Question

why do petrol road tankers usually have a length of metal chain hanging down to touch the ground

Answer 1

to avoid blast. ..Thats why chain touch the roaf

Answer 2

Answer:

Due to air friction, the body of the tankers also gets charged in addition to the charge developed on tires. In case, the charge accumulation becomes large, it may prove hazardous to the vehicle carrying inflammable materials.

So that it does not happen, metallic chains from the vehicles are made to touch the ground. Due to this, the charge on the tanker leaks to the earth.