MI 3200 TeraOhm 10 kV
Measurements
17
Withstanding voltage test
Some standards allow the use of a DC voltage as an alternative to AC withstanding
voltage testing. For this purpose the test voltage has to be present across the insulation
under test for a specific time. The insulation material only passes if there is no
breakdown or flash over. Standards recommend that the test starts with a low voltage
and reaches the final test voltage with a slope that keeps the charging current under the
limit of the current threshold. The test duration normally takes 1 min.
Withstanding voltage test or dielectric test is usually applied for:
-
Type (acceptance) tests when a new product is being prepared for manufacture,
-
Routine (production) tests for the verification of safety on each product,
-
Maintenance and after service tests for any equipment where the insulation system
can be exposed to degradation.
Some examples for DC withstanding test voltage values:
Standard (only sample values)
Voltage
EN/IEC 61010-1 CAT II 300 V basic insulation
1970 V
EN/IEC 61010-1 CAT II 300 V double insulation
3150 V
IEC 60439-1 (clearance between live parts…), withstanding impulse
voltage 4 kV, 500 m
4700 V
IEC 60598-1
2120 V
Humidity and insulation resistance measurements
When testing outside the reference ambient conditions, the quality of the insulation
resistance measurements can be affected by humidity. Humidity adds leakage paths
onto the surface of the complete measuring system, (i.e. the insulator under test, the test
leads, the measuring instrument etc). The influence of humidity reduces accuracy
especially when testing very high resistances (i.e. Tera ohms). The worst conditions
arise in environments containing high condensation, which can also reduce safety. In the
case of high humidity, it is recommended to ventilate the test areas before and during
the measurements. In the case of condensed humidity the measuring system must dry
and it can take several hours or even few days to recover.
5.2. Guard terminal
The purpose of the GUARD terminal is to lead away potential leakage currents (e.g.
surface currents), which are not a result of the measured insulation material itself but are
a result of the surface contamination and moisture. This current interferes with the
measurement i.e. the Insulation Resistance result is influenced by this current. The
GUARD terminal is internally connected to the same potential as the negative test
terminal (black one). The GUARDs test clip should be connected to the test object so as
to collect most of the unwanted leakage current, see the figure
Fig. 8
below.
