MI 3205 TeraOhm XA 5kV
Measurements
45
Note:
When determining Riso (1 min) pay close attention to the capacitance of the test
objects. It has to be charged-up in the first time section (1 min). Approximate
maximum capacitance using:
V
s
F
U
t
C
10
3
max
where:
t ...................................... period of first time unit (e.g. 1 min).
U .................................... Test voltage.
Analyzing the change in the measured insulation resistance over time and calculating
the DAR and PI are very useful maintenance tests of an insulating material.
5.7.3 Dielectric Discharge Testing (DD)
DD is the diagnostic insulation test carried out after the completion of the Insulation
Resistance measurement. Typically the insulation material is left connected to the test
voltage for 1
30 min and then discharged before the DD test is carried out. After 1
minute a discharge current is measured to detect the charge re-absorption of the
insulation material. A high re-absorption current indicates contaminated insulation
(mainly based on moisture):
F
C
V
U
nA
Idis
DD
min
1
where:
Idis 1 min ........................ Discharging current measured 1 min after regular discharge.
C .................................... Capacitance of test object.
U .................................... Test voltage.
A high re-absorption current shows that the insulation has been contaminated, usually
by moisture. Typical values of dielectric discharge are shown in table.
DD value
Tested material status
> 4
Bad
2 – 4
Critical
< 2
God
