Mechanical sparks are another ignition source in vehicle fires. There is a limited amount of information to help the investigator determine the likelihood of mechanical sparks as an ignition source in specific cases.

The following definition for a spark was given in NFPA 921:

•  1.3.113 Spark: A small, incandescent particle.

 Such sparks can occur through the following means:

• Collision (metal to metal or metal on pavement).

• Dragging metal object on pavement.

Test Results

Drag tests of metals on concrete have shown the following characteristics for mechanical spark ignition [1-3]:

Steels

• Consistent ignition of gasoline when drag speeds were 10 mph and contact pressures were 20 psi and higher.

• Inconsistent ignition of kerosene, though it becomes more consistent at drag speeds of 20 mph and contact pressures of 50 psi.

Aluminum

• No ignition of gasoline or kerosene at drag speeds of up to 45 mph and contact pressures of greater than 1455 psi.

Pavement factors

• There was a lower ignition potential on asphalt than concrete.

When mechanical sparking occurs, minimum ignition energy of the fuel must be achieved for the spark to be considered a valid ignition source.

In general, mechanical sparks are rather poor ignition sources because of their small size in relation to their temperature, which gives them little energy. Titanium and magnesium have higher energy sparks than steel or aluminum and have a higher propensity for ignition of a fire. However, both titanium and magnesium are rarely used in any quantity on modern road vehicles. A dragging metal component can heat to high temperatures which also raises the potential for autoignition of flammables.

References

1. Mowrer, F.W., "Heavy Truck Fuel System Safety Study: Final Report," DOT HS 807 484, 1989.

2. Johnson, N., “Spilled Fuel Ignition Sources and Countermeasures," DOT HS S01 727, 1975.

3. Arndt, S., "Motor Vehicle in the Post-Crash Environment, An Understanding of Ignition Properties of Spilled Fuels," SAE 1999-01-0086, 1999.