Hioki IR3455, Instruction Manual

Page: 82 / 184

3.2 Measuring Insulation Resistance
76
When measuring the same object repeat-
edly, the insulation resistance or leakage
current indications may differ. This is caused
by polarization*, which occurs when a volt-
age is applied to an insulating material.
An insulating material is represented by an
equivalent circuit as shown by the diagram
on the previous page.
Absorption current due to relatively slow
polarization is represented by I
A
, as shown
in the diagram above. It takes time for the
polarization caused by the previous mea-
surement disappear. Until it does, electric
charge remains in C
A
as shown in the dia-
gram. The electric charge level in C
A
differs
at the start of previous measurement and at
the start of next measurement and thus
absorption current I
A
differs, too. Further, the
combined leakage current and insulation
resistance vary from measurement to mea-
surement. This will be become more appar-
ent for higher insulation resistance values.
To ensure reproducibility of measurement,
leave a sufficient time interval between mea-
surement sessions. Further, the ambient
temperature and humidity should not vary.
*Polarization: the phenomenon in which positive
and negative charges on the atoms of
a material move in opposite direc-
tions causing a shift of the center
when an electric field is applied to the
material.
Reproducibility of insulation resistance
measurement