Ignition temperature of a substance, whether solid, liquid, or gaseous, is the minimum temperature required to initiate or cause self-sustained combustion independently of the heating or heated element.

Ignition temperatures observed under one set of conditions may be changed substantially by a change of conditions. For this reason, ignition temperatures should be viewed only as approximations: Ignition temperatures under one set of conditions may be changed substantially by a change of conditions. Some of the variables known to affect ignition temperatures are percentage composition of the vapor or gas-air mixture; shape and size of the space where the ignition occurs; rate and duration of heating; kind and temperature of the ignition source, catalytic or other effect of materials that may be present; and oxygen concentration. Another variable is the many differences in methods and conditions of testing ignition temperature (size and shape of containers, method of heating, and ignition source).

5.3.5 FLAMMABLE (EXPLOSION) LIMITS

As mentioned in Section 5.3.1.1, in the case of gases or vapors that form flammable mixtures with oxygen, there is a minimum concentration of gas or vapor in air or oxygen below which 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 pressurepiling 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 ft apart, depending on the conduit size, to minimize the effects of pressure-piling. Sealing fittings are required.

4. At each entrance to an enclosure housing with an arcing or sparking device when used in Class I Division 1 and 2 hazardous locations, seals must be as close as practicable to and in no case more than 18 in. from such enclosures.

5. At each 2-in. or larger entrance to an enclosure or fitting housing terminals, splices, or taps when used in Class I Division 1 hazardous locations, seals must be as close as practicable to and in no case more than 18 in. from such enclosures.

6. Seals must be located in conduit systems when the conduit leaves the Class I Division 1 or 2 hazardous locations.

7. Seals must be located in cable systems when the cables either do not have a gastight or vaportight continuous sheath or are capable of transmitting gases or vapors through the cable core when these cables leave the Class I Division 1 or Division 2 hazardous locations.

NEC Section 502-5 requires the use of seals in Class II locations under certain conditions. Any approved sealing fittings can be used to meet this requirement.