If a fault occurs, the external metal of an appliance will become live and the voltage with respect to earth will be anything up to 240 volts, depending on which part of the internal circuit touches the casing. If the metal isn't grounded (earthed) and someone touches the appliance, current will travel through their body to ground.
If they are lucky and have rubber-soled shoes and are standing on a dry floor, they may just experience a tingling sensation. However, in most cases, if conditions are damp, they have wet hands and standing outdoors, they are more likely to experience a severe shock. If one hand touches the appliance and the other touches a grounded object (e.g pipework, poles, radiators or whatever), current will travel across their heart, a more dangerous scenario. If the person is unlucky or has a heart condition, this can kill.
The resistance of the main earthing conductor, measured between the main earthing terminal/connection or bar and the earth electrode, including the connection to the earth electrode, shall be not more than 0.5 Ohm.
Refer to the Australian wiring rules: AS/NZS 3000 - Clause 5.5.1.4
Refer to the Australian wiring rules: AS/NZS 3000 - Clause 8.3.5.2 (b)
