Resistance Heating

Heat is created by the flow of electric current in a conductive material. According to Ohm’s Law:

 Voltage=Current (measured in amperes) X Resistance (measured in ohms)

 And because

 Power=(Current)² X Resistance,

if there is too much current for the size of the wire and/or excessive resistance (such as from a poor connection), undesired heating can occur. This heating can cause ignition of nearby combustibles, including wiring insulation, vapors from flammable liquids, or other solids. Light bulb filaments, typically tungsten, are designed to emit light through resistance heating. Filaments operate at temperatures as high as 2550 degrees F. while contained in a sealed, evacuated glass bulb [1] . Should the glass break from a collision, the filament will oxidize and usually will fail in a short time, though exposure to flammable vapors during this time can cause ignition. Other automotive electrical components, including certain fan motor speed control devices, use resistors in the circuitry that may generate sufficient heat to ignite combustibles should they come into contact with each other.

 Automotive components most likely to cause ignition from resistance heating include:

• Batteries and battery cables
• Other high-current electrical cables, including those to the alternator and starter motor
• Any wiring that does not have proper current overload protection
• Broken light bulb filaments
• Resistor packs used for motor controls
• Resistance heating elements
• Poor connections of conductors, including loose and oxidized terminals
  (For more detail regarding loose connections, click here)
• Overloaded or stalled electric motors

References

1.    NFPA 921, Guide for Fire and Explosion Investigations, 2020 Edition.