Fire extinguishers stop flames either by dousing them in water or by dousing them in water or by excluding the oxygen which a fire needs in order to burn.
There are three main kinds of fires. First are those occurring in ordinary materials like paper and wood for which the quenching and cooling effects of water or water solutions are the most effective. Second come those involving inflammable liquids or greases for which a blanketing or smothering effect is essential. Finally there are the fires occurring in “live” electrical equipments where a special extinguishing agent must be used.
The most common extinguisher for the first type of fire is a bucket of water, or a manufactured extinguisher with water containing a chemical. The chemical reaction expels the water which puts out the fire.
For the second kind of fire the most common method is to use a chemical extinguisher to blanket the burning material, excluding oxygen and thus putting out the fire. When oil or grease is burning you should not use a foam-type extinguisher, as it may contain a certain amount of water. However these foam-type extinguishers are safe in most other cases, except electrical fires.
For oil, grease or electrical fires a powder extinguisher must be used, never water or foam, as these could conduct electrical current.
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