How to Use a Megohmmeter: A Beginner’s Complete Guide

The first time you pick up a megohmmeter, it looks familiar but behaves differently. It has probes and a display like a multimeter, but the numbers it produces are completely different. Why is 500 MΩ good on some equipment and 5 MΩ good on others? Why does the reading keep climbing after you apply voltage? Why does pressing “Test” sometimes trip the protection on nearby equipment?

This guide covers everything you need to know to use a megohmmeter correctly from your first test. It’s written for someone who already knows their way around basic electrical work but has never used a megger before. If you follow this guide, your first tests will be safe, accurate, and meaningful.

What a Megohmmeter Actually Does

A megohmmeter measures very high resistance values by applying a controlled DC voltage and measuring the tiny current that leaks through. Ohm’s law does the rest: Resistance = Voltage ÷ Current.

What you’re actually measuring is the quality of electrical insulation. Good insulation resists the applied voltage almost completely — only nanoamperes of current flow, so resistance appears as hundreds or thousands of megohms. Damaged insulation allows more current to flow, so resistance drops.

The output of a megger is insulation resistance in megohms (MΩ) or gigohms (GΩ):

• 1 megohm = 1,000,000 ohms
• 1 gigohm = 1,000,000,000 ohms (1,000 MΩ)

A reading of 500 MΩ means you can use a stronger current before the insulation gives way. A reading of 0.5 MΩ means the insulation is barely doing its job.

How It’s Different from a Multimeter

A multimeter measures resistance using a low voltage — typically 3 to 9 volts DC. That’s fine for measuring wire resistance, component resistance, or checking continuity. But it’s completely inadequate for testing insulation.

Why? Because insulation that is partially broken down may still pass a continuity test at 9 volts. The fault only shows up when normal operating voltage (240 V, 480 V, or higher) is applied. A multimeter can’t reveal this.

A megohmmeter solves this by applying a stress voltage — typically 250 V, 500 V, 1,000 V, 2,500 V, or 5,000 V DC depending on the equipment. At those voltages, any insulation defect shows up as measurable leakage current.

Quick comparison

Feature	Multimeter	Megohmmeter
Test voltage	3–9 V DC	50–10,000 V DC
Typical use	Measuring low resistance, continuity, voltage	Testing insulation integrity
Resistance range	0 to ~200 MΩ	0 to 10,000+ GΩ
Can detect incipient insulation faults?	No	Yes

The core takeaway: A multimeter in resistance mode will tell you if a wire is connected. A megohmmeter tells you if the insulation around that wire is still doing its job.

The Parts of a Megger

A typical handheld digital megohmmeter has these main components:

Display

Shows the resistance reading, usually in MΩ or GΩ. Better instruments also show the applied test voltage and the elapsed test time.

Test voltage selector

A rotary dial or buttons that let you choose the test voltage — 250 V, 500 V, 1,000 V, 2,500 V, etc. On most instruments, you must set this before starting the test.

Test terminals (leads)

Three terminals on most modern instruments:

• LINE (sometimes called L, +, or HIGH) — The high-voltage output. Connect this to the conductor under test.
• EARTH (sometimes called E, −, or GND) — The return path. Connect this to ground or the equipment frame.
• GUARD (sometimes called G) — Used to eliminate surface leakage currents. Not needed for basic tests, but essential for accurate readings in damp conditions.

Test button

Initiates the test. On some instruments, you press and hold it for the duration of the test. On most digital models, you press once to start, once to stop.

Mode selector

Lets you choose IR (basic insulation resistance), PI (polarization index — 10-minute test), DAR (dielectric absorption ratio — 60-second test), V (voltage measurement), or continuity. The modes vary by instrument.

Selecting the Right Test Voltage

This is the question beginners most often get wrong. The test voltage must match the equipment’s rated voltage — too low and the test won’t reveal defects, too high and you may damage healthy insulation.

