Vanguard IRM-5000P - Manual (Page 56)

IRM-5000P USER’S MANUAL REV 2
52
APPENDIX A - DIELECTRIC DISCHARGE RATIO TEST DESCRIPTION
When a dielectric is subjected to a voltage gradient, some charge will slowly permeate into the
dielectric. The effect is referred to as Dielectric Absorption. Since the dielectric is by intent an
insulator, one can imagine that the movement of these small charges has a very long time
constant. While virtually all dielectrics exhibit the effect to some extent, poorer dielectrics or
dielectrics that have undergone degradation will absorb more charge.
The IRM-5000P’s Dielectric Discharge test uses a standard test voltage of 500V and a standard
test duration of 30 minutes. During the testing period some charge migrates into the dielectric
material under test, as described above. At the completion of the test, the insulation system is
discharged to zero potential. At the zero point, the absorbed charge will start to migrate out of
the insulator’s dielectric. After discharging the dielectric for one minute, the Dielectric
Discharge current flow is measured. As can be seen in the equation below, the Dielectric
Discharge Ratio is proportional to the current measured:
In general, higher ratios indicate a poorer insulation system. As one would expect, a higher test
voltage will produce a higher electric field across the insulator’s dielectric material, and more
charge can be expected to be absorbed by the dielectric. Thus, doubling the test voltage can be
expected to double both the absorbed charge as well as the discharge current measured at the
end of the test. To make the Dielectric Discharge Ratio independent of this natural
phenomenon, the formula for the Dielectric Discharge Ratio is divided by the test voltage.
An insulation system that has longer lengths of insulated conductors can naturally be expected
to absorb more charge, due to the longer length of the conductors and dielectric. Of course,
such systems will also have a higher capacitance, as the capacitance of a pair of conductors in a
cable is proportional to the length of the cable. Thus, a system with twice as much cable will
absorb, in general, twice as much charge, but will also have twice as much capacitance.
Dividing the discharge current by the measured capacitance in the DD Ratio formula above will
normalize the effect.
e Capacitanc Voltage Test
(mA) minute 1 after Current
RATIO DD
Ч
=