ABC powder is a combination of monoammonium phosphate and ammonium sulphate. The extinguisher powder interferes with the chemical reactions that take place in the combustion zone, thus the flame ceases to exist.
The powder also has a low melting point and on application the particles fuse and swell to form a barrier which excludes oxygen and completes the extinguishing process and prevents re-ignition.
The powder is non conductive and may be used in the presence of al electric current. ABC powder will extinguish class A (carbonaceous) fires, class B (flammable liquids) fires and class C (flammable gas) fires.