Fire Extinguisher Classes
Authority: NFPA 10: Standard for Portable Fire Extinguishers
NFPA 10 classifies fires by fuel type: Class A covers ordinary combustibles like wood and paper, Class B covers flammable liquids and gases, Class C covers energized electrical equipment, Class D covers combustible metals, and Class K covers cooking oils and fats. The UK/EU system uses Class F instead of K.
| Class | Fuel / Fire Type | Typical Extinguishing Agent | Key Note |
|---|---|---|---|
| A | Ordinary combustibles: wood, paper, cloth, rubber, many plastics | Water, foam, or multi-purpose dry chemical | The most common fire class in homes |
| B | Flammable liquids and gases: gasoline, oil, grease, solvents, propane | Foam, CO2, or dry chemical | Never use a direct water stream -- it scatters burning liquid and spreads the fire |
| C | Fires involving energized electrical equipment | CO2, dry chemical, or another non-conductive agent | The agent must not conduct electricity back to the operator; once power is cut, the underlying fire is really Class A or B |
| D | Combustible metals: magnesium, titanium, sodium, lithium, potassium | Specialized dry powder agents | Water and standard agents can react violently with burning metal |
| K | Cooking oils and fats in cooking appliances | Wet chemical extinguisher | The agent saponifies the hot oil, forming a foam blanket that cools and smothers the fire |
Why the Wrong Extinguisher Is Dangerous
Fire classes exist because putting the wrong agent on a fire can make it worse rather than stopping it. Water sprayed on a Class B liquid fire can scatter burning fuel across a wider area instead of extinguishing it. Water sprayed on a Class C electrical fire can conduct current back through the stream toward the person holding the extinguisher. Water poured on a Class K grease fire can cause the hot oil to violently splatter, similar to what happens when water hits a hot deep fryer, spreading flames rather than smothering them.
NFPA 10, the standard that defines these classes in the United States, exists specifically so extinguishers, training, and fire response can be matched to the fuel actually burning. Multi-purpose extinguishers rated for more than one class, such as an ABC-rated dry chemical extinguisher common in homes, are designed to be safe across several fuel types at once, but they are still not effective or safe on every class -- a standard ABC extinguisher is not rated for the specialized metal fires of Class D.
How the UK/EU System Differs
The United States NFPA system and the UK/EU system (based on BS EN 2) classify ordinary combustibles, flammable liquids, and flammable gases the same way as Class A and Class B. They diverge on cooking fires and electrical fires. What NFPA 10 calls Class K -- fires involving cooking oils and fats -- is labeled Class F under the UK/EU system.
Electrical fires are handled differently, too. The UK/EU system does not maintain a separate letter class for electrical fires the way older classifications once did with a Class E designation, because electricity is treated as an energy source that can be present in a fire of any class rather than a fuel type in its own right. Instead, UK/EU extinguishers intended for use near live electrical equipment are marked separately to indicate they are safe up to a specified voltage.