The National Fire Protection Association (NFPA) has classified five general types of fires, based on the combustible materials involved and the kind of extinguisher needed to put them out. The five fire classifications are A, B, C, D, and K. Each classification has special markings, and A through D have special symbols and color identification. The five are as follows:

Class A. This type of fire is the most common. The combustible materials are wood, cloth, paper, rubber and plastics. The common extinguisher agent is water, but dry chemicals are also effective. Carbon dioxide extinguishers and those using sodium or potassium bicarbonate chemicals are not to be used on this type of fire.

Class B. Flammable liquids, gases and greases create class B fires. The extinguishers to use are foam, carbon dioxide and dry chemical. Also, water fog and vaporizing liquid extinguishers can be used.

Class C. Class C fires are electrical fires and a nonconducting agent must be used. Carbon dioxide and dry chemical extinguishers are to be used. Never use foam or water-type extinguishers on these fires.

Class D. Combustible metals, such as magnesium, titanium, zirconium and sodium fires are class D. These fires require specialized techniques to extinguish. None of the common extinguishers should be used since they can increase the intensity of the fire by adding an additional chemical reaction.

Class K. Class K fires involve cooking appliances and combustible cooking media (vegetable or animal oils and fats). Wet chemical-based extinguishers are recommended.

There are only two dry chemical extinguishers that can be used on A, B, and C fires, and those are multi-purpose ABC extinguishers, either stored pressure or cartridge operated. Multi-purpose extinguishers (ABC) will handle all A, B, and C fires. All fire extinguishers are labeled with either ABC, or A, or B, or C.

It is important to know what type of fire is in progress. If you use a fire extinguisher, be sure to use one only on fires for which that fire extinguisher is designed. Using the wrong agent on a fire may increase the intensity of the fire. Check the label on the fire extinguisher; it should list the fire class(es) it is meant to put out.

Key definitions — Fire protection

After-flame: The time a test specimen continues to flame after the flame source has been removed.

Aqueous film forming foam (AFFF): A fluorinated surfactant with a foam stabilizer which is diluted with water to act as a temporary barrier to exclude air from mixing with the fuel vapor by developing an aqueous film on the fuel surface of some hydrocarbons which is capable of suppressing the generation of fuel vapors.

Approved: Acceptable to OSHA under the following criteria:

• If it is accepted, or certified, or listed, or labeled or otherwise determined to be safe by a nationally recognized testing laboratory; or
• With respect to an installation or equipment of a kind which no nationally recognized testing laboratory accepts, certifies, lists, labels, or determines to be safe, if it is inspected or tested by another federal agency and found in compliance with the provisions of the applicable National Fire Protection Association (NFPA) Fire Code; or
• With respect to custom-made equipment or related installations which are designed, fabricated for, and intended for use by its manufacturer on the basis of test data which the employer keeps and makes available for inspection to OSHA.

For the purposes of this definition, equipment is listed, labeled, accepted, or certified if it meets the criteria listed in 1910.155(c)(3). Also, refer to 1910.7 for definition of nationally recognized testing laboratory.

Automatic fire detection device: A device designed to automatically detect the presence of fire by heat, flame, light, smoke, or other products of combustion.

Buddy-breathing device: An accessory to self-contained breathing apparatus which permits a second person to share the same air supply as that of the wearer of the apparatus.

Carbon dioxide: A colorless, odorless, electrically nonconductive inert gas (chemical formula CO2) that is a medium for extinguishing fires by reducing the concentration of oxygen or fuel vapor in the air to the point where combustion is impossible.

Class A fire: A fire involving ordinary combustible materials such as paper, wood, cloth, and some rubber and plastic materials.

Class B fire: A fire involving flammable or combustible liquids; flammable gases, greases, and similar materials; and some rubber and plastic materials.

Class C fire: A fire involving energized electrical equipment where safety to the employee requires the use of electrically nonconductive extinguishing media.

Class D fire: A fire involving combustible metals, such as magnesium, titanium, zirconium, sodium, lithium and potassium.

Dry chemical: An extinguishing agent composed of very small particles of chemicals, such as, but not limited to, sodium bicarbonate, potassium bicarbonate, urea-based potassium bicarbonate, potassium chloride, or monoammonium phosphate supplemented by special treatment to provide resistance to packing and moisture absorption (caking) as well as to provide proper flow capabilities. Dry chemical does not include dry powders.

Dry powder: A compound used to extinguish or control Class D fires.

Education: The process of imparting knowledge or skill through systematic instruction. It does not require formal classroom instruction.

Enclosed structure: A structure with a roof or ceiling and at least two walls which may present fire hazards to employees, such as accumulations of smoke, toxic gases and heat, similar to those found in buildings.

Extinguisher classification: The letter classification given an extinguisher to designate the class or classes of fire on which an extinguisher will be effective.

Extinguisher rating: The numerical rating given to an extinguisher which indicates the extinguishing potential of the unit based on standardized tests developed by Underwriters' Laboratories, Inc.

Fire brigade (private fire department, industrial fire department): An organized group of employees who are knowledgeable, trained, and skilled in at least basic firefighting operations.

