METREL MI 3205, Instruction Manual

Page: 44 / 59

MI 3205 TeraOhm XA 5kV Measurements
44
5.7.1 Dielectric Absorption Ratio (DAR)
DAR is ratio of Insulation Resistance values measured after 30 s and after 1 minute. The
DC test voltage is present during the whole period of the test (also an Insulation
Resistance measurement is continually running). At the end, the DAR ratio is displayed:
 
 
) 30 _( 1
min) 1 _( 2
s Timer
Timer
DAR
R
R
iso
iso

Some applicable values for DAR (Timer1 = 30 s and Timer2 = 1 min):
DAR value Tested material status
< 1 Bad insulation
1 ≤ DAR ≤ 1.25 Acceptable insulation
> 1.4 Very good insulation
Note:

When determining Riso (30 s) pay close attention to the capacitance of the test
objects. It has to be charged-up in the first time section (30 s). Approximate
maximum capacitance using:
 
 
 
V
s
F
U
t
C
10
3
max



Where:
t ...................................... period of first time unit (e.g. 30 s).
U .................................... Test voltage.
5.7.2 Polarization Index (PI)
PI is the ratio of Insulation Resistance values measured after 1 minute and after 10
minutes. The DC test voltage is present during the whole period of the measurement (an
Insulation Resistance measurement is also running). On completion of the test the PI
ratio is displayed:
 
 
min) 1 _( 2
min) 10 _( 3
Timer
Timer
PI
R
R
iso
iso

Some applicable values for PI (Timer2 = 1 min and Timer3 = 10 min):
PI value Tested material status
1 – 1.5 Not acceptable (older types)
2 – 4 Considered as good insulation (older types)
4 Modern type of good insulation systems