How are ignition sources categorized in source models?

• A. Primary sources; probability depends only on energy.
• B. Primary (likely to ignite), secondary, and remote sources; probability of initiation depends on energy, duration, and fuel concentration.
• C. Remote sources only; probability depends on humidity.
• D. Ignition sources are categorized by flame color.

Answer: (B)

Ignition sources in source models are treated as three tiers: primary sources, which are most likely to ignite a mixture; secondary sources, which can ignite under more favorable conditions; and remote sources, which are less likely but still capable in the right circumstances. The likelihood of initiation isn’t a fixed value—it depends on energy, how long the energy is applied, and the fuel concentration available for ignition. More energy or longer exposure makes ignition more probable, and a higher fuel concentration means there’s more combustible material to sustain ignition. This combination of categories and dependency on energy, duration, and fuel concentration is what the correct answer describes. The other options oversimplify or misstate the factors (for example, focusing only on energy, or humidity, or claiming ignition only comes from remote sources, or mentioning flame color, which isn’t how ignition sources are classified).