minute period. Readings for contaminated insulation are fairly constant
because any absorption effects are masked by high leakage currents.
On completion of the test the polarisation index is displayed under the
segment heading ‘PI’.
Polarisation Index (PI) = R
10 min
R
1 min
The ‘Dielectric Absorption Ratio’ is the term applied to the polarisation
index using other time intervals set by T1 and T2.
If timers T1 and T2 are both set, then the insulation resistance measured
at these times is also recorded. These are displayed together with
the calculated dielectric absorption ratio under the T1, T2, and DAR
segments respectively.
The instrument can be made to display either the insulation resistances,
insulation currents or ratios plus capacitance measurements by using the
Ω
/I toggle button. See section ‘
Ω
/I button’ for cycling through results
and toggling units.
Dielectric Absorption Ratio (DAR) = R
T2 min
R
T1 min
Dielectric discharge ‘DD’ test
The ‘DD’ 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, so the internal condition
of the insulation is tested, largely independent of any surface
contamination. On discharge the capacitive component of the discharge
current decays from a high value with a relatively short time constant of
17
the ‘A
’ symbol is flashing on the display. Use the timer
s
and
t
buttons to set the resistance threshold between the limits of 10 k
Ω
and
15 T
Ω
. Release the function button to save the current limit. Adjusting
the alarm threshold level automatically activates the alarm limit mode.
This is indicated by the ‘A
’ symbol turning solidly on.
To toggle the alarm limit mode on/off, hold down the function button,
then press and release the mode button repeatedly until the ‘A
’ symbol
is flashing on the display. Release the function key to toggle.
Step voltage ‘SV’ test
This is a test based on the principle that an ideal insulator will produce
identical readings at all voltages, while an insulator which is being over
stressed, will show lower insulation values at higher voltages. The main
timer and test voltage settings can be adjusted if desired from their
default values of 5 minutes and 1 kV respectively. During the test the
applied test voltage incrementally steps by one fifth of the test voltage
setting (final value) every minute, for 5 minutes, taking successive
measurements until the final voltage is reached. Readings for the first 4
recorded values are displayed under the consecutive segment headings
‘1m’ to ‘4m’. The 5 minute reading is displayed by the main display.
If the range of measured insulation between readings is too wide for the
instrument to display, then those readings too small compared to the
final reading will be represented by ‘---’.
Polarisation index ‘PI’ test, and dielectric absorption ratio
‘DAR’ test
The ‘PI’ test is a particular example of a time/resistance method, which
takes the ratio of the insulation measured at 1 minute and at 10 minutes.
Good insulation generally shows an increase in resistance over a 10
