Insulation Resistance Test (Megger Test) Explained

Why the insulation resistance test is done

Insulation in motors, generators, transformers, cables and switchgear degrades with heat, moisture, dirt, vibration and age. An IR test is a simple, non-destructive way to:

• check new installations before they are energised,
• check equipment after repair, flooding or long storage,
• track insulation condition over time in a maintenance programme.

A single reading is useful, but the trend of readings taken under similar conditions is far more informative.

What the tester measures

When a DC voltage is applied across insulation, the total current has three components:

The tester divides the test voltage by the total current. As the charging and absorption currents die away, the indicated resistance rises towards a value set by the leakage current. Moisture and contamination increase leakage current, so they keep the reading low and flat.

Choosing the test voltage

The test voltage should stress the insulation without exceeding what it can safely withstand. IEEE 43 recommends DC test voltages for rotating machine windings based on rated voltage:

For low voltage installations, IEC 60364-6 specifies the test voltages and minimum values for verification of circuits. Electronic equipment, surge protective devices and drives must be disconnected first, because the test voltage can damage them.

Polarization index and dielectric absorption ratio

Because the reading rises with time, ratios of readings taken at two times are used to judge insulation quality independently of size and temperature:

$$\text{PI} = \frac{IR_{10\,\text{min}}}{IR_{1\,\text{min}}}, \qquad \text{DAR} = \frac{IR_{60\,\text{s}}}{IR_{30\,\text{s}}}$$

IEEE 43 recommends a minimum PI of 2.0 for windings with class B, F and H insulation, and 1.5 for class A. A PI close to 1 means the leakage current dominates, which usually indicates moisture or contamination. IEEE 43 also notes that PI is not a meaningful indicator when the 1 minute reading is already above 5000 MΩ.

Minimum insulation resistance for machines

IEEE 43 gives recommended minimum 1 minute values, corrected to 40 °C, for rotating machines:

Effect of temperature

Insulation resistance falls as temperature rises. A common rule of thumb is that it roughly halves for every 10 °C increase, although the exact relationship depends on the material. Readings should therefore be corrected to a reference temperature, such as 40 °C in IEEE 43, before comparing them with limits or earlier results. Humidity also lowers readings, especially through surface leakage.

Keeping useful records

The value of insulation testing comes from comparing results over time. Each record should include the date, equipment identity, test voltage, readings at 30 seconds, 1 minute and 10 minutes where taken, winding or ambient temperature, humidity, the instrument used and the test connections. Plotting temperature-corrected readings for the same machine over several years shows gradual deterioration long before a failure, so repairs can be planned during a shutdown.

Safe test procedure

1. Isolate the equipment, lock it off and prove it is dead.
2. Disconnect sensitive electronics, surge arresters and capacitors that could be damaged or affect the result.
3. Clean and dry terminals to reduce surface leakage.
4. Connect the tester: line lead to the conductor, earth lead to the frame or earth. Use the guard terminal to remove surface leakage if needed.
5. Apply the test voltage for the required time and record the readings, temperature and humidity.
6. After the test, discharge the insulation by earthing it. Large windings and long cables can hold a dangerous charge; IEEE 43 recommends earthing for at least four times the test duration.

Testing cables

Insulation resistance of cables is tested between each core and the other cores connected to earth, and between each core and the armour or screen. Points to remember:

• Length matters. A long cable has more insulation in parallel, so its resistance is lower than a short cable of the same type. Results are sometimes converted to MΩ·km for comparison.
• Capacitance matters. Long cables take time to charge, so readings should be taken after the reading stabilises, and the cable must be discharged afterwards.
• Disconnect both ends. Equipment at either end, such as drives, meters or surge arresters, must be disconnected or the result will reflect that equipment instead of the cable.

For medium and high voltage cables, an IR test is usually followed by withstand tests such as very low frequency (VLF) or DC tests according to the cable standard, and by sheath tests for cables with an outer sheath.

Interpreting results

• High and rising: insulation is in good condition.
• Low but rising: often surface moisture or dirt; cleaning and drying usually help.
• Low and flat or falling: moisture absorbed into the insulation, contamination or damage; investigate before energising.
• Sudden drop between routine tests: a warning sign even if the value is still above the minimum.