Overview

A fire extinguisher is a man-portable device, usually weighing between 5 and 20 pounds (2–10kg), intended to stop a small or smoldering fire from growing into a serious threat. These incipient stage fires pose little danger if caught early and some basic safety rules are followed, assuming the fire extinguisher is of the proper type to fight that kind of fire. However, if the fire grows too large for a single fire extinguisher to put it out, or if attempting to fight it is in any way dangerous, the job should be left up to professional firefighters.

When to Fight and When to Run

Not every fire should be fought by an amateur. Although an immediate reaction could save property and prevent a larger, more dangerous fire, your safety and the safety of the people around you are most important. Do not attempt to fight any fire that is too large to be extinguished with a single fire extinguisher, and do not put yourself into any potential danger. Attempting to fight a fire with the wrong type of fire extinguisher could make the situation worse, as well. Some paper smoldering in a garbage can, or a grease fire on a stove, are usually safe to fight. However, walking into a room and seeing the couch, curtains, and part of the wall ablaze means you should get out and call the fire department immediately.

    Basic safety rules:
  • Call the fire department or pull the fire alarm before you try to fight the fire, or assign someone to do this for you. Even if you put it out, they will make sure the area is safe and the fire will not re-ignite. If you assign someone, pick a specific person and make eye contact, do not simply yell "Someone call 911!".

  • Evacuate anyone else from the area. Make sure everyone else is safe before attempting to fight the fire except for one person, if available, to stand at a safe distance and be available to help if necessary.

  • Make sure you have a clear path to the exit. Never let the fire come between you and the exit. If you can't put it out easily, you will need to get out of there as quickly as possible. Remember that fires can unexpectedly flare up and become much larger and more dangerous in a very short time.

  • Do not attempt to fight a fire with thick smoke or dangerous fumes. The professionals have the proper equipment to handle these.

  • Do not attempt to fight the fire unless you are certain you can put it out, and with only one fire extinguisher. Any time a fire extinguisher is discharged, even partially, it will need to be fully recharged, so always use the whole charge.

  • Do not attempt to fight a fire with the wrong type of fire extinguisher.

P.A.S.S.

P.A.S.S. is an acronym intended to help people remember the proper use of a fire extinguisher.

P - Pull the pin
Fire extinguishers come with a safety pin which holds the handle open and prevents it from being activated accidentally. The first thing you need to do is pull this pin out so it can be used.
A - Aim
Fire extinguishers come with either a straight nozzle or a flexible hose for expelling their fire extinguishing media. Point this at the base of the fire, where the fuel is, for maximum effectiveness.
S - Squeeze
Squeeze the handle of the fire extinguisher to release the contents. Adjust your aim if necessary. Do not stop until the extinguisher is completely empty.
S - Sweep
The contents will be ejected in a continuous stream. Sweep this stream back and forth across the base of the fire, making sure to spray or coat all of the exposed fuels feeding the flames. Empty the entire contents of the extinguisher on the fire.

The Fire Triangle

Fire requires three elements in order to burn. If any of them are missing, combustion will not take place. Fire extinguishers are designed to remove one or more legs of the fire triangle.

Heat - A source of heat such as a match, cigarette, electrical spark, or exothermic chemical reaction. Spontaneous combustion is a special case resulting from decomposing vegetable matter or volatile chemicals breaking down in situations where they cannot lose their heat to the environment. Once the fire starts, it produces its own heat to continue burning.

Fuel - The fuel is the actual substance that is burning, and is used to classify the fire into A, B, C, D, or K categories. This will determine how to fight the fire. Certain fuels do not burn, they melt instead. Others are capable of exploding violently if the right conditions are present, which is basically burning suddenly and all at once, resulting in a pressure wave from expanding gasses.

Oxygen - Combustion is an exothermic chemical reaction which oxidizes the fuel source, so it requires a ready supply of oxygen to burn. However some rare fires are capable of supplying their own oxygen, and can therefore burn underwater. Usually oxygen is simply present in the atmosphere at 21% concentration, although this level can be artificially raised with oxidizers or venting oxygen storage tanks. More oxygen creates a hotter, faster burning fire that is easier to start. In some cases a fire can simply burn itself out if allowed to use up all the oxygen in a sealed environment.

