Types of Fire

The National Fire Protection Association has classified fires into three basic types:

a. Class A fires - as fires in ordinary combustible materials such as wood, cloth, paper, upholstery materials, etc.

b. Class B fires - as fires in flammable petroleum products or other flammable or combustible liquids, greases, solvents, paints, etc.

c. Class C fires - as fires involving energized electrical equipment where the electrical nonconductivity of the extinguishing media is of importance.

In most cases where electrical equipment is deenergized, extinguishers suitable for use on Class A or B fires may be employed effectively.

A fourth class of fire, Class D fire, is defined as fire in flammable metal. Class D fires are not considered a basic type since they are generally caused by a Class A, B, or C fire. Usually these fires involve magnesium in the shop or in aircraft wheels and brakes.

Any one of these types of fires can occur during maintenance or operations. There is a particular type extinguisher which is most effective for each type of fire.

Fire Extinguishment
 

Three things are required for a fire. Fuel - something that will in the presence of heat combine with oxygen, thereby releasing more heat and as a result reduces itself to other chemical compounds. Heat - can be considered the catalyst which accelerates the combining of oxygen with fuel, in turn releasing more heat. Oxygen - element which combines chemically with another substance through the process of oxidation. Rapid oxidation, accompanied by a noticeable release of heat and light is called combustion or burning (figure 11-9). Remove any one of these things and the fire goes out.

Types of Fire vs. Extinguishing Agent

Class A fires respond best to water or water type extinguishers which cool the fuel below combustion temperatures. Class B and C extinguishers are effective but not equal to the wetting/cooling action of the Class A extinguisher.

Class B fires respond to carbon dioxide (CO2), halogenated hydrocarbons (Halons) and dry chemicals, all of which displace the oxygen in the air thereby making combustion impossible. Foam is effective, especially when used in large quantities. Water is ineffective on class B fires and will cause the fire to spread.

Class C fires involving electrical wiring, equipment, or current respond best to carbon dioxide (CO2) which displaces the oxygen in the atmosphere making combustion improbable. The CO2 equipment must be equipped with a nonmetallic horn to be approved for use on electrical fires. Two reasons for this must be considered:

1. The discharge of CO2 as through a metal horn can generate static electricity. The static discharge could reignite the fire.

2. The metal horn if in contact with the electrical current would transmit that current to the extinguisher operator.

Halogenated hydrocarbons are very effective on Class C fires. The vapor reacts chemically with the flame to extinguish the fire. Dry chemicals are effective but have the disadvantage of contaminating the local area with powder. Also, if used on wet and energized electrical equipment, it may aggravate current leakage.

Water, wet water or foam are not acceptable agents for use on electrical equipment fires.

Class D fires respond to application of dry powder, which prevents oxidation and the resulting flame. Application may be from an extinguisher or scoop or shovel. Special techniques are needed in combating fires involving metal. Manufacturers recommendations should be followed at all times. Areas which could be subjected to metal fires should have the proper protective equipment installed. Under no conditions use water on a metal fire. It will cause the fire to burn more violently and can cause explosions.

Fire Extinguisher Periodic Check List

1. Appropriate extinguisher located in proper place.
2. Safety seals unbroken.
3. Remove all external dirt and rust.
4. Gauge or indicator in operable range.
5. Check for proper weight.
6. No nozzle obstruction.

Fire Extinguishing Agents

A. Water and water based agents.

Water may be combined with antifreeze compounds or wetting agents (accelerate penetration of materials by water). Water is used on carbonaceous fires. It extinguishes fires by cooling the fuel below the combustion temperature.

1. Soda-acid and foam act on a fire the same as water by lowering the temperature. Foam has some effect on a petroleum base fire by preventing oxygen from getting to the fire.

2. Loaded stream contains an antifreeze as well as a flame retardant.

B. Dry Chemical.

Four types of chemicals are used:

1. Sodium bicarbonate (Formula H). For ordinary risk class B and C fires.

2. Ammonium phosphate (Multipurpose). For multiple risk class B and C fires.

3. Potassium bicarbonate (Purple K). For high risk class B and C fires.

4. Multipurpose dry chemical (ABC). For use on Class A, B, and C fires. The dry chemicals extinguish a fire by smothering it, cutting off oxygen, and the blanket of dry chemicals prevents reflash fires. It also affords the operator some protection from the heat. All dry chemicals are nonconductors of electricity.

C. Gas.

1. Carbon dioxide (CO2) has a toxicity rating (Underwriter's Laboratory) of 5A especially recommended for use on class B and C fires. Extinguishes flames by dissipating oxygen in the immediate area.

2. Halogenated hydrocarbons (commonly called freon by the industry), are numbered according to chemical formulas with Halon numbers.

Carbon tetrachloride (Halon 104). Chemical formula CCl4. UL toxicity rating of 3. It is poisonous and toxic. Hydrochloric acid vapor, chlorine and phosgene gas are produced whenever carbon tetrachloride is used on ordinary fires. The amount of phosgene gas is increased whenever carbon tetrachloride is brought in direct contact with hot metal, certain chemicals, or continuing electrical arcs. It is no longer approved for any fire extinguishing use.

Methyl bromide (Halon 1001). Chemical formula CH3Br - a liquified gas, UL toxicity rating of 2. Effective but very toxic and also is corrosive to aluminum alloys, magnesium and zinc. Not recommended for aircraft use.

Chlorobromomethane (Halon 1011). Chemical formula CH2ClBr - a liquified gas, UL toxicity rating is 3. Not recommended for aircraft use.

Dibromodifluoromethane (Halon 1202). Chemical formula CBr2F2. UL toxicity rating of 4. Not recommended for aircraft use.

Bromochlorodifluoromethane (Halon 1211). Formula CBrClF2 - a liquified gas with a UL toxicity rating of 5. It is colorless, noncorrosive and evaporates rapidly leaving no residue whatever. It does not freeze or cause cold burns and will not harm fabrics, metals, or other materials it contacts. Halon 1211 acts rapidly on fires by producing a heavy blanketing mist that eliminates air from the fire source, but more importantly interferes chemically with the combustion process. It has outstanding properties in preventing reflash after the fire has been extinguished.

Bromotrifluoromethane (Halon 1301). Chemical formula CF3Br is also a liquified gas with a UL toxicity rating of 6. It has all the characteristics of Halon 1211. The significant difference between the two is: Halon 1211 forms a spray similar to CO2, while Halon 1301 has a vapor spray that is more difficult to direct.

D. Powder.

Dry powder for metal fires. Fires in metal require special handling. If water is used on a magnesium fire the burning is accelerated. Special dry powders are available for use wherever metal fires are possibilities. These are normally applied by scoop or shovel. Multipurpose (ABC) dry chemicals have a limited use on metal fires such as fires in wheel brakes or in magnesium fires. (See figure 11-10 and figure 11-11).
 

RECOMMENDED MARKINGS TO INDICATE EXTINGUISHER SUITABILITY

(From NFPA Standard #10)

The following recommendations are given as a guide in marking extinguishers, and/or extinguisher locations, to indicate the suitability of the extinguisher for a particular class of fire.

Markings should be applied by decalcomanias, painting or similar methods having at least equivalent legibility and durability.

Where markings are applied to the extinguisher, they should be located on the front of the shell above or below the extinguisher nameplate. Markings should be of a size and form to give easy legibility at a distance of 3 feet.

Where markings are applied to wall panels, etc., in the vicinity of extinguishers, they should be of a size and form to give easy legibility at a distance of 25 feet. (See figure 11-12 and figure 11-13).