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5.6 Dielectric Discharge Mode:
Dielectric discharge test is a diagnostic insulation test that allows ageing, deterioration,
and voids in the insulation to be assessed. The result is dependent on the discharge
characteristic which tests the internal condition of the insulation and is largely independent of
surface contamination.
The insulator must be charged until the only remaining component of current is leakage
current. On discharge the capacitive component of the discharge current decays from a high
value with a relatively short time constant of a few seconds. The released reabsorption
current decays from a lower value with but has relatively long time constant of up to several
minutes.
The DD timer (t1) defaults to 30 minutes of charge, which is generally sufficient time for
full absorption to take place in an insulation material. The discharge timer (t2) defaults to 1
minute. Timer settings t1 and t2 are adjustable. On completion of the test, the instrument
uses the current, the test voltage and calculated capacitance to produce a figure of merit
indicating the quality of the insulation. The Dielectric discharge of an insulator is given by the
formula:
DD = I /(V x C)
1min
where I is the discharge current in mA one minute after removal of the test voltage V in
1min
Volts and C is the capacitance in Farads.
DD value >7 indicates that insulation condition is bad, DD value of 4-7 indicates that
insulation condition is poor, DD value of 2-4 indicates that the insulation condition is
questionable and DD value of <2 indicates that the insulation condition is good and DD value
of 0 indicates homogenous insulation.
The steps to take a test in DD mode are as follows:
To enter the DD mode press the
icon on the home screen and following screen will be
displayed.
