14
Measurements
General Information about DC High voltage testing
The purpose of insulation tests
Insulating materials are important parts of almost every electrical product. The material’s proper-
ties depend not only on its compound characteristics but also on temperature, pollution, mois-
ture, aging, 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
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
Riso
Cpi
Rpi
Ciso
Riss1
Riss2
material
surface
Itest
+
-
Guard
I
PI
I
Ciso
I
Riso
I
Riss
Itest
Fig. 10
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