Basic guideline

The test voltage should generally be at or slightly above the equipment’s working voltage:

Equipment Rating	Test Voltage (DC)
Up to 50 V (SELV/PELV)	250 V
50–500 V (household, general industrial)	500 V
500–1,000 V (industrial machinery)	1,000 V
1,000–2,500 V	1,000 V or 2,500 V
2,500–5,000 V	2,500 V or 5,000 V
Above 5,000 V	5,000 V or higher

What the standards say

Different standards give slightly different values:

• IEC 60364-6 (electrical installations): 500 V DC for circuits up to 500 V, minimum IR ≥ 1 MΩ
• IEC 60204-1 (machinery): 500 V DC, minimum IR ≥ 1 MΩ (or ≥ 0.5 MΩ for heating circuits)
• IEEE 43-2013 (rotating machinery): Table 1 specifies test voltages by motor rating
• IEC 60255-5 (protection relays): 500 V DC ± 10%, minimum IR ≥ 100 MΩ

When in doubt, use 500 V DC. It’s the most common test voltage and works for the vast majority of industrial and building electrical installations.

What to avoid

Don’t exceed the manufacturer’s maximum test voltage. Some instrument cables and electronic components are rated only for 500 V test voltage. Applying 1,000 V or 2,500 V can damage them.

Don’t use 50 V or below for bulk insulation testing. Low voltages don’t stress insulation enough to reveal defects. The minimum standard test voltage for most equipment is 250 V.

The Complete Test Procedure

Here’s the step-by-step procedure that works for any basic insulation test. I’ll use testing a 400 V motor as the example, but the same sequence applies to cables, transformers, and control panels.

Step 1: De-energize and isolate

Turn off the breaker or disconnect feeding the equipment. Apply lockout/tagout. Remove fuses if applicable. Verify with a voltmeter that the equipment is truly dead — test phase-to-phase and phase-to-ground.

Step 2: Disconnect from everything else

Disconnect the motor (or equipment under test) from its feeder cable at the motor terminal box. Remove wiring to VFDs, PLCs, relays, surge protection devices, and any electronic equipment. These can all be damaged by the test voltage.

This step matters. A common beginner mistake is testing with the VFD still connected. A 500 V test will destroy a typical VFD’s input rectifier and DC bus capacitors in seconds.

Step 3: Discharge stored energy

Short all conductors to ground for at least 60 seconds. This removes any residual charge from recent operation. Even a motor that’s been off for an hour can hold measurable charge.

Step 4: Record conditions

Note the ambient temperature, equipment temperature if possible, and relative humidity. These affect the reading. A motor reading 50 MΩ at 30°C and a motor reading 50 MΩ at 15°C are in very different condition — you can’t compare them without recording the temperature.

Step 5: Set up the instrument

1. Check the megger’s batteries and calibration date.
2. Inspect the leads for cracked insulation or damaged clips.
3. Turn on the instrument.
4. Select the correct test voltage (500 V for this example).
5. If the instrument has an auto-discharge feature, verify it’s enabled.

Step 6: Make the connections

• Connect the LINE lead (red) to the equipment terminal you want to test. For a motor: to the three phase terminals (T1, T2, T3) connected together. For a cable: to the conductor.
• Connect the EARTH lead (black) to the equipment frame or ground terminal.
• If the instrument has a GUARD terminal and you’re testing in damp conditions, connect the guard lead to intercept surface leakage. Otherwise, leave it disconnected for a basic test.

Step 7: Make sure the area is clear

Before pressing the test button, verify nobody is touching the equipment. Post a warning if there are other workers in the area. The test voltage appears on every conductor connected to the circuit — not just where your leads are.

Step 8: Run the test

Press the TEST button. The instrument applies voltage. The reading will climb over several seconds as the instrument stabilizes and the insulation absorbs charge.

• Wait at least 60 seconds before recording the reading. The 60-second reading is what you compare to the standards.
• For a PI test, let the voltage run for the full 10 minutes and record readings at 30 seconds, 1 minute, and 10 minutes.

Step 9: Record the reading

Write down: the IR value, the test voltage, the test duration, the ambient temperature, the equipment temperature, the humidity, the date, and the equipment ID. This data is how you’ll trend the results over time.

