Insulation Resistance Tester
A specialized high-voltage test instrument used to measure the insulation resistance of AGL cables, transformers, and electrical equipment. Also known as a "megohmmeter" or "megger," this tool is essential for detecting insulation degradation, moisture ingress, and potential breakdown risks before they cause system failures.
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What Is It?
An Insulation Resistance Tester (commonly referred to as a megger or megohmmeter) is a test instrument that applies a high DC voltage to an electrical cable or equipment and measures the resulting leakage current to determine the insulation resistance. In AGL systems, it is used to assess the health of:
- Primary and secondary AGL cables – detecting insulation breakdown, moisture ingress, and mechanical damage
- Isolating transformers – verifying primary-to-secondary and winding-to-ground insulation
- Light fixtures – checking internal insulation integrity
- Junction and distribution boxes – ensuring insulation between conductors and earth
- CCRs and power equipment – verifying insulation of windings and bus bars
The tester typically offers multiple test voltages (250V, 500V, 1000V, 2500V, 5000V) to suit different cable ratings and applications. Results are displayed in megohms (MΩ) or gigohms (GΩ), with higher values indicating better insulation quality.
Purpose & Function
The insulation resistance tester exists to catch the kind of insulation breakdown — from moisture ingress, mechanical damage, or age-related degradation — that won't necessarily show up as an obvious fault yet, but represents a developing safety and reliability risk. A cable with insulation resistance quietly dropping over months of service isn't causing problems yet, but it's heading toward a leakage path or short that eventually will, and this test is what catches that trend early.
Applying a range of test voltages lets the instrument match the stress level to what a given cable or component is actually rated for, giving a meaningful health reading whether it's testing a low-voltage fixture circuit or a transformer's high-voltage winding insulation — making it one of the most routine tools for verifying that buried and hard-to-access AGL cabling remains safe and functional between visual inspections.
Where Is It Used?
Insulation resistance testing is a critical part of AGL system commissioning, maintenance, and fault diagnosis:
- Installation Commissioning: Testing newly installed cables and equipment to verify insulation integrity before energizing.
- Preventive Maintenance: Periodic testing to detect gradual insulation degradation due to aging, moisture, or thermal stress.
- Fault Diagnosis: Identifying cables with low insulation resistance that may cause nuisance tripping or earth faults.
- Post-Repair Verification: Confirming that cable splices, terminations, and repairs have restored insulation integrity.
- Transformer Testing: Verifying the insulation resistance of isolating transformer windings to detect internal faults.
- Safety Verification: Ensuring that equipment is safe to operate and does not present a shock hazard.
Testing is performed on de-energized circuits, with proper safety precautions including lockout/tagout and discharge procedures.
Storage & Handling
After each test, the cable or equipment under test is fully discharged before the leads are disconnected, since insulation resistance testing can leave a stored charge on the tested conductor that presents a shock hazard if handled immediately. Test leads are coiled loosely and stored in the case's dedicated slots, kept separate from other test leads to avoid confusing high-voltage-rated leads with those from lower-voltage instruments. The instrument is kept dry and within its rated storage conditions, with batteries removed for long periods of disuse, and is protected from the moisture and dust typical of buried cable testing environments during transport between test locations. Calibration is checked against the manufacturer's schedule, particularly given that this instrument's readings are often used as a pass/fail commissioning record where measurement accuracy needs to be demonstrably current.
How Does It Work?
The insulation resistance tester operates by applying a known DC voltage between the conductor under test and ground (or between two conductors). The tester measures the tiny leakage current that flows through the insulation. Using Ohm's Law (R = V/I), it calculates and displays the resistance. Key principles include:
- Test Voltage Selection: The appropriate voltage is selected based on the cable's rated voltage (typically 500V for 600V cables, 1000V for higher ratings).
- Leakage Current Measurement: The tester measures currents in the microampere range to calculate resistance in megohms.
- Timed Tests: Many testers support timed tests (e.g., 1-minute or 10-minute) to assess absorption characteristics and polarization index.
- Automatic Discharge: After testing, the instrument automatically discharges the cable to ensure safety before disconnecting.
- Data Logging: Advanced testers store results with timestamps for trend analysis and reporting.
The polarization index (PI) – the ratio of 10-minute to 1-minute insulation resistance – is a valuable diagnostic parameter that indicates the condition of insulation independent of temperature and humidity variations.
