ABC dry chemical

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An ABC Dry Chemical Fire Extinguisher made by Amerex

Monoammonium phosphate, ABC Dry Chemical, tri-class or multi-purpose dry chemical is a dry chemical extinguishing agent used on class A, B, and C fires. It uses a specially fluidized and siliconized monoammonium phosphate powder. ABC dry chemical is usually a mix of monoammonium phosphate and ammonium sulfate, the former being the active one. The mix between the two agents is usually 40–60%, 60-40%, or 90-10% depending on local standards worldwide. The USGS uses a similar mixture, called Phos Chek G75F.[1]


Class A fires[edit]

It insulates Class-A fires by melting at approximately 350–400 degrees F. Class "A" fires are caused by the burning of common combustible materials, such as wood, paper, or trash.[2]

Class B fires[edit]

The powder breaks the chain reaction of Class-B fires by coating the surface to which it is applied. Class B fires consist of flammable liquid or gas which include gasoline, oil, propane, and natural gas.

Class C fires[edit]

It is safe and effective for Class-C fires since it is a non-conductor of electricity. Class C fire deals with live electrical equipment and need to be put out with equipment that will not conduct its energy back to the user as in the case with water. Electricity can also cause Class A and B fires.

Inappropriate Uses[edit]

ABC dry chemical is inappropriate for chlorine or oxidizer fires.[3] The resulting chemical reaction can cause an explosion or a breakdown of the chemicals releasing toxic gases. Water should be used.[4][5] ABC dry chemical is inappropriate for certain metal fires (Class-D) as well as cooking oil fires (Class-K). Due to the corrosive properties of ABC Dry chemical, it is not recommended for use around aircraft or sensitive equipment.

Chimney bombs[edit]

Chimney bombs are zip-lock bags or other small bags filled with ABC dry chemical powder. Chimney bombs are used by fire service personnel to help extinguish chimney fires. Creosote, which is the by-product of the incomplete burning of wood (typically due to chronic combustion-air insufficiency), is extinguished by the chain reaction caused by the chimney bombs. Chimney bombs work by first being dropped directly down a chimney, where upon contact with the flue bottom and heat of the fire, they explode, thereby releasing the powder. Then, the natural chimney draft will carry the dry chemical powder up the shaft of the chimney, thus coating the creosote and eventually neutralizing the fire. Use of multiple chimney bombs may be necessary, depending on how severe the fire is. Chimney bombs are also ineffective if they are opened and then dropped down the chimney. It should be kept in mind that, in order for chimney bombs to be effective, it may be necessary to first unclog the chimney.[6]

Fire Class Geometric Symbol   Pictogram Intended Use Mnemonic compatibility
A Class A fire triangle.svg Fire type A.svg Ordinary solid combustibles A for "Ash" Compatible
B Class B fire square.svg Fire type B.svg Flammable liquids and gases B for "Barrel" Compatible
C Class C fire circle.svg Class C fire icon.svg Energized electrical equipment C for "Current" Compatible
D Class D fire icon.svg (none) Combustible metals D for "Dynamite" Not compatible
K Class K fire hexagon.svg Class K fire icon.svg Oils and fats K for "Kitchen" Not compatible


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