If your device or a cable in the device is defect, it is possible that sparking will occur under some circumstances. In the case of 3-pole devices, the earth connection ensures that the current is safely led away and that there is no danger of an electric shock. The earth connection is leading, that means the plugs have been constructed so that the earth connection is made before the plug pins make a connection with the current source, in other words protection has been provided before electrical current can even flow.