ABC power is monommonium phosphate and/or ammonium sulphate. As well as suppressing the flame in the air, it also melts at a low temperature to form a layers of slag which excludes the gas and heat transfer at the fuel surface.
For this reason it can also effective against class A fires. ABC power is usually the best agent for fires involving multiple classes.
This type of power can also be used on electrical fires, but provide a significant cleanup and corrosion problem that is likely to make the electrical equipment unsalvageable.