DC Motors Used in Hazardous Applications

When DC motors are used in hazardous locations, special considerations must be taken in specifying these motors to ensure that they not only solve the motion control needs of the application, but also can be operated safely and not become an ignition source for a hazardous event. The term ‘hazardous location’ has a very specific meaning in this context. The National Electrical Code (NEC) defines hazardous locations as “those areas where fire or explosion hazards may exist due to flammable gases or vapors, flammable liquids, combustible dust, or ignitable fibers or flyings.”1 Fire or explosive hazards require not only the presence of a flammable substance but also an oxidizer and a source of ignition.2 Since electrical equipment are a common ignition source, the NEC classifies and sets the standards for the safe use of these equipment in hazardous locations. DC motors accepted for use in hazardous locations are a special type of motor rated as “explosion proof” since they are housed in a “motor enclosure that can withstand an explosion of a specified gas, vapor or dust within it, and prevent [an] internal explosion from igniting any gas, vapor or dust surrounding it. In addition, they are UL and/or CSA approved and marked to show the class, group, and operating temperature for which they are approved.”3

Causes of Fire and Explosions in Hazardous Locations

The most common hazardous locations are in the agricultural, aviation, mining, industrial, chemical processing and petroleum industries. But the cause of fires or explosions in these industries varies. The NEC4 classifies hazardous locations in three ways:

  • Class I: flammable gases or vapors in the air such as natural gas or gasoline vapor. (Ex. Petroleum refineries and gasoline storage).
  • Class II: combustible dust or finely pulverized material. (Ex. Grain elevators or Flour and feed mills).
  • Class III: easily ignitable fibers or flyings that are collected around machinery or on lighting fixtures. (Ex. Textile mills or cotton gins)

Each class is associated with a type of condition called a “division.” Division I hazards are under normal conditions, while Division II hazards occur in abnormal or fault conditions.

Grain elevators, rice mills, dust pellet mills, flour mills, feed mills, etc. commonly have combustible dusts in layers or suspended in the air. When oxygen and ignition sources are present, flames can ignite surrounding the ignition source and, if not contained, will spread out omni-directionally with a shock wave causing a chain reaction resulting in an explosion.5 Gasoline pumping stations (Ex. aviation) that have gas vapors in the presence of air can be ignited from static electricity, which “depends on residual impurities dissolve water, the linear velocity of the fuel through the piping systems and the type of filter and water separator used,” as well as the quantity of fuel being pumped.6 Coal dust explosions7 are well documented and can occur in “thermal dryers, cyclones, bag houses, pulverized-fuel systems, grinding mills, and other process or conveyance equipment” due to small particle coal dusts suspended in the air that are ignited.8 Fire and explosion hazards are found in dye works due to the flammable solvents and dyestuffs used in the processes.9 Soap manufacturing can lead to explosive atmospheres due to the alcohol in scent perfumes, while food manufacturers can experience explosions from dusts, flour or powdered gelatin.10

Explosion Proof Motors: Eliminating A Source of Ignition

While flammable gases, combustible dusts, vapors, or fibers in the presence of air provide the fuel and oxidizer for a fire or an explosion, an ignition source is needed to ignite the reaction to create an explosive atmosphere.11 There are many types of ignition sources. They include:

  • Sparks or arcs from electrical equipment or wiring
  • Open flames (welding)
  • Chemical reactions or biological processes
  • Oxygen levels or temperatures
  • Lightning
  • Ionizing radiation
  • Compression and shock waves
  • Static electricity

Regarding electrical ignition sources, explosion proof DC motors and associated electrical devices (i.e., wiring, conduits, connectors, etc.) are utilized to deprive a potential hazardous event its ignition source. Explosion proof DC motors can be in any wiring configuration: brushed or brushless, permanent magnet or electromagnetic field. What makes these motors safe for use in hazardous locations is an explosion proof motor enclosure that can withstand, prevent or isolate an explosion so it cannot spread into the surrounding atmosphere.12

To meet explosion proof requirements, explosion proof motors are designed13 with the following specifications:

  • Limit the maximum temperature of the motors so any flames that happen to escape the motor are cooled to a level that cannot ignite the external environment. This is accomplished by creating extra long flame paths and special clearances.
  • Set definite length paths, air gaps, widths and tight clearances with motor parts, rotating and stationary, so rubbing and arcing is avoided.
  • Ability to withstand an internal explosion without suffering damage.
  • Pressure testing of motor housing, end bells, terminal boxes, and covers prior to use.
  • Eliminate the use of light metals on external surfaces to avoid friction-based arcing.

Explosion proof motor enclosures are designed to meet standards set by different organizations throughout the world. In North America, explosion proof enclosures are designed to meet the NEC/NFPA and Canadian Electric Code (CEC) requirements, based on a class/division system. They are tested and listed with Underwriters’ Laboratories (UL) and/or the Canadian Standards Association (CSA) when they meet the requirements. In Europe, the European Committee for Electrotechnical Standardization (CENELEC) and the International Electrotechnical Commission (IEC) sets explosion proof standards, based on a zone system, for Europe and international adoption. The European Community (EU) has set standards for explosion proof motors in its ATEX directive. Countries outside of North America or Europe typically choose to set standards based on European or North American requirements.14

  1. OSHA. Hazardous (Classified) Locations. Occupational Health and Safety Administration, OSHA Office of Training and Education, 1996.
  2. Peter Schram, Robert Benedetti, and Mark Earley. Electrical Installations in Hazardous Locations. Jones & Bartlett Learning, 2009. Page 12.
  3. Claire Soares and Heinz P Bloch. Process Plant Machinery, Second Edition. Elsevier, 1998. Page 2.
  4. OSHA. Hazardous (Classified) Locations. Occupational Health and Safety Administration, OSHA Office of Training and Education, 1996.
  5. Michigan Occupational Safety & Health Administration. Onsite Consultation Abatement Method Advice for:GRAIN ELEVATORS. Michigan Department of Licensing and Regulatory Affairs, 2012.
  6. Peter Schram, Robert Benedetti, and Mark Earley. Electrical Installations in Hazardous Locations. Jones & Bartlett Learning, 2009. Page 94.
  7. Rolf K. Eckhoff. Dust Explosion in the Process Industries. Gulf Professional Publishing, 2003. Page 136.
  8. Clete R. Stephan, P.E. Coal Dust Explosion Hazards. Mine Safety And Health Administration, Pittsburgh, PA, 2012.
  9. Jeanne Mager Stellman. Encyclopaedia of Occupational Health and Safety: Industries and occupations, Volume 3. International Labour Organization, 1998. Page 89.18
  10. Rolf K. Eckhoff. Dust Explosion in the Process Industries. Gulf Professional Publishing, 2003. Page 136.
  11. Alan McMillan. Electrical Installations in Hazardous Areas. Butterworth-Heinemann, 1998. Page 3.
  12. NFPA. NEC ARTICLE 500. National Fire Protection Association, 2001. Page 319.
  13. K. C. Agrawal. Industrial Power Engineering Handbook. Newnes, 2001. Page 7/180.
  14. Rockewell Automation. Class/Division Hazardous Location, Publication 800-WP003A-EN-P. Rockwell Automation, 2001.
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