In the lower horsepower ratings, most motors are "duallabeled"designed and approved for both Class I (usually Group D) and Class II (Groups F and G most often). Although Class II service does not involve confining an internal explosion, because no gas or vapor is present, the bearing assemblies must still be sealed to keep out dust that could contaminate the lubricant. As the grease hardens, the bearing begins to overheat, accelerating the process of lubricant deterioration. Grain explosions
In a grain handling facility, the results are often catastrophic. Explosions in grain dust killed 75 persons in Japan between 1962 and 1975; Britain suffered 20 major losses in one year alone. During 1977, four grain elevator explosions in the U.S. took 55 lives.
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Suppose the AHJ concludes that the location is not a Division 1 area? Here's how the NEC defines Division 2 (for Class I atmospheres): ". . . a location . . . in which volatile flammable liquids or flammable gases are handled, processed, or used, but in which . . . [they] . . . will normally be confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown of such containers or systems, or in case of abnormal operation of equipment; or . . . in which ignitible concentrations. . . are normally prevented by positive mechanical ventilation, and which might become hazardous through failure or abnormal operation of the ventilating equipment; or . . . that is adjacent to a Class I, Division 1 location, and to which ignitible concentrations. . might occasionally be communicated unless such communication is prevented by adequate positive-pressure ventilation from a source of clean air, and effective safeguards against ventilation failure are provided."
That's an even bigger mouthful-one that has caused a great deal of controversy. We usually sum up the distinction between Divisions 1 and 2 by simply saying, "In Division 1 the hazard can be expected to be present any time; in Division 2 it's present only when something goes wrong." But careful reading of the Code indicates that this is an oversimplification. Words like "normally," "effective," or "occasionally" can be subjectively interpreted.
Nevertheless, the most common view is that in a Class I Division 2 atmosphere a motor need not be explosion-proof. For official guidance, we must turn to Section 501-8 of the NEC. There, the Code specifically allows the use of motors in "open" enclosures-provided the design does not include "brushes, switching mechanisms, or similar arc-producing devices." In addition, the exposed surfaces of space heaters must not reach temperatures above 80% of the ignition temperature of the gas or vapor involved.
What about the temperatures attained by the electrical parts of the motor itself? In a "Fine Print Note," the Code says only that "It is important to consider" those temperatures.
That has been one of the causes of concern about use of any open motor in "Div 2" areas. Because of uncertainty concerning the risk of ignition, many motor users have standardized on labeled explosion-proof motors even in Division 2 locations. That position has been supported by NEMA MG2, the "Safety Standard for Construction and Guide for Selection, Installation, and Use of Electric Motors and Generators."
In Para. 3.5, that document says this: ". . . the user has two possibilities when selecting a motor for Class I, Division 2 applications. The recommended approach. . . is to select an explosion-proof motor, which in accordance with Underwriters Laboratories Inc. requirements, shall not exceed the specified external surface temperature under any operating condition. As an alternative, the user may select an open or non-explosion-proof enclosed motor for submission to the local authority for approval . . . the user should consider the temperature of external and internal surfaces of the motor to which the surrounding atmosphere has access."
What's wrong with that statement? That phrase "under any operating condition." As we have already seen, the surface temperature limitation applies only under "normal operating conditions," not under "any operating condition." Moreover, that vague exhortation to "consider" surface temperatures of a non-explosion-proof motor is of negligible value.
Motor temperature
Of greatest concern in application of non-explosion-proof motors to Class I Division 2 service is the surface temperature of the rotor. During those Code-defined "normal running conditions," rotor and stator surface temperatures are about the same (although no extensive body of data shows precise relationships).
We know, however, that during starting-particularly of a high inertia or high torque load-rotor bars and end rings may get much hotter than the stator. Typical design limits for bar temperature are 300 deg C to 400 deg C. Yes, the Code considers starting (or even service factor overload) to be "abnormal." But NEMA standards define ranges of allowable starting frequency; "severe starts" are far from uncommon in motor usage, no matter how "abnormal" the Code may consider them; and no motor can reach "normal running conditions" without first having been started. Ignoring the high rotor temperature resulting from acceleration hardly seems realistic, even though the Code may not be violated.
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Because a hazard can therefore exist despite full compliance with the NEC, engineers have come to accept this truth, from a 1994 IEEE paper on Division 2 applications: "Good experience based on present practice is not sufficient. Sound engineering judgment is required in selecting and applying motors in Class I, Division 2 locations?"
Recognizing the uncertainties involved in that process, UL attempted to develop a standard for this application a decade ago. When the proposed document (UL 1836) was reviewed by the IEEE Petroleum & Chemical Industry Committee, however, numerous deficiencies were pointed out. A few examples: Motor ambient temperature was limited to -13oF-unrealistic in the northern U.S.; resistive devices such as space heaters were required to be of Division 1 construction, a requirement not in the NEC (such devices are not normally available anyway); the usual 90 deg rotation capability of motor terminal boxes appeared to be prohibited.