System Architecture & Control Logic
Because the currents being measured are tiny — nanoamps on good insulation — the instrument's architecture is built around keeping that signal clean from generation through to display. A regulated DC-DC converter steps the battery voltage up to the selected test voltage (250 V to 5 kV depending on model) and holds it stable under the near-open-circuit load that healthy insulation presents. The return current flows through a precision, low-noise current-to-voltage converter rather than a simple shunt, since a shunt's own leakage would swamp a nanoamp-scale signal, and this stage is typically guarded — surrounded by a driven shield at the same potential as the sense input — to stop surface leakage across the test leads or terminals from being counted as insulation leakage. The MCU digitises this signal, converts it to resistance via Ohm's law, and for timed tests runs a background clock that samples resistance at 1 minute and 10 minutes to compute the polarization index without operator intervention. When the test ends, the MCU switches the output relay from the HV source to an internal discharge resistor and holds the "test in progress" interlock active until the residual voltage decays to a safe level, only then releasing the leads for handling.
Timed Test & Polarization Index
Physical Construction
An insulation resistance tester is built as a rugged, hand-held instrument housed in an impact-resistant polymer case, typically rated for drop protection and sealed against dust and light moisture ingress for field use around airfield equipment. Internally, a high-voltage DC generator circuit — either battery-powered with a step-up converter or hand-cranked in older designs — produces the selectable test voltage, isolated from the low-voltage measurement and display electronics by internal insulation barriers rated well above the instrument's maximum test voltage. The test leads are heavy-duty, high-voltage-rated cable with insulated probes or crocodile clips, designed to be handled safely at the instrument's full output voltage. The front panel houses a backlit LCD display, rotary or push-button voltage selection, and connection terminals recessed or shrouded to prevent accidental contact during a test. A rechargeable battery pack or replaceable cells power the unit, and the case is typically fitted with a carrying strap or holster along with a soft case or hard shell for transport and storage in a maintenance vehicle.
Technical Parameters
| Parameter | Typical Value |
|---|---|
| Test Voltages | 250V, 500V, 1000V, 2500V, 5000V (selectable) |
| Insulation Resistance Range | 0.1 MΩ to 10 TΩ (depending on voltage) |
| Accuracy | ±5% of reading ± 2 digits (typical) |
| Test Current | 1 mA to 5 mA (short-circuit) |
| Display | LCD with backlight, digital and analog bar graph |
| Polarization Index (PI) | Automatic calculation (10 min / 1 min ratio) |
| Dielectric Absorption Ratio (DAR) | Automatic calculation (60 sec / 30 sec ratio) |
| Data Storage | 1000+ test records with time/date stamp |
| Interface | USB, Bluetooth, or SD card for data export |
| Power Supply | Rechargeable Li-ion battery or 6x AA batteries |
| Safety Rating | CAT III 600V, CAT IV 300V |
| Operating Temperature | -10°C to +50°C |
| Standards Compliance | IEC 61557-2, EN 61010-1, IEEE 43 |
Applicable Standards
IEC 61557-2
Insulation resistance test equipment for low-voltage systems
IEEE 43
Recommended practice for testing insulation resistance of rotating machinery
EN 61010-1
Safety requirements for electrical test and measurement equipment
FAA AC 150/5340-26
Maintenance of Airport Visual Aid Facilities — recommends regular insulation resistance (megger) testing of AGL series circuits (note: AC 150/5345-42 covers light bases/transformer housings and is a different document)
Note on figures: the values on this page are typical reference figures drawn from the standards listed above and from manufacturers’ published data. Requirements differ between ICAO, FAA and national regimes, and between aerodrome code numbers, approach categories and equipment types. Always design, install, commission and maintain to the project specification, the manufacturer’s manual and the standard in force at the aerodrome.
Setup Requirements
Circuit De-Energisation & Isolation
Pre-UseConfirm the circuit or equipment under test is fully isolated and de-energised before connecting test leads.
Test Voltage Selection
Pre-UseSelect the test voltage appropriate to the rating of the cable or equipment being tested.
Lead & Probe Inspection
Pre-UseInspect leads and probes for damage and confirm continuity before connecting to the circuit under test.
Battery/Charge Check
Pre-UseVerify sufficient battery charge for the planned test duration before starting.
Calibration Currency Check
Pre-UseConfirm the instrument's calibration certificate is current before relying on the readings.
Discharge Path Confirmation
Pre-UseConfirm the instrument's built-in discharge function is available and enabled before testing begins.
Testing & Commissioning
Visual & Physical Inspection
Receipt InspectionCheck the instrument on receipt for case damage, cracked test leads, and correct accessory set before it is placed into service.
Battery & Self-Test Verification
Receipt InspectionConfirm battery condition and run the instrument's built-in self-test to verify internal circuitry initialises without fault.
Calibration Verification Against Reference Standard
Calibration VerificationCompare readings against a traceable resistance standard across the instrument's measurement range and confirm results fall within the manufacturer's accuracy specification.
Test Voltage Accuracy Check
Calibration VerificationVerify each selectable test voltage (250 V to 5 kV depending on model) is within tolerance using a calibrated high-voltage reference.
