Fire Extinguisher Classes: When to Use Each Type of Fire Extinguisher
What types of fire extinguishers live in your house or office? Do you know?
Can they be used on any kind of fire? Are you sure?
Not all fire is created equal, and not all extinguishers are the same, either. In fact, the wrong extinguisher can make a fire worse and put your life in danger.
This knowledge doesn't matter at all until it matters a lot. You might not need to walk around with a working understanding of fire all the time but learning enough to prepare properly now could save your life later.
Fire extinguishers are divided by the class(es) of fire they work on and how the extinguisher does its job. When you're choosing the right type of extinguisher for any location, you need to consider both.
Fire Extinguisher Classes
Fires are divided into five different classes, based on the type of material and fuel for combustion. Different combustion materials require different extinguishing methods, so fire extinguishers are marked with the class(es) they're safe and effective on.
It's worth noting that the way fires are classified varies a little between countries. In this article, we're referring to the U.S. system.
Class A Fires
Class A fires are probably the first thing that comes to mind when you imagine a fire. They consist of ordinary combustibles, like paper, cloth, and some plastics. When you burn trash or light a campfire, you're setting a Class A fire.
Water is safe and effective on Class A fires, as well as chemicals foams or powders that coat and smother the fire.
Class B Fires
Class B fires, in the U.S. system, originate from flammable liquids or gases, like oil-based paint, gasoline, alcohol, or natural gas.
Water can cause the liquid or gaseous fuel of a Class B fire to scatter, which spreads the fire. Instead, Class B fires call for a method that will smother the fire or interrupt the chemical chain reaction.
Class C Fires
In the U.S. system, Class C fires involve potentially energized electrical equipment. The fire may be caused by electricity, but that's not a requirement. In other words, any kind of fire where live electrical equipment could be present is Class C.
This matters because any substance that conducts electricity can't be used on a Class C fire, including water or foam. These extinguishing agents could carry electricity and cause electrical shocks to anyone nearby.
It's worth noting that if the power is turned off, or electrical equipment is otherwise de-energized, it's no longer a Class C fire and would be classified by its fuel source, instead.
Class D Fires
Class D fires involve combustible metal, like sodium, lithium, magnesium, or titanium. Most people aren't familiar with this type of fire because combustible metals need a lot of heat to ignite. However, metal shavings are a fire risk because they combust more rapidly than larger pieces.
Most normal extinguishing agents will make Class D fires worse. They need to be smothered with a dry powder (different from dry chemical!) extinguisher.
Different metals require different chemical agents, and they won't all work the same. That means anywhere with risk factors for a Class D fire needs to select their extinguisher carefully.
Class K Fires
Class K fires, in the U.S. system, are often referred to as "kitchen fires" or "grease fires." They involve cooking oils or fats.
Specialized extinguishers exist for commercial kitchens, but ABC powder extinguishers will work on a kitchen fire at home. Just remember that water will make the fire worse. You can smother the flame with a metal container large enough to cover the fire. Baking soda also works.
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Fire Extinguisher Types and Uses
Fire extinguishers can also be divided by the makeup of the extinguishing agent and how it gets applied. Both of these things impact the potential uses of extinguishers.
Water Spray Extinguishers (Class A Fire Extinguishers)
Water spray extinguishers have significant downsides – you have to get close, they're heavy, and they're dangerous for most fires. On the other hand, they don't contain dangerous chemicals.
They work on Class A fires (organic solid combustibles), but they're usually reserved for specific circumstances where other extinguishers are dangerous.
For example, chlorine and oxidizers react with the chemicals in other extinguishers, resulting in explosions or toxic gas. That's why NFPA regulations call for only water extinguishers to be installed where pool chemicals and similar compounds are stored.
Foam and Water Extinguishers (Class A Fire Extinguishers)
Sometimes just called "foam extinguishers," it's important to remember that the chemical foam contains some water content, which makes them dangerous for some fires.
