Inflammable material carrying vehicles

[phymathlover]

Special rubber tires are made slightly conducting, in aircrafts, to enable them to pass on charge (produced by friction) to the ground, as too much charge may result in a spark and hence cause a fire. This is apparently the same reason why some vehicles usually have metallic ropes touching the ground during motion especially when they carry inflammable materials. But I don't get how exactly the charge causes the spark.

 

[UltrafastPED]

Sparks are what you see when there is an air breakdown due to the strength of the electrostatic field (volts/meter) between two objects.

Providing a conductive path (drag chains, conductive tires, etc) allows the electrostatic charges to be drained ... thus the vehicle should not build up a static charge strong enough to spark. This makes it safer when refueling.