Power Network Application Trainer
– Insulation Analysis
198
Figure 110-3: Time diagrams of good and bad insulation tested with the time-rise method
The result of this measurement is polarization index (PI), which is defined as the ratio
of measured resistance in two time slots (typically the ratio is 10 min value to 1 min
value at a continuous measurement).
PI
value Tested material status
1 - 1.5
Not acceptable (older types)
2
– 4 (typically 3)
Considered as good insulation (older types)
4
Very good insulation. Modern type of good insulation systems
Table 66-3: Typical values of polarization index
The results of this method don’t depend on temperature and the method can give a
conclusive information without comparing records of past tests.
Dielectric absorption ratio (DAR) is similar to the polarization index method. The only
difference are periods for capturing the results which are usually 30 s (or 15 s) and 1
minute.
DAR value
Tested
material
status
< 1
Bad insulation
1 ≤ DAR ≤ 1.25
Acceptable insulation
> 1.4
Very good insulation
Table 67-3: Typical values for dielectric discharge
Summary of Contents for MI 3298
Page 55: ...Power Network Application Trainer Earthing Grounding network impedance measurement 55 ...
Page 56: ...Power Network Application Trainer Earthing Grounding network impedance measurement 56 ...
Page 124: ...Power Network Application Trainer Earthing Grounding network impedance measurement 124 ...
Page 228: ...MI 3290 Earth Analyser 228 ...
Page 229: ...MI 3280 Digital Transformer Analyser 229 ...
Page 230: ...MI 3280 Digital Transformer Analyser 230 ...
Page 231: ...MI 3250 MicroOhm 10A 231 ...
Page 232: ...MI 3205 TeraOhmXA 5kV 232 ...