Fire extinguishers are distinguished based on the types of fires on which they are effective. These fires are classified by their fuel source and assigned identifying letters as follows:
“A” class – Fires that result from ordinary combustibles, such as wood and paper.
“B” class – Fires that result from combustible liquids, such as kerosene, gasoline, oil, and grease.
“C” class – Fires of an electrical nature. These result from the combustion of circuit breakers, wires, outlets, and other electrical devices and equipment. Extinguishers designed to handle this type of fire cannot use chemicals that are conductive since conductive agents increase the risk of electric shock to the operator.
“D” class – Fires resulting from combustible metals, such as sodium, potassium, titanium, and magnesium. These fires occur mostly in chemical laboratories and are rare in most other environments.
“K” class – These types of fires consume vegetable oils, animal fats, and generally happen in kitchens.
*Note* Although, technically, the letter rankings listed above refer to fire types, these symbols can also be used to identify the extinguishers themselves. For instance, an extinguisher that uses CO2 can be called a “CO2 extinguisher” or a “BC extinguisher."
No fire extinguisher can be safely and effectively used for every type of fire. Some contain chemicals that are ineffective in certain situations and can even cause harm to the operator if misapplied. To prevent confusion, extinguishers are classified by the type of chemical agents they contain. A few of the most common extinguisher types are listed below:
Dry Chemical – There are two types of fire extinguishers that use a dry chemical. One is called “multi-purpose dry chemical” and uses ammonium phosphate as the extinguishing agent, which is effective on “A,” “B,” and “C” class fires. This chemical is corrosive and must be scrubbed from surfaces after use. These types of extinguishers are very common and are found in schools, homes, hospitals and offices. Sodium bicarbonate is used in extinguishers known as “regular dry chemical,” which are capable of handling “B” and “C” class fires. These extinguishers are found in garages, kitchens and laboratories. Sodium bicarbonate is easy to clean and non-toxic.
Carbon Dioxide – These extinguishers contain liquid CO2 that is expelled as a gas. They are effective against “B” and “C” class fires. Unlike other chemicals, CO2 does not leave a harmful residue and is environmentally friendly. It also poses very little danger to electronics and is effectively employed in laboratories, computer rooms, and other areas with sensitive equipment.
Water Extinguishers – These extinguishers are most suited for “A” class fires. However, they cannot be used in “B,” “C” or "D" class fires. In "B" and "D" class fires, the water will spread the flames. In a "C" class fire, the water is conductive and poses a risk of electric shock to the operator. However, the misting nozzle of a "Water Mist" extinguisher breaks up the stream of deionized water so that there is no conductive path back to the operator. Since the agent used is water, these types of extinguishers are inexpensive and environmentally friendly.Wet Chemical Fire Extinguishers – These devices are designed to combat “K” class fires and commonly use potassium acetate. They are appropriately employed in commercial kitchens and restaurants, especially around deep fryers. The chemical is emitted as a fine mist that does not cause grease to splash onto other surfaces. They can also be used in “A” class fires.
InterNACHI inspectors should:
- check that a portable fire extinguisher exists within a 30-foot travel distance of commercial-type cooking equipment that uses cooking oil or animal fat.
- check that a portable fire extinguisher is within 75-feet of travel on every floor.
- check for the presence of portable extinguishers, and determine that they are located in conspicuous and readily available locations immediately available for use, and not obstructed or obscured from view.
- confirm that access to extinguishers is not obstructed.
- make sure that the hose (if so equipped) is intact and not obstructed.
- make sure the pressure dial reads in the green or “charged” area. It should also be clear and readable.
- check that the pull pin is securely fastened within the handle and held in place by the tamper seal.
- check for visible dents or cracks in the extinguisher body.
- check that the extinguisher is in its proper location and mounted correctly.
- check for modifications that might reduce the extinguisher’s functionality.
- make sure that the fire extinguisher has a label and that is is legible.
- test fire extinguishers.
- determine the adequate number of fire extinguishers needed or their ratings.
- ignite or extinguish fires.
The National Fire Protection Agency (NFPA) recommends that extinguishers should be tested every twelve years or five years, depending on the type. The standard method of testing, “hydrostatic,” is conducted underwater where the cylinders are subjected to pressures that exceed their ratings. Vessels that fail the test are condemned and destroyed, while the rest are reassembled and put back into service.
According to the NFPA, extinguishers should be destroyed if any of the following conditions are present (they should not be tested):a. where repairs by soldering, welding, brazing, or use of patching compounds exist.
b. where the cylinder threads are worn, corroded, broken, cracked or nicked.
c. where there is corrosion that has caused pitting, including pitting under a removable nameplate or nameband assembly.
d. where the fire extinguisher has been burned in a fire.
e. where a calcium chloride-type of extinguisher agent was used in a stainless steel fire extinguisher.
f. where the shell is of copper or brass construction joined by soft solder or rivets.
g. where the depth of a dent exceeds 1/10 of the greatest dimension of the dent if not in a weld, or exceeds 1⁄4 in. (0.6 cm) if the dent includes a weld.
h. where any local or general corrosion, cuts, gouges or dings have removed more than 10 percent of the minimum cylinder wall thickness.
i. where a fire extinguisher has been used for any purpose other than that of a fire extinguisher.
When should a fire extinguisher be used?
Small fires can be controlled through the use of household or commercial fire extinguishers. A household extinguisher can often completely douse a very small fire and prevent the need for professional assistance. Even if a fire cannot be completely doused, a homeowner can potentially control a blaze long enough with an extinguisher for firefighters to arrive. Fire extinguishers should not be used if the operator is not sure they have the proper type of extinguisher, if they are not sure how to use it, or if they cannot avoid smoke or are in imminent danger. If the operation of an extinguisher will place building occupants in danger, they should evacuate the building and wait for fire crews to arrive.
What is on an extinguisher’s label? You'll find:
- essential information about the types of fires they can combat. Newer devices have pictures that correspond directly to the fire types listed above. Older models have letters serve the same purpose.
- a numerical rating that designates the extinguishing potential for that particular model (class “A” and “B”).
- instructions for operation.
- a tag that indicates if and when an inspection occurred.
Fire extinguishers expire and they do this for a few different reasons. One common way is that, over time, the seal on the neck will weaken and allow compressed gas to escape. Extinguishers that have lost much of their pressure will not operate. Pressure within an extinguisher can be conveniently checked through a pressure gauge. “ABC” class extinguishers (ammonium phosphate) have the tendency to fail due to solidification of the chemical in the canister base. Homeowners and inspectors can delay this process by periodically shaking the extinguisher. Expensive extinguishers that have expired, especially those designed for commercial use, can be refilled and resealed by companies who specialize in this service. Inexpensive models are disposable.
Unfortunately, an expiration date cannot be fully trusted and there is no foolproof way to know if an extinguisher is no longer functional. Due to the extremely destructive potential of fires and the relatively low cost of extinguishers, it is advisable to replace or recharge questionable extinguishers.
In summary, extinguishers are classified based on their chemical ingredients, all of which have their own strengths and limitations. It is important to know what type of extinguisher combats what type of fire. Fire extinguishers are critical indoor components that must be maintained and inspected regularly.