HT instruments HT7052 - Manual (Page 12)

HT7052
EN - 10
6 HOW TO PERFORM THE MEASUREMENTS
6.1 THEORY OF INSULATION RESISTANCE MEASUREMENT
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 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 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
required. 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
The represents the equivalent electrical circuit of an insulating material
R iso
C pi
R pi
C iso
R iss1
R iss2
m aterial
surface
Itest
+
-
G uard
I
PI
I
Ci so
I
Ri so
I
Ri ss
Ites t
Fig. 7: Equivalent electrical circuit Fig. 8: Current graphs