Insulation resistance test (megger test): procedure, minimum values and PI calculator

What an insulation resistance test is

The tester applies a steady DC voltage, typically 500 V to 5 kV, and measures the small current that leaks through and across the insulation. Resistance is voltage divided by current, so good insulation reads hundreds or thousands of megohms; damp, dirty or damaged insulation reads low.

The test is non-destructive at the right voltage, quick and repeatable, which is why it is the first electrical check on almost every item. It does not prove the insulation will withstand a high voltage: that is the job of a withstand (hipot) test.

When it is done

• After cable installation: every power and control cable, conductor to conductor and conductor to earth, usually on a construction (A) ITR.
• Before energising: motors, transformers, switchgear and busbars, on pre-commissioning (B) ITRs.
• Before a motor solo run, and again after long storage or if the equipment has been wet.
• In maintenance, where the trend over the years matters more than a single value.

See the ITR guide for how these records fit into completion and commissioning.

Test voltages

For rotating machines, IEEE 43 recommends the DC test voltage by the rated voltage of the winding:

For low voltage installations, IEC 60364-6 (BS 7671 in the UK) gives the test voltage and the minimum together:

Project specifications and manufacturers often ask for higher values than these minimums. Use the figure in your test procedure.

How to do the test

1. 01
   Isolate and prove dead

   Work under the permit and isolation procedure. Lock off, test for dead, and earth the circuit until the tester is connected.

2. 02
   Disconnect what the test could damage

   Drives, soft starters, electronic relays, surge arresters, capacitors and instruments. Open neutral links where needed.

3. 03
   Connect the tester

   Between each conductor and earth, and between conductors where the procedure calls for it. On a motor, test the winding to earth with the other phases connected to earth, or all three phases together.

4. 04
   Apply the voltage and read

   Read at 1 minute for a spot reading. For a polarization index, keep the voltage on and read again at 10 minutes.

5. 05
   Record the conditions

   Winding or cable temperature, humidity, test voltage, readings and the tester serial number and calibration date.

6. 06
   Discharge

   Earth the conductor after the test, for several times as long as the voltage was applied. Long cables and large windings hold a dangerous charge.

Minimum values for machines (IEEE 43)

IEEE 43 gives a recommended minimum for the 1 minute reading, corrected to 40 °C:

A reading above the minimum does not prove a winding is good, and a single low reading does not prove it is bad. The trend and the polarization index tell you more.

Polarization index and dielectric absorption

When DC is applied, the current through good insulation keeps falling for several minutes as the insulation polarises, so the resistance keeps rising. Damp or contaminated insulation leaks steadily and the reading barely changes.

• Polarization index (PI) = reading at 10 minutes / reading at 1 minute. IEEE 43 recommends at least 2.0 for class B, F and H insulation, and 1.5 for class A.
• Dielectric absorption ratio (DAR) = reading at 60 seconds / reading at 30 seconds. A quicker check, often used on smaller equipment.
• If the 1 minute reading is above 5,000 MΩ (5 GΩ), the PI may not be meaningful and IEEE 43 does not require it.

Because PI and DAR are ratios taken at the same temperature, they need no temperature correction.

Temperature correction

Insulation resistance falls as temperature rises, so readings are corrected to a common base, 40 °C for machines in IEEE 43. A common rule of thumb is that resistance halves for every 10 °C rise:

R at 40 °C = R measured x 0.5 ^ ((40 - T) / 10), where T is the winding temperature in °C. A reading of 800 MΩ at 20 °C corrects to 200 MΩ at 40 °C.

The rule of thumb is an approximation. For trending, take readings at similar temperatures or use the correction curve from the manufacturer.

Insulation resistance calculator

Enter the rated voltage, the winding type, the readings and the temperature. The calculator gives the IEEE 43 test voltage, corrects the reading to 40 °C, compares it with the recommended minimum and works out the polarization index.

Record readings against the tag, with low values flagged before sign-off: Deskely Verify →

Recording the result

An insulation resistance test is only useful if it can be found and trusted later. The record, usually an ITR, should hold:

• The tag of the cable, motor or switchboard, and its system and subsystem.
• The test voltage, the readings for each phase to earth and phase to phase, and the PI where taken.
• The temperature and humidity at the time of the test.
• The tester, its serial number and calibration due date.
• The acceptance value from the procedure, the result, and the signatures.

In Deskely, these readings are entered on the tablet against the tag, and Deskely Verify flags values outside range or tolerance before the record is signed, so a low reading becomes a punch item on site rather than a surprise at energisation.