Fire Extinguisher Classifications: What They Mean and Why They Matter
Trying to fight a fire with the wrong type of extinguisher can have dangerous and deadly results.
Your workplace needs fire extinguishers, but you can't just buy the first one you come across. With six types of extinguisher and five different classes of fire, it's important to know a thing or two about them before making your selection.
What’s the difference between a class A fire and a class C fire? What types of fires are water extinguishers appropriate for? How do you care for and maintain your extinguishers?
We’re going to answer those questions and more. Keep reading to find out what type of fire extinguisher your workplace needs.
Understanding Fire Hazards in Your Workplace
You can’t just pick up any old extinguisher to put out any old fire. In fact, doing so could make the fire worse and put you in serious danger.
Every extinguisher is specifically rated for use on a particular class of fire, and should only ever be used for that class. Before purchasing an extinguisher, it’s imperative that you understand the types of fires that your workers could face on the job.
- Class A. Extinguishers rated for class A fires are acceptable for dealing with ordinary combustibles, including wood, paper, textiles, and some plastics. To safely extinguish class A fires, you require the heat-absorbing effects of water or the coating effects of (some) dry chemicals. All extinguishers suitable for this type of fire are labeled with a triangle containing the letter A.
- Class B. Extinguishers marked with a square containing the letter B are acceptable to use with flammable liquid and gas fires, including oil and gasoline. These extinguishers work by depriving the fire of the oxygen it requires to keep burning and inhibiting the release of combustible vapors.
- Class C. Class C extinguishers are appropriate for fires involving live electrical equipment and are identifiable by a circle containing the letter C. Once you’ve de-energized the electrical equipment, you may use class A or B extinguishers.
- Class D. Extinguishers rated for class D fires are intended to handle combustible metals like magnesium and titanium. These types of fires require an extinguisher that doesn’t react with burning metal, and are recognizable by the letter D inside a five-point star.
- Class K. These fires involve cooking fats, greases, and oils used in commercial cooking sites like restaurants. Extinguishers rated for class K fires use a process called saponification, which applies alkaline mixtures (like potassium acetate or potassium carbonate) to fatty acids (like burning cooking oil) to crate a foam that extinguishes the fire. Extinguishers rated for class K fire are marked with the letter K.
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Selecting the Right Type of Extinguisher
In addition to being rated for specific classes of fires, extinguishers come in different types, categorized by the way they extinguish fires.
1. Water Extinguishers
Water extinguishers are the most common and least expensive, but they are only acceptable for use on class A fires (ones involving paper, wood, straw, coal, rubber, and solid plastic). These extinguishers only contain water, and put out fires by soaking them with water and absorbing heat from the burning objects.
This type of extinguisher is best for offices, retail shops, schools, hotels, warehouses, and domestic premises.
2. Water Mist Extinguishers
Water mist fire extinguishers are smaller and more expensive, but more powerful than simple water extinguishers. With their ultra-fine mist of de-mineralized water particles, they cool the fire and reduce oxygen supply. They’re acceptable for use on class A, B, C, and K fires, as well as for electrical fires involving equipment up to 1,000 volts (like printers and computers). Water mist extinguishers must not be used on class D fires.
3. Foam Extinguishers
Foam extinguishers are more expensive than water-based ones. They're also far messier, leaving a residue that must be cleaned up. They work by smothering the fire and are acceptable for use on class A and B fires. They should not be used on burning fats or cooking oils (class K), and may only be used on electrical fires if they’ve been tested and are fired from at least one meter away.
4. Dry Powder Extinguishers
These extinguishers form a crust that smothers the fire and prevents it from spreading. They’re generally inexpensive and good for use on burning solids, liquids, and gases (class A, B, and C fires), and some are specifically designed to handle class D fires (combustible metals).
Dry powder extinguishers have a few notable disadvantages. Most notably, the powder is hazardous if inhaled and therefore unacceptable for use in offices and homes. It also doesn’t cool the fire, so there’s a chance of re-ignition. Finally, the powder requires a significant amount of cleanup and may damage soft furnishing and machinery.
5. CO2 Extinguishers
These extinguishers contain pressurized carbon dioxide (CO2), which suffocates the fire. They don’t leave any residue or cause damage to electrical items, so they're appropriate for use on class B and electrical fires (for example, large computer equipment). These extinguishers, however, are expensive and don’t prevent re-ignition, so fires may start again once the CO2 has dissipated.
6. Wet Chemical Extinguishers
Wet chemical extinguishers are ideal for handling class K fires and, therefore, usually found in kitchens with deep fat fryers. They use a solution of alkali salts in water to create a fine mist that cools flames and prevents splashing. Although these extinguishers are more expensive than water and water mist extinguishers, they may also be used on class A and B fires.
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Maintaining Your Extinguishers
An extinguisher isn't much good if it doesn't work properly when you need it to. Conduct regular inspections and maintenance to keep your fire extinguishers in good working condition.
All portable fire extinguishers should undergo visual inspection on a monthly basis. Look for the following things:
- Extinguisher is in the correct place and isn’t blocked or hidden
- All extinguishers are correctly mounted in accordance with NFPA standards
- Extinguisher pressure gauges have adequate pressure readings
- Pin and seals are intact
- No visual signs of damage
- Nozzles are free of blockage
In addition to monthly visual inspections, employers should conduct testing and maintenance at least once per year. Record the date on which the equipment was services and keep the records in a safe place for one year.
Complying with OSHA and NFPA Standards
OSHA and the NFPA have specific requirements when it comes to fire extinguisher locations. They should be conspicuous and easily accessible in case of a fire. Normal paths of travel and exits are excellent locations for extinguishers.
Travel distances to the nearest extinguisher vary depending on the fire class:
- Class A: 75 feet or less
- Class B: 50 feet or less
- Class C: 50-75 feet, depending on class A or B hazard
- Class D: 75 feet
- Class K: no requirement, but typically located at potential point of ignition
Extinguishers that weigh under 40 pounds should be installed so that the top of the unit is no more than 5 feet above the floor. Heavier extinguishers must be installed lower, with the top of the unit sitting no more than 3.5 feet above the floor.
Per OSHA regulations, employers must conduct training to familiarize employees with fire extinguisher use and potential risks. Employees who are designated to use the extinguishers in case of an emergency must receive comprehensive training on a yearly basis.
Fire extinguishers of all classes look similar, so understanding their classifications and types is critical to ensuring they are used properly and safely. Selecting the wrong extinguisher for the class of fire that could break out in your workplace can be a deadly mistake – one that is completely preventable with the right education and training.
Written by Jessica Barrett
Jessica is a freelance writer and editor from Toronto, Canada. She specializes in creating content for nonprofits and has written for organizations working in human rights, conservation, education, and health care. She loves traveling and food, speaks Spanish, and has two dogs, one of whom she rescued while living in Mexico.