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Classification Basics

Classification Terminology

Definitions

The following terms that are critical to understanding area classification are defined in NFPA 497 and NFPA 499: Auto ignition temperature Auto ignition temperature
The minimum temperature required to initiate or cause self-sustained combustion of a solid, liquid or gas independently of the heating or heated element. Flash point Flash point
the minimum temperature at which a liquid gives off vapor in sufficient concentration to form an ignitable mixture with air near the surface of the liquid, as specified by test. Ignitable mixture Ignitable mixture
A combustible material that is within its flammable range. Flammable range Flammable range
This term is not defined in NFPA 497 or 499, but the flammable range is the range of mixtures of a specific material in air that will form an ignitable mixture. This range is defined by the LFL (lower flammable limit) and the UFL (upper flammable limit) of a material. The LFL is expressed as a percentage, and is the lowest percentage of the material in air that will form an ignitable mixture. The UFL is the highest percentage of the material in air that will form an ignitable mixture. For example, the %LFL and %UFL for gasoline are 1.4 and 7.6 respectively.
Combustible material Combustible material
A generic term used to describe a flammable gas, flammable liquid produced vapor, or combustible liquid produced vapor mixed with air that may burn or explode.
Flammable liquid Flammable liquid
A liquid designated as Class I, having a flash point of 100 o F (37.8 o C) and having a vapor pressure not exceeding 40 psia at
100 o F (37.8 o C). (Class I liquids are subdivided into Classes IA, IB, and IC and should not be confused with Class I classified areas.
Combustible liquid Combustible liquid
A liquid having a flash point at or above 100 o F (37.8 o C). (Combustible liquids are subdivided into Classes II, IIIA, and IIIB and should not be confused with Class II and Class III classified areas.)
Combustible dust Combustible dust
A finely divided solid material 420 microns or less in diameter (i.e., material passing through a U.S. No. 40 Standard Sieve) that presents a fire or explosion hazard when dispersed.
NOTE: This is an unformatted excerpt from our online fire protection training library.

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