Recently we provided more information about what fire is, but what kind of firefighters would we be if we didn’t also share a little knowledge on what we have at our disposal to fight those fires. This story will examine the types of fires the Alpha Volunteer Fire Company has to face year to year. These fires are separated into “classes” based on what is burning and thus, what is needed to put the fire out. Fire extinguishers used to fight fires are also ranked by those classes. For review, in order for fire to break out, three elements must be present: oxygen, fuel and heat. In recent years, we’ve also added “chemical reaction” as a fourth element necessary for fire to break out to encompass a broader array of potential fires. The chemical chain reaction refers to the reaction of the burning material with the atmosphere (rapid oxidation). In some cases, this chain reaction refers to a chemical reaction — making it an actual ‘chemical’ fire. Fire extinguishers act on one — or more —  of the four elements to:

1. Eliminate oxygen.
2. Remove heat: Cool fuel below it’s ignition point (this is what water really does, mostly).
3. Remove fuel: Usually not so easy to do. This includes, for example, removing unburned brush ahead of a brush fire; turning off a valve to stop the flow of flammable gas, etc.
4. Break the chemical chain reaction (which is what a fire actually is) by disruption. Chemicals within fire extinguishers do this in a variety of ways; a process occurring at the molecular level.

Types And Classes Of Fire And Fire Suppression

Class A Fires: These fires would be considered most familiar to us, started with materials like paper, wood or dry grasses or leaves. A “Class A” fire extinguisher will often hold pressurized water,which mostly works by removing heat, or a dry chemical powder or CO₂. Dry chemical extinguishers mostly work by disrupting the chemical reaction that is fire, while CO₂ smothers fire by removing oxygen and heat. Class B Fires: Flammable liquids such as gasoline and lighter fluid are the causes of Class B fires, and should never have water used on them due to the potential increase of the chemical reaction that fuels this type of fire, and the potential splashing of the fuel itself. Class B fire extinguishers contain a special dry chemical powder (typically monoammonium phosphate, in case you wondered) designed to both suppress the oxygen of the fire as well as remove the heat and chemical reaction of the fire. For large fires. chemical foam agents can be added to water to make an effective suppression mixture. Class C Fires: Fires sparked by an electrical reaction – a short circuit in wiring or inside an appliance, for example – are a little trickier than other fires because with the addition of electricity as a cause, you cannot EVER use water to extinguish them because water conducts electricity. As such, there are specific “Class C” fire extinguishers that contain a dry powder or inert gas (such as CO2) to extinguish the fire. Class D Fires: Generally considered “industrial” fires caused by flammable metals such as magnesium, Class D fires are uniquely difficult to exterminate. Class D fire extinguishers contain an alkali metal designed to smother the active chemical reaction. As you might have guessed, water should never be used on a metals fire—as it would greatly spread the chemical reaction. Class K Fires: Commonly known as cooking grease or cooking oil fires, Class K fires should be suppressed by using saponification** to smother the fire. Many commercial or industrial kitchens require a special “K-Guard” system to be installed that can deliver specific Class K fire suppression. Under no circumstances should a grease or oil fire ever be extinguished using water. (Dry chemical extinguishers are effective on grease fire and cooking oil fires, but they make more mess.) Class K is only suited to organic cooking greases and oils; not petroleum based lubrication grade oils. **S0 you don’t have to look it up: Saponification is a chemical reaction that occurs when a vegetable foil or animal fat is mixed with a strong alkali. 

Read The Label. Or Talk To A Firefighter.

The types of fire suppression used by firefighters can differ from the extinguishers you might use in your home. (Some of our vehicles carry Class A or Class B extinguishing foam, for example—but one class cannot be used on the other). Be absolutely certain your home extinguishers are current, charged and rated for the types and kinds of fires that may break out in your home. READ THE LABEL and heed the posted warnings on home fire suppression! Remember—our articles, informed as they are, are for only that; general information. We are making no claims or recommendations for your own personal choices for home-based fire suppression. Our goal is to simply provide insight into, and create interest for, the generations-old tradition of volunteer firefighting. Hopefully, learning about the classes of fire provides such insight!