"...I have seen more than one instance where a vehicle caught fire and burned up completely after a primary line fell on it. Usually the tires will track enough current to heat up and catch fire...."
I have heard this many times. I don't doubt the fires, but the mechanism is suspect. We do medium voltage testing (12.47/7.2 kV) and have had opportunity to test truck tires and small car tires. In each case we tied the 7.2 kV line to the metallic rims (either directly or indirectly through the car body). The conventional wisdom was that leakage current (particularly through steel-belted radials) would cause resistive heating, which would eventually melt the tires or set then on fire (or cause the air in the tire to expand to the point that it overpressured the tire and blew it out).
When we energized the line, with the tires on the ground, nothing happened. We let this situation persist for a long time. Afterwards, there was not even any apparent heat-up of the tires where they touched the ground.
If we had so much as a blade of grass that was tall enough to touch the undercarriage, however, it was a different story. The vegetation was conductive enough to conduct but the vegetation quickly burned up. The resulting smoke in the air reduced the impedance of the air path so that an arc was created directly between the metallic undercarriage and the ground. The process fed on itself, with the arcing causing more fire and the fire causing more smoke-inducing arcing. The tires eventually caught fire from the grass fire and/or from the intense heat of the arcing, and as a result, the tires finally melted/blew-out.
The end result is much the same (blown tires, truck sitting on rims, electrical and fire hazard) but the mechanism was interesting to observe.
Carl
