Vanguard IRM-5000P - Manual (Page 57)

REV 2 IRM-5000P USER’S MANUAL
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APPENDIX B - GUARD TERMINAL APPLICATION
When testing an insulation system, there may be times where the measured leakage current is
not necessarily a function of a poor insulation system, but rather of contamination at the
connection point, which allows leakage currents to flow along the surface of the connection
point. These readings can cause the test operator to incorrectly assume that the insulation
system under test has failed and must be replaced.
To exclude these surface effects and get an accurate measurement of the insulation system
itself, it is often advisable to perform the Insulation Test with a guard terminal attached as
shown in the figure below.
Surface leakage currents, which may flow along the surface of the exposed wires, will flow from
the Positive HV Test lead to the Guard lead. These currents are not measured by the IRM-
5000P. The cable itself will also have some inherent leakage current that will flow from the
Positive HV lead, through the cable, and out the Negative HV lead. The current that will be
used for the Insulation Resistance measurements.
The guard terminal is at very nearly the same voltage potential as the Negative Test lead. For
the same reason, the guard connection should be positioned to avoid arcing from the Positive
Test lead. As shown in the illustration, by placing the guard near the Negative Test lead, the risk
of arching can be avoided.