Halons are very effective in extinguishment type “B”& “C” fires.
For type “A” fires, halons extinguish the residual flames, but only rarely causes complete extinction of gas to diffuse into the solids into deep seated “A” type fires. It is therefore seen that in the case of a burning solid, early detection is necessary during the period where there is a surface fire.
Some fuels are not extinguished by halons; these are:

Fuels whose molecules contains all or part of the oxygen required for their combustion;-such as nitrates, explosives.
Metals in general;-alkali metals, aluminum, heavy metals.
Certain very reactive chemicals or explosives, metal hydrides, peroxides.hydrazine etc.