Discharge Function Verification
Functional AcceptanceConfirm the instrument correctly discharges a charged test capacitor after a measurement, since a failed discharge function is a shock hazard in the field.
Test Lead Continuity & Insulation Check
Functional AcceptanceVerify each test lead is continuous end to end and its own insulation is intact, since a compromised lead corrupts every subsequent field measurement.
Documentation & Certification Sign-off
Final AcceptanceRecord calibration results, test voltage accuracy, and next due date on the instrument's calibration certificate before it is released for field use.
Inspection & Maintenance
Calibration Check
AnnualSend the tester for calibration against a traceable resistance standard to confirm accuracy of insulation resistance readings.
Discharge Function Test
QuarterlyVerify the automatic discharge function fully dissipates residual charge after each test to prevent shock or tester damage.
Test Lead Inspection
QuarterlyCheck test leads for damage or exposed wire that could introduce measurement error or a safety hazard.
Battery Check
QuarterlyVerify battery condition, since low voltage can affect the tester's applied test voltage and reading accuracy.
Functional Self-Test
Before each useTest against a known resistance value or the unit's built-in check function before conducting cable insulation tests.
Common Field Issues
Moisture Effects on Readings
High humidity or condensation on cable surfaces can significantly lower insulation resistance readings, leading to false indications of insulation failure. Testing should be performed under dry conditions, or readings should be corrected for humidity using standard correction factors.
Temperature Dependence
Insulation resistance decreases with increasing temperature. For consistent comparison, test results should be corrected to a standard reference temperature (typically 20°C) using temperature correction factors provided by cable manufacturers or standards.
Insufficient Discharge Time
Failing to fully discharge the cable after testing can result in electric shock or damage to the tester. Always allow the tester's automatic discharge function to complete, and verify that the cable is discharged before disconnecting test leads.
Troubleshooting Guide
Insulation Resistance Reading Unexpectedly Low
Reading Does Not Match a Previous Test on the Same Cable
Technician Reports a Shock or the Tester Behaves Erratically After a Test
Failure Modes & Root Causes
Calibration Drift
Incorrect Test Voltage Selection
Test Lead or Probe Contact Degradation
Residual Charge Discharge Failure
Battery / Internal Power Source Degradation
Safety & Operational Notes
High-Voltage Test Precautions
Electrical SafetyTreat the tester's output as a genuine high-voltage source even though its current is limited; keep test leads and probes clear of personnel and confirm the correct test voltage before applying it to a circuit.
Circuit De-Energisation Before Testing
Electrical SafetyConfirm the circuit under test is fully de-energised and isolated from its supply before connecting the tester; insulation testing an energised circuit can damage the instrument and endanger the operator.
Discharge Residual Charge After Test
Electrical SafetyAllow the tester's automatic discharge function to complete, or manually discharge the circuit, before disconnecting leads; cable capacitance can retain a hazardous charge briefly after the test voltage is removed.
Calibration Currency Verification
OperationalCheck the tester's calibration due date before use and do not rely on results from an instrument that is out of calibration for pass/fail compliance decisions.
PPE / Insulated Tools
Personnel SafetyUse insulated gloves and tools rated for the test voltage in use, and follow the site's electrical safety procedures for working near circuits that will be energised for testing.
Engineering & Expert Notes
Always Normalise to Reference Temperature Before Comparing Trends
MethodologyComparing raw megohm readings taken on a cold winter morning against a reading from a hot summer afternoon can look like dramatic insulation degradation when it's actually just temperature effect. Apply the manufacturer's temperature correction before logging any value meant for trend comparison.
Surface Moisture Can Mimic a Genuine Insulation Fault
MethodologyA cable that's electrically sound but has surface condensation or dew on its jacket can produce a misleadingly low reading that looks like real insulation breakdown. Where practical, test in dry conditions or note ambient humidity so a low reading can be correctly attributed before condemning a cable.
Never Skip the Discharge Step, Even Under Time Pressure
SafetyCables and transformer windings retain a stored charge after a megger test that can deliver a genuine shock if the tester's discharge cycle is interrupted or bypassed to save time. Always confirm full discharge before disconnecting leads, regardless of schedule pressure.
Match Test Voltage to the Component, Not a Default Setting
Test VoltageApplying a higher test voltage than a component is rated for — a lamp fixture rated for 500V tested at 2500V, for example — can itself cause insulation breakdown that wasn't present before the test. Confirm rated test voltage for each component type before testing, don't default to the highest available setting.
A Single Low Reading Is Less Useful Than a Declining Trend
TrendingOne low insulation resistance reading can be a measurement artifact from moisture or temperature, but a steadily declining trend across successive tests on the same cable is a much more reliable indicator of genuine insulation degradation. Log and compare against prior results rather than judging each test in isolation.
Record Test Voltage and Duration Alongside Every Result
DocumentationA megohm reading without its test voltage and dwell time recorded is difficult to compare meaningfully against a future test taken under different settings; standardise and log the full test parameters, not just the final number.
Frequently Asked Questions
Short answers about the Insulation Resistance Tester: what it is, how it works, typical specifications, standards, maintenance and common faults.
Q1What is an Insulation Resistance Tester?
An Insulation Resistance Tester (commonly referred to as a megger or megohmmeter) is a test instrument that applies a high DC voltage to an electrical cable or equipment and measures the resulting leakage current to determine the insulation resistance.
Q2What is the purpose of an Insulation Resistance Tester?
The insulation resistance tester exists to catch the kind of insulation breakdown — from moisture ingress, mechanical damage, or age-related degradation — that won't necessarily show up as an obvious fault yet, but represents a developing safety and reliability risk. A cable with insulation resistance quietly dropping over months of service isn't causing problems yet, but it's heading toward a leakage path or short that eventually will, and this test is what catches that trend early.
Q3Where is an Insulation Resistance Tester used?
Insulation resistance testing is a critical part of AGL system commissioning, maintenance, and fault diagnosis: Installation Commissioning: Testing newly installed cables and equipment to verify insulation integrity before energizing. Preventive Maintenance: Periodic testing to detect gradual insulation degradation due to aging, moisture, or thermal stress. Fault Diagnosis: Identifying cables with low insulation resistance that may cause nuisance tripping or earth faults. Post-Repair Verification: Confirming that cable splices, terminations, and repairs have restored insulation integrity.
Q4How does an Insulation Resistance Tester work?
The insulation resistance tester operates by applying a known DC voltage between the conductor under test and ground (or between two conductors). The tester measures the tiny leakage current that flows through the insulation. Using Ohm's Law (R = V/I), it calculates and displays the resistance. Key principles include: Test Voltage Selection: The appropriate voltage is selected based on the cable's rated voltage (typically 500V for 600V cables, 1000V for higher ratings).
Q5How should an Insulation Resistance Tester be stored and handled?
After each test, the cable or equipment under test is fully discharged before the leads are disconnected, since insulation resistance testing can leave a stored charge on the tested conductor that presents a shock hazard if handled immediately. Test leads are coiled loosely and stored in the case's dedicated slots, kept separate from other test leads to avoid confusing high-voltage-rated leads with those from lower-voltage instruments.
Q6What are the typical technical parameters of an Insulation Resistance Tester?
Typical values are — Test Voltages: 250V, 500V, 1000V, 2500V, 5000V (selectable); Insulation Resistance Range: 0.1 MΩ to 10 TΩ (depending on voltage); Accuracy: ±5% of reading ± 2 digits (typical); Test Current: 1 mA to 5 mA (short-circuit); Display: LCD with backlight, digital and analog bar graph; Polarization Index (PI): Automatic calculation (10 min / 1 min ratio); Dielectric Absorption Ratio (DAR): Automatic calculation (60 sec / 30 sec ratio); Data Storage: 1000+ test records with time/date stamp; Interface: USB, Bluetooth, or SD card for data export; Power Supply: Rechargeable Li-ion battery or 6x AA batteries; Safety Rating: CAT III 600V, CAT IV 300V.
Q7Which standards apply to an Insulation Resistance Tester?
The main references are IEC 61557-2 — Insulation resistance test equipment for low-voltage systems; IEEE 43 — Recommended practice for testing insulation resistance of rotating machinery; EN 61010-1 — Safety requirements for electrical test and measurement equipment; FAA AC 150/5340-26 — Maintenance of Airport Visual Aid Facilities — recommends regular insulation resistance (megger) testing of AGL series circuits (note: AC 150/5345-42 covers light bases/transformer housings and is a different document). Always verify the current edition and your aerodrome's approved requirements.
Q8How often should an Insulation Resistance Tester be inspected and maintained?
Typical maintenance tasks and intervals are: Calibration Check (annual); Discharge Function Test (quarterly); Test Lead Inspection (quarterly); Battery Check (quarterly); Functional Self-Test (before each use). Your airport's approved maintenance programme takes precedence.
Q9How do I troubleshoot common Insulation Resistance Tester faults?
Common symptoms covered are: Insulation Resistance Reading Unexpectedly Low; Reading Does Not Match a Previous Test on the Same Cable; Technician Reports a Shock or the Tester Behaves Erratically After a Test. For example, for "Insulation Resistance Reading Unexpectedly Low": Likely Cause: High humidity or surface condensation on the cable was present at the time of the test, artificially depressing the reading. Diagnostic Steps: Check ambient humidity and cable surface condition at the time of test; repeat the test under dry conditions or apply the standard humidity correction factor. Corrective Action: Retest once conditions are dry, or apply the appropriate correction factor before comparing the result against acceptance criteria.
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