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Fire Extinguishing Agents
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When selecting the appropriate type of fire extinguisher, it is important to think about extinguishing agents. Each class of fire is best fought by a specific extinguishing agent. You will find a color-coded box on your fire extinguisher identifying which classes of fire it can be used for, and the type of fire extinguishing agent it contains.

The following is a list of commonly used fire extinguishing systems and their corresponding classes of fire. The classes are indicated in parentheses such as (A, B, C):

Multi-Purpose Dry Chemical (A, B, C)
A dry chemical agent called mono ammonium phosphate. The chemical is non-conductive and can be mildly corrosive if moisture is present. In order to avoid corrosion, it is necessary to scrub and thoroughly cleanup the contacted area once the fire is out. A dry chemical fire extinguisher is usually used in schools, general offices, hospitals, homes, etc.

Regular Dry Chemical (B, C)
A dry chemical agent called sodium bicarbonate. It is non-toxic, non-conductive and non-corrosive. It is easy to cleanup, requiring only vacuuming, sweeping or flushing with water. Extinguishers with sodium bicarbonate are usually used in residential kitchens, laboratories, garages, etc.

Carbon Dioxide (B, C)
Carbon dioxide removes oxygen to stop a fire but has limited range. It is environmentally friendly and leaves no residue, so cleanup is unnecessary. Extinguishers with carbon dioxide are usually used in contamination-sensitive places such as computer rooms, labs, food storage areas, processing plants, etc.

Halotron (A, B, C)
A vaporizing liquid that is ozone friendly and leaves no residue. Because it requires no cleanup, fire extinguishers with halotron are ideal for computer rooms, telecommunication areas, theaters, etc.

Foam (A, B)
Foam floats on flammable liquids to tame the fire and helps prevent reflashes. To cleanup the affected area, it must be washed away and left to evaporate. Fire extinguishers with foam are usually used in garages, homes, vehicles, workshops, etc.

Purple K Dry Chemical (B, C)
A dry chemical called potassium bicarbonate. It is non-conductive and non-corrosive. Clean up requires vacuuming, sweeping or flushing with water. Extinguishers with potassium bicarbonate are usually used in military facilities, oil companies, vehicles, etc.

Water (A)
The most common agent is water; however, it cannot be used for class B or C fires because it is conductive. Water-based fire extinguishers are usually used in stockrooms, schools, offices, etc.

Fuel Source

Class of Fire

Type of Extinguisher (Extinguishing Agent)

Ordinary combustibles
(e.g. trash, wood, paper, cloth)

A

Water; chemical foam; dry chemical*

Flammable liquids
(e.g. oils, grease, tar, gasoline, paints, thinners)

B

Carbon dioxide (CO2); halon**; dry chemical; aqueous film forming foam (AFFF)

Electricity
(e.g. live electrical equipment)

C

CO2; halon; dry chemical

Combustible metals
(e.g. magnesium, titanium)

D

Dry powder (suitable for the specific combustible metal involved)

* Dry chemicals, CO2 and halon can be used on Class A fires, but may not be effective on their own. They need to be supplemented with water.

** Halon extinguishers are no longer made but some may still be in use. Dangerous gases are formed when halon is used to put out fires. Wear proper respiratory equipment, particularly in enclosed spaces. After use, do not allow anyone to enter the area until it has been well ventilated.

Reference: www.fire-extinguisher101.com/prevention.html