Classes of Fire — A, B, C, D, and K

Here's what each class means and what's burning:

• Class A — ordinary combustibles: wood, paper, cloth, cardboard, rubber, and most plastics.
• Class B — flammable and combustible liquids and gases: gasoline, diesel, oil, grease, solvents, paint, propane.
• Class C — energized electrical equipment: live wiring, panels, motors, tools, and machinery still under power.
• Class D — combustible metals: magnesium, titanium, sodium, potassium, lithium, and zirconium, often as chips, turnings, or powder.
• Class K — cooking media: vegetable and animal oils and fats in commercial kitchen equipment.

The class isn't trivia — it changes what's safe to put on the fire:

• Water on a Class B liquid fire spreads the burning liquid; on Class C it can conduct current back to you.
• Water or a standard extinguisher on Class D can react violently with the hot metal.
• Class K grease fires re-ignite unless you use an agent that cools and seals the surface.
• A Class C fire becomes a Class A or B fire once you kill the power — de-energizing is often the first real move.

So the first question at any fire isn't "where's an extinguisher" — it's "what's burning," because that's what tells you which extinguisher will actually work.

On a job site you'll usually be sizing this up in seconds. Ordinary trash and lumber is A. Anything from a fuel can, a solvent drum, or a gas line is B. If it's plugged in or energized, treat it as C until the power's off. Grinding dust or shavings from magnesium or titanium is D. Fryer or cooking oil is K.

When more than one fuel is involved — say a fuel spill near energized equipment — you need an extinguisher rated for every class present, which is exactly why multipurpose ABC extinguishers are so common on site. The next topic covers the extinguisher types that cover these classes.