To pick the right extinguisher you need to know what is burning. Using the wrong one can spread a fire or cause an electric shock.
What a fire needs
A fire needs four things:
- fuel, something that can burn;
- heat, enough to ignite it;
- oxygen, usually from the air;
- a chemical chain reaction that keeps the fire going.
Take away any one and the fire goes out. Extinguishers do this. Water cools. Foam and some powders smother. Carbon dioxide pushes away oxygen. Dry chemicals interrupt the chain reaction.
Stages of a fire
- Incipient stage: the fire is small and just starting. OSHA defines it in 29 CFR 1910.155 as a fire that can be controlled with portable extinguishers or small hose systems without protective clothing or breathing apparatus.
- Growth: the fire spreads, smoke thickens and the heat rises.
- Fully developed: the whole room burns. Nobody should try to fight it.
Extinguishers are meant for the incipient stage only. It can pass in a minute or two.
The fire classes
Extinguishers are labeled by the class of fire they can fight. The classes are set by the fuel:
- Class A: ordinary combustibles, such as wood, paper, cloth, cardboard and many plastics.
- Class B: flammable liquids and gases, such as gasoline, oil, paint, solvents and propane.
- Class C: fires involving energized electrical equipment, such as live panels, motors and appliances.
- Class D: combustible metals, such as magnesium, titanium, sodium and lithium metal.
- Class K: cooking oils and fats in commercial kitchen appliances such as deep fryers. Class K is part of NFPA 10, and is common in restaurants and cafeterias.
OSHA's standard speaks of classes A, B, C and D. NFPA 10 adds K.
Class C is about the power
A fire is Class C only while the equipment is energized. Once the power is switched off, it is usually treated as Class A or B depending on what burns. Never use water on live electrical equipment. Water conducts electricity and can shock you.
Mixed fires
Many real fires involve more than one fuel. A burning office computer has plastic (A) and electricity (C). A kitchen fire may involve grease (K), a wooden cabinet (A) and a gas line (B). A multipurpose extinguisher rated for several classes helps here, but you must still check what is burning.
Hazards that call for caution
- Metal fires (Class D) can react violently with water. Only a special dry powder extinguisher made for metals will do. Most staff should leave.
- Cooking oil fires can flare up if sprayed with water or a regular extinguisher that blows the oil around. Class K is designed for them.
- Gas fires: do not put out a gas fire unless the supply can be shut off. Otherwise the gas can build up and explode.
- Lithium-ion batteries in tools, e-bikes and devices can catch fire and re-ignite. A standard extinguisher may not stop them. Leave and call 911.
Key points
- Fire needs fuel, heat, oxygen and a chain reaction.
- Extinguishers are for the incipient stage only.
- Classes: A ordinary, B flammable liquids and gases, C energized electrical, D metals, K cooking oils.
- Never use water on live electrical or grease fires.
- If you cannot tell what is burning, leave.