3.2 Measuring Insulation Resistance
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3.2.7 Use of GUARD Terminal
A GUARD terminal is used to prevent the
surface electrical resistance of an insulating
material affecting measurement, enabling
correct measurement of the entire volume of
the material.
When testing the insulation of a cable, as
shown in the diagram above, wind a bare
conductor around the surface of the insulat-
ing material and connect the conductor to
the GUARD terminal. This prevents the
leakage current on the surface of the insu-
lating material flowing into the current
detector, which enables the actual resis-
tance of the entire volume of the insulating
material to be measured.
Measurement unaffected by surface
electrical resistance
Metallic shielding layer
Bare conductor
Insulating material
Cable
Core
Current
detector
Power supply
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