The earth tester generates an a.c. signal which is fed into the system under test. The instrument then checks the status of the circuits for good connection and noise. If either of these variables is out of specification then the operator is informed.
Having checked that the conditions for test are met, the instrument automatically steps through its measurement ranges to find the optimum signal to apply.
Measuring the current flowing and the voltage generated the instrument calculates and displays the system resistance in the range of 0.001 to 20,000 ohms.
There are three basic test methods as noted below.
Fall-of-potential method, or three-terminal test.
Dead Earth method (two-point test).
Clamp-on test method