Amprobe AMB-110 Insulation-Resistance-Tester - Manual (Page 15)

10
I
PI
I
C i s o
I
R i s o
I
R i s s
Itest
Fig. 6
The Fig. 6 shows typical currents for that circuit.
I
test
= overall test current (I
test
= I
PI
+ I
RISO
+ I
RISS
)
I
PI
= polarization absorption current
I
RISO
= actual insulation current
I
RISS
= surface leakage current
Some application examples for using Teraohm 10 kV
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 (R
iso
) 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 (see the description of Guard terminal). This becomes critical when measured
values are in the G
e
range.
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.
Time dependence test – Diagnostic test