M4000 Insulation Analyzer User Guide
72A-1230 Rev. F 4-43
July 22, 2005
Continuity The center conductor and shield should each have less than 1-ohm resistance.
The center conductor’s resistance is measured between the clip and Pin 3 of
the inboard connector. In order to check the shield, penetrate the outer
insulation of the lead (approximately 1-2 inches from the outboard clip) using
a thin/sharp pointed electrode (pointed ohmmeter probes are suitable for this
purpose). The shield resistance is measured between Pins 1 or 2 of the inboard
connector and the ohmmeter probe penetrating the insulation.
If either conductor’s resistance is greater than 1 ohm, replace that Low Voltage
Test Lead.
DC Insulation
Resistance
The DC insulation resistance of a lead, measured with a low-voltage
instrument, between the center conductor and shield should be 100 megohms
or greater. This measurement is made by connecting one probe of the DC
instrument to the clip at the lead’s outboard end, and the other probe
connected to the shield (Pins 1 and 2 of the lead’s inboard connector).
If abnormally low insulation resistance is observed, the condition of the
insulation between the center conductor and shield at the clip end of the lead
should be checked.
If any insulation resistance is less than 100 megohms, replace that Low
Voltage Test Lead.
Checking M4100 Guard-to-Ground Insulation
The DC insulation resistance between the Guard and Ground circuits of the
M4100 should be in the range 2.5-4 megohms. The measurement is made
between the blue and silver terminal posts on the front panel, with the M4100
turned off and the AC power cord removed. A low-resistance path between
these points is a frequent cause of large negative values of GST watts,
especially when running the Field Recalibration Test diagnostic.
If a short or low resistance is detected between these points on the front panel,
perform the internal checks described in ”Guard-to-Ground Shorting
Problems” on page 4-51.
NOTE The short circuit may be intermittent, so it may be necessary to move or
shake the instrument in order to stimulate the short.
Checking Instrument Accuracy in the Field
In lieu of elaborate laboratory checks, it has been found that the Doble Liquid
Insulation Cell, properly cleaned and dried, is a reliable standard that can be
used in the field for a quick, simple accuracy check.
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