A fire extinguisher is an active fire protection device used to extinguish or control small fires, often in emergency situations. It is not intended for use on an out-of-control fire, such as one which has reached the ceiling, endangers the user, or otherwise requires the expertise of a fire brigade. Typically, a fire extinguisher consists of a hand-held cylindrical pressure vessel containing an agent which can be discharged to extinguish a fire. They are small metal canisters that contain compressed gas (usually nitrogen) that, when activated, propel a directed spray of flame-retardant chemicals. Fire extinguishers manufactured with non-cylindrical pressure vessels also exist, but are less common and pricy.
*Note* Although, technically, the letter rankings listed above refer to fire types, these symbols can also be used to identify the extinguishers themselves. For instance, an extinguisher that uses CO2 can be called a "CO2 extinguisher" or a "BC extinguisher."
Dry Chemical – There are two types of fire extinguishers that use a dry chemical. One is called "multi-purpose dry chemical" and uses ammonium phosphate as the extinguishing agent, which is effective on "A," "B," and "C" class fires. This chemical is corrosive and must be scrubbed from surfaces after use. These types of extinguishers are very common and are found in schools, homes, hospitals and offices. Sodium bicarbonate is used in extinguishers known as "regular dry chemical," which are capable of handling "B" and "C" class fires. These extinguishers are found in garages, kitchens and laboratories. Sodium bicarbonate is easy to clean and non-toxic.
Carbon Dioxide – These extinguishers contain liquid CO2 that is expelled as a gas. They are effective against "B" and "C" class fires. Unlike other chemicals, CO2 does not leave a harmful residue and is environmentally friendly. It also poses very little danger to electronics and is effectively employed in laboratories, computer rooms, and other areas with sensitive equipment.
Water Extinguishers – These extinguishers are most suited for "A" class fires. However, they cannot be used in "B," "C" or "D" class fires. In "B" and "D" class fires, the water will spread the flames. In a "C" class fire, the water is conductive and poses a risk of electric shock to the operator. However, the misting nozzle of a "Water Mist" extinguisher breaks up the stream of deionized water so that there is no conductive path back to the operator. Since the agent used is water, these types of extinguishers are inexpensive and environmentally friendly.
Foam Extinguishers – Foam fire extinguishers can be used on Class A & B fires. A Class A fire is a solid fire, such as paper, wood or plastic. A Class B fire is flammable liquids, such as petrol and oil. It is not recommended to use foam on electrical fires.