METREL TeraOhm 5kV Plus - Measurements; General Information about High DC Voltage Testing

MI 3201 TeraOhm 5 kV Plus Measurements
15
5 Measurements
5.1 General Information about High DC 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
Typical insulation tests
In general, insulation resistance tests consist of the following possible procedures:
- Simple insulation resistance measurement also called a spot test;
- Measurement of the relationship between voltage and insulation resistance;
- Measurement of the relationship between time and insulation resistance;
- Test of residual charge after the dielectric discharge.
The results of this test can indicate whether the replacement of the insulation system is
needed.
Typical examples of 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
Fig. 7 represents the equivalent electrical circuit of an insulating material
Riso
Cpi
Rpi
Ciso
Riss1
Riss2
material
surface
Itest
+
-
Guard
I
PI
I
Ciso
I
Riso
I
Riss
Itest
Fig. 7. Fig. 8.