The useful answer is simple: higher insulation resistance is generally better, but a “good” Megger reading is one taken safely, at the correct test voltage, under the governing installation rules, and interpreted in context. For many IEC-style low-voltage circuits, 1 MΩ at 500 V DC is the familiar minimum—not an all-purpose safety guarantee.
A good Megger reading is not one universal number. It is a correctly performed insulation-resistance test that meets the rule governing that installation, uses a voltage suitable for the circuit and connected equipment, and produces a result that is credible when compared with similar circuits and earlier records. For many IEC/BS 7671-style low-voltage wiring tests, the familiar baseline is at least 1 MΩ at 500 V DC for circuits up to and including 500 V. That is a minimum verification value—not a blanket declaration that any circuit reading 1.01 MΩ is healthy or safe in every setting.
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“Megger” is commonly used as a verb, but the accurate generic name for the instrument is an insulation-resistance tester. It applies DC voltage across insulation and measures the tiny leakage current. The instrument then reports resistance, usually in megohms (MΩ), using Ohm’s law: R = E ÷ I. In general, higher insulation resistance means less measured leakage through or across insulation.
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What counts as a good insulation-resistance reading?
There are three different questions hidden inside “Is this Megger reading good?” They should not be confused.
- Compliance: Does the test meet the minimum value and method required by the adopted code, project specification, or authority having jurisdiction?
- Condition: Is the result stable, sensible for this circuit, and comparable with equivalent circuits and prior readings?
- Diagnosis: If the result is low or unusual, what is causing it?
An insulation-resistance test can help answer the first two questions. A single number rarely identifies the exact failed cable, junction box, appliance, or contamination path. Nor does a high reading prove that continuity, polarity, earthing, protective-device operation, or every other required electrical test has passed.
The commonly cited IEC test voltages and minimum values
IEC 60364-6 is a low-voltage-installation verification framework used as the basis for the following widely cited values. They are useful context, but they are not a substitute for the locally adopted code edition, project requirements, or equipment instructions.
| Installation/circuit context | DC insulation-test voltage | Minimum insulation resistance |
|---|---|---|
| SELV/PELV | 250 V DC | 0.5 MΩ |
| Circuits up to and including 500 V, including FELV | 500 V DC | 1 MΩ |
| Circuits above 500 V | 1,000 V DC | 1 MΩ |
In UK practice, 1 MΩ is the familiar BS 7671 minimum for the applicable insulation-resistance test. The IET model forms also reflect that 500 V is normally used unless the circuit could be damaged. That does not make 1 MΩ at 500 V a universal rule for the United States, Canada, Australia, or any other jurisdiction. The qualified person must follow the requirements actually adopted for the work.
Where surge-protection devices or other connected equipment could affect the result or be damaged, the relevant equipment should normally be disconnected before the test. In the cited IEC framework, where disconnection is not reasonably practicable, the circuit test voltage may be reduced to 250 V DC while retaining a 1 MΩ requirement. This is a specific testing accommodation, not permission to test a modern occupied circuit with all loads left connected.
Minimum compliance is not the same as healthy wiring
A minimum value is a floor for a defined test method. It does not mean that every result above the floor deserves the same conclusion.
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For example, if several comparable dry branch circuits consistently read far above the meter’s practical range but one reads 1.2 MΩ at the same voltage, that circuit may meet a 1 MΩ minimum while still deserving investigation. Check the circuit arrangement, disconnected loads, moisture exposure, contamination, cable damage, and comparison records before accepting it without comment.
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How to interpret common results
High, stable readings
High resistance is generally desirable. It indicates low measured leakage under the selected test conditions. The result is most useful when the circuit was correctly isolated, the correct conductors were selected, connected equipment was handled according to procedure, and the reading is consistent with comparable circuits or previous records.
Readings close to the applicable minimum
Do not simply write “pass” and move on. Recheck the test setup and test voltage. Confirm whether lighting drivers, controls, appliances, surge protection, electronic power supplies, or other equipment remain connected. Review ambient and building conditions, especially recent water intrusion or high humidity. Compare the result with similar circuits and with any historical results taken under similar conditions.
Low, falling, fluctuating, or unstable readings
Low insulation resistance can be associated with dampness, dirt, conductive contamination, aged or damaged insulation, wiring errors, or a path to earth. A changing reading may be equally significant. The appropriate next step is controlled isolation and fault finding—not guessing from one display value.
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A qualified technician may divide the circuit at accessible, documented points; retest the separated sections; and inspect or test connected equipment under the governing procedure. The goal is to narrow the affected section without applying unsuitable test voltage to electronics or defeating required safety controls.
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Insulation resistance and continuity are opposites in one important respect
These two tests are often confused because both report resistance.
| Test | What the intended path should do | Generally desirable result |
|---|---|---|
| Continuity or bonding | Carry current reliably through a deliberate conductive path | Low resistance |
| Insulation resistance | Prevent current from escaping to another conductor or earth | High resistance |
A low continuity reading can be good. A low insulation-resistance reading can indicate an unintended leakage path. Neither result should be interpreted outside its test purpose and method.
Selecting the test voltage without damaging the installation
Start with the installation documentation and manufacturer instructions. The test voltage must suit both the circuit and the equipment that remains connected. An insulation tester must never be connected to an energized circuit, and its output can damage solid-state equipment such as electronic power supplies, PLCs, variable-speed drives, UPS systems, battery chargers, controls, and similar devices.
