Almost all fires are small in their incipient stage and can be put out quickly if the proper firefighting equipment is available and the person discovering the fire has been trained to use the equipment at hand. Most facilities turn to portable fire extinguishers for fighting incipient stage fires. The requirements for portable fire extinguishers in general industry are governed by the National Building Code (NBC).
To be effective, according to NBC, portable fire extinguishers must be:
Underwriters’ Laboratories classifies fire extinguishers by the type of fire that they will extinguish.
Class A fire extinguishers are used for ordinary combustibles such as wood, paper, some plastics and textiles. This fire class requires the heat-absorbing effects of water or the coating effects of certain dry chemicals. Extinguishers suitable for Class A fires should be identified by a triangle containing the letter “A.” If in color, the triangle should be green.
Class B fire extinguishers are used for flammable liquid and gas fires such as oil, gasoline, etc. These fire extinguishers deprive the fire of oxygen and interrupt the fire chain by inhibiting the release of combustible vapors. Extinguishers suitable for Class B fires should be identified by a square containing the letter “B.” If in color, the square should be red.
Class C fire extinguishers are used on fires that involve live electrical equipment that require the use of electrically nonconductive extinguishing agents. Once the electrical equipment is de-energized, extinguishers for Class A or B fires may be used. Extinguishers suitable for Class C fires should be identified by a circle containing the letter “C.” If in color, the circle should be blue.
Class D fire extinguishers are used on combustible metals such as magnesium, titanium, sodium, etc., that require an extinguishing medium that does not react with the burning metal. Extinguishers suitable for Class D fires should be identified by a five-point painted star containing the letter “D.” If in color, the star should be yellow.
Class K fire extinguishers are used on fires involving cooking media (fats, grease and oils) in commercial kitchens. Due to the higher heating rates of vegetable oils in commercial cooking appliances, the National Fire Protection Association (NFPA) Standard for Portable Fire Extinguishers (NFPA 10) includes a Class K extinguisher. These fire extinguishers work on the principle of saponification, which takes place when alkaline mixtures such as potassium acetate, potassium citrate or potassium carbonate are applied to burning cooking oil or fat. The alkaline mixture combined with the fatty acid creates a soapy foam on the surface that holds in the vapors and steam and extinguishes the fire. These extinguishers are identified by the letter K.
Portable fire extinguishers are labeled so users can quickly identify the classes of fire on which the extinguisher will be effective. The marking system combines pictographs of both recommended and unacceptable extinguisher types on a single identification label. Following are examples of typical labels.
Extinguisher for Class A, B and C
Extinguisher for Class B and C
Extinguisher for Class A and B
Extinguisher for Class A
Also located on the fire extinguisher label is the UL rating, which is broken down into Class A and Class B:C ratings. These numerical ratings allow you to compare the relative extinguishing effectiveness of various fire extinguishers. For example, an extinguisher that is rated 4A:20B:C indicates the following:
When analyzing these ratings, note that there is not a numerical rating for Class C or Class D fires. Class C fires are essentially either a Class A or Class B fire involving energized electrical equipment where the fire extinguishing media must be non-conductive. The fire extinguisher for a Class C fire should be based on the amount of the Class A or Class B component. For extinguisher use on a Class D fire, the relative effectiveness is detailed on the extinguisher nameplate for the specific combustible metal fire for which it is suggested.