ABC extinguisher

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fire extinguishers

Extinguishers that are effective on one kind of burning material don't necessarily work well on another. To indicate the kinds of fire for which an extinguisher is suitable, it will usually be labeled with an Underwriters Laboratory (UL) rating using the code letters A, B, C, D or K.

UL classes

Class A

Burning wood, paper, cloth, and other everyday materials. Enough water can extinguish such fires.

Class B

Burning liquids, such as kitchen grease, gasoline, oil, and other flammable liquids. Putting water on such fires may simply spread them.

The code letters A and B are prefixed with numbers indicating the relative size of the fire the extinguisher can put out. For example, a 2-A extinguisher could put out a paper fire twice the size of the biggest fire a 1-A extinguisher could handle.

Class C

Fires involving electrical equipment. Spraying such a fire with water might not be a good idea.

The UL ratings capture almost everything one needs to know about the extinguisher. The one exception is the extent of the damage done by the extinguisher itself. Most modern extinguishers spray powder, in many cases baking soda, in others ammonium phosphate. In most situations if the powder is promptly cleaned up afterwards no harm is done. But if the powder gets into a computer or other electronic gear, the equipment will probably be ruined. Halon extinguishers, formerly used for fighting fires in such circumstances, are no longer allowed except for very special circumstances (airplane cockpits), because Halon is a fluorocarbon that destroys the ozone layer. A carbon dioxide extinguisher might be a useful compromise.

A good size for each floor of a home is a 3-A:40-B:C, unless residents are too frail to use one this heavy (eight to ten pounds). The kitchen needs a small extinguisher of its own, one specifically designed for grease fires (that is, no A rating is needed). Inexpensive 5-B:C models are sold.

Class D

Burning combustible metals, such as magnesium and sodium. This is not a problem in the home; D-class extinguishers are special items.

Class K

K for kitchen, a class introduced in the 1998 edition of NFPA-10. These are used in places where there is a large amount of combustible fat or oil, such as a deep far fryer, mainly industrial and commercial kitchens. Usually sold to be used after an fixed fire suppression system built into a stove hood, for example, has been activated. Bth dry chemical and wet chemical model exist. A requirement is that the chemicals must be non-toxic. Dry models use potassium carbonate. The wet versions usually spray a fine mist of potassium citrate and potassium acetate, which turns into a suffocating foam.  See the remarks of Leif Jenkinson under contributors, below.

Tags on extinguishers

Fire departments conduct regular inspections of extinguishers required in commercial and industrial settings. Basically, they check the tag to see if it has expired. Over time fire extinguishers can go bad. They can lose pressure; their plumbing can become clogged; the powdered fire suppresent can stick together.

An extinguisher sold with a tag will generally meet all legal requirements for 12 months. An extinguisher sold without a tag must be inspected and tagged by a competent authority before it is placed in service.

In the home, no one is inspecting tags. But the homeowner needs to be aware that an extinguisher can lose its fire-fighting ability, even if a pressure gauge on the extinguisher indicates adequate pressure. Services can recharge your extinguisher. For the householder, however, it is often cheaper to buy a new extinguisher than to recharge an old one.

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