Dd (Dielectric Discharge Index); Dd Measurement - AEMC 6550 User Manual

4.7 DD (dielectric discharge index)

In the case of multilayer insulation, if one of the layers is defective but the resistance of all the others is high,
neither the quantitative insulation measurement nor the calculation of the PI and DAR quality ratios will reveal
the problem.
This makes it judicious to perform a dielectric discharge test, from which the DD term can be calculated. This
test measures the dielectric absorption of heterogeneous or multilayer insulation and disregards parallel-sur-
face leakage currents.
The dielectric discharge test is especially well suited for measuring the insulation of revolving machines and
more generally for measuring the insulation on heterogeneous or multi-layer insulating materials containing
organic substances.
It involves applying a test voltage for long enough to electrically "charge" the insulation to be measured. At
the end of the measurement, the instrument induces rapid discharging, during which the capacitance of the
insulation is measured, then, one minute later, it measures the residual current flowing in the insulation.
The DD term is then calculated as follows:
DD = current measured after 1 minute (mA)/[test voltage (V) x measured capacitance (F)]

4.7.1 DD Measurement

Press the CONFIG button.
Use the▲▼ arrow buttons to select Manual Stop + DD or Timed Run + DD (manual or automatic measurement).
Total Run Time
Manual Stop
Manual Stop + DD
Timed Run (m:s)
Timed Run + DD
DAR (s/s)
PI (m/m)
To set the duration of the measurement, place the cursor on Timed Run (m:s). Then use the ◄ ► and▲▼ arrow
buttons to set the minutes and seconds. The minimum setting is 0:01 but a duration below 30 seconds is hardly
useful because acquiring a stable resistance result requires some time.
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Total Run Time
CONFIG
CONFIG
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2:00
30/60
1.0/10
ShopAEMC
www.
CONFIG
Manual Stop
Manual Stop + DD
Timed Run (m:s)
Timed Run + DD
DAR (s/s)
PI (m/m)
CONFIG
Total Run Time
Manual Stop
Manual Stop + DD
Timed Run (m:s)
Timed Run + DD
DAR (s/s)
PI (m/m)
.ca
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2:00
30/60
1.0/10
00:03:00
2:00
30/60
1.0/10
Megohmmeter Models 6550/6555
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