Appendix A. Utilizing The Guard Terminal - AEMC 5070 User Manual

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APPENDIX A. UTILIZING THE GUARD TERMINAL

Guard terminals are useful when measuring high resistance values and for
stabilizing readings.
Surface leakage is fundamentally a low resistance path in parallel with
the most important resistance path through the body of the insulation. 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 insulation.
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,
particularly 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.
Figure A-1 - Shielded Cable
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
surface. (Note: If a shield is not available and you do not make up a shield
around "B" and connect to the guard terminal (-), the measurement will be
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