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DOE-HDBK-1092-2004
propagation of flame cannot occur on contact with a source of ignition. There is also a maximum
concentration of vapor or gas in air above which propagation of flame cannot occur. These
boundary-line mixtures of vapor or gas with air, which if ignited will just propagate flame, are
known as the lower and upper flammable or explosion limits and are usually expressed in terms
of percentage by volume of gas or vapor in air.
In popular terms, a mixture below the lower flammable limit is too lean to burn or explode and a
mixture above the upper flammable limit is too rich to burn or explode.
5.3.6
FLASHPOINT
The flashpoint of a flammable liquid is the lowest temperature at which the liquid gives off
sufficient vapor to form, with the air near its surface or within the vessel used, an ignitable
mixture. An ignitable mixture is a mixture that is within the flammable range (between upper and
lower explosive limits) that is capable of propagating flame away from the source of ignition
when ignited. Some evaporation takes place below the flashpoint but not in sufficient quantities
to form an ignitable mixture. This term applies mostly to flammable and combustible liquids,
although there are certain solids, such as camphor and naphthalene, that slowly evaporate or
volatilize at ordinary room temperature or liquids, such as benzene, that freeze at relatively high
temperatures and, therefore, have flashpoints while in the solid state.
5.4
ELECTRICAL EQUIPMENT FOR CLASS I, II, AND III AREAS
A wide variety of explosion-proof, ignition-proof electrical equipment is available for Class I, II,
and III areas. Selection of such equipment shall fully comply with current NFPA requirements.
Excellent references of manufacturers' electrical equipment available and used in hazardous
areas is the Crouse-Hinds ECM Code Digest, or the Appleton NEC Code Review which are
based on the current NEC.
5.4.1
SEALS AND DRAINS
Seals are to be provided in conduit and cable systems to minimize the passage of gases or
vapors from one portion of the system to another. The seals also keep an explosion from being
transmitted and ignition from traveling between sections of the system.
5.4.1.1 SEALS
The following are uses and requirements for seals:
1. They restrict the passage of gases, vapors, or flames from one portion of the electrical
installation to another at atmospheric pressure and normal ambient temperatures.
2. They limit explosions to the sealed-off enclosure and prevent precompression or pressure-
piling in conduit systems.
3. While it is not a code requirement, many engineers consider it good practice to divide long
conduit runs into sections by inserting seals not more than 50 to 100 feet apart, depending
on the conduit size, to minimize the effects of pressure-piling. Sealing fittings are required.
5-24


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