Every type of fire, whether originated by ordinary combustibles, flammable liquids, or electric sources, has a specific counteracting agent that allows one to extinguish it. Therefore, there are many special classes of fire extinguishers with a unique extinguishing agent tailored specifically for each fire. This allows fires to be extinguished quicker and more easily than if there was just one-fire-fits-all extinguishing agent. To help people find the right fire extinguisher, the National Fire Protection Association (NFPA) highlights the standard for the selection, use and maintenance of a fire extinguisher.
Classification of Portable Fire Extinguishers:
The basic classes of fire extinguishers in the US are classified A, B, C, D and K. In addition to that, multi-purpose fire extinguishers are also prevalent, such as those labeled "A-B-C" (multipurpose) or "B-C" (standard), which can be used on two or more types of fires.
Apart from the type of fires they dissolve, fire extinguishers are also numerically rated for the size of the fire they can control. Standard ratings serve as quick identifiers for the scale of fire which can be handled.
To start with, numerical ratings only apply to classes “A” and “B”. A rating like this would appear on the fire extinguisher label –“2-A:10-B:C.” Here, the numerical rating precedes the class letter, in this case, “2.” When the numeral rating is multiplied by 1.25, it’s the equivalent capacity in gallons of water. Therefore, a 2-A rating extinguisher is equivalent to 2.5 gallons of water. Therefore, larger numbers indicate a capacity to handle larger fires. The number prefixed to B, in this case, “10,” specifies the size (in square feet) that the extinguisher is capable of expelling. The letter “C” is not accompanied by any rating, as its sole purpose is to show that the extinguishing agent does not conduct electricity.
Fire-extinguishing capacity is rated according to ANSI/UL 711, Rating and Fire Testing of Fire Extinguishers. Other details about the extinguisher such as class category, are mentioned on fire extinguisher labels—without which, determining the right fire extinguisher would be impossible. In addition to labels, proper fire extinguisher signs should be erected, and in fact, are a safety requirement by OSHA. According to OSHA regulation 1910.157(c)(1) “The employer shall provide portable fire extinguishers and shall mount, locate and identify them so that they are readily accessible to employees without subjecting the employees to possible injury.”] Signs help people “locate” and “identify” fire extinguishers quicker in times of emergency.
Previously, extinguishers were marked with colored geometric symbols (see table below), and is still common to see some extinguishers feature both symbols.
|SYMBOLS & COLORS FOR EXTINGUISHER CLASSES BASED ON TYPE OF FIRE FUELS
||INTENDED FIRE EXTINGUISHER PURPOSE
||TYPE OF FIRE EXTINGUISHING AGENT(s) REQUIRED
||Class A Extinguishers – For ordinary combustibles like wood, cloth, plastic, paper, rubber etc.
||Water, Foam, Dry Chemical
||Class B Extinguishers – For fires due to flammable liquids like oil, gasoline, oil-based paints, petrol etc.
||Foam, Dry Chemical, Carbon Dioxide
||Class C Extinguishers – For fires generating from equipment or appliances connected to electricity.
||Dry Chemical, Carbon Dioxide
||Class D Extinguishers – For flammable metal. Needs special extinguishing agents. Found typically in factories.
||Class K Extinguishers – For combustible cooking oils like vegetable oils, fats, animal oils & more. In general meant for commercial kitchens.
||Foam, Carbon Dioxide
The most common types of fire extinguishers are classified by agent:
Water: The most common and widely used agent is water. Water based extinguishers are ideal for schools, small offices, stockrooms etc. Due to their electric conductivity, however, these extinguishers are not suited for B and C fires. Moreover, they may freeze in cold climates. Overall, they are an inexpensive choice and non-toxic.
A Water Based Fire Extinguisher
Dry Chemical: The standard dry chemical extinguisher is best suited for Class B and C fires. They function by interrupting the flow of chemical reactions, and are non-toxic. The sodium bicarbonate-based dry chemical is more effective than water on Class B (flammable liquid) and Class C (electrical) fires.
A Dry Chemical Based Fire Extinguisher
Foam: Foam creates a separating layer between the flame and the fuel source – ideal for use on flammable liquids. Types of foam agents include Aqueous Film-Forming Foam (AFFF), Alcohol-Resistant Concentrates (ARC), Fluoroprotein Foam Concentrates, Protein Foam Concentrates and High Expansion Foam Concentrates.
Foam Based Fire Extinguisher
Carbon Dioxide: Effective in reducing oxygen down to less than 15%. Plus, these extinguishers leave no residue because it dissipates rapidly, thereby eliminating any messy clean-up. Bear in mind, however, it is toxic. Suitable for food storage, processing plants, labs, industrial plants, etc.
An example of a Co2 Fire Extinguisher