TeraOhm 5kV
13
R
iss1
and R
iss2
- the 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 right figure 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 5 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
Ω
)
,
R
iso
usually dominates. The results are adequate and stabilize quickly.
-
The voltage, time and limit are usually given in the appropriate standard or
regulation.
-
The suggested test voltage is often defined as 1.7 times the value of line voltage of
the tested device (e.g. motor), i.e. the test voltage for a 600 V device would be 1020
VDC. Measuring time should be set to 60 s or the minimum time required for the Ciso
to be charged up.
-
Sometimes it is required to take into account the ambient temperature to 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 measuring values in the G
Ω
range.
Some examples:
Standard*
Test voltage / time
End value
EN/IEC 60204
500 V
1 M
Ω
EN/IEC 60335-1
2000 V / 15 min
EN/IEC 60349-1
500 V
1000
Ω
/V
EN/IEC 60598-1
500 V / 1 s
2 M
Ω
* Only sample values are listed!
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 such as 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 withstanding voltage.
If the results of successive tests show a reduction of the tested insulation resistance the
insulation should be replaced.
