MI 3202 GigaOhm 5 kV
Measurements
14
5 Measurements
5.1 Generally about DC High voltage testing
The purpose of insulation tests
Insulating materials are important parts of almost every electrical product. The
material’s properties depend not only on its compound characteristics but also on
temperature, pollution, moisture, ageing, electrical and mechanical stress, etc. Safety
and operational reliability require the regular maintenance and testing of the insulation
material to ensure it is kept in good operational condition. High voltage tests are used to
test insulating materials.
DC vs. AC testing voltage
Testing with a DC voltage is widely accepted as being as useful as testing with AC and /
or pulsed voltages. DC voltages can be used for breakdown tests especially where high
capacitive leakage currents interfere with measurements using AC or pulsed voltages.
DC is mostly used for insulation resistance measurement tests. In this type of test, the
voltage is defined by the appropriate product application group. This test voltage is
lower than the voltage used in the withstanding voltage test so the tests can be applied
more frequently without stressing the test material.
Electrical representation of insulating material
The following figure (
Fig.10
) represents the equivalent electrical circuit of insulating
material
Riso
Cpi
Rpi
Ciso
Riss1
Riss2
material
surface
Itest
+
-
Guard
I
PI
I
Ciso
I
Riso
I
Riss
Itest
Fig. 10
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
