ABC or multi-purpose dry chemical is a dry chemical extinguishing agent. It uses a specially fluidized and siliconized monoammonium phosphate powder. ABC dry chemical is usually a mix of monoammonium phosphate and ammonium sulfate, the former being the active one. The mix between the two agents is usually 40–60%, 60-40%, or 90-10% depending on local standards worldwide. The USGS uses a similar mixture, called Phos Chek G75F. [1]
It insulates Class-A fires by melting at approximately 350–400 degrees F. The powder breaks the chain reaction of Class-B fires by coating the surface to which it is applied. It is safe and effective for Class-C fires since it is a non-conductor of electricity.
Sizes of these handheld fire extinguishers range in US customary sizes from 2½ lb., 5 lb., 6 lb., 10 lb., 20 lb. and "Fast Flow" and cartridge operated 30 lb. extinguishers. Wheeled extinguishers used in high hazard areas range from 150 lb. – 350 lb. For stationary handline systems the capacities range from 500 lb. – 3000 lb. units. In EN 3 European standards, the range is 1 kg to 12 kg; wheeled models containing 30, 50, 75 and 100 kg are also used.
ABC dry chemical is inappropriate for chlorine or oxidizer fires.[2] The resulting chemical reaction can cause an explosion or a breakdown of the chemicals releasing toxic gases. Water should be used.[3][4] ABC dry chemical is inappropriate for certain metal fires (Class-D) as well as cooking oil fires (Class-K). Due to the corrosive properties of ABC Dry chemical, it is not recommended for use around aircraft or sensitive equipment.