Abstract: A method for testing whether an excessively high electrical current caused or was caused by a fire. A melt bead from an electrical conductor is selected from the region of a fire. Its relative oxygen concentration-depth profile is measured by chemical surface analysis, recorded and plotted. The oxygen concentration is low and its peak is below about 200 Angstroms when the fire causes the short because the melt bead was formed in a relatively oxygen poor ambient atmosphere. The oxygen concentration is higher and peaks between 200 and 2000 Angstroms when the short causes the fire because the melt bead was formed in a relatively oxygen rich ambient atmosphere. The oxygen concentration is still greater and remains high to a substantially greater depth of, for example, 20,000 to 40,000 Angstroms, when the conductor had been subjected to an overload current for a substantial time period.