Step 10: Discharge the equipment

This is the critical safety step. Stop the test. Keep the leads connected. Most modern meggers have an auto-discharge feature — you’ll see the voltage drop to zero on the display. For a 60-second test, discharge for at least 4 minutes. For a 10-minute PI test, discharge for at least 40 minutes.

Before removing the leads, verify with a voltmeter that the voltage across the terminals is below 50 V.

Step 11: Disconnect and restore

Remove the megger leads. Reconnect the motor cables and any other equipment you disconnected. Remove the lockout/tagout. The equipment is ready to return to service.

What the Numbers Mean

Your megger will give you a number. Here’s how to interpret it for the most common equipment types.

For general electrical installations (IEC 60364-6)

IR Reading at 500 V DC	Assessment
Above 100 MΩ	Excellent
10–100 MΩ	Good
1–10 MΩ	Marginal, but meets minimum
Below 1 MΩ	Fail — do not energize

For motors (IEEE 43-2013, at 40°C)

Motor Type	Minimum IR
Random-wound (small motors below 1 kV)	5 MΩ
Form-wound (medium and high-voltage motors)	100 MΩ
Windings made before 1970	(kV rating + 1) MΩ

For protection relays (IEC 60255-5)

Minimum IR at 500 V DC: 100 MΩ for new relays.

Interpreting a single reading

One reading tells you whether the equipment is above the pass/fail threshold today. It doesn’t tell you how quickly the insulation is degrading, or whether it’s going to fail soon. For that, you need multiple readings over time.

The dedicated article on this topic is How to Read Insulation Test Results, which covers interpretation in much more depth.

Common Beginner Mistakes

I’ve seen all of these during training sessions. Avoid them and your tests will be much more reliable.

Not disconnecting connected equipment

Testing with a VFD, PLC, or surge protection device still connected destroys the connected equipment and gives a false low reading. Always disconnect everything before applying test voltage.

Forgetting to discharge before and after

Before the test: residual charge creates errors in the first few seconds. After the test: stored charge in the insulation is a shock hazard. Discharge both times — before for at least 60 seconds, after for at least 4× the test duration.

Wrong test voltage

Testing a 24 V control circuit at 500 V can damage the components. Testing a 4,160 V motor at 500 V won’t reveal insulation defects that only show at operating voltage. Match the test voltage to the equipment — or when in doubt, use 500 V.

Not recording conditions

A 50 MΩ reading in January doesn’t mean the same thing as a 50 MΩ reading in July. Without recording temperature and humidity, you can’t trend the results. Record everything — even if you feel like you’re over-documenting.

Touching the leads during the test

The test voltage is on both leads during the test. Even at 500 V DC, the current is low but uncomfortable. At 2,500 V or higher, it can cause serious injury. Hands off during testing.

Comparing readings from different equipment types

A cable reading 50 MΩ and a motor reading 50 MΩ are not comparable. Equipment size, insulation type, and operating conditions all affect what’s acceptable. Use equipment-specific minimums, not generic rules.

Ignoring the 1-minute wait

The first few seconds of a test show capacitive charging current, which gives an artificially low reading. Wait the full 60 seconds for the reading to stabilize. If you read at 10 or 20 seconds, the number will be lower than the true insulation resistance.

Reading Your Megger’s Display

Different instruments display information differently, but these elements are common:

Main resistance value — The big number in the center. Usually in MΩ by default, switching to GΩ for higher values. Some instruments auto-range; others require manual range selection.

Test voltage indicator — Shows the actual voltage being applied. Some instruments display both the requested voltage and the actual voltage — they can differ under load.

Timer — Shows elapsed test time. Important for knowing when to take the 60-second reading.

Mode indicator — IR, PI, DAR, step voltage, or continuity. Make sure you’re in the right mode before starting.

Battery indicator — Low battery can cause incorrect readings. Replace or recharge before testing.

Range indicator — Some instruments show the active measurement range (e.g., “GΩ” or “MΩ”). Make sure the range is appropriate for the expected value.

When the reading doesn’t stabilize

If the reading keeps climbing slowly for minutes, that’s normal on good insulation — it means the insulation is absorbing charge. The 60-second reading is your reference value.

