Question

vehicle carrrying inflamable  material  usually have metalic rope touching the ground during motion .why?

Answer 1

The metallic body of trucks gets charged due friction between them and the air rushing past them
This charge being large can produce a spark (it may)
And this is dangerous
So they have metal chain dragging along the ground
The charges produced pass to the ground through the chain 
Even nowadays tyres contain  some carbon compounds in their rubber which makes the produced charges leak to the ground 

Answer 2

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

Tell why ?

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> This is done just to avoid the charging process occuring .

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During motion,

The body of the vehicle gets charged due to friction of air. The tyres also accumulate the charge due to friction between the tyres and the road.

Due to charge accumulation inflammable materials inside the vehicle may catch fire.

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So, to avoid this, the accumulated charge is made to flow from body to the ground through metallic ropes.