The earth wire (green/yellow wire) = 0Volts = there for safety. In the event of a fault, the live current passing through the case will follow this pass to the ground instead of to electrocuting the person. If the earth wire was faulty and the live wire becomes loose, there is a danger that the live wire will touch the case.
The loose live wire will have made the case 230 V, a person touching the case is also touching the ground and is, therefore, at 0 V. The large potential difference between the hand, on the case, and the feet, on the ground, will push a current through the person's body and shock them.
SO YOU MUST HAVE THE EARTH WIRE CONNECTED PROPERLY.
