How to Select the Proper Explosion-Proof Motor for Your Fan Application
by Ken Kuntz |
A variety of exhaust fans with spark-resistant designs that are suitable for contaminated and flammable environments are available in the marketplace. Many of these environments can use totally enclosed (TE) motors since the motors are not in the contaminated airstream and pose no threat to ignite any flammable gasses. But there are cases when the motor is exposed to fumes and other flammable contaminants because of the environment around the fan. For those cases, there are hazardous duty and explosion-proof (EXP) motors.

The National Electric Code standard NEC 500 defines hazardous locations as those areas where fire or explosion hazards may exist due to the presence of substances that are flammable, combustible, or ignitable. The code breaks down these locations further into classes and divisions, defined with groups and temperature classifications indicating the required level of protection for the application. This information must be available when selecting the appropriate motor; otherwise, the motor may not meet the location requirements or be over-designed, costing more money.

Here’s a quick overview of how hazardous locations are defined by class, division, group, and temperature classification.

Class: Defines the type of hazard present

Class I – Created by the presence of flammable gases or vapors in the air, or flammable liquids, in sufficient quantities to be explosive or ignitable.

Class II – Created by the presence of combustible dust, suspended in the air, in sufficient quantities to be explosive or ignitable.

Class III – Areas where there are easily ignitable fibers or flyings present. These include cotton lint, flax, and rayon as examples. The fibers in a Class III area are not likely to be in the air but can collect around machinery or on lighting fixtures. (generally, not used in our industry)

Division: Defines the probability of a hazard

Division 1 - Normal conditions. Hazard is present in everyday production operations or during frequent repair and maintenance activities. Explosion Proof Motor Needed

Division 2 - Abnormal conditions. Hazard is confined in closed containers or closed systems (ventilation) and will be present only through accidental rupture, breakage, or unusual faulty operation. Hazardous Duty Motor Needed.

Class Groups:
Specifically defines the hazard present

Class I Groups

Group      Gas
A              Acetylene
B              Hydrogen, etc.
C              Ethylene, etc.
D              Hydrocarbons, Propane, fuels, solvents, etc. (Most common in HVAC industry)

Class II Groups

Group      Dust Type
E              Metal dust
F              Coal, carbon dust
G             Grain, sugar, plastic, or chemical dust

Temperature Classification “T-Codes”:
Table 1 identifies the maximum absolute surface temperature developed by the motor under all conditions.

Maximum motor surface temperature
Table 1

To correctly specify and select a motor, all the above information is needed.

Class (I, II, III)
Division (1 or 2)
Group (A, B, C, D, E, F, G)
Temperature Classification (Txx)

Class I, Div. 1, Group D, T3A (Gas/vapor, normal operation, Hydrocarbon, 356F) 
Class II, Div. 1, Group F, T3A (Dust, normal operation, coal/carbon, 356F)

Many times, this information is not all available.  Fan manufacturers can provide motors suited for the most common needs.   For example, Greenheck provides HVAC hazardous duty and EXP motors with ratings for Class I or II, Division 1, Groups D, F&G with temperature classification of T3 or better in NEMA frame sizes.

Motor manufacturers offer a wide variety of hazardous duty and explosion-proof motors. It is important to know that equipment suitable for all hazard locations may not exist just because an NEC hazard location is defined. Table 2 uses the Greenheck example again, showing general EXP or hazardous duty motor availability.


Hazardous duty and EXP motor availability
Table 2: Hazardous duty and EXP motor availability
The hazardous duty and Explosion Proof motor options in Table 2 cover what is commonly used in the HVAC industry for situations when a totally enclosed motor will not meet the requirements. Categories in the green area are available, while categories colored in yellow require a special design request. The design request may include modifications by the motor manufacturer or a suitable substitution.  Categories in grey are not available.  There may be applications involving contaminants outside of these specifications.  In those cases, our recommendation is to consult with the manufacturer’s fan application staff or motor vendor to determine the appropriate enclosure and ensure proper fit on the exhaust fan.”

Join the discussion...

Leave a comment
Reader Comments
Comment by
Ken Kuntz
Ken Kuntz
Ken Kuntz
Ken Kuntz is the fume exhaust and tubular inline (FETI) engineering manager for Greenheck. He has with Greenheck for 15 years. Ken earned a mechanical aerospace engineering degree from the University of Missouri – Columbia. He is a voting member on ASHRAE’s TC 9.10-Laboratory Systems and is the subcommittee chair for the ASHRAE Laboratory Design Guide publication. Ken also is a voting member on AMCA 204-20, Balance Quality and Vibration Levels for Fans and is active on AMCA 260, Laboratory Methods of Testing Induced Flow Fans for Rating and AMCARC committees. Prior to joining Greenheck, Ken spent 14 years with Harley-Davidson and is named on two patents for powertrain design.
Other Blog Topics
Other Blog Topics