- Class I—Class I covers gases, vapors and liquids that are explosive or pose a threat as ignitable mixtures. Facilities and environments where Class I locations can typically be found include but are not limited to ethanol and petroleum refineries, spray finishing areas, gas plants and gasoline storage or dispensing areas.
- Class II—Class II covers electrically conductive metallic and nonmetallic dusts. This classification exists when combustible dust in the air is in sufficient concentration to be explosive or ignitable. Some examples of Class II locations include but are not limited to grain elevators, flour and feed mills and starch production plants.
- Class III—Class III covers easily ignitable fibers and combustible flyings. These materials are often heavier and settle more rapidly than fine particles, but they are flammable and have the potential to create hazardous conditions when in close proximity to electrical equipment. These locations do not normally require hazardous location motors. Some common Class III locations include textile mills, flax processing plants and any facilities where high quantities of sawdust are present.
- Group—Class I and II locations are further divided into groups, which are based on how the hazardous material behaves after it has been ignited. Class I locations encompass Groups A, B, C and D. Groups E, F and G fall under Class II areas. Acetylene is the only substance in Group A. Ethanol is designated Class I, Group D, while corn, wheat, wheat flour, soy and sugar are designated as Class II, Group G.
- Division—Hazardous locations are broken down further into Division 1 and Division 2. Division 1 environments exist when ignitable substances are likely to be present during normal operations. Division 2 environments exist when combustible materials are handled and/or stored in a manner that allows the substance to escape in the event of spill or failure of containment. As a simple rule of thumb, all Class I and Class II motors meet Division I requirements, which means they can be installed in both Division 1 and Division 2 locations.
Hazardous Location Temperature Codes
After a hazardous location has been classified, the next piece of information that is needed to determine if a motor is qualified is its temperature code (i.e., T-code). All motors manufactured after February 1975 possess a T-code designation, which is often listed on the nameplate. The T-code informs installing personnel of the maximum absolute motor surface temperature (i.e., skin temperature) that will develop when the motor is in operation. This includes its temperature under normal and abnormal operating conditions, including during overload and burnout.
For the electric motor to be qualified for a location, its T-code must correlate with the autoignition temperature (AIT) of the substances that are present in the operating environment. In simple terms, AIT is the minimum temperature at which a substance can ignite. Take, for example, a Class II, Division 1 location where wheat flour is present. Wheat flour is a Group G material and has an AIT of approximately 716 F. For a motor to be used in this environment, it must have a T-code rating of at least T2, which correlates with a maximum motor surface temperature of 572 F. Use of a motor with a T1 designation, which correlates with a maximum motor surface temperature of 842 F, introduces the risk of combustion if the wheat flour comes in contact with the motor enclosure.
While AIT values for most substances can be found in literature, they do not always translate to the operating environment. This is primarily due to the fact that AIT can change depending on atmospheric variables. Additionally, many hazardous location environments contain a mixture of different substances, which makes determining AIT virtually impossible. In these situations, it is common practice to use the lowest AIT of all the known substances. In every case, published AIT values should serve as a useful aid for hazardous duty motor selection and not a defining parameter.
Considerations for Variable Frequency Drives
In recent decades, the use of electric motors operated with variable frequency drives (VFDs) has increased substantially. These systems offer higher efficiencies and enable better control; however, they also pose an added risk in the presence of combustible materials due to potential excessive heating.
Operating motors with VFDs contributes to heat rise on the surface of the motor in two ways. First, since most motors in grain facilities are totally enclosed fan cooled (TEFC) motors, the fan that cools the motor is attached to the shaft of the motor and turns at the same speed as the output shaft of the motor. When the motor is operated at slow speeds, there could be insufficient airflow over the motor, and the motor surface could get too hot.
Second, the electrical wave from most VFDs is not a smooth sine wave. It is a pulse width modulated wave form that switches voltage on and off thousands of times per second to approximate a sine wave of varying frequency to change the speed of the motor (hence the name).
This rapid cycling of the voltage onto the motor windings, which is electrically an inductor, causes voltage spikes much higher than the line voltage.
These spikes cause the windings to heat up. Inverter-duty motors are designed and built to withstand the higher voltage spikes produced by VFDs and can run at slow speeds without overheating and are labeled accordingly.