Fixed extinguishing system: A permanently installed system that either extinguishes or controls a fire at the location of the system.

Flame resistance: The property of materials, or combinations of component materials, to retard ignition and restrict the spread of flame.

Foam: A stable aggregation of small bubbles which flow freely over a burning liquid surface and form a coherent blanket which seals combustible vapors and thereby extinguishes the fire.

Gaseous agent: A fire extinguishing agent which is in the gaseous state at normal room temperature and pressure. It has low viscosity, can expand or contract with changes in pressure and temperature, and has the ability to diffuse readily and to distribute itself uniformly throughout an enclosure.

Halon 1211: A colorless, faintly sweet smelling, electrically nonconductive liquefied gas (chemical formula CBrC1F2) which is a medium for extinguishing fires by inhibiting the chemical chain reaction of fuel and oxygen. It is also known as bromochlorodifluoromethane.

Halon 1301: A colorless, odorless, electrically nonconductive gas (chemical formula CBrF3) which is a medium for extinguishing fires by inhibiting the chemical chain reaction of fuel and oxygen. It is also known as bromotrifluoromethane.

Helmet: A head protective device consisting of a rigid shell, energy absorption system, and chin strap intended to be worn to provide protection for the head or portions thereof, against impact, flying or falling objects, electric shock, penetration, heat, and flame.

Incipient stage fire: A fire which is in the initial or beginning stage and which can be controlled or extinguished by portable fire extinguishers, Class II standpipe or small hose systems without the need for protective clothing or breathing apparatus.

Inspection: A visual check of fire protection systems and equipment to ensure that they are in place, charged, and ready for use in the event of a fire.

Interior structural firefighting: The physical activity of fire suppression, rescue, or both, inside of buildings or enclosed structures which are involved in a fire situation beyond the incipient stage.

Lining: A material permanently attached to the inside of the outer shell of a garment for the purpose of thermal protection and padding.

Local application system: A fixed fire suppression system which has a supply of extinguishing agent, with nozzles arranged to automatically discharge extinguishing agent directly on the burning material to extinguish or control a fire.

Maintenance: The performance of services on fire protection equipment and systems to assure that they will perform as expected in the event of a fire. Maintenance differs from inspection in that maintenance requires the checking of internal fittings, devices, and agent supplies.

Multipurpose dry chemical: A dry chemical which is approved for use on Class A, Class B, and Class C fires.

Outer shell: The exterior layer of material on the fire coat and protective trousers which forms the outermost barrier between the fire fighter and the environment. It is attached to the vapor barrier and liner and is usually constructed with a storm flap, suitable closures, and pockets.

Positive-pressure breathing apparatus: Self-contained breathing apparatus in which the pressure in the breathing zone is positive in relation to the immediate environment during inhalation and exhalation.

Pre-discharge employee alarm: An alarm which will sound at a set time prior to actual discharge of an extinguishing system so that employees may evacuate the discharge area prior to system discharge.

Quick disconnect valve: A device which starts the flow of air by inserting of the hose (which leads from the facepiece) into the regulator of self-contained breathing apparatus, and stops the flow of air by disconnection of the hose from the regulator.

Sprinkler alarm: An approved device installed so that any waterflow from a sprinkler system equal to or greater than that from single automatic sprinkler will result in an audible alarm signal on the premises.

Sprinkler system: A system of piping designed in accordance with fire protection engineering standards and installed to control or extinguish fires. The system includes an adequate and reliable water supply, and a network of specially sized piping and sprinklers which are interconnected. The system also includes a control valve and a device for actuating an alarm when the system is in operation.

Standpipe systems:

• Class I standpipe system means a 21/2″ (6.3 cm) hose connection for use by fire departments and those trained in handling heavy fire streams.
• Class II standpipe system means a 11/2″ (3.8 cm) hose system that provides a means for the control or extinguishment of incipient stage fires.
• Class III standpipe system means a combined system of hose which is for the use of employees trained in the use of hose operations and which is capable of furnishing effective water discharge during the more advanced stages of fire (beyond the incipient stage) in the interior of workplaces. Hose outlets are available for both 11/2″ (3.8 cm) and 21/2″ (6.3 cm) hose.

Small hose system means a system of hose ranging in diameter from 5/8″ (1.6 cm up to 11/2″ (3.8 cm) which is for the use of employees and which provides a means for the control and extinguishment of incipient stage fires.

Total flooding system: A fixed suppression system which is arranged to automatically discharge a predetermined concentration of agent into an enclosed space for the purpose of fire extinguishment or control.

Training: The process of making proficient through instruction and hands-on practice in the operation of equipment, including respiratory protection equipment, that is expected to be used and in the performance of assigned duties.

Vapor barrier: Material used to prevent or substantially inhibit the transfer of water, corrosive liquids, and steam or other hot vapors from the outside of a garment to the wearer’s body.

Key definitions — Fire prevention

Fire prevention plan (FPP): A plan required by certain OSHA regulations to address how an employer will prevent fires and control their damage.