“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."
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.
When should a fire extinguisher be used?
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.