INSULATION RESISTANCE VS CONTINUITY
Two complementary electrical tests used across AGL circuits — an Insulation Resistance (IR) test checks whether insulation is electrically sound and preventing unwanted current leakage, while a Continuity test checks whether the intended conductor path is electrically complete. Neither test can replace the other.
At a Glance
| Feature | Insulation Resistance Test | Continuity Test |
|---|---|---|
| Main purpose | Check insulation integrity | Check conductor continuity |
| What it detects | Leakage paths / insulation deterioration | Open circuits / broken conductors |
| Test principle | Applies DC test voltage and measures resistance | Applies a small test current/voltage and checks resistance/path |
| Typical instrument | Insulation resistance tester (Megger) | Continuity tester / multimeter / low-resistance ohmmeter |
| Result | Resistance, usually in MΩ/GΩ | Resistance, usually in Ω |
| Desired result | High resistance | Low resistance |
| Open circuit | May show high IR, but doesn't prove continuity | Detected |
| Insulation failure | Detected | Not reliably detected |
| Typical AGL application | Primary series-circuit cable insulation | Cable/connection continuity |
Insulation Resistance
The test checks the resistance between conductors and earth/ground, or between conductors, as applicable.
CONDUCTOR
───────────────
│
│ INSULATION
│
───────────────
EARTH / SCREEN
↓
APPLY DC TEST
VOLTAGE
↓
MEASURE RESISTANCE
↓
HIGH Ω = GOOD
The higher the measured insulation resistance, generally, the better the insulation condition, subject to the applicable acceptance criteria.
Continuity
Continuity checks whether the intended electrical path is complete.
CABLE
A ───────────────── B
│
▼
TEST PATH
│
▼
LOW RESISTANCE
↓
CONTINUITY ✓
A broken conductor instead gives an open circuit: A ────✕──── B → OPEN CIRCUIT.
High Resistance vs Low Resistance
This is the easiest distinction to remember.
GOOD INSULATION
↓
HIGH IR
↓
LOW LEAKAGE
GOOD CONTINUITY
↓
LOW RESISTANCE
↓
COMPLETE PATH
Easy memory: Insulation Resistance wants a high number. Continuity wants a low number.
Applying Both Tests to a Primary Cable
Consider an AGL primary series cable running from the Constant Current Regulator (CCR) to an isolation transformer:
CCR │ ═════════════════ PRIMARY CABLE ═══════════════ │ ISOLATION TRANSFORMER
Two different tests can be performed on this same cable.
Continuity Test
CCR ───────────────────── Transformer
COMPLETE PATH?
Result: low resistance / continuous path.
Insulation Resistance Test
PRIMARY CONDUCTOR
══════════════════
│
│ INSULATION
│
▼
EARTH
Result: high resistance between the conductor and earth, according to the specified test method and acceptance criteria.
Fault Coverage Comparison
| Condition | Continuity Test | IR Test |
|---|---|---|
| Broken conductor | ✅ | Not its primary purpose |
| Open circuit | ✅ | Not its primary purpose |
| Good conductor path | ✅ | Not its primary purpose |
| Insulation deterioration | ❌ | ✅ |
| Leakage to earth | ❌ | ✅ |
| Short between conductors | May indicate low resistance | Can help identify insulation failure |
| Poor insulation | ❌ | ✅ |
Two Failure Cases
Continuity fails immediately. IR may show high resistance because the circuit is open — but this does not mean the cable insulation is good.
A passing IR reading on an open circuit is meaningless; the two tests cannot replace each other.
The conductor may still be fully continuous, so a continuity test can pass. IR drops low because current leaks through the damaged insulation to earth.
Continuity can pass while insulation resistance fails — this is exactly why both tests are performed.
Testing a Full Series Circuit
CCR
│
▼
════════ PRIMARY CABLE ════════
│
▼
ISOLATION
TRANSFORMER
│
▼
LIGHT
Continuity Test
Verifies the intended electrical path: CCR → Cable → Transformer → Circuit Path.
IR Test
Verifies insulation integrity: Primary conductor ↕ Earth, or between appropriate conductors, depending on the approved test procedure.
Megger vs Multimeter
IR Test
Typically uses an Insulation Resistance Tester, commonly called a Megger. The tester applies a specified DC test voltage and measures insulation resistance.
Continuity Test
- Multimeter
- Continuity tester
- Low-resistance ohmmeter
The instrument checