Fires involve ordinary combustibles, like wood, paper, cloth, trash, and plastics. They do not contain metals, combustible liquids, or electricity. (Class A fires generally leave Ashes.)
Class A fires can be extinguished with water. A Class A extinguisher is typically either a pressurized can of water that can be sprayed on a fire or a container of water with a pump mechanism. Class A extinguishers are for use only on Class A fires. A Class A fire extinguisher is marked with a green triangle containing the letter A. An alternative method of extinguishing a Class A fire is to smother it with a blanket, a gloved hand, or other material.
Fires involve flammable liquids. Typical flammable liquids are gasoline, oil, grease, paint, and acetone. Class B fires can be very difficult to control because they involve burning nonmetals in a liquid state. This classification also includes flammable gases. (Class B fires generally involve materials that Boil or Bubble.)
Electricity is not present in Class B fires. A Class B fire is extinguished with carbon dioxide (CO2) contained in a heavy cylinder and under pressure. CO2 is very cold when under pressure and displaces the oxygen around the fire, causing the fire to be extinguished. It should not be sprayed on people or animals. The use of a Class B extinguisher usually results in very little mess and no damage to property. However, avoid breathing CO2. Class B extinguishers may be used to extinguish both Class A and Class B fires. A Class B fire extinguisher is marked with a red square containing the letter B.
Fires involve electrical equipment. Thus, electricity is always present. It is often combined with combustible materials. An additional hazard of a Class C fire is the potential for electric shock while fighting the fire. If possible, always turn off the source of electricity before fighting an electrical fire. The fire may be extinguished, but if the electricity is not turned off, the fire may rekindle. (Class C fires deal with electrical Current.)
A Class C fire is extinguished with a dry chemical, which does not conduct electricity. The chemical is a very fine powder that smothers the fire when applied. A disadvantage is the mess the chemical makes when ejected from the extinguisher. Avoid breathing dry chemical extinguisher powder. Class C extinguishers may be used to extinguish Class A, Class B, and Class C fires. A Class C fire extinguisher is marked with a blue circle containing the letter C.
Fires involve combustible metals. Potassium, sodium, aluminum, and magnesium burn at extremely high temperatures. Unless you work in a laboratory or in an industry that uses these materials, it is unlikely you’ll have to deal with Class D fires. They are uncommon in agriculture. Burning metal is very difficult to extinguish, and only Class D extinguishers are recommended on burning metal. Class D fire extinguishers are not used on any other class of fire. The material in a Class D extinguisher is a foam product that puts out the fire by replacing the oxygen near it. Class D fire extinguishers are the most expensive. Class D extinguishing equipment is marked with a yellow star containing the letter D.