ABC 40 powder is a combination of Mono-ammonium phosphate and ammonium sulphate.
On Class A fires the particles fuse and swell to form a barrier which excludes oxygen and prevents re-ignition. On Class B fires the powder interferes with the chemical reactions, thus the flame ceases to exist.
The powder is nonconductive.
ABC 40 powder will extinguish Class A (carbonaceous) fires, Class B (flammable liquid) fires and Class C (flammable gas) fires.