Besides knowing what type of fire you're likely to encounter, it's also important to know what kind of extinguishing agent your fire extinguisher should have in it. This is because certain extinguishments will work only on certain kinds of fires. Others can actually aggravate the situation, causing an even more critical situation.

For example, a WATER agent should only be used on Class A fires. This is because they put out fires by cooling, soaking and penetrating combustible materials. A REGULAR DRY CHEMICAL agent should only be used on Class B and C fires. This is because they attempt to interrupt the fire's flame by interfering with the fuel-oxygen-heat triangle instead of simply cooling a fire. A MULTIPURPOSE DRY CHEMICAL, however, can be used on Class A, B, and C fires. This is because the extinguishing agent melts when it's exposed to heat, forming an oxygen barrier over the burning materials.

CARBON DIOXIDE, another popular extinguishment, should also be used on Class B and C fires. They put out fires by cutting off the air supply, virtually choking a fire. FOAM agents also cut off a fire's oxygen supply, but they should only be used on Class A and B fires.

DRY POWDER agents are good only on Class D fires. Do not use dry powder for Class A, B, and C fires. This is because they specifically form a coating agent over the burning metals, suffocating the flame.

To assure that people know the classification of an extinguisher they're about to use, all portable fire extinguishers are marked at the factory with an A inside a triangle for Class A, a B inside of a square for Class B, a C inside of a circle for Class C, and a D inside of a five- sided star for Class D (source: U.L. Canada).

Be sure the fire extinguishers you buy are rated for the kinds of materials found in your home. You may have to purchase more than one type of extinguisher if you intend to cover all the areas in your home. For example, a Class A extinguisher will put out fires in the main part of your house, like the living room and bedrooms. A Class B extinguisher can put out a fire in a kitchen or garage because there are both burning liquids and wood/cloth materials in these locations. And a Class C extinguisher should be used in the electrical room in the basement.

Obviously, most of the time there are more than one type of materials found in each area of a home. For instance, in a basement you're likely to find cloth; wood; an electrical panel box; electric, oil or a gas furnace; and an electric or gas hot water tank. In this case, buying a Class A/B/C extinguisher would be advisable.

Another example is the kitchen area where you're likely to find flammable liquids (grease) and gases (propane or natural gas). In addition, you'llusually find various electrical appliances. In this situation, it is probably more economical to buy Class B/C extinguisher instead of keeping separate Class B and C extinguishers in these areas. Class A extinguishers also are good for the main living areas of a home, such as the bedrooms, bathrooms, den and living room.

In situations like this, it's sometimes more economical to buy all Class A/B/C extinguishers than separately rated units. This is because they can be placed in almost any area of a home and they can readily be interchanged.

For more information on portable fire extinguishers, contact the National Fire Protection Association. Call them at: (617)770-3000 or write them at: 1 Batterymarch Park, Quincy, MA 02269. Also, contact Underwriters' Laboratories of Canada, 7 Crouse Rd., Scarborough, Ontario, M1R 3A9, Canada.

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