Vehicles carrying inflammable materials usually have metallic ropes touching the ground during motion. Why ? 

Ans: When vehicles move through air, they get charged due to friction. Charge will be developed on tyres also. If the change is sufficient, they will produce spark. The vapour which escapes from the inflammable materials carried by the vehicle may catch fire. To prevent this metallic ropes touching the ground are suspended. Through this conducting metal the charge flows to the earth.