Earthing means connecting any non-current carrying conductor part of an electrical system with general mass of earth in such a manner that there is an immediate discharge of electrical energy to the earth in the event of electrical potential developed at that part of the system. For example, metallic frame work of electrical appliances, metallic covering of electrical cables, the earth terminal of three pin socket outlets, stay wires and also neutral point of single phase and three phase supply systems must be properly earthed.
Earthing is done to ensure that no current carrying part of the system rises to be potential beyond its normal value, no non-current carrying conducting part of a system rises to a potential beyond earth potential that is zero. Proper earthing also helps to avoid electrical shock to the human beings also to avoid the chance of fire hazard due to leakage current through unwanted path.
Properly designed, constructed and installed electrical equipments and appliances should not have any of the non-current carrying conducting parts which is in contact with any current carrying part. But accidentally may be due to failure of insulation between current carrying and non-current carrying conducting parts of the equipment/appliance, if any of the non-current carrying conducting parts comes in contact with any of the current carrying parts of the equipment/appliance, there will be a static electrical charge developed in the non-current carrying conducting part.
Now if any human being touches that non-current carrying conducting part of the equipment or appliance, the accumulated static charge will get a path to the earth through his body and hence it is discharged immediately, as a result he gets an electrical shock. But if the non-current carrying conducting parts that are metallic frameworks parts of the equipment or appliance properly earthed, then at the occurrence of touching between any current carrying part or live part to the non-current carrying part of the equipment/appliance, the live part of the equipment/appliance gets low impedance path to the earth through the properly earthed metallic frameworks and hence there will be a huge current drawn from source, passing to the earth through this path. As a result the circuit breaker or MCB, or fuses associated with this equipment/appliance will immediately break to discontinue the supply to the equipment/appliance. Thus proper earthing of non-current carrying metallic parts of electrical equipments and appliances provides safety of operation.