The Fire Triangle and Classes of Fire

Picture a triangle. Each side is one thing a fire has to have:

• Fuel — anything that burns: wood, paper, fuel, solvents, insulation, dust.
• Heat — the ignition source: a spark, flame, hot surface, or friction.
• Oxygen — normally the air around us, about 21%.

Break any side and the fire goes out. Water cools it (removes heat). A blanket or CO2 smothers it (removes oxygen). Shutting a fuel valve starves it (removes fuel). Dry-chemical extinguishers add a fourth control — they interrupt the chemical chain reaction that keeps the flame going, which is why the triangle is sometimes called the fire tetrahedron.

Prevention works the same way: keep ignition sources away from fuels and you never complete the triangle in the first place.

Fires are grouped by what's burning. Extinguishers are rated for the classes they can handle:

• Class A — ordinary combustibles: wood, paper, cloth, cardboard, most plastics, trash.
• Class B — flammable liquids and gases: gasoline, diesel, oil, grease, solvents, propane.
• Class C — energized electrical equipment: panels, cords, motors, tools that are still powered.
• Class D — combustible metals: magnesium, titanium, sodium, lithium.
• Class K — cooking oils and fats, mostly in kitchens.

The class tells you what's safe to use. Water is great on Class A, but it's dangerous on the others. Throw water on a grease fire (Class B) and it flashes and spreads the burning liquid. Hit an energized panel (Class C) with water and you can get shocked through the stream. A combustible-metal fire (Class D) can react violently with water.

Most site extinguishers are ABC dry chemical because they cover the three you're most likely to meet. But before you grab anything, know what's on fire — that decides whether you fight it or get out and let it burn.