The upshot was that UL 1836 was withdrawn from consideration without being published. In 1993, however, UL began looking at the Division 2 issue again, primarily from the rotor surface temperature standpoint. With the widespread use of adjustable-speed drives (ASDs), a motor's "normal running conditions" now include many instances of possibly high surface temperatures caused by low speed and harmonic currents. The values will depend upon the nature of the inverter used, and the way the drive is used. No easy solution to this problem has appeared because of the wide variation in those conditions.
Nevertheless, the NEC does not impose limits on internal (stator or rotor) surface temperatures for an open motor in a Division 2 area. A hazard could exist as a result, and that has been widely recognized by petrochemical engineers within the IEEE, which has set up a Working Group to develop its own standard, No. 1349, under development for several years now, to be titled "Guide for Application of Electric Motors in Class I Division 2 Hazardous (Classified) Locations."
Extensive motor testing is under way to establish a firmer basis for application than existed for UL 1836. Although much work remains to be done, one of the conclusions thus far is that ignition temperature of a gas or vapor in motion, within a running motor, may be as much as 200 deg C higher than with the gas at rest in a lab sample. "Movement inhibits ignition," and such movement exists by definition when the NEC's "normal running conditions" are present. Requirements for TEFC motors may well differ from those for open machines. Diffusion of the potentially hazardous atmosphere into-and out of-an idle motor is being examined in detail.
So, will you be finding UL labels on Division 2 motors? Certainly not soon. If the motor is labeled for Division 1, of course, it's automatically suitable for Division 2. But the reverse will not be true.
For Class II service (not to be considered in IEEE 1349), the problem becomes still more complex. Dust can readily accumulate on the stator windings of an open machine. That can create a hazard in two ways. First, the dust blanket can interfere with heat dissipation such that the winding burns out, leading directly to fire or explosion that immediately involves the surrounding atmosphere. Second, the dust can create surface tracking paths, leading to local arcing that produces the same result. If, however, the motor is totallyenclosed (even though not of dust-ignition-proof design), internal dust accumulation isn't a problem. Rotor overheating caused by dust buildup isn't likely for any type of motor enclosure.
A difficulty with Class II areas is that the dust present may be a material that is not ordinarily combustible. Hydrocarbon gases and vapors are recognized as fire or explosion hazards even by those untrained in fire protection. But starch, sugar, flour, and many other materials never used as fuels or solvents are seldom thought of as an explosion hazard.
A good example is ammonium nitrate, a common fertilizer. Here's a complaint submitted to an electrical trade magazine by a designer of industrial facilities: "Over the years. . . we have always used TEFC motors. . . in our fertilizer blending plant construction .... These plants blend potash, ammonium nitrate and other inert materials. The dusts produced by these operations. . . are not of an explosive or combustible nature. But a local electrical inspector recently stated that the equipment in such plants should be Class II, Group G rated.... We do not agree. ..."
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In finely powdered form, many other ordinary or "nonflammable" materials become deadly explosives, not only more powerful than dynamite but far less predictable. A chewing gum factory in New York was blown apart by explosion of magnesium stearate dust used as a non-stick coating on the gum; six workers died. Cause of the blast was machinery failure leading to high vibration, followed by a bearing/shaft failure producing friction sparks.
Resist any temptation, then, to cut corners by assuming that a dusty atmosphere should be safe just because you aren't aware of any particular hazard involved with the material. And beware of anyone in plant operations who says "it's OK" to put an unlabeled motor in place of an explosion-proof unit, even temporarily. Don't listen when you're told that some other motor is "just as good, but doesn't happen to have a UL label."
Is UL the only agency that can certify motor safety in hazardous areas? What about Factory Mutual, or some other organization? Since 1973, the Occupational Safety and Health Administration has been empowered to adopt suitable product-certification procedures as adjuncts to workplace safety. That has resulted in an OSHA program to accredit "nationally recognized testing laboratories" or NRTLs, permitted to certify product safety.
Several other certifying or testing agencies have since been recognized as NRTLs and have begun listing products formerly certified only by UL. Often, however, the certification standards-the design/test criteria-have remained those of UL, as being "the only game in town."
User specifications still sometimes call for motors to meet the requirements of the "NBFU"-the National Board of Fire Underwriters. That body went out of existence more than 30 years ago, merged into the American Insurance Association and later becoming the Insurance Services Office, which rates municipal fire protection agencies and has nothing to do with electrical equipment or hazardous areas of application.
As far as motors are concerned, then, the "UL label" remains the basic sign of hazardous area acceptance. If another agency tests or certifies a motor, the procedures will probably continue to be those first arrived at by UL testing. In any event, the motor user must satisfy the AHJ first.
Copyright Barks Publications Nov 1997
Provided by ProQuest Information and Learning Company. All rights Reserved
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