Earthing, Protective Bonding and Fault Loop Impedance
Earthing questions combine three ideas: protective earthing (exposed conductive parts connected to the earthing system so fault current trips protection), equipotential bonding (main and supplementary bonding to remove dangerous potential differences), and fault loop impedance (the total impedance path that limits fault current and hence disconnection time).
Understand the supply arrangements - MEN (multiple earthed neutral) is standard in Australia, with the neutral earthed at the transformer and at each installation, and the protective earth and neutral separated downstream of the main switchboard. In MEN, the main earthing conductor, the neutral bar link and the earth electrode work together; removing or misplacing the MEN link is a serious defect. Supplementary bonding is required where potential differences could exist, such as bathrooms, metallic pipework and equipotential zones.
Traps: confusing main earthing conductor sizing with bonding conductor sizing; thinking an earth electrode provides a low-impedance fault path on its own (it does not - the MEN return path does); forgetting that increasing conductor length or adding a high-resistance connection raises Zs and lengthens disconnection time; and assuming RCDs remove the need for earthing (they do not).
Worked example: If the measured fault loop impedance Zs at a socket is 1.2 Ω and the protective device requires disconnection within 0.4 s, check the device's maximum permitted Zs from the standard's tables. If the table limit is 1.0 Ω, the circuit fails and you must improve the earth return path (larger CPC, shorter run, better connections) or change protection.