Insulation resistance testing is the single most important electrical health check you can perform. A cable, motor, or transformer that looks perfectly fine on the outside can have insulation that has degraded to the point of danger — and the only way to know is to test it. This guide covers everything: how the test works, correct voltages, procedure, and how to read the results.
What Insulation Resistance Testing Measures
Every electrical conductor is surrounded by insulation material — PVC, rubber, XLPE — that prevents current from leaking to adjacent conductors or to earth. Over time, heat, moisture, mechanical damage, and chemical contamination degrade this insulation. The test applies a high DC voltage and measures the tiny current that leaks through the insulation. Using Ohm's Law (R = V ÷ I), it calculates the insulation resistance in megohms (MΩ) or gigohms (GΩ).
Figure — Megohmmeter applies HV between conductor and earth, measures leakage current
Correct Test Voltages
Using the wrong test voltage invalidates the result
Too low a voltage won't stress the insulation enough to reveal weak spots. Too high a voltage can damage good insulation. Always use the voltage specified for the equipment rating.
| Equipment Rating | Test Voltage | Minimum IR (1 min) |
|---|---|---|
| LT cables and wiring (up to 1000 V) | 500 V DC | > 1 MΩ |
| Motors and generators (< 1 kV) | 500 V DC | > 1 MΩ (new), > 0.5 MΩ (service) |
| HT cables and equipment (1–33 kV) | 1000–2500 V DC | > 100 MΩ |
| Switchgear and busbars | 1000 V DC | > 100 MΩ |
| Transformers (HV winding) | 2500 V DC | > 500 MΩ |
Test Procedure — Step by Step
Isolate and discharge before testing
The equipment must be completely de-energised, isolated, locked out, and discharged before connecting the megohmmeter. Capacitive equipment (motors, long cables) will store charge from the test voltage — always discharge via the instrument's built-in discharge function or a discharge resistor after testing.
Isolate and verify zero voltage
Switch off, lock out, and use a voltage tester to confirm zero volts on all phases and to earth.
Disconnect equipment under test
Disconnect cables from switchgear, motors from drives, etc. Test leads of other instruments connected in parallel give false low readings.
Connect megohmmeter leads
Connect the Line (L) terminal to the conductor under test. Connect Earth (E) to the equipment frame or earth. Some tests also use a Guard terminal to eliminate surface leakage.
Select correct test voltage
Use the table above. Most mains-voltage installations use 500 V DC.
Apply voltage for 60 seconds
Press the test button. Note the reading at 15 seconds and at 60 seconds. A rising reading (R60 > R15) indicates good insulation. A falling or stable reading may indicate moisture or contamination.
Record and discharge
Note the 1-minute reading. Immediately use the discharge function or hold the test leads connected for 60 seconds after removing voltage to discharge stored charge safely.
The Polarisation Index (PI)
The Polarisation Index is the ratio of the 10-minute reading to the 1-minute reading. It is a more reliable indicator of insulation condition than a single reading, because it filters out surface effects.
Interpreting Your Results
A single megohm reading in isolation means little — trend analysis over time is far more valuable. Key rules:
A reading that is consistent with previous records and above the minimum = insulation is acceptable.
A rising reading during the test (R at 60s > R at 30s) = insulation is absorbing charge normally = healthy polarisation.
A reading 30–50% below previous records = investigate immediately, even if it is above the minimum threshold.
A reading that falls during the test = surface leakage or moisture = dry out and retest before energising.
Zero or very low reading (< 0.5 MΩ on a 500V test) = definite fault. Do not energise.
CIE manufactures hand-operated and motorised insulation testers for LT and HT applications — from the compact CIE-555 to the high-voltage CIE-777 series. Contact our team to select the right model for your application.