HT instruments HT7052 - Manual (Page 14)

HT7052
EN - 12
6.1.1 Time dependence test – Diagnostic test
Diagnostic test is a long duration test for evaluating the quality of the insulation material
under test. The results of this test enable the decision to be made on the preventive
replacement of the insulation material
DIELECTRIC ABSORPTION RATIO (DAR)
DAR is ratio of Insulation Resistance values measured after 15s 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:
 
 
s
DAR
R
R
iso
iso
15
min 1

Some applicable values:
DAR value Tested material status
< 1.25 Not acceptable
< 1.6 Considered as good insulation
> 1.6 Excellent
When determining Riso (15s) pay attention to the capacitance of the test object. It has to
be charged-up in the first time section (15s). Approximate maximum capacitance using:
 
 
 
V
s
F
U
t
C
10 3
max
.
 
where:
t = period of first time unit (e.g. 15s)
U = test voltage.
To avoid this problem, increase the DIAG. Starting time parameter in CONFIGURATION
menu, because start of timer in the DIAGNOSTIC TEST functions depends on the test
voltage. The timer begins to run when test voltage reaches the threshold voltage, which is
product of the DIAG. Starting time and nominal test voltage (Unominal)
Using filters (fil1, fil2, fil3) in the DAR function is not recommended!
Analysing the change in the measured insulation resistance over time and calculating the
DAR and PI are very useful maintenance tests of an insulating material