Fire classification

Demonstration of using the wrong suppressant on a grease fire

Fire classification is a system of categorizing fires with regard to the type(s) of combustible material(s) involved, and the form(s) of suitable extinguishing agent(s). Classes are often assigned letter designations, which can differ somewhat between territories.

Standards

• International (ISO): ISO 3941 Classification of fires
• Australia: AS/NZS 1850
• Europe: DIN EN2 Classification of fires
• United States: NFPA 10 Chapter 5.2.1-5.2.5

Materials and hazards

Flammable solids: Class A (US/EU/AU)

Fires involving ordinary flammable solids fall under Class A. This includes wood, paper, fabric, rubber, and some types of plastics. Such fires may be extinguished by water, wet chemical suppression, or dry chemical powder.

Flammable liquids: Class B (US/EU/AU)

A carbon dioxide fire extinguisher rated for flammable liquids and gases

Fires involving flammable liquids or liquefiable solids fall under Class B. Examples may include petrol/gasoline, oil, paint, some waxes & plastics, though cooking fats and oils are explicitly excluded (discussed and categorised separately below).

A solid stream of water should never be used to extinguish this type of fire because it can cause the fuel to scatter, spreading the flames. The most effective way to extinguish a liquid fire is by inhibiting the chemical chain reaction of the fire, which can be done by dry chemical or Halon extinguishing agents. Smothering with CO₂ or foam is also effective. Halon has fallen out of favor in recent times (except for aircraft fire extinguishing systems) because it is an ozone-depleting material (the Montreal Protocol declares that Halon should no longer be used). Chemicals such as FM-200 are now the recommended halogenated suppressant.

Flammable gases: Class B (US) / Class C (EU/AU)

Fires involving flammable gases fall under Class C in the European/Australian system, and Class B (along with flammable liquids) in the US system.