Earthing is required in a circuit when
1. the appliance draws high current. High power devices draw large current.(Current =power/voltage).
2. the appliance has outer metallic case.
Why is echo not heard in our living rooms? To hear echo, the minimum distance between the source and the reflector must be 17m. When this distance is more than 17m, the reflected sound can be distinguished from the original. As the length of our rooms is less than 17m, we don't hear echo.
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