If the reading fluctuates wildly, you may have:

• A loose connection (tighten it)
• Surface leakage in a humid environment (use the guard terminal)
• A failing instrument (check with a known-good reference resistor)
• Induced voltage from nearby energized equipment (uncommon but possible in substations)

If the reading shows “OL” or “∞” or “infinity” — the resistance exceeds the instrument’s measurement range. This usually means the insulation is in excellent condition. You may need an instrument with a higher range to get a specific number.

If the reading shows zero or very near zero — the insulation has failed, or you have a direct short in your test setup. Check your connections first.

When to Use a Megger and When Not To

Use a megger for

• Commissioning new installations per IEC 60364-6
• Maintenance testing of motors, cables, transformers
• Troubleshooting suspected insulation faults
• Verifying electrical safety after repairs or modifications
• Building a trending database for predictive maintenance

Don’t use a megger for

• Live circuits (the megger is not a voltmeter — testing a live circuit can damage the instrument)
• Electronic equipment (PLCs, drives, sensors — will destroy them)
• Very small resistance measurements (below 1 MΩ — use a multimeter)
• Measuring voltage or current (use dedicated instruments)

When you need something more than a basic megger

• Full diagnostic testing: look into the PI test, DAR, and step voltage testing
• Cable fault location: a TDR (time domain reflectometer) is a better tool
• Power factor / tan delta: specialized equipment beyond a standard megger

Your First 10 Tests

Here’s a practical learning sequence for a beginner. Each test builds on skills from the previous ones.

1. A known-good insulated wire — Connect both ends, verify reading is in the GΩ range. Builds familiarity with the instrument.
2. A deliberately short-circuited wire — Confirms that the instrument reads near-zero for failed insulation.
3. An office lighting circuit (de-energized, all lamps removed) — First real installation test per IEC 60364-6.
4. A small single-phase pump motor (500 V DC) — First motor test. Practice disconnection and terminal box work.
5. A three-phase motor, bulk test (all three phases together) — Build on single-phase skills.
6. The same motor, phase-by-phase — Learn to compare phase readings and spot imbalance.
7. A short length of cable (under 50 meters) — Practice cable testing with both ends disconnected.
8. A longer cable run — Discover how cable length affects the reading and why normalization matters.
9. A motor with a suspected problem — Practice troubleshooting. What does a motor with contaminated windings look like?
10. A PI test on a form-wound motor — 10-minute test. Learn the timing, the patience, and what rising resistance looks like.

By the end of this sequence, you’ll have the core skills for most field work.

FAQ

Key Takeaways

• A megohmmeter measures insulation resistance by applying high DC voltage (typically 500–5,000 V) and measuring the tiny leakage current.
• Multimeters can’t do this job. Their low test voltage (under 10 V) won’t stress insulation enough to reveal faults.
• Match the test voltage to the equipment. 500 V DC for most general-purpose industrial work.
• Always de-energize, disconnect, and discharge before testing. Electronic equipment must be removed from the circuit.
• Wait the full 60 seconds for the reading to stabilize. Short reading times give artificially low values.
• Discharge after the test for at least 4× the test duration. Long tests at high voltages store significant charge.
• Record everything — reading, temperature, humidity, test voltage, equipment ID. Single readings are snapshots; trending is the real value.
• Start with the 10-test learning sequence. Build skills on known-good equipment before testing anything critical.

Standards Referenced in This Article

Standard	Key Content
IEEE 43-2013	Motor testing: test voltages (Table 1), minimum IR (Clause 12.1), PI methodology
IEC 60364-6	Electrical installations: test voltages (250/500/1000 V DC), minimum IR ≥ 1 MΩ (Clause 6.4.3.3)
IEC 60204-1	Machinery: 500 V DC test, ≥ 1 MΩ minimum (Clause 18.3)
IEC 60255-5	Protection relays: 500 V DC ± 10%, ≥ 100 MΩ (Clause 6.2.2)
IEC 61010-1	Safety requirements for electrical test equipment
NFPA 70E	Electrical safety in the workplace: PPE, LOTO, arc flash