Urgent physics question?

In the secondary coil of the transformer, current flows from the upper wire to the middle (110 v) or lower wire (220 v) and vice versa (because it is an ac supply). This makes a complete circuit, which is essential for current to flow.

If the person touches ANY ONE of the wires in the secondary coil, current cannot return to either one of the other two wires. Hence, there is no complete circuit, and no current can flow.

In (bi), the LIVE wire of the PRIMARY COIL of the transformer carries a potential, and the other wire (the NEUTRAL wire) is earthed. Current thus flows from the LIVE wire to earth. This completes the circuit.

When the person touches the LIVE wire, current can flow from the LIVE wire through the person's body and then to earth (this is where the NEUTRAL wire is connected). This also completes a circuit, thus giving an electric shock to that person.

The protection given in (bii) is that none of the wires is earthed. Should any one of the wires is earthed, the person would surely get an electric shock when he touches one of the other two wires (except the earthed wire itself, of course).

The situation given in (bii) without earthing any wire is similar to the case of a bird staying on a high-tension (i.e. high voltage) power line. The bird will not get any electric shock because no part of its body touches the earth.