- Polarization Index Definition: The polarization index is defined as a test that determines the condition of insulation in electrical equipment by measuring the resistance over time.
- How PI Test Works: The PI test involves applying a DC voltage to the insulator and measuring resistance at 1 minute and 10 minutes.
- Current Components: During the test, currents through the insulator include capacitive, conductive, surface leakage, and polarization components.
- Importance of PI: A high polarization index indicates healthy insulation, while a low value suggests potential problems.
- Ideal PI Values: A polarization index above 2 is good; below 1.5 is hazardous.
The Polarization Index Test (PI Value Test) is a timed form of the Insulation Resistance Test (IR Value Test). It is used on de-energised electrical equipment to assess how insulation resistance changes under a constant DC test voltage. The PI test can reveal moisture, contamination or deterioration, but it cannot establish insulation condition from one ratio alone.
For an insulation resistance test, isolate the equipment and discharge its insulation before connecting the tester. The instrument applies a specified DC voltage, measures the resulting current through the electrical insulator and reports resistance from Ohm’s law.
A field PI test should use a purpose-built insulation-resistance tester rather than a separate DC source, voltmeter and ammeter. The tester controls the voltage and measures very small current. It is not a potentiometer; Megger is one manufacturer name commonly used for this class of tester.
A Megger insulation tester supplies DC test voltage and displays resistance in MΩ or GΩ. Select the test voltage from the equipment manufacturer’s instructions and the standard applicable to the winding, cable or apparatus. Values such as 500 V, 2.5 kV and 5 kV are tester ranges, not a universal rule based only on nominal voltage. A high voltage transformer can require a different procedure from a rotating machine.
The insulation behaves partly as a capacitor, so an initial charging current flows when DC voltage is applied to the electrical insulator. That current decays rapidly but should not be described as exactly zero after a fixed time. The standard PI reading uses measure insulation resistance values at 1 minute and 10 minutes. A 30-second or 60-second reading belongs to a separate dielectric-absorption-ratio method.
Measuring insulation resistance requires recorded winding temperature because absolute resistance changes strongly with temperature. Compare readings at a common corrected temperature and under consistent test conditions.
The polarity index test, more correctly called the PI value test, reduces some sensitivity to absolute resistance because both readings occur during one test at nearly constant temperature. The PI test still needs trend data, the one-minute resistance and the acceptance criteria for the equipment.
The measured current has four useful components:
- Capacitive component.
- Conductive component.
- Surface leakage component.
- Polarization component.
Each component changes differently during the timed test.
Capacitive Component
Applying DC voltage charges the capacitance of the insulation system. The resulting current is initially high and then decays rapidly. Its time constant depends on equipment capacitance and the test circuit, so fixed claims of 10 or 60 seconds do not apply to every test object.
Conductive Component
The conductive component is the relatively steady volume leakage through the insulation. Real insulating materials have finite resistance, so a small electric current remains while the DC test voltage is applied. Temperature and material condition affect its magnitude.
Surface Leakage Component
Dust, moisture, salts and other surface contamination can create a leakage path across exposed insulation. Guard terminals can exclude selected surface leakage from some measurements, but the connection must follow the tester and equipment instructions.
Polarization Component
Insulation absorption or polarisation current comes from slower dielectric processes under an applied electric field, including dipole orientation and charge migration. These processes occur in many solid insulating materials and are broader than moisture or contaminant effects. Energy for these processes comes from the voltage source and appears as a time-dependent electric current. As the dielectric approaches equilibrium, this current declines while the steady leakage components remain. Moisture can change both absorption and leakage behaviour in the electric field.
The 10-minute megger reading is taken after much of the charging and absorption current has decayed, not after polarisation has disappeared completely.
The 1-minute reading normally excludes most initial capacitive charging current. The 10-minute reading includes the remaining leakage and absorption currents. The resistance usually rises with time when the absorption current decays and steady leakage is low.
Polarisation index is the 10-minute insulation resistance divided by the 1-minute insulation resistance.
The equations below show the current components used in the simplified polarization index test model.
Let I be the total current during the PI test.
IC is capacitive charging current.
IR is volume conduction current.
IS is surface leakage current.
IP is absorption or polarisation current.
The 1-minute insulation-resistance value is:
The 10-minute insulation-resistance value is:
The polarization index test result is:
If steady leakage represented by (IR + IS) dominates the time-dependent absorption current IP, resistance changes little and PI approaches 1. A low ratio can indicate moisture, contamination, conductive leakage or an unsuitable test condition. The absolute IR and IS values also matter.
When (IR + IS) is small and IP decays during the test, the 10-minute resistance rises and PI is higher. The resistive leakage IR should still be judged from the absolute resistance and trend, not the ratio alone.
Interpret the result for the equipment and insulation system. IEEE 43 criteria apply to covered rotating-machine windings and use different minimum PI values for different insulation thermal classes. Other equipment may use different limits, and some very high-resistance modern windings make PI less useful. A value below 1 means resistance fell during the test and requires investigation; the page’s universal claims that every value above 2 is good or every value below 1.5 is hazardous are not valid. After testing, keep the leads connected for automatic discharge and verify zero voltage before touching the electrical insulator.
