Sources of Ignition

DOE-HDBK-1092-98

The electrical installation shall prevent accidental ignition of flammable liquids, vapors, and dusts

in the atmosphere. In addition, because portable electrical equipment is often used outdoors or in

corrosive atmospheres, its material and finish should be such that maintenance costs and shutdowns

are minimized. (See Figure 5-2.)

NEC Article 500

Figure 5-2. Arcs and sparks are sources of ignition that produce enough heat to cause an explosion if the air

and gas mixture is between the lower and upper flammable limits of the liquid involved.

5.2.1 SOURCES OF IGNITION

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

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