Information's on Electrical Testing:
Insulation Resistance ‘IR’ test
This test mode measures insulation resistance continuously at the selected voltage. The test voltage may be varied during an ‘IR’ test by changing setting buttons. 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) = Rins x Cins
If timers T1 and T2 are set, the instrument will calculate and display the DAR value under the segment ‘DAR’. On completion of the test, the instrument will display the insulation resistance measured at these times under the segment symbols ‘T1’ and ‘T2’. 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’.
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 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 toggling units.
Dielectric Absorption Ratio (DAR) = RT2 min
RT1 min
SV 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 and recorded under the consecutive segment headings ‘1m’ to ‘4m’. The 5 minute reading is displayed by the main display during the testing.
DD 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 but can be done at a higher voltage. 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) = I at 1 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).
AR Absorption Ratio
This test allows checking the current absorption and determining the condition of insulation. This test is performed at the same time than PI (Polarisation Index), the measurement are all recorded inside the test bench and downloaded on the computer with USB port, all data are incorporated in Microsoft EXCEL software and arranged to see the form of the absorption current curve.