They're useful for Class A fires (and lighter than regular water extinguishers).
Liquid fires (Class B) are trickier – it depends on the foam. Some foam extinguishers are safe for liquid fires, and some are not.
ABC Powder Extinguishers
Also called ABC dry chemical extinguishers, ABC powder extinguishers are useful for most environments. They're effective for (you guessed it) most A-, B-, and C-class fires (the exception being chlorine/oxidizer fires). They're also safe for Class K fires.
It's the most common type of extinguisher, due to its versatility.
ABC extinguishers spray a fine chemical powder that smothers Class A and liquid fires, can break the chain reaction for liquid and gaseous fires, and is safe for Class C fires because it doesn't conduct electricity.
They do have significant downsides, including the risk of chemical inhalation in confined spaces, tough cleanup, and damage to soft materials and electronics. It also doesn't cool things down like water and foam.
CO2 Extinguishers (Class C Fire Extinguishers)
CO2 extinguishers are especially useful for live electrical fires (Class C) when you're hoping to salvage some equipment, afterward.
They contain pressurized CO2 only. When you spray this gas on the fire, it displaces the oxygen and suffocates the flames. It can reach places within the equipment that other fire extinguishers can't, and there's no residue to cause damage or make electrical equipment short-circuit.
These also work on burning liquids but aren't suitable for any other type of fire (including gas).
There are also a few safety risks to the user. The nozzle can become extremely cold as CO2 is dispensed, and you could burn your hands. If you use them in a confined space, you could asphyxiate.
Wet Chemical Extinguishers (Class K Fire Extinguishers)
Wet chemical extinguishers are specialized for kitchen fires (Class K).
They contain a liquid chemical mist that both cools the fire and undergoes a chemical reaction with the cooking medium that results in a thick soap-like substance. It's called saponification and it prevents reignition of the cooking medium.
They're also safe to use on Class A fires, so commercial kitchens only need the one extinguisher.
Dry Powder Extinguishers (Class D Fire Extinguishers)
The right kind of dry powder extinguisher is your only option for combustible metal fires (Class D). They don't work on any other kind of fire, so their use is very specialized.
In fact, the chemical agent needs to be specific for the type of metal that's burning.
Dry powder extinguishers are typically reserved for facilities that cut, grind, or otherwise process metals.
Water Mist Extinguishers (ABCK Fire Extinguishers)
These extinguishers are very versatile – safe for Classes A, B, C, and K. In other words, they're useful for everything except combustible metal fires, which are extremely rare in most settings.
We did say water extinguishers are only safe for Class A, but water mist extinguishers are a different animal.
First of all, water mist extinguishers use deionized water, which is non-conductive. That eliminates the shock risk for live electrical equipment.
They also release water in a microscopic, fog-like form, rather than a spray. This mist decreases the level of oxygen in the air to suffocate most fires, while the water also reduces heat.
Since the mist is so fine, it won't cause Class B or K fires to spread like a stream of water might. They also don't leave everything wet, and there are no chemicals to damage property or your health.
Why aren't these more common than ABC powder extinguishers? They're new, and at the moment, they're still much more expensive. For now, they're reserved for environments where the benefits outweigh the costs, like hospitals, clean rooms, museums, and server rooms.
Fire extinguishers are an important part of home safety, but they're critical – and legally required by OSHA – in the workplace. Stocking the type of fire extinguisher that's appropriate is the top priority, but periodical training on how to use them is also essential.
That's true of most workplace safety measures: step one is to raise awareness of the hazard, step two is to put the right tools and protocols in place, and step three is periodic training so that people know what to do when the time comes.
OSHA's safety training requirements exist to raise awareness and ensure adequate training. Just like choosing the right type of fire extinguisher, training needs to suit the specific circumstances of each workplace to be effective.
Check out our large catalog of OSHA-authorized online safety coursework to build the best training for your business!