“My life experience confirms that the U.S. government frequently overclassifies data.”  — Michael Hayden

When it comes hazardous (classified) locations—electrical classification—even seemingly sophisticated technical facilities often get it wrong. The issue isn’t with the class, or the group, but with the division. Getting the division wrong, though, can render a facility more hazardous than it should be, or result in unnecessary capital and operating expenses.

Why does this happen?

Hazardous Area Classifications

The National Fire Protection Association (NFPA), developer and keeper of the standards about hazardous (classified) locations, states that area classifications are “concerned entirely with the proper application of electrical equipment to avoid ignition of combustible materials.” While that may be true for the NFPA, it hasn’t kept other organizations from using hazardous area classification for their own purposes. In 29 CFR 1910.178(c)(2), for instance, OSHA uses hazardous area classification to designate where different types of fork trucks may be used.

But mostly, hazardous area classifications are about ignition by electrical equipment.

Distinguishing between classes is easy. Classes are based on the type of material. Class I is for flammable gases or vapors. Class II is for combustible dusts and Class III is for ignitable fibers. In the process industries, most hazardous (classified) locations are the result of flammable gases or vapors, so are Class I. However, combustible dusts are not unheard of, so some process facilities may have Class II locations. We’ve never seen a Class III location in a process facility.

Distinguishing between groups is not very hard, either. NFPA sorts Class I into Group A, Group B, Group C, and Group D and sorts Class II into Group E, Group F, and Group G. (There are no groups for Class III.)

  • Group A consists solely of acetylene.
  • Group B consists of gases and vapors with minimum igniting current ratio (MIC) of less than 0.4 when compared to methane. Examples include ethylene oxide and hydrogen. In fact, the NFPA list of 268 flammable gases and liquids includes only 8 Group B compounds.
  • Group C consists of gases and vapors with an MIC of less than 0.8. Examples include carbon monoxide, ethylene, and hydrazine.
  • Group D consists of gases and vapors with an MIC of more than 0.8. By definition, methane has an MIC of 1, so is Group D. Most flammable gases and vapors are in Group D, a list that includes alkanes, alcohols, and ketones.

Within Class II, combustible dusts, sorting into groups is even easier.

  • Group E is metal dusts.
  • Group F is carbonaceous dusts, e.g., charcoal dust, coal dust, and lamp black.
  • Group G is a combustible dust that is not Group E or Group F. The list includes flour, plastic dusts, powdered sugar, and sulfur dust.

The real challenge is in deciding the Division to which a hazardous location belongs.

Hazardous Area Divisions

There are essentially three divisions: Division 1, where mixtures that can burn may be present under normal circumstances; Division 2, where mixtures that can burn may be present only under abnormal circumstances or as a transition from Division 1 to unclassified locations; and unclassified locations, where the hazard of ignition by equipment no longer exists.

One of the points that NFPA makes repeatedly is that “normal” doesn’t necessarily mean operating correctly. If it is routine for a piece of equipment to fail, the area must be evaluated in terms of that equipment being in its failed state and during repair. Another point that NFPA goes to great pains to make is that locations that are not Division 1 or Division 2 are not called “non-hazardous”. They are called “unclassified.”

So, how does one decide where the boundaries of Division 1, Division 2, and unclassified areas belong? NFPA has produced two excellent documents:

NFPA 497, Recommended Practice for the Classification of Flammable Liquids, Gases or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas


NFPA 499, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installation in Chemical Process Area.

The most important feature of these two documents are the classification diagrams in Chapter 5 of NFPA 497 and Chapter 6 of NFPA 499. Commonly called “bubble diagrams”, they explicitly show how far hazardous locations should extend, how the boundaries should be determined, and the impact of design features on the extent of hazardous locations.

These are not the only two guidance documents that are available. FM Global has a Property Loss Prevention Data Sheet, FMDS 07-32, Ignitable Liquid Operations, that is far more useful for small quantities.

Divisions Have Limits

It should be obvious that hazardous (classified) locations do not extend infinitely from a source of material. Some people will simply classify an entire room with a source as being Division 1 or as Division 2. Not because it is required, but because it is easy. The disadvantage of that approach is that it often results in overclassification, and the costs associated with over classification.

The boundaries of a division depend on more than the Class and Group. They also depend on the physical layout and construction of the space and the equipment in it, the ventilation of the space, and the amount of hazardous material in the space. NFPA 497 observes that well-ventilated areas where flammable liquids are contained in suitable, well-maintained, closed piping systems do not need to be classified, and if the ventilation is inadequate, those areas do not need to be classified when the piping system is without valves, fittings, flanges, and similar fittings.

Ventilation also makes a difference in other ways. Figure 1 shows how adequate ventilation within a room or building can completely transform the hazard classifications of the space.

Figure 1. Comparison of hazardous classification in a Room without and with adequate ventilation. (from NFPA 497)

Also, remembering that the purpose of hazardous classification is to avoid introducing sources of ignition, it should make sense that locations with open flames or hot surfaces do not need to be classified. Instead, a safer operation must be achieved by avoiding flammable or combustible mixtures.

Classified Electrical Equipment

Overclassification is one problem. Its consequences are unnecessary expense. Underclassification is another problem, and its consequences are far more dire. Underclassification can result from not recognizing that an area even needs a hazard classification. It can result from an illiberal application of division boundaries. It can result from the mistaken idea that Class I and Class II equipment are interchangeable or hierarchical.

The most common reason for underclassification, however, is the installation of electrical equipment that is not rated for the hazard classification of the area. For instance, there is no such thing as a Class I, Division 2 coffee maker, and Class II, Division 1 laptop computers are extremely expensive specialty devices not likely to be found in any process facility. It’s always ironic to see a control room equipped with Class I, Division 1 lighting fixtures and power outlets, and with a window fan from Walmart plugged in with an adaptor someone rigged up in the shop.

Hazardous Classification is a Process Safety Question

Once an area is properly classified, expertise in electrical equipment is essential in choosing and installing appropriate devices. Classifying an area, on the other hand, is not about choosing equipment but about understanding process conditions. What are the hazards of the materials being used and to what Class and Group do they belong? What is normal and what is abnormal? These are process safety questions, questions that will need to be answered again and again, as operating conditions, practices, and equipment change.

Do You Comply?

Determining where hazardous (classified) locations are in a facility is often a question left to the facility architect, or someone at an engineering design firm. They will make their best effort but will be working with limited information. The classification is unlikely to be correct, however, if those responsible for operating and maintaining the facility are not involved. Then, after the facility is running and the designer and constructor are long gone, it will still be those responsible for operating and maintaining the facility that will need to adjust the classification as necessary when operating conditions, practices, and equipment change.

OSHA requires documentation for hazardous (classified) location documentation in 29 CFR 1910.307. This is not just a requirement of the Process Safety Management Standard, 29 CFR 1910.119, but for any facility where there is a flammable or combustible concentration of flammable vapors, liquids, or gases, combustible dusts, or ignitable fibers. Make sure that your facility has that documentation, that it continues to accurately reflect your facility, and that your equipment still complies.


  • Mike Schmidt

    With a career in the CPI that began in 1977 with Union Carbide, Mike was profoundly impacted by the 1984 tragedy in Bhopal and has been working on process safety ever since.