Arcing

Arcing results in high temperatures, sometimes in the range of several thousand degrees, easily sufficient to ignite a flammable mixture under the right conditions.

A minimum of 350 volts is required for an arc to jump even the smallest gap [1]. Although most vehicles have 12  or 24 volt electrical systems, which would be insufficient to cause an arc to jump across a gap, ignition can occur from arcing in high energy electrical components such as the secondary ignition wires (spark plug wires), ignition wire connectors, coil(s), or distributor.

A parting arc is a brief electrical discharge that can occur when the current in an energized circuit is interrupted by the separation of conductors. They can occur at lower voltages, including in a 12 volt automobile electrical system. Parting arcs in vehicles can occur from:

• Wiring that is severed or disconnected, such as during a collision
• Damaged insulation on wiring (worn, cracked, burned or missing), resulting in unintended electrical contact and separation
• Switch contacts as the switch is turned on or off
• Brushes and commutators in electric motors
• Actuation of electromechanical control devices such as relays and solenoids

Parting arcs also occur when short circuited wires melt through, resulting in a brief arc as the circuit is broken. Under some circumstances, small particles of hot, molten metal (electrical sparks) can also be emitted from a short circuit when a parting arc occurs.

 Option: For more information about the potential for ignition by electric motor sparks, click here.

References

1. NFPA 921, Guide for Fire and Explosion Investigations, 2004 Edition.