Combustion Principles

DOE-HDBK-1092-2004

5.2.2

COMBUSTION PRINCIPLES

The following three basic conditions are necessary for a fire or explosion to occur:

1. A flammable liquid, vapor, or combustible dust is present in sufficient quantity.

2. A flammable liquid, vapor, or combustible dust mixes with air or oxygen in the proportion

required to produce an explosive mixture.

3. A source of energy is applied to the explosive mixture.

In applying these principles, the quantity of the flammable liquid or vapor that may be liberated

and its physical characteristics are taken into account. Also, vapors from flammable liquids have

a natural tendency to disperse into the atmosphere and rapidly become diluted to

concentrations below the lower explosion limit, particularly when there is natural or mechanical

ventilation. Finally, the possibility that the gas concentration may be above the upper explosion

limit does not ensure any degree of safety since the concentration first passes through the

explosive range to reach the upper explosion limit.

5.2.3

EVALUATION OF HAZARDOUS AREAS

Each area that contains gases or dusts that are considered hazardous shall be carefully

evaluated to make certain that the correct electrical equipment is selected. Many hazardous

atmospheres are Class I Group D or Class II Group G. However, certain areas may involve

other groups, particularly Class I Groups B and C. Conformity with the NEC requires the use of

fittings and enclosures approved for the specific hazardous gas or dust involved. The

determination of the area classification wiring and equipment selection for Class I, II, and III

areas should be made by a person cognizant of the requirements. The determination of the area

classification, wiring, and equipment selection for Class I, Zone 0, 1, and 2 areas shall be under

the supervision of a qualified registered professional engineer.

5.2.4

INTRINSICALLY SAFE EQUIPMENT

The use of intrinsically safe equipment is primarily limited to process control instrumentation

because these electrical systems lend themselves to the low energy requirements. The

installation rules are covered in Article 504 of the NEC. The definition of intrinsically safe

equipment and wiring is: "Equipment and wiring that are incapable of releasing sufficient

electrical energy under normal or abnormal conditions to cause ignition of a specific hazardous

atmospheric mixture in its most easily ignited concentration." UL and Factory Mutual list several

devices in this category. The equipment and its associated wiring shall be installed so they are

positively separated from the nonintrinsically safe circuits. Induced voltages could defeat the

concept of intrinsically safe circuits.

5.2.5

ENCLOSURES

In Class I Division 1 and 2 locations, conventional relays, contactors, and switches that have

arcing contacts shall be enclosed in explosion-proof housings, except for those few cases

where general-purpose enclosures are permitted by the NEC. By definition, enclosures for these

locations must prevent the ignition of an explosive gas or vapor that may surround it. In other

words, an explosion inside the enclosure shall not start a larger explosion outside. Adequate

strength is one requirement for such an enclosure. For an explosion-proof enclosure, a safety

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