Some people consider this to be a fire tetrahedron, counting the actual chemical chain reaction taking place as a separate issue. This is useful for understanding the Purple-K fire extinguisher, which is unusual in that it affects the chemical reaction directly.

Classifications of Fires

Fires are classified according to what fuel is burning. These classifications are:

A - Green Triangle or burning trashcan and campfire symbol
Wood, paper, cloth, plastic, rubber, etc. These are what people think of as ordinary fires. They are best combatted with water, but dry chemical is also good. The biggest risk is that smoldering material could re-ignite even if you think the fire is out.

B - Red Square or burning gasoline can symbol
Flammable liquids or vapors. These are grease fires, gasoline fires, and the like, and are sometimes the result of spontaneous combustion. They are best combatted with foam or carbon dioxide, but dry chemical is also good. The biggest risk is that they spread quickly and easily as the liquid flows and spills. Using water on these fires will do nothing but spread it around.

C - Blue Circle or burning electrical outlet and plug symbol
Electrical fires. These result from overheated wires, short circuits, or damaged insulation, and are usually burning insulation or other material around the source of electricity. These must be combatted with non-conductive media such as carbon dioxide, dry chemical, or Halon. The biggest risk is electric shock, so always turn off the circuit breaker before fighting an electrical fire. Using water on these fires may result in electric shock or further short circuits.

D - Red Star or letter D inside a yellow star symbol
Burning metals such as powdered aluminum, magnesium, sodium, lithium, and hafnium. These fires are extremely difficult to put out, even for professionals, and many of them are capable of burning completely underwater. Extinguishers that require special training must be used for these, none of the standard ABC types will work. Some models coat the surface with a waxy substance, others with a sodium chloride solution. Lithium is a special case and can only be extinguished with a copper powder mixture developed by the US Navy which not only suffocates the fire, but also acts as a heat sink to pull heat away from it. These should never be attempted without training.

K - Letter K or burning frying pan symbol
Commercial grease and kitchen fires. These are a separate classification from Class B fires because cooking oil and grease tend to burn hotter and are more difficult to put out than other liquids found in Class B.

Types of Fire Extinguishers

There are several types of fire extinguishers on the market, each rated for a different kind of use. Dry chemical ABC fire extinguishers are the most common for home use, since they can be used to fight almost every common household fire. If a specific type of fire threat exists in some location, it is a good idea to have available the specific fire extinguisher which is best for combatting that type of fire.

Water - Class A only
Pressurized water fire extinguishers are used to soak the burning material with cool water, robbing the fire of its heat to extinguish it. Water is also an excellent heat sink and helps prevent smoldering blazes from re-igniting. Water-based chemical fire extinguishers act in a similar manner.

CO2 - Class B, C
Pressurized carbon dioxide gas is heavier than oxygen, and asphyxiates a fire by displacing the oxygen from the area. It also robs the fire of heat because rapid de-pressurization of gasses results in a drop in temperature, a concept used in the the refrigeration cycle.

Dry Chemical - Class A, B, C
Dry chemical fire extinguishers coat the surface of the material with a nonflammable powder, removing the fuel from the fire. These are popular because they are rated for all three common fire classifications, but there are some circumstances in which they are inappropriate. They are usually an adequate second-best catch-all for situations with multiple potential fire hazards, such as homes. Because it is lightweight and floats, dry chemicals are easily washed away by flowing, burning liquids, reducing their effectiveness.

Halon - Class B, C
Halon gas fire extinguishers are very similar to carbon dioxide gas in that they suffocate the fire by displacing oxygen. Halon is considered to be better for use around expensive electrical equipment because, unlike carbon dioxide, it does not cause condensation or otherwise damage electrical components.

Foam - Class A, B
These contain a lightweight chemical foam which coats the surface of the burning material, removing the fuel from the fire. These are particularly effective against Class B fires.

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