HT instruments HT7052 - Manual (Page 16)

HT7052
EN - 14
DIELECTRIC DISCHARGE RATIO (DD)
An additional effect of polarization is the recovered charge (from Cpi) after the regular
discharging of a completed test. This can also be a supplementary measurement for
evaluation of the quality of insulating material. This effect is generally found in insulating
systems with large capacitance Ciso. The polarisation effect (described in “Polarisation
Index”) causes a capacitance to form (Cpi). Ideally this charge would dissipate
immediately a voltage was removed from the material. In practice, this is not the case
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 10
 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
mA Idis
DD
.
min 1

where:
Idis 1min = discharging current measured 1 min after regular discharge
U= test voltage
C= capacitance of test object
General applicable values:
DD Value Tested material status
> 4 Bad
2 - 4 Critical
< 2 Good