Classes of Fire

Four elements, (1) heat or energy for ignition, (2) fuel vapors, (3) oxygen and (4) free radicals are necessary for a sustained hydrocarbon fire. Without all these four elements, a sustained fire is not possible. See the Fire Tetrahedron.

In addition, the first three elements must be available in the proper balance, for a sufficient period of time, for ignition to occur. This time increases near the flammability limits and the ignition energy limits.

The three first elements (1), (2) or (3), may be independently varied with ignition occurring only under a certain combination of conditions. A change in one element may affect the other two elements. A stronger ignition source (heat) may lower the amount of oxygen required (lower oxygen concentration) and/or reduce the amount of fuel vapors necessary at ignition. Increasing the amount of oxygen available may lower the energy necessary for ignition (heat) and/or decrease the amount of fuel vapors needed. But if we remove totally one of the elements fire cannot exist.

Fire is successfully extinguished by removing any one of the four elements necessary to sustain it.

Flash point (FP) temperature is the lowest temperature where a fuel will give off sufficient vapors for ignition under ambient conditions. It is an estimate of the lower flammability limit.

Fires are classified according to the material, which is being burned. The four classes of fires, with the American and International symbols, are as follows:

Class A:

Ordinary Combustibles - Cloth, Wood, Paper, Rubber, many plastics.

Extinguisher:
Pressurized water (it removes Heat) suitable for use on Class A only. Dry chemical: mono-ammonium phosphate, (it removes contact between Oxygen and Fuel), rated for Class A, B, and C fires.
Extinguishers suitable for Class A fires should be identified by a green triangle containing the letter "A" and the pictograph shown above.

Class B:

Flammable Liquids - Gasoline, Oil, Oil-based paint, Cooking Oil

Extinguisher:
1) Carbon dioxide (it displaces Oxygen but dissipates quickly; the combustible surface, if hot, may re-ignite).
2) Dry Chemical (it removes Oxygen from the Fuel by coating the surface inhibiting the release of combustible vapors): mono ammonium phosphate, rated for Class A, B, and C fires; Sodium Bicarbonate and Potassium Bicarbonate, for Class B and C, preferred for cooking oil fires.
3) Halon: it interferes with the fire chemical reaction by quenching free radicals. Production has been banned (Montreal, 1998) because Halon has been found to be an ozone-depleting substance.
Extinguishers that are suitable for Class B fires are identified by a red square containing the letter B and the pictograph shown above.

Class C:

Energized electrical equipment, including appliances, wiring, circuit breakers, and fuse boxes.

Extinguisher:
1) Carbon dioxide (it removes Oxygen but dissipates quickly; the combustible surface, if still hot, may re-ignite).
2) Dry Chemical (it removes Oxygen from the Fuel by coating the surface and inhibiting the release of combustible vapors): mono Ammonium Phosphate, rated for Class A, B, and C fires; Sodium Bicarbonate and Potassium Bicarbonate, for Class B and C, preferred for cooking oil fires.
3) Halon: it interferes with the fire chemical reaction by quenching free radicals. Production has been banned (Montreal, 1998) because Halon has been found to be an ozone-depleting substance.
Extinguishers suitable for Class C fires are identified by a blue circle containing the letter C and the pictograph shown above.

Class D:

Combustible metal such as Mg, Na, Li, powdered Al, etc.

Extinguisher:
Extinguishers rated for class D fires have a label, which list the types of metal, on which the extinguisher may be used. The extinguishing medium must not react with the burning metal. Extinguishers suitable for Class D fires are identified by a yellow star containing the letter D.
At Stockton, sand buckets are available in all laboratories to extinguish metal fires.

Fire Extinguishers: Types and Effectiveness

Fire extinguishers are labeled with the letters and pictographs identifying the class of fire they can effectively extinguish. The UL (Underwriters Laboratory) rating is also shown on the label and describes the degree of effectiveness of the particular fire extinguisher.
The UL rating is applied to Class A and Class B:C, as follows.

The A rating is a water equivalency rating. Each A is equivalent to 1 ¼ gallons of water.
So, 4A = 5 gallons of water.

The B:C rating is equivalent to the amount of square footage that the extinguisher can cover when handled by a professional. 20 B:C = 20 sq. ft. of coverage.

Class C equivalency is the same amount as when the particular fire extinguisher is used on a Class B fire. The main difference is that for a Class C fire, the nature of the extinguishing media must be non-conductive.

The relative effectiveness of a particular Class D extinguisher is detailed on its label for the specific combustible metal fire recommended.