A protective system for electrical apparatus mounted in a vault which has an opening to the atmosphere. Overheating of the electrical apparatus is distinguished from tobacco smoke, engine exhaust fumes, and other air borne particulates, by an ion chamber monitor and a thermally activated material in the vault which decomposes at a predetermined elevated temperature indicative of overheating. The material is selected to decompose above 300.degree. C. and produce a gas which decreases the self-recombination rate of the ion in the ion detector monitor to increase the ion current. An alarm is generated on a predetermined increase in ion current, instead of upon a decrease, such as caused by particulates.