KYORITSU 3128 - Manual (Page 73)

KEW3128
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KEW3128 6. Measurement
6. 11 Other Functions
6.11.1 Use of Guard Terminal
When measuring insulation resistances of a cable, leakage currents flowing
on the surface of cable jacket and the currents flowing inside the insulator
are mixed and may cause error in readings. In order to prevent such error,
wind a conductive wire around the point where leakage currents flow.
Then connect it to the Guard terminal as shown in the below figure.
This is to move out the surface leakage resistance of the cable insulation
to measure only the volume resistance of insulator. Use the Guard cord
supplied with this instrument to connect the instrument and the Guard
terminal.
G Terminal Earthing Procedure
The G Terminal Earth System is a measurement method using a Guard
Terminal which is appropriate to measure the whole electrical paths
including high-voltage cable with the other high-voltage devices.Connect
the Guard Terminal to the Earth Electrode of the measured object and the
shielded wire of the cable to the Earth Terminal. In this case, disconnect the
shielded wire of the cable from the Earth Electrode.
To use this measurement method, the insulation resistance of sheath
(between the shielded wire and the ground) should be 1MΩ or more.