Dry Chemical/ABC Extinguishers

Dry Chemical extinguishers are qualified to be used on A type, B type, and C types of fires.

Class A of fires are known as ordinary combustibles, such as cloth, rubber, trash, wood, plastics, & paper. It uses an green background triangle, with an A. A trash can and stack of wood on fire is used as the symbol.

Class B of fires are combustible liquids, flammable liquids, petroleum greases, tar, oils, oil-based paints, solvents, lacquers, alcohol, and flammable gas, (vegetable/animal oils, and fat is K Class). It uses a red background square, with a B. A puddle on fire, with a gas can is used as the symbol.

Class C of fires are energized electrical equipment. It uses a blue background circle, with a C. A plug and outlet on fire is used as the symbol.

The size, and nozzle are how ABC extinguishers are rated. To achieve a specific rating, NFPA 10, Standard for Portable Fire Extinguishers 2010 edition 4.1.1(1) requires the extinguisher to pass a series of tests set forth by (United States) ANSI/UL 711 Standard for Rating and Fire Testing of Fire Extinguishers, and (Canada) CAN/ULC-S508 Standard for Rating and Testing of Fire Extinguishers. Below is a table that gives a bit more information about the possible classifications for each size, and ranges.

As you can see, the capacity is directly tied to the classification. Within each cylinder, there can only be a certain amount of powder, to allow for the space of nitrogen. Most Multipurpose/ABC/Dry Chemical/Dry Powder extinguishers are pressurized to around 195 psi. The Gauges have a green triangle around the desired 195 psi pressure that allows for a small deviation of pressure. This deviation is to allow for the pressure to be between roughly 170-220 psi, due to temperature changes. The temperature range permitted for normal extinguishers is between -65 to 120 degrees Fahrenheit. When pressurized, it is required to be pressurized using Nitrogen with a dew point of -60°F or -51°C. This lessens the chance of humidity to cause the powder to clump, or the cylinder to corrode.

For example the range of weight on a 5 lb extinguisher from Ansul (Sentry) is between 9 lbs. 5 ounces, and 10 lbs 1 ounce, for the fully assembled cylinder, and pressurized with powder. The weight of 5 lbs is the amount of powder within the cylinder. This range allows for the variation of either a bit more/less of powder, and a bit more/less of nitrogen. As long as they are within the specifications, they are permitted to maintain their classification, and will be able to put out the specified range of fires. (this 5 lb Ansul is a 3-A:40:B-C, putting it on the second tier of the above table)