When flammable gases or combustible dust are mixed in the proper proportion with air, a source of energy is all that is needed to touch off an explosion. One prime source of energy is electricity. During normal operation, equipment such as switches, circuit breakers, motor starters, pushbutton stations or plugs, and receptacles can produce arcs or sparks when contacts are opened and closed, which can easily cause ignition. Other energy hazards are devices that produce heat, such as lighting fixtures and motors. Surface temperatures of these devices may exceed the safe limits of many flammable atmospheres. Finally, many parts of the electrical system can become potential sources of ignition in the event of insulation failure. Included in this category are wiring (particularly splices), transformers, impedance coils, solenoids, and other low-temperature devices without make-or-break contacts.

Nonelectrical sources such as sparks from metal can also easily cause ignition: a hammer, file, or other tool dropped on masonry or on a nonferrous surface could be a hazard unless it is made of 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 9131988 and ANSUISA 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 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.