Fire Extinguisher Basics
To understand how a fire extinguisher works, you need to understand a little about fire. Fire is a very rapid chemical reaction between oxygen and a combustible material, which results in the release of heat, light, flames, and smoke.
Fire Triangle
For fire to exist, the following four elements must be present at the same time:
- Enough oxygen to sustain combustion.
- Enough heat to raise the material to its ignition temperature.
- Some sort of fuel or combustible material.
- The chemical reaction that is fire.
How a Fire Extinguisher Works:
Portable fire extinguishers apply an extinguishing agent that will either cool burning fuel, displace or remove oxygen, or stop the chemical reaction so a fire cannot continue to burn. When the handle of an extinguisher is compressed, agent is expelled out the nozzle.
All portable fire extinguishers must be approved by a nationally recognized testing laboratory 
to verify compliance with applicable standards. [29 CFR 1910.157(c)(2)] Equipment that passes the laboratory’s tests are labeled and given an alpha-numeric classification based on the type and size of fire it will extinguish.
Let’s take a look at the label pictured. The classification is:
1-A:10-BC
The letters (A, B, and C) represent the type(s) of fire for which the extinguisher has been approved.
The number in front of the A rating indicates how much water the extinguisher is equal to and represents 1.25 gallons of water for every unit of one. For example, a 4-A rated extinguisher would be equal to five (4 x 1.25) gallons of water.
The number in front of the B rating represents the area in square feet of a class B fire that a non-expert user should be able to extinguish. Using the above example, a non-expert user should be able to put out a flammable liquid fire that is as large as 10 square feet.
Don’t forget to inspect your fire extinguishers, learn how to here.
Types of Fire Extinguishers
Different types of fire extinguishers are designed to fight different types of fire. The three most common types of fire extinguishers are: air pressurized water, CO2 (carbon dioxide), and dry chemical. The following table provides information regarding the type of fire and which fire extinguisher should be used.
|
Extinguisher Type |
Type of Fire |
|---|---|
|
Ordinary Combustibles
Fires in paper, cloth, wood, rubber, and many plastics require a water type extinguisher labeled A.  |
OR
|
Flammable Liquids
Fires in oils, gasoline, some paints, lacquers, grease, solvents, and other flammable liquids require an extinguisher labeled B.  Electrical Equipment Fires in wiring, fuse boxes, energized electrical equipment, computers, and other electrical sources require an extinguisher labeled C.  |
|
Extinguisher Type |
Type of Fire |
|---|---|
|
Ordinary Combustibles, Flammable Liquids, or Electrical Equipment
Multi-purpose dry chemical is suitable for use on class A, B, and C.  |
|
Class D |
Metals
Fires involving powders, flakes or shavings of combustible metals such as magnesium, titanium, potassium, and sodium require special extinguishers labeled D. |
| Kitchen Fires
Fires involving combustible cooking fluids such as oils and fats.  NOTE: Your present fire extinguishing equipment may not put out a fire involving vegetable oil in your deep fat fryer. |
Read OSHA’s Fire Extinguisher Guidelines here
Common Fire Extinguishers for small businesses
Water – Air-pressurized Water Extinguishers (APW)
Water is one of the most commonly used extinguishing agents for type A fires. You can recognize an APW by its large silver container. They are filled about two-thirds of the way with ordinary water, then pressurized with air. In some cases, detergents are added to the water to produce a foam. They stand about two to three feet tall and weigh approximately 25 pounds when full.
APWs extinguish fire by cooling the surface of the fuel to remove the “heat” element of the fire triangle.
APWs are designed for Class A (wood, paper, cloth, rubber, and certain plastics) fires only.
Important:
- Never use water to extinguish flammable liquid fires. Water is extremely ineffective at extinguishing this type of fire and may make matters worse by the spreading the fire.
- Never use water to extinguish an electrical fire. Water is a good conductor and may lead to electrocution if used to extinguish an electrical fire. Electrical equipment must be unplugged and/or de-energized before using a water extinguisher on an electrical fire.
CO2 or Dry Chemical – Carbon Dioxide Extinguishers
This type of extinguisher is filled with Carbon Dioxide (CO2), a non-flammable gas under extreme pressure. These extinguishers put out fires by displacing oxygen, or taking away the oxygen element of the fire triangle. Because of its high pressure, when you use this extinguisher pieces of dry ice shoot from the horn, which also has a cooling effect on the fire.
You can recognize this type of extinguisher by its hard horn and absent pressure gauge.
CO2 cylinders are red and range in size from five to 100 pounds or larger.
CO2 extinguishers are designed for Class B and C (flammable liquid and electrical) fires only.
Important:
- CO2 is not recommended for Class A fires because they may continue to smolder and re-ignite after the CO2 dissipates.
- Never use CO2 extinguishers in a confined space while people are present without proper respiratory protection.
Locations:
Carbon dioxide extinguishers will frequently be found in industrial vehicles, mechanical rooms, offices, computer labs, and flammable liquid storage areas.
Multi-purpose – Dry Chemical Extinguishers
Dry chemical extinguishers put out fires by coating the fuel with a thin layer of fire retardant powder, separating the fuel from the oxygen. The powder also works to interrupt the chemical reaction, which makes these extinguishers extremely effective.
Dry chemical extinguishers are usually rated for class B and C fires and may be marked multiple purpose for use in A, B, and C fires. They contain an extinguishing agent and use a compressed, non-flammable gas as a propellant.
ABC fire extinguishers are red in color, and range in size from five pounds to 20 pounds.
Dry Chemical extinguishers will have a label indicating they may be used on class A, B, and/or C fires.
or
Locations:
These extinguishers will be found in a variety of locations including: public hallways, laboratories, mechanical rooms, break rooms, chemical storage areas, offices, commercial vehicles, and other areas with flammable liquids.
To learn more about how to locate and place your fire extinguishers, check out this post