Hi Russel Bull,
Many thanks for your reply. That was very informative.

So that answers, why many of the instruments provide a signal ground/earth terminal along with the signal wires for RS422/RS485.
Some instruments such as temperature controllers in an injection moulding machine, do NOT give the Earth Terminal at all. Maybe because they are always supposed to be mounted inside the control panel and there is no risk of lightening; but there are other high voltage/high current carrying cables which might effect the RS485 cables. In this case the routing of the RS485 cable inside the control panel is very crutial; and insist on using a shielded cable for such applications. Am I correct ?

Your post also informs that, if I am able to keep the voltages on the RS485 within common-mode range, then it is just fine.
For RS485 implementations where lightening (and places where high current and high voltage carrying cables are running close to the RS485 Wires), I must insist on using shielded cable.

Thanks for the information Russell Bull.
From,
Kiran V. Sutar.