Hioki Super megohm SM7110 - Appx. 2 Changes in Current Running through Insulator

Appx.2
Changes in Current Running through Insulator
Appx. 2 Changes in Current Running through
Insulator
When measuring insulation resistance, a large current ows as a voltage is applied, and gradually the current
becomes smaller, not reaching a constant value.
This is caused by charge current, absorption current, and leak current, and is generally called dielectric
absorption. The equivalent circuit of an insulator is shown in Figure 1. In this gure, applying a voltage
generates charge currents owing through the capacitance C
0
, C
1
, C
2
, . . . , C
n
. First, these currents charge C
0
,
and then C
1
, C
2
, . . . , C
n
in order. As the charging progressed, the currents become smaller, resulting in only a
leak current owing through R0 remaining. (See Figure 2.)
R
0
R
1
C
1
R
2
C
2
R
3
R
n
C
3
C
n
C
0
Figure 1 Equivalent Circuit of the Insulator
Current
Time
Charge current
Applying
voltage
(Charging)
Electrodes
short-circuited
(Discharging)
Absorption current
Leak current
0
Figure 2 Dielectric Absorption
R
0
is the insulation resistance. Because the high resistance R
1
, R
2
, . . . , R
n
are connected in series
respectively with C
1
, C
2
, . . . , and C
n
, it is di󰀩cult to measure only R
0
separately.
It is generally said that the time until convergence can be several hours to several days. Therefore, the
resistance 1 minute after applying voltage is considered as the insulation resistance. It is called 1-minute
insulation resistance, and is widely used in standards.
When measuring the 1-minute insulation resistance, the rst value di󰀨ers from the second or third
consecutively measured values, so be sure to discharge the object with a voltage applied before a
measurement.
It also depends on the amount of voltage across C
0
shown in Figure 1; however, discharging the capacitance
generally requires a period 5 to 6 times as long as the voltage-applying time during the measurement.