33
34
3.2.5 Reviewing other test data
Insulation resistance
3.2.5.1 When ‘HOLD’ symbol is not displayed before a resistance test or after setting a test voltage, with every push
on ‘SELECT’ button, the displayed value will switch in the following order:
GUARD terminals are used in order to prevent the surface resistance of insulation materials from affecting the measurement
so that all materials can be measured correctly; a diagram for cable measurement is shown below:
3.2.7 Using GUARD terminal
Leak current
DAR 1 min/15 s
DAR 1 min/30 s
PI
Insulation resistance
Leak current
3.2.7.1 Diagram for using GUARD terminals in cable tests
3.2.6 Principle of insulation resistance test
1. Leak current (I) will be generated when a high DC voltage (V) is applied on the test object. The insulation resistance tester
measures applied voltage and the generated current, and calculate the insulation resistance.
2. When repeating measurements on the same test object, it is possible that each measurement results in a different
insulation resistance value and leak current value. This is caused by polarization effect which occurs when applying
voltage on the insulation material. Insulation material can be represented by an equivalent circuit, as shown in the diagram
below: IA stands for the absorbing current generated by slow polarization; it takes some time for the polarization resulted
from last test to disappear. There is still charge remaining in CA until the polarization disappears; the charge in CA during
last test is different from that at the beginning of the test that follows, and therefore the absorbing current (IA) is different,
too. Therefore, every test gives different combined leak current and insulation resistance; In order to ensure reproducible
measurements, please leave sufficient time between every two tests, and additionally, keep ambient temperature and
humidity relatively stable.
3.2.5.2 With every push on ‘SELECT’ button during a measurement, the displayed value will switch in the following
order
Insulation resistance
Insulation resistance
3.2.5.3 With every push on ‘SELECT’ button when results are held after the test, the displayed value will switch in the
following order
Insulation resistance
Leak current
DAR 1 min/15 s
DAR 1 min/30 s
PI
Insulation resistance
