1 · The Basics
How fire works
The fire triangle
A fire needs three things at the same time:
- Fuel: something to burn (wood, paper, oil, gas, plastic).
- Heat: enough to get it burning and keep it burning.
- Oxygen: normally from the air, which is about 21% oxygen.
Take away any one of them and the fire goes out. Every system you'll service works by attacking one or more sides of the triangle.
flowchart LR F["🔥 FIRE"] Fu["Fuel"] --- F H["Heat"] --- F O["Oxygen"] --- F H -.->|"Water, sprinklers, deluge, hose reels: COOL"| X1["Remove heat"] O -.->|"CO₂, inert gas, foam blanket: SMOTHER"| X2["Remove oxygen"] Fu -.->|"Isolate supply, foam blanket: STARVE"| X3["Remove fuel"]
The fire tetrahedron (the fourth side)
Scientists add a fourth element: the chemical chain reaction that keeps the flames going. Some agents, such as dry powder and some clean agent gases, work mainly by breaking that chain reaction rather than by cooling or smothering. That's why a small amount of powder can knock down a fire so fast.
How each system fights fire
| System | Main way it works |
|---|---|
| Water extinguisher, hose reel, sprinkler, deluge | Cools the fuel below the temperature it burns at |
| Foam (extinguisher or system) | Smothers by forming a blanket over a burning liquid, and cools |
| CO₂ (extinguisher or system) | Smothers by pushing oxygen out |
| Inert gas systems (e.g. IG-541, IG-55) | Smothers by lowering oxygen to around 12–15%, still enough for people to breathe for a short time |
| Dry powder | Mainly breaks the chain reaction, and blankets |
| Wet chemical | Cools and forms a soapy crust on burning cooking oil (called saponification) |
| Clean agent (chemical) gases (e.g. FK-5-1-12) | Mainly absorb heat, and some chemical action |
| Fire doors | Don't put fire out. They contain it to give people time to escape |
| Fire alarms | Don't put fire out. They detect and warn, and often trigger other systems |
Classes of fire: the first BS vs NFPA difference
Fires are grouped into classes by what's burning, so you can pick the right extinguisher.
| What's burning | UK / Europe BS (BS EN 2) | US NFPA (NFPA 10) |
|---|---|---|
| Ordinary solids: wood, paper, cloth, plastics | Class A | Class A |
| Flammable liquids: petrol, solvents, oils | Class B | Class B |
| Flammable gases: propane, natural gas | Class C | Class B (included with liquids) |
| Live electrical equipment | Not a class: shown with an electrical symbol | Class C |
| Combustible metals: magnesium, sodium, lithium | Class D | Class D |
| Cooking oils and fats in deep-fat fryers | Class F | Class K |
Key takeaways
- Fire needs fuel, heat and oxygen, plus a chain reaction. Remove one and it goes out.
- Every system works by cooling, smothering, starving or breaking the chain, or by containing / warning.
- The letters of fire classes differ between BS and NFPA. C and F/K are the ones to watch.