DESCRIPTION OF TESTS
The description of tests contained within this user guide are instructions for the use of the DET family of
instruments by competent persons.
Suitable applications
Where there is doubt about a particular application, reference should be made to the advice and guidance
contained in the publication, “Getting Down to Earth” which is available from Megger, part number 21500-
072.
The DET range of instruments can be used in agricultural locations (as per IEC 61557-5). In these
circumstances, the output voltage should be switched to 25 V in order to comply with the standard.
The DET range of instruments makes resistance measurements using a switched DC signal at a frequency
of 128 Hz. The DET4TC2 and DET4TCR2 provide the user with the option to select 94 Hz,
105 Hz, 111 Hz or 128 Hz as the test frequency.
Note about measurements: when connecting the instrument to electrodes, ensure that any leads and
cables are fully unwound and laid out without loops.
Principle of operation (two-terminal resistance measurement)
This technique is used to measure the resistance between two earth points, e.g. between an earth
electrode of unknown resistance and a known “good” ground connection such as metallic underground
pipework or building steelwork.
The DET injects an a.c. current of known magnitude into the system under test and measures the voltage
developed across it as shown in Figure 3. The system resistance is a simple ratio as per Ohm’s Law, i.e.
R=V/I.
Figure 3: schematic for two-terminal resistance measurement
The DET instruments automatically link the C1-P1 and C2-P2 terminals when a two-terminal test is
selected.
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