Operation Manual

TeraOhm 5kV
12
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. For testing the insulation
materials high voltage test methods are applied.
DC vs. AC testing voltage
Testing with DC voltage is widely accepted as being useful as well as AC and / or
pulsed. DC voltage can be used for breakdown tests especially where high capacitive
leakage currents interfere with measurements using AC or pulsed voltage. It is mostly
applied for measurement tests of the insulation resistance. The voltage in this type of
test is defined by the appropriate product application group. It is lower than the voltage
for the withstanding test so the tests can be applied more frequently without stressing
the tested material.
Typical insulation tests
In general, insulation resistance tests consist of the following possible procedures:
- Simple insulation resistance measurement also called spot test;
- Measurement of voltage dependence of the insulation resistance;
- Measurement of time dependence of the insulation resistance;
- Test of recovered charge after the dielectric discharge.
The results of this test can indicate whether the replacement of the insulation system is
needed.
Typical examples, where testing insulation resistance and its diagnosis are
recommended, are transformer and motor insulation systems, cables and other
electrical equipment.
Electrical representation of insulating material
The following figure 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
R
iss1
and R
iss2
- the surface resistivity (position of optional guard connection)