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• Guard terminals are useful when measuring high resistance values and for sta-
bilizing readings.
• Surface leakage is fundamentally a low resistance path in parallel with the most
important resistance path through the body of the insulation. The guard termi-
nal can be used to stop the effects of the surface leakage from influencing the
measurement of resistance through the insulation. In the example of a cable,
shown in the diagram below, connecting the guard terminal of the Model 5070 to
the surface of the insulation will redirect surface leakage currents away from the
true measured value of leakage current from the conductor through the insula-
tion.
• The use of the guard terminal is most important when there is a significant
exposed surface in a cable. When testing the insulation at the end of a cable,
it is necessary to eliminate the error from surface leakage which occurs, par-
ticularly at high resistance values. The guard terminal provides a third terminal
within the path of the surface leakage. Connect the instrument as shown in
Figure A-1.
A
Conductor
Line (-)
Terminal
• Connect the black (-) test lead to the conductor at A
• Connect the red (+) test lead to the shield layer of the cable at C
• Connect the blue Guard (G) lead to the insulation layer of the cable at B
If there is no shield, use a copper wire wound several times around the exposed
Megohmmeter Model 5070
APPENDIX A
UTILIZING THE GUARD TERMINAL
B
C
Exposed
Surface
Insulation
to
to
Guard
Earth (+)
Terminal
Terminal
Figure A-1 - Shielded Cable
Outer jacket
The guard terminal
is useful when
measuring very
high resistance
Shield
values.
to
87
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