Insulation Resistance ‘IR’ test:

This is the default mode in which the instrument powers up. This test mode measures insulation resistance continuously at the selected voltage. The test voltage may be varied during an ‘IR’ test by pressing the test voltage s or t buttons (depending of testing material). The test duration can be set using the ‘main timer’. The test will finish automatically after this time has elapsed. On test completion, insulation capacitance and the time constant associated with it is calculated and displayed.

Time Constant (TC) = R ins x C ins
The instrument will normally calculate and display the DAR value under the segment ‘DAR’. On completion of the test, the instrument will normally display the insulation resistance measured at these times under the screen. If the test runs for longer than 10 minutes, the instrument will calculate the ‘polarisation index’. This value will be displayed under the display segment symbol ‘PI’. On completion of the test, the instrument will display the insulation resistance recorded at 1 minute and 10 minutes under the segment symbols ‘1m’ and ‘10m’.

The capacitance reading is indicated by its units of either ‘nF’ or ‘µF’.

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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 (depending of testing material). 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 ‘---’.

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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 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) = R10 min/R1 min
The ‘Dielectric Absorption Ratio’ is the term applied to the polarisation index using other time intervals set by the test bench.
If timers are both set (depending of testing material), then the insulation resistance measured at these times is also recorded. These are displayed together with the calculated dielectric absorption ratio under the screen segments respectively.
The instrument can be made to display either the insulation resistances, insulation currents or ratios plus capacitance measurements by using the toggling units.
Dielectric Absorption Ratio (DAR) = RT2 min /RT1 min

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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 a few seconds. The other current component, comprising the released absorption current, decays from a lower value with a relatively long time constant of up to several minutes. If this component of the discharge current is large (>7 @ 500 V test voltage) then the insulation condition is poor.
The main timer will default to 30 minutes, which is normally sufficient time for full absorption to take place in an insulation material. The default test voltage is set to 500 V. The ‘DD’ test requires the instrument to measure the discharge current 1 minute after the removal of the test voltage. At this time the capacitive current should be insignificant compared with the released absorption current. On completion of the test, the instrument uses this measurement along with the test voltage and calculated capacitance to produce a figure of merit indicating the quality of the insulation.
Dielectric Discharge (DD) = I1 min/V x C
where I is the measured current expressed in milliamps (mA), V is the test voltage in Volts (V), and C is the measured capacitance in Farads (F).