Class A Fires - ordinary combustibles: The most common fire, called Class A, involves burning wood, cloth, paper and plastic. These fires start every day and have the ability to spread to other similar materials. Waste paper cans filled to over-flowing, and piles of old newspapers are a frequent fuel for Class A fires. Water is an excellent extinguishing agent for Class A fires.
Class B Fires - flammable liquids: Class B fires involve flammable and combustible liquids. Burning liquids can flow rapidly and spreading a fire. For example, if toluene spills from a broken bottle, it can spread out rapidly, evaporate and ignite. Water will usually not extinguish this type of fire. The burning liquid will float on the surface of the water and spread the fire further.
Class C Fires - electrical equipment: Fires in energized electrical equipment, called Class C, are especially dangerous to fight. In addition to the dangers associated with the fire, the potential for electrocution exists. Never attempt to put out a Class C fire with water.
Class D Fires - burning metals: Burning metals, called pyrophoric metals, such as magnesium and sodium are the toughest to put out, and are called Class D. By their very nature, they are unresponsive to conventional fire extinguishers. Special extinguishing agents must be used or the fire may be smothered with dry sand.
Types of Fire Extinguishers
There are recommended fire extinguishers for each class of fire. The following describes some of the common types that can be used:
Class A type: Class A fire extinguishers can be used for ordinary combustibles. Water-based Class A fire extinguishers can include water, loaded stream, antifreeze, wetting agents, and aqueous film-forming foam (AFFF) types. Never use these on electrical fires.
Class ABC Multipurpose Dry Chemical: Most areas contain a variety of ordinary combustibles, solvents and electrical equipment. A Class ABC multipurpose dry chemical fire extinguisher can be used to handle any of A, B, or C class fires. This multipurpose fire extinguisher discharges a stream of monoammonium phosphate (or other dry chemical) which extinguishes most types of fires you would encounter on campus.
Class ABC Halon 1211: Halon 1211 fire extinguishers, if large enough, are considered Class ABC and can be used in place of Class ABC multipurpose dry chemical fire extinguishers. They do not have the disadvantage of the clean-up and corrosiveness of the monoammonium phosphate types. These types of extinguishers are not recommended due to human safety issues.
Class BC Carbon Dioxide: Unless substantial amounts of Class A materials are involved, a Class BC carbon dioxide (CO2) fire extinguisher can be used. This extinguisher releases a cloud of carbon dioxide to displace air and cut off the fire's oxygen (caution: you need oxygen too!). It is not as effective outdoors in large open areas. Caution should be exercised with this extinguisher - contents are extremely cold upon release and can cause freeze burns (stay away from the nozzle).
Fire extinguishers are also assigned Class A and B numerical ratings based on their relative effectiveness in extinguishing a particular type of fire. For example, a 4A rating can put out twice the size class A fire than a 2A rating can. The size of rating needed will depend on the relative amounts of the flammable materials present.
BASIC SAFETY PRINCIPLES FOR PORTABLE EXTINGUISHER USE
- Do not attempt to fight a fire without sufficient facilities to combat it effectively.
- Remember every fire gives off toxic gases.
- Use extreme caution when an electrical appliance or wiring is involved in the fire (don't use water extinguishers for these types of fires).
- Never allow the fire to get between you and your means of escape.
- If outdoors, always attack the fire with the wind to your back.
- Never turn away from the fire; back out, keeping a watchful eye on the extinguished fire.
- Most extinguisher contents are under pressure and need to be treated with respect.