The Biggest Misconception in Electrical Fires
One of the most common shortcuts in fire reports is to write off the cause of a fire as simply an 'electrical fault' the moment a burned or melted wire is spotted in the debris. Yet electrical wiring can short-circuit when the flames melt its insulation (the plastic sheathing) and the conductors touch one another, even if the fire started for an entirely different reason (such as a knocked-over candle or sabotage). One of fire forensics' most critical tasks is to scientifically determine whether this arc occurred before the fire (its cause) or during the fire (its effect).
Types of Arcing: Primary Arc vs. Secondary Arc
The electrical melting marks found on copper or aluminum conductors fall into two main categories, depending on when they occurred:
A short circuit that occurs before the fire has begun, under normal room temperature and normal atmospheric conditions. This initial spark — caused by a wiring fault, overload, or cable wear — ignites the surrounding combustible materials and starts the fire.
A short circuit that occurs when a fire that has already started and grown for another reason melts the plastic insulating sheath outside the wires, causing the conductors to touch one another. This arc is a direct consequence of the fire.
How Is the Scientific Distinction Made in the Laboratory?
Melted wire ends (beads) collected during field work are subjected to microscopic (SEM — Scanning Electron Microscope) and metallographic analysis in metallurgy laboratories. The distinction rests on the following scientific evidence:
During a short circuit, copper reaches thousands of degrees within milliseconds and melts. If the arc occurred before the fire (in a normal room), the molten copper cools rapidly, trapping oxygen from the air and forming smooth, microscopic gas voids within the bead.
If the arc occurred during the fire (secondary), the surrounding environment already contains dense smoke, carbon monoxide, and high heat. In this chaotic environment, the resulting bead's crystalline structure shows heavy carbon infiltration and a thick copper-oxide layer. This proves the arc occurred in an oxygen-depleted, smoky, hot environment.
The Legal and Financial Importance of the Analysis
This distinction has a revolutionary importance in determining who is at fault in judicial cases and in insurance companies' damage-compensation payments.
If the analysis reveals the arc was 'secondary' (an effect), the property owner's or tenant's defense of 'the fire started from electrical fault, it's not my fault' collapses, and the search for the fire's true, hidden cause (negligence, sabotage, etc.) continues.