Fundamental Classes
The Fundamental Classes domain covers how fuels are classified and how their physical properties drive fire behavior. It is the vocabulary layer the rest of the exam assumes, and questions often turn on exact thresholds and units.
Fire classes (NFPA 10)
- Class A: ordinary combustibles such as wood, paper, cloth, and many plastics.
- Class B: flammable and combustible liquids and gases.
- Class C: energized electrical equipment - a hazard condition, not a fuel.
- Class D: combustible metals such as magnesium, titanium, and sodium.
- Class K: commercial cooking oils, fats, and greases.
Extinguisher ratings carry numerals: a 2-A:10-B:C unit has twice the A rating of a 1-A and ten times the B rating of a 1-B.
Liquid classification (NFPA 30)
Class I liquids have flash points below 37.8 °C (100 °F): IA below 22.8 °C (73 °F) with boiling point below 37.8 °C; IB below 22.8 °C with boiling point at or above 37.8 °C; IC from 22.8 °C to below 37.8 °C. Class II runs 37.8–60 °C (100–140 °F); Class IIIA 60–93 °C (140–200 °F); Class IIIB at or above 93 °C.
Properties worth memorizing
Flash point (ignitable vapor forms), fire point (sustained burning), autoignition temperature (no external ignition source), vapor density (propane about 1.5, heavier than air), specific gravity, lower and upper flammable limits, heat release rate. Fire growth stages run incipient, growth, fully developed, decay.
Worked example
A liquid has a closed-cup flash point of 105 °F. Reasoning: 105 °F is above 100 °F, so it is not Class I; it falls in the 100–140 °F band, so it is Class II. Reporting it as Class IB would be wrong on two counts - wrong band and wrong test basis, since open- and closed-cup values are not interchangeable.
Traps that cost marks
Treating Class C as a fuel type rather than an electrical involvement; confusing flash point with autoignition temperature; mixing closed-cup and open-cup values; applying water to a Class D metal fire; forgetting that 100 °F separates flammable from combustible while the class boundaries do not fall neatly on that line.