HT instruments HT7052 - Manual (Page 13)

HT7052
EN - 11
R
iss1
, R
iss2
= surface resistivity (position of optional guard connection)
R
iso
= the actual insulation resistance of material
C
iso
= capacitance of material
C
pi
, R
pi
= represents polarization effects
The Fig. 8 shows typical currents for that circuit, where:
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
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 § 5.2.). This becomes critical when measured values are in the G

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. If the results of successive tests show a reduction in the tested
insulation resistance the insulation should be replaced
In this function the instrument measure the insulation resistance by considering 5 equal
time intervals with the test voltage divided from 1/5 of nominal value to the set nominal
value (see Fig. 9)
Fig. 9: Insulation measurement with step voltage test