How Good Should the Earth Be?
This is a difficult question to answer; in general the impedance of the earth connection must be low enough to ensure that sufficient current can flow through the protective device so that it disconnects the supply quickly (< 0.4 second) and that voltage on the earth connection does not rise more than 50V. RCDs operate at much lower fault currents than fuses and so can provide much better protection against shock. RCDs are therefore recommended whatever the method of earthing but where the electricity company cannot supply an effective earth and a local earth electrode is used, an RCD (30 mA trip) must be used. Measuring the resistance of an earth electrode is not easy and is really outside the scope of most d-i-y'ers.
