Question

what is significance of black wire in domestic electric circuit​

Answer 1

The purpose of the green wire is to provide a path to ground for a circuit's electric current if a live wire within the circuit happens to touch metal or some other conductive material. In the event of a fault, it could carry significant current, so treat green wires cautiously.

Answer 2

Black electrical wire is used for power in all circuits. Any circuit's black wire should be considered hot or live. Black wire is never used for a ground or neutral wire and should be used as the power feed for a switch or an outlet. A black wire is often used in a circuit as a switch leg, the connection that runs from the switch to the electrical load.