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- the circuit’s nominal voltage and the verification standard that applies;
- all connected loads, controls, protective devices, surge-protection devices, and power-electronic equipment;
- whether the required method calls for components to be disconnected or conductors to be tested in a particular grouping;
- the tester’s rated test voltages, category rating, live-circuit warning or inhibit function, and discharge capability; and
- any manufacturer instruction that limits testing or requires a particular procedure.
The highest available test voltage or the largest number printed on a meter is not automatically better. Use the voltage called for by the applicable method and the actual equipment configuration.
A safe high-level testing sequence
The exact procedure must come from the applicable safety program, installation rules, and instrument instructions. At a high level, qualified personnel should:
- Plan the test. Identify all sources of energy, circuit boundaries, backfeed possibilities, connected equipment, and the required test configuration.
- Isolate and secure the circuit. Apply the required lockout/tagout and energy-control measures. Switching a breaker off alone is not proof of de-energization.
- Verify absence of voltage. Use appropriate test equipment and check for induced voltage or unrelated backfeed as required by the formal procedure.
- Protect equipment. Disconnect or isolate sensitive electronics, surge-protection devices, and other components the method requires to be removed from the test.
- Perform the specified test. Use the required voltage, lead arrangement, duration, and conductor grouping. Keep clear of exposed conductors and test leads.
- Allow discharge and confirm safe condition. Keep the tester connected until its discharge process is complete and account for residual charge before touching, reconnecting, or reassembling the circuit.
- Restore only when ready. Remove temporary controls according to the approved procedure, reconnect equipment correctly, and re-energize only after the installation has been verified ready.
Why weather, dirt, and time matter
Insulation resistance is affected by temperature, humidity, moisture, and surface contamination. A reading taken in a dry, conditioned building may not be directly comparable with one taken after a wet construction phase, a flood event, or prolonged high humidity. The result can include leakage over insulation surfaces as well as through the insulating material itself.
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Useful records identify the circuit or equipment, test voltage, conductor configuration, test duration where relevant, meter used, result, date, and notable environmental or equipment conditions. For maintenance work, a consistent decline under roughly comparable conditions often tells more than a single isolated number.
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Timed readings and polarization index
For suitable maintenance applications, a one-minute spot reading allows charging current to settle. A polarization index (PI) compares insulation resistance at 10 minutes with resistance at one minute:
PI = resistance at 10 minutes ÷ resistance at 1 minute
These timed techniques can be useful for evaluating insulation behavior over time on equipment for which they are appropriate. They should not be treated as mandatory for every residential circuit, routine fixed-wiring verification, or small installation. Follow the procedure and equipment guidance governing the job.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choosing an insulation-resistance tester
Search for an insulation resistance tester rather than relying on “Megger” as a generic product description. Select a model around the work, not around the most dramatic maximum-resistance specification.
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- Installation safety rating: Confirm the meter and leads carry the category rating appropriate for the actual installation. The Fluke 1507 is specified for CAT IV 600 V.
- Live-circuit protection: Favor clear live-circuit warning or inhibit behavior, but never use it as a substitute for proper isolation and voltage verification.
- Measurement range and resolution: A wide range can be valuable for maintenance trending. The Megger MIT480/2 series, for example, is specified for handheld insulation and continuity testing up to 500 V with a 100 GΩ range and gated 500 V access.
- Discharge behavior and leads: Understand how the instrument discharges capacitive circuits and use properly rated, intact leads and accessories.
- Procedure compatibility: The instrument must support the approved test method; a capable meter does not authorize a test configuration that the installation or manufacturer prohibits.
A lockout/tagout kit is a useful related accessory, but it does not replace an energy-control plan, voltage verification, competent personnel, or an insulation-resistance tester.
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What to record on a test report
- Circuit, cable, panel, or equipment identification
- Date, tester identification, and calibration status where required
- Test voltage and test duration
- Conductors tested and any temporary disconnections made
- Measured value, including an honest over-range notation such as “>” value
- Temperature, humidity, visible moisture, contamination, or unusual site conditions when relevant
- Comparison with prior or similar-circuit results
- Any limitations, corrective work, retests, and final disposition
The practical takeaway
For ordinary IEC-style low-voltage verification, 1 MΩ at 500 V DC is the well-known minimum for circuits up to 500 V. It is a useful reference point, but not a universal shortcut. A genuinely good Megger result is obtained safely at the correct voltage, with sensitive equipment properly handled, meets the governing requirement, and makes sense in the context of the circuit, environment, and test history.
Frequently Asked Questions
Is 1 MΩ a good Megger reading?
It is a common minimum in IEC/BS 7671-style verification for circuits up to and including 500 V when tested at 500 V DC. It is not a universal pass/fail rule for every country, circuit, or equipment configuration.
Can I Megger a circuit with appliances and electronics connected?
Not necessarily. A tester applies DC voltage that may damage surge-protection devices and solid-state equipment, including electronic power supplies, PLCs, VSDs, UPS systems, and battery chargers. The qualified person must follow the equipment instructions and approved test procedure.
Does a high insulation-resistance result prove the wiring is safe?
No. A high result indicates low measured leakage at the selected voltage and range; it does not prove continuity, polarity, earthing, protective-device performance, or the absence of every electrical defect.
What should happen if the Megger reading is low or unstable?
Treat it as a condition requiring controlled investigation. Confirm the test setup and test voltage, consider moisture or contamination and connected equipment, then isolate sections in accordance with the approved procedure to narrow the issue.
The Bottom Line
Bottom line: High insulation resistance is generally good, but “good” means more than clearing a single number. Use the correct test method and voltage, verify the circuit is safely de-energized, protect connected electronics, and interpret the result against the applicable rule and comparable records.
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