IET Labs, Inc. 1864-1644, User And Service Manual

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1864-1644 Positive Polarity Megohmmeter
APPLICATIONS
13
Chapter 4
APPLICATIONS
4.1 Insulation Testing
The insulation resistance of electrical machinery,
transducers, etc, is one of several parameters that
may indicate the condition of the insulation. Routine
measurement of capacitance, dissipation factor, and
leakage resistance provides useful data for monitor-
ing the condition of the insulation and for guarding
against incipient breakdown.
A routine test that has been widely adopted for
insulation testing calls for the measurement of the
apparent leakage resistance after a test voltage has
been applied for one minute and again after the test
voltage has been applied for 10 minutes. The ratio
of the indicated resistances, sometimes referred to
as the Polarization Index, can have some relation to
the condition of the Insulation. The results of such
a measurement are apt to be more dependent on the
dielectric absorption of the insulator than on its true
leakage resistance measured at equilibrium. A com-
plete charge-current-vs-time plot will provide more
useful information.
The 1864-1644 Megohmmeters can be used for ei-
ther true leakage measurements or for measurements
at 1-or 10-minute intervals following the operating
procedure described in Section 3.
MIL-STD-202C gives procedures for insulation-
resistance measurements of various components.
On large machinery, one terminal must usually be
grounded. The 1864-1644 Megohmmeter is designed
so that the binding post grounding strap should be
connected between the ground terminal and the
-UNKNOWN terminal.
To determine the charge current, divide the test
voltage by the indicated resistance. At the start of
a charge-current-vs-time plot, the meter will be off
scale. The resistance in series with the insulator is
the reading of the upper dial multiplier divided by
500. Table 4-1 lists dial readings and resistor values.
2G