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Have series resistors R
0
, R
1
, R
2
,
. . . .
R
n
. Therefore, a measurement
of R
0
only is very difficult. I t is said that, with some insulation resistance
measurements, it takes several hours to a few days for the leakage curre nt
to stabilize. This is not practical.
To avoid this problem, a method is customarily used in the insulation
resistance measurement for convenience to read the resistance value one
minute after charg ing the test voltag e to the sample. This value is called
minute rate value for the resistance value of an insulator, and is widely
employed among a variety of electrical standards.
In the 1-minute rate insulation resistance measurement, the measured
values may vary when a measurement is repeated once or twice with the
same samp le. To minimize such a d eviation, it is imp ortant to comp letely
discharge the sample before the start of each measure ment. The required
discharge time mainly depends upon the charging voltage and the size of
C
0
in Fig . 6 .7.1 , but, generally it can be said to be 5 to 6 times longer than
the time of test voltage charging .
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