Combustion of Principles

DOE-HDBK-1092-98

nonsparking material. For this reason, portable electrical equipment is usually made from aluminum

or other material that will not produce sparks if it is dropped.

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. (See NEC Article 5001 for more

information.) The determination of the area classification wiring and equipment selection for Class

I, II, and III areas (NEC Articles 500-504) 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 (NEC Article 505) 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. ANSI/UL 913-

1988 and ANSI/ISA RP12.6 provide information on the design test and evaluation. 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

See Appendix D, Reference Matrix

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