How to use a Megger to test insulation resistance on cables
A Megger is a specialised insulation resistance tester used to assess the condition of electrical cables, motors, switchboards and other equipment. It applies a controlled direct-current voltage and measures how effectively insulation prevents current leakage between conductors and earth. The result is normally displayed in megohms, or MΩ, where a higher reading generally indicates better insulation.
Testing insulation resistance can reveal moisture ingress, contamination, crushed insulation, ageing, thermal damage and faults created during installation. It is commonly carried out during commissioning, preventive maintenance, fault finding and after cable repairs. A sound-looking cable can still contain insulation damage that will only become apparent under a test voltage.
The work must be planned carefully. A megohmmeter can generate potentially hazardous voltages, and the circuit must be isolated, proved dead and disconnected from sensitive equipment before testing begins. In Australia, testing should be performed by an appropriately licensed electrical worker in line with applicable state or territory requirements, AS/NZS 3000, AS/NZS 3017 and site procedures.
What a Megger measures
Insulation resistance is the resistance offered by the insulating material around a conductor. When a test voltage is applied, a small leakage current flows through or across the insulation. The instrument uses that current to calculate resistance, commonly according to the relationship between voltage and current. The displayed value may be expressed as MΩ or GΩ on higher-range instruments.
A cable with healthy, dry and undamaged insulation should produce a stable, high reading. A low reading suggests that current may be able to travel through the insulation, between cores, from a core to earth, or through surface contamination. Falling readings can be significant too, especially when comparing phases, cores or repeated tests over time.
The test voltage must suit the circuit and the equipment connected to it. Portable testers may provide settings such as 250 V, 500 V, 1,000 V or higher. Selecting a voltage without checking the cable rating, connected devices and applicable standards can damage electronic components or produce an unsafe test condition. The manufacturer’s instructions and the relevant installation standard take priority over a generic testing routine.
Prepare the cable and work area
Begin by identifying the correct cable, its origin and destination, conductor arrangement, nominal voltage and connected loads. Review drawings, isolation procedures and previous test records. At an industrial site such as a processing plant in Gladstone, Newcastle or the wider Hunter region, cable routes may pass through several switchboards, marshalling panels and field devices, so a visual trace and documentation check are essential.
Isolate every possible source of energy, including normal supplies, standby generators, photovoltaic systems, batteries, capacitors and backfeeds from interconnected equipment. Lock out and tag the isolation points, then use an approved voltage detector to prove the conductors are de-energised. The tester itself should be checked on a known proving source before and after the dead test where site procedures require it.
Disconnect sensitive equipment before applying the insulation test. Variable-speed drives, programmable logic controllers, control instruments, surge protection devices, electronic power supplies, LED drivers and communication equipment may be affected by the test voltage. Where a cable remains connected to a motor or transformer, follow the equipment manufacturer’s instructions and confirm whether the winding, neutral, screen or protective devices should be included.
Wear the required personal protective equipment and establish an exclusion area. Use insulated test leads in good condition, keep exposed conductors controlled and prevent other workers from touching the circuit. A clear test plan is particularly important during shutdowns in Queensland, Western Australia and New South Wales, where several contractors may be working around the same electrical assets.
Connect the Megger correctly
For a multicore cable, test each conductor to the others joined together and to earth, unless the applicable procedure specifies a different arrangement. You may also test individual core-to-core combinations. For example, on a three-core cable, possible tests include core 1 to cores 2 and 3 plus earth, core 2 to cores 1 and 3 plus earth, core 3 to cores 1 and 2 plus earth, and each pair of cores separately.
Connect the instrument’s line or positive lead to the conductor under test. Connect the earth or negative lead to the cable screen, armour, protective conductor or a verified earth reference. If the cable has a metallic screen, bonding arrangement or concentric neutral, record how it was connected because this affects the meaning of the result.
A guard terminal may be available on some professional insulation testers. It can divert surface leakage from the measurement circuit, helping distinguish contamination on the outside of insulation from leakage through the insulation itself. The guard function is useful for long cables, damp environments and high-resistance measurements, but it must be connected according to the tester manufacturer’s instructions.
Keep untested conductors isolated and secure. Do not hold the leads or touch the cable while the test is running. Press the test control for the required duration, commonly one minute for a timed diagnostic reading, and allow the tester to discharge the cable when the test ends. Long cables can retain a charge, so never assume that the circuit is safe immediately after releasing the test button.
Choose the test voltage and interpret readings
There is no universal pass value for every cable. The acceptable result depends on the installation voltage, cable type, length, operating environment, standard, manufacturer’s data and condition of the connected equipment. A short new low-voltage cable may produce an extremely high reading, while a long, humid or older cable may show a lower value without automatically indicating immediate failure.
The following guide provides a practical comparison, but it is not a substitute for the applicable Australian standard, project specification or equipment instructions.
| Test situation | Typical approach | What the result may indicate |
|---|---|---|
| Low-voltage cable with sensitive electronics disconnected | Select the approved lower or standard test voltage | Protects equipment while checking for basic insulation leakage |
| New or recently repaired power cable | Use the voltage specified by the cable and installation requirements | A high, stable reading supports commissioning; investigate unexpected imbalance |
| Long industrial feeder | Test at the specified voltage and allow adequate stabilisation time | Cable length, temperature and moisture can influence the value |
| Motor cable with a variable-speed drive | Disconnect the drive before testing unless its manual permits the test | Prevents damage to semiconductors and control electronics |
| Cable with a metallic screen or armour | Connect the reference lead as specified and document the arrangement | Helps identify core-to-screen or core-to-earth leakage |
| Low or rapidly falling reading | Stop, isolate the defect and retest sections separately | May indicate water, contamination, crushing, heat damage or an incorrect connection |
Record the test voltage, duration, conductor combination, temperature, humidity, cable identification and measured resistance. Temperature has a material effect on insulation resistance, so comparisons are more useful when conditions are similar. A result that is acceptable at 20°C may differ considerably from one recorded on a hot Western Australian summer afternoon or in a damp coastal environment near Wollongong.
When a reading is low, do not repeatedly increase the test voltage in an attempt to force a pass. First confirm that all equipment has been disconnected, the correct conductors are being tested and the leads are clean and dry. Then divide the circuit into sections and retest. This process helps locate whether the problem is in the cable, a termination, a joint, a gland or connected equipment.
Test cables safely in an industrial environment
Industrial cables may be exposed to chemicals, vibration, heat, salt air, oil, dust and mechanical movement. In a food-processing plant, a washdown area can introduce moisture around glands and terminations. In a mining or resources facility near Perth or Port Hedland, dust, high ambient temperatures and long cable runs can influence both the test method and the result.
Before testing, inspect the cable route and terminations for cracked glands, loose covers, damaged outer sheaths, corrosion, tracking or signs of water entry. Insulation resistance testing measures electrical leakage, but it does not replace visual inspection, continuity testing, earth-fault loop checks or verification of correct polarity and phase identification.
Hazardous areas require additional controls. Equipment installed in classified locations may have special certification, isolation requirements and restrictions on test equipment. Gas, vapour or dust hazards must be controlled before opening enclosures or disconnecting cables. Follow the site permit system and consult the responsible electrical supervisor when the cable serves a hazardous-area installation.
After the test, discharge the conductors through the instrument or an approved discharge method. Confirm with the voltage detector that no residual voltage remains before touching the cable. Remove temporary connections, restore screens and earths, check covers and glands, and complete the reinstatement and lockout removal process under the site’s authorisation system.
Diagnose low or inconsistent insulation resistance
A low reading can come from several sources. Moisture is common around outdoor junction boxes, underground joints, cable trenches and poorly sealed glands. Dirt, conductive dust, salt deposits and chemical residue can create a leakage path across a termination without the cable insulation itself being damaged. Cleaning and drying may improve the reading, but the cause of contamination should still be addressed.
Mechanical damage can result from excessive bending, pulling tension, crushing, sharp edges or poorly supported cable trays. Heat from furnaces, motors, steam lines and overloaded conductors can accelerate insulation ageing. In Australia’s industrial and mining sectors, ultraviolet exposure and large temperature changes can also affect exposed cable sheaths and accessories.
Compare individual cores and test sections where practical. If one core produces a significantly lower value than the others, inspect its terminations and route. If all cores are low to earth but core-to-core readings are higher, the fault may involve the sheath, screen, armour or moisture on the outside. If core-to-core readings are low, there may be damage between conductors or a connected component that was not removed.
A rising reading during the test can occur as insulation polarises and absorbs charge. A falling reading may indicate moisture, contamination or insulation breakdown under stress. Some testers provide polarisation index or dielectric absorption ratio functions, but those measurements must be used only when suitable for the equipment and test duration. They are more relevant to certain machines and larger assets than to every ordinary cable test.
Record results and build a maintenance history
A useful test record identifies the asset, cable number, circuit origin and destination, conductor configuration, test voltage, test duration, resistance value, ambient conditions and person conducting the work. Note whether motors, drives, instruments or surge devices were disconnected. Include any cleaning, drying, repair or replacement completed before the final test.
Trend records can reveal deterioration before a cable fails in service. A gradual decline across several maintenance periods may justify closer inspection or planned replacement, even when the latest result remains above the minimum requirement. Consistent records also help maintenance teams distinguish a genuine change in cable condition from differences caused by test setup or weather.
Digital insulation testers often store readings and export reports, while simpler instruments rely on manual forms. Either approach is effective when the information is complete and traceable. Keep records with the site’s electrical asset management system and make them available to the authorised person responsible for commissioning or returning the equipment to service.
Training should cover practical connection methods, safe isolation, instrument limitations and interpretation of results. Facilities such as HCF CATCH provide realistic process-plant environments where learners can practise industrial electrical and safety procedures using equipment and scenarios that reflect workplace conditions. For employers, hands-on assessment can help verify that workers understand the procedure rather than simply knowing how to press the test button.
Apply the results to commissioning and maintenance
For a new installation, insulation resistance testing forms part of a wider verification process. Confirm conductor identification, continuity, protective earthing, polarity, phase sequence and correct operation of protective devices as required by the installation design and applicable standards. A high insulation reading alone cannot prove that the circuit has been wired correctly or that it is safe to energise.
For existing equipment, compare the result with previous records and the manufacturer’s guidance. A sudden change should be investigated before the circuit is returned to service. Depending on the defect, the response may include replacing a termination, drying a junction box, repairing a cable section, improving gland sealing or isolating a damaged feeder until a planned shutdown.
Keep test boundaries clear. A megger test is designed to assess insulation resistance, not to prove cable capacity, fault-current withstand, thermal performance or mechanical integrity. It should sit within a documented electrical maintenance program that includes inspection, functional testing and risk-based replacement.
Australian workplaces may use different forms, permit systems and regulator expectations across states and territories, but the essential principles remain consistent: isolate, prove dead, test with the correct voltage, discharge, record and reinstate safely. Competent supervision and current knowledge of AS/NZS requirements are essential when the test involves high-energy industrial equipment.
A reliable insulation test is the product of correct planning, suitable equipment and disciplined interpretation. HCF CATCH supports employers, apprentices and technical teams with practical industrial training, safety qualifications and hands-on learning in a live-process-plant setting. Explore relevant electrical and safety training options to strengthen cable testing capability and support safer maintenance across your workplace.