Fire extinguishers are tested and rated by the Underwriters Laboratories of Canada (ULC in Canada and UL in the USA) for the class and size of fire that they will extinguish.
Class "A" fire extinguishers are to be used on ordinary combustibles such as wood, paper, cloth, rubber, straw, etc. An example would be a 2.5 U.S. gallon pressurized water extinguisher which has a 2-A rating.
The number is used to identify the amount of fire extinguishing potential of the different types of extinguisher suitable for use on Class "A" fires. For example, a 4-A extinguisher can be expected to extinguish approximately twice as much fire as a 2-A extinguisher.
Class "B" fire extinguishers are used on flammable or combustible liquids such as oil, gas, paint, thinners, grease, etc. The extinguishment required is one that excludes air, inhibits the release of combustible vapours, or interrupts the combustion chain reaction.
The number serves two purposes:
- It indicates the relative fire extinguishing potential of various sizes of the different types of extinguishers suitable for Class "B" fires.
- It is also an approximate indication of the square-foot area of deep-layer flammable liquid fire that the average trained operator can extinguish.
For example, a 10-B unit can be expected to extinguish 10 times as much fire as a 1-B unit and should successfully extinguish 10 square feet of a flammable liquid fire when used by a trained operator.
The Class "C" rating has no number and issued to indicate that the extinguishing agent does not conduct electricity. A Class "C" extinguisher is required for fires involving energized electrical wiring and equipment or appliances. Whenever possible, electrical equipment should be de-energized before attacking a Class "C" fire.
There is also a Class "D" rating. These are specialized extinguisher used to extinguish fires involving combustible metals such as magnesium, sodium, zirconium, etc.
Your extinguisher is a multipurpose fire extinguisher and is good for all the types of fire found in your average home office. It is also the size that is most often recommended.