NFPA 499: Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas

What is NFPA 499?

NFPA 499 provides information on the classification of combustible dusts and of hazardous (classified) locations for electrical installations in chemical process areas and other areas where combustible dusts are produced or handled. It provides information on combustible dusts as it relates to the proper selection of electrical equipment in hazardous (classified) locations in accordance with NFPA 70.

A “hazardous area” is defined as an area in which the atmosphere contains, or may contain in sufficient quantities, flammable or explosive gases, dusts, or vapors.

Combustible Dusts

A combustible dust is any material (finely divided solid particles) that has the ability to disperse in air and catch fire and explode when exposed to an ignition source. Combustible dust may include materials that are in the physical states of powders, flakes, fines, fibers, etc. Combustible dust layers also have the potential to be ignited, but depending on the speed of the burning, the dust could also be dispersed into the air as a cloud resulting in a flash fire.

Stonehouse Process Safety has prepared several articles on Combustible Dusts on topics such as dust explosion prevention, dust explosion protection, and combustible dust testing. See for example:

Classification of Combustible Materials

Article 500 of NFPA 70 establishes the basis for classifying locations where fire or explosion hazards can exist due to flammable gases, flammable liquid–produced vapors, combustible liquid–produced vapors, combustible dusts, or ignitable fibers/flyings.

NFPA 70 defines a Class II hazardous (classified) location as one that is hazardous because of the presence of a combustible dust.

Class II hazardous (classified) locations are further subdivided into either Class II, Division 1 or Class II, Division 2. More explanation of Class/Division and Zone Groups are available from our article here .

Classification of Combustible Dust Locations

The decision to classify an area as hazardous should be based on the probability that a combustible dust could be present. This defines the ‘combustible dust condition’ as per NFPA 70. Once the NFPA 70 ‘combustible dust condition’ has been defined, the next step should be to determine the degree of hazard — that is, whether the area is Division 1, Division 2, or unclassified.

For further information on Classification of Combustible Dust Locations, please see one of our Stonehouse Explainer articles here.

Can we help you?

If you would like help in classifying your hazardous areas, please contact us on 609-455-0001 or email us at for a complementary discussion with one of our HAC specialists.