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207
Change in the Current Flowing Through Insulators
11.5 Change in the Current Flowing Through
Insulators
When insulation resistance measurement is performed, a high current flows as soon as a voltage is
applied, however, the current gradually drops and may not be stabilized.
This is due to charging current, absorption current, and leakage current and this condition is
generally called dielectric absorption. The equivalent circuit with insulators is shown in Fig. 1 below.
In this figure, a charging current flows through C
0
, C
1
, C
2
........, and C
n
when a voltage is applied.
C
0
is charged first, then C
1
, C
2
......, and C
n
in that order. As the insulators are charged, the current
becomes smaller and drops to the leakage current with R
0
at the end. (See Fig. 2.)
C
n
R
n
C
3
C
2
C
1
R
3
R
2
R
1
C
0
R
0
Fig. 1 Insulator equivalent circuit
0
Absorption current
Leakage current
Voltage
applied
(Charge)
Electrode
short circuit
(Discharge)
Time
Charging current
Current
Fig. 2 Dielectric absorption
R
0
is the insulation resistance. Actually, high resistance R
1
, R
2
......., and R
n
are connected in series
with C
1
, C
2
....., and C
n
and it is difficult to measure R
0
only.
In general, it is assumed that it takes a few hours to a few days for the measured value to stabilize.
Therefore, the resistance value measured when 1 minute has elapsed after a voltage is applied
is used as the insulation resistance of an insulator for convenience. This value is used widely for
standards as a 1-minute value of the insulation resistance.
When a 1-minute value of the insulation resistance is measured, the initially measured value is
different from the values of consecutive measurements. The DUT with a voltage applied needs to
be fully discharged before measurement.
Depending on the magnitude of voltage “C
0
” in Fig. 1, generally a discharge time that is 5 to 6 times
the time for which a voltage is applied during measurement is required.
Appendix
HIOKI BT5525A961-00
Содержание BT5525
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