MI 3205 TeraOhm XA 5kV
Measurements
32
5.2 Some application examples
5.2.1 Basic Insulation resistance test
Virtually every standard concerning the safety of electrical equipment and installations
requires the performance of a basic insulation testing. When testing lower values (in the
range of M
), the basic insulation resistance (Riso) usually dominates. The results are
adequate and stabilize quickly.
It is important to remember the following:
The voltage, time and limit are usually given in the appropriate standard or
regulation.
Measuring time should be set to 60 s or the minimum time required for the
Insulation capacitance Ciso to be charged up.
Sometimes it is required to take ambient temperature into account and adjust the
result for a standard temperature of 40
C.
If surface leakage currents interfere with the measurements (see Riss above) use
the guard connection. This becomes critical when measured values are in the G
range.
5.2.2 Voltage dependence test – Step Voltage Test
This test shows if the insulation under test has been electrically or mechanically
stressed. In this instance the quantity and size of insulation anomalies (e.g. cracks, local
breakdowns, conductive parts, etc). is increased and the overall breakdown voltage is
reduced. Excessive humidity and pollution have an important role especially in the case
of mechanical stress.
The test voltage steps are usually close to those required in the DC withstanding
test.
Sometimes it is recommended that the maximum voltage for this test should not
be higher than 60 % of the withstanding voltage.
If the results of successive tests show a reduction in the tested insulation resistance the
insulation should be replaced.
5.2.3 Time dependence test – Diagnostic Test
5.2.3.1 Polarization Index – PI
The purpose of this diagnostic test is to evaluate the influence of the polarization part of
insulation (Rpi, Cpi). After applying a high voltage to an insulator the electric dipoles
distributed in the insulator align themselves with the applied electrical field. This
phenomenon is called polarization. As the molecules polarize, a polarization (absorption)
current lowers the overall insulation resistance of the material.
The absorption current (IPI) typically collapses after a few minutes. If the overall
resistance of the material does not increase, this means that other currents (e.g. surface
leakages) dominate the overall insulation resistance.
PI is defined as the ratio of the measured resistances in two time slots. The most
typical ratio is 10 min value to 1 min value but this is not a rule.
