What are the different types of fires and fire extinguisher classes?

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Answered by: Kia, An Expert in the Safety Category
Understanding fire can be a tricky business. While most are taught that water is the key to putting out fires, that is not always the case. There are different classes of fires that can crop up, depending on what caused the fire. This also means that there are different fire extinguisher classes that can be used to put out said fires.

First off, one has to understand how fires start before you can go on to discuss the classes of fires. Traditionally people have seen fire as a combination of three things which need to be present in order to cause a fire. These three things are oxygen, fuel and heat. Without any one of these three things, a fire cannot start. Similarly, if you take one of these things out of the equation when the fire is burning, you will be able to put out the fire. As of late however, more industries are switching to the ‘quadrangle’ view of fire, which includes the three elements from before, plus the fourth element, being a chemical reaction.

Now that that the basics of fire and combustion are understood, a discussion of the classes of fire can begin. There are five classes of fire, which correspond to five fire extinguisher classes. These are class ‘A’, ‘B’, ‘C’, ‘D’ and ‘K’.

Class ‘A’ fires and extinguishers

Class ‘A’ fires are fires that are started with ‘traditional’ materials, such as wood and paper. These fires are the only fires that on their own can be extinguished with water. Thusly, class ‘A’ fire extinguishers will often hold water, though others may have a dry chemical powder inside them that will also do the trick. CO2 extinguishers will also extinguish these classes of fire. This is due to suppressing the oxygen around the fire, removing that piece of the equation, as discussed in the fire basics section.

Class ‘B’ fires and extinguishers

Class ‘B’ fires are fires that are started with flammable liquids, such as gasoline. These fires should never have water poured on them, as that can exaggerate the chemical reactions that are taking place. Instead, one should use a class ‘B’ extinguisher, which contains a dry chemical that is made to completely extinguish the fire. This happens through both removal of the oxygen, and the removal of the heat and chemical reaction around the fire.

Class ‘C’ fires and extinguishers

Class ‘C’ fires are fires that are started by an electrical stimulus, such as a toaster fire. These fires are slightly different than the other classes that have been reviewed previously. There are specific class ‘C’ extinguishers that can be used to extinguish these fires. These have dry powder within them that will suppress the oxygen and the chemical reaction. However, if it is safe to do so, removing the power source from the electrical fire will cause the fire to do one of two things. The fire may go out, as the source of its energy has been removed. On the other hand, the fire may continue to burn, but not that there is no electrical current running through it, it will change classes and become an A class fire that can be extinguished with water. However, never under any circumstance pour water of an electrical fire that still has current running through it. Water is a conductor, which means that the electricity could run through the water and into you causing you to get electrocuted.

Class ‘D’ fires and extinguishers

Class ‘D’ fires are fires that are started by flammable metals, such as magnesium. These fires require a special type of extinguisher that normally contains an alkali metal. These metals are good at smothering the chemical reaction going on and stopping the fire. Never put water on these fires, as that will simply create a larger chemical reaction.

Class ‘K’ fires and fire extinguishers

Class ‘K’ fires are fires at are started by kitchen oils and greases. These kitchen fires require a large extinguisher system, most commonly known as a K-Guard system. These systems are set up over the fryers in most commercial kitchens, and contain a liquid foam. If the event of a grease fire, the foam will be released through nozzles causing it to settle on the fryers, cutting off al the oxygen from the fire and thus extinguishing it. Never under any circumstance put water on these fires, as water on a grease fire can cause a huge explosion that could maim or possibly kill.

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