KYORITSU 3123A - Manual (Page 9)

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4-5. Use of Guard Terminal
When measuring the insulation resistance of a cable, Ieakage
current flowing on the surface of cable jacket and the current
flowing inside the insulator are mixed and may cause error in
insulation resistance value. In order to prevent such error, wind a
conductive wire around the point where leakage current flows. Then
connect it to the Guard terminal as shown in below Fig.2. This is
to move out the surface leakage resistance of the cable insulation
to measure only the volume resistance of insulator. Make sure to
use the Guard cord supplied with this instrument to connect the
instrument to Guard terminal.
Fig.2