A fire extinguisher is a device used to put out a fire, often in an emergency situation. They consist of a pressurised container of chemicals that when discharged can put out a fire. It is important to familiarise yourself with the use of fire extinguishers in your vicinity, as improper or untimely use may be counterproductive. Fire extinguishers can be divided into four classifications: Class A, Class B, Class C, and Class D. Each class corresponds to the type of fire the extinguisher is designed for, and, thus, the type of extinguishing agents used.

•Type A is for fighting fires involving paper, wood, fabric, or plastic. A fire extinguisher containing water and pressurized gas should be adequate for small Class A fires, but nothing else. Before buying any fire extinguisher, look for the letter designation (A, B, C, D or a combination) or a picture depicting the precise type(s) of fire it is designed to fight. A water-based fire extinguisher is only rated A for common combustible materials. Never use it on grease or electrical fires.

•Type B is for fires involving grease and flammable liquids such as gas. A fire extinguisher for a Class B fire may contain a supply of carbon dioxide which smothers the flame without spreading it. This type of fire extinguisher may also work on electrical fires, but it won't smother Class A fires effectively. A Class B fire extinguisher can emit dry ice during use, so avoid skin contact.

•Type C is for fires involving live electronics. A Class C fire extinguisher either uses a dry chemical powder or another form of carbon dioxide which does not conduct electricity. Class C fires actually involve the same fuels as A and B, except electrical current supplies the ignition source needed for burning. Take away the current, and any remaining fire falls into one of the other two classes.

•Type D fires are generally caused by reactive metals such as magnesium or sodium, so most home fire extinguishers are not rated to handle them.

Which class should I pick?

Most residential extinguishers are BC- or ABC- rated. For most homeowners, ABC extinguishers are the best choice. BC extinguishers use carbon dioxide or sodium bicarbonate as the extinguishing agent and are most effective on B and C fires, though they can have some effect on an A fire. ABC extinguishers, however, work on all three fire classes. Often referred to as "multipurpose," ABC units use ammonium phosphate. The downside to this chemical: it leaves behind a fine, yellowish powder that can coat floors, furniture, and appliances, and wreak havoc on electrical and electronic equipment if not quickly cleaned up.

•Do not test a home extinguisher unless the manufacturer's instructions say otherwise. Squeezing the trigger for even a second releases some of the pressure.

•If you have never used a fire extinguisher, call your local fire department and ask them if they have a fire safety program you can take. If not, consider buying a fire extinguisher and using it just so you can see how it handles and how long it lasts.

•Refillable extinguishers have pressure gauges to tell you the pressure level. Some one-use models come with a pressure-check button.

Other fire extinguisher considerations include ease of use and storability. A fire extinguisher should have clear instructions on its proper use and easy access to the trigger and safety pin. Special holding brackets may be included to secure the fire extinguisher to a wall or inside a cabinet. You'll want easy access to the extinguisher during an emergency, but you may not want to move it constantly to get at other items. Find an inconspicuous area out of the main line of traffic and mount a holding bracket there.

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