Electrical systems are present in nearly every room of every building, which makes them both a frequent ignition source and a frequent source of error in fire investigations. Wiring damaged by fire can look like wiring that caused the fire, and a conclusion that "it was electrical" is sometimes reached simply because wiring was found in the area of origin. The 2022 Utah Chapter IAAI conference focused on this subject, with sessions on how electricity can serve as an ignition source, including concepts of voltage, a morning of electrical theory, and electrical effects demonstrations at the Wendover Airport followed by a review of electrical effects from the burn cells. This article summarizes the core concepts.
Electricity Basics Every Investigator Needs
Four quantities describe most of what matters:
- Voltage (V) is the electrical "pressure" that drives current through a circuit. Typical U.S. residential branch circuits operate at 120 volts, with 240 volts for larger appliances.
- Current (I), measured in amperes, is the flow of electric charge.
- Resistance (R), measured in ohms, is opposition to current flow.
- Power (P), measured in watts, is the rate at which energy is delivered or converted, often into heat.
Ohm's law (V = I × R) links these values, and the heat produced in a conductor or connection is proportional to the square of the current multiplied by the resistance (P = I² × R). That relationship explains why both excess current and localized high resistance can create dangerous heating.
How Electricity Becomes an Ignition Source
Resistance heating at connections
Poor connections at terminals, splices, receptacles and switches can develop high resistance. Even at normal current, heat concentrated at a small point can be enough to ignite nearby combustibles over time. Loose screws, backstabbed receptacle connections, corrosion and incompatible materials are classic contributors.
Overcurrent and overload
When more current flows than a conductor is designed to carry, the entire conductor heats. Properly sized circuit breakers and fuses are designed to interrupt overcurrent, so investigators must evaluate whether protection was present, correctly sized and functional. Overloaded extension cords and undersized cords feeding heaters are common scenarios.
Arcing
An arc is a high-temperature luminous discharge across a gap. Parallel arcing occurs between conductors of opposite potential, such as line-to-neutral or line-to-ground, often after insulation fails. Series arcing occurs across a break in a single conductor, such as a partially severed cord. Arcs can reach very high temperatures, but brief arcs may not ignite surrounding materials; whether ignition is possible depends on the energy, duration and nearby fuel.
Arcing through char
When insulation is charred, whether by an external fire or by overheating, the char can become conductive and allow current to flow between conductors. This is common during fires and is one of the reasons arc sites are useful for mapping fire spread.
Appliances and equipment
Heat-producing appliances such as space heaters, cooking equipment and lighting can ignite combustibles through normal operation if materials are too close, or through failure of controls and safety devices. Lithium-ion batteries in tools, phones, bikes and vehicles add another category, including thermal runaway.
What Electrical Evidence Can Tell You
Arc sites and arc mapping
If a circuit is energized when fire attacks it, insulation fails and arcing often occurs at the point first attacked. Because an arc typically causes the circuit protection to trip or severs the conductor, downstream portions of the circuit may be de-energized. By systematically locating arc sites along each circuit and recording them on a diagram, a technique known as arc mapping, the investigator can help identify areas where fire attacked energized conductors earliest. NFPA 921 describes arc mapping as one of the tools used to determine origin, together with witness information, fire patterns and fire dynamics.
Arcing versus fire melting
Fire can also melt copper and aluminum conductors without any electrical activity, and alloying can occur when molten aluminum or other metals contact copper. Distinguishing arc damage from fire melting requires careful examination, and visual appearance alone can be misleading. When the distinction matters to the conclusion, consider laboratory examination by a qualified expert. Note that the 2024 edition of NFPA 921 relocated its discussion of fire effects on electrical systems to Chapter 6.
Circuit protection devices
Breaker positions after a fire can be affected by fire damage and by firefighters, so their meaning must be interpreted carefully and documented before anything is operated.
Avoiding Common Mistakes
- Presence is not proof. Finding wiring or an appliance in the area of origin does not make it the cause. The hypothesis must be tested against all data, as described in NFPA 921 and the scientific method.
- Consider the first fuel. An electrical ignition hypothesis must explain what ignited, how, and why the fire developed as observed.
- Do not rely on myths. Visual indicators that were once widely taught have been re-examined by research; stay current through training and the literature.
- Know your limits. Complex cases may require an electrical engineer, especially where product failure is alleged.
Safety First
Before examining electrical systems, confirm that power has been disconnected at the service and verify de-energization. Remember alternative sources such as generators, solar arrays and battery storage. Our fire scene safety guide covers these hazards in more detail.
Documenting and Collecting Electrical Evidence
Photograph the service panel, breaker positions, branch circuit routing and each arc site before disturbing anything. Label conductors to show which circuit and which end they came from, and diagram their routing. When products or equipment may be the subject of civil claims, consider notifying interested parties and arranging a joint examination before removal or destructive testing. See evidence collection and preservation and fire scene photography.
Vehicles and Emerging Technologies
Electric and hybrid vehicles, home battery systems and other lithium-ion technologies are changing electrical fire investigation. The 2024 edition of NFPA 921 expanded its guidance on hybrid and electric vehicle fires. Our article on vehicle fire investigation discusses high-voltage safety at vehicle scenes.
Continue Learning
Electrical fire investigation rewards continuous study and hands-on practice. Watch the Training page for future conference topics, read about the Utah IAAI annual conference, and explore the online electrical modules listed on our Links page.
This article is general educational information and does not replace formal training, NFPA 921, or the advice of a qualified electrical engineer.