Question

why the vehicles containing flammable material have long metallic chains touching the ground

Answer 1

static charges are built up in any object,this may cause a spark and
make the container burst.

metal is a conductor of electricity ,when they touch the ground they carries the charges safely to the ground.

Answer 2

Answer:

The vehicles containing flammable material have long metallic chains touching the ground because the body of the vehicle in motion is charged due to air friction.

If the charge accumulation is large, it can produce a spark and the inflammable material may catch fire.

To prevent this mis-happening metallic ropes are hung from the vehicle to touch the ground so that any charge accumulated on the vehicle leaks to the earth.

Explanation:

Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field.