Question

Earthing is done only in the outer part of the electric appliances. What is the reason?

Answer 1

Because the user is warned not to disassemble the appliance while power is applied.

Explanation:

Earthing is done in the outer part of the electric appliances to keep the user safe. Provided that the user does not try to disassemble the appliance while electrical power is applied, that is sufficient safety practice.

I hope this helps,
Steve

Answer 2

Earthing (also known as "Grounding") is done to the outer part of electrical appliances because it protects the user from electric shock. The engineers and scientists who design these appliances must make decisions that are a compromise between manufacturing cost and safety. Their research and testing have shown that more grounding (for example, some form internal ground shield) does not make the appliance any safer under normal use and greatly increases manufacturing cost.