Monoammonium phosphate, ABC Dry Chemical, ABC Powder, tri-class, or multi-purpose dry chemical is a dry chemical extinguishing agent used on class A, class B, and class C fires. 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 component. 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. Uses Common use Dry chemical powder is used on all classes of fires. Dry chemical powder puts out the fire by coating the burning material with a thin layer of dust, thereby separating the fuel from the oxygen in the air. The powder also works to interrupt the chemical reaction of fire, so these extinguishers are extremely effective at putting out the fire. Pressure is generated by gas cartridges stored inside the cylinder. Its force will last for 45 seconds and can reach . Common combustible solids It insulates Class A fires by melting at approximately . Class A fires are caused by the burning of common combustible materials, such as wood, paper, or most plastics. Burning liquids and gases The powder breaks the chain reaction of liquid and gas fires by coating the surface to which it is applied. These fires (Class B in the American system; Classes B and C in the European and Australian systems) include the burning of gasoline, oil, propane, and natural gas. Electrical fires It is safe and effective for electrical fires since it is a non-conductor of electricity. Fires involving live electrical equipment (Class C in the American system; Class E in the Australian system) need to be put out with equipment that will not conduct its energy back to the user as is the case with water. Electricity can also cause fires of the other classes. Chimney bombs Chimney bombs are zip-lock bags or other small bags filled with ABC dry chemical powder. Chimney bombs are used by fire service personnel to help extinguish chimney fires. Creosote, which is the by-product of the incomplete burning of wood (typically due to chronic combustion-air insufficiency), is extinguished by the chain reaction caused by the chimney bombs. Chimney bombs work by first being dropped directly down a chimney, where upon contact with the flue bottom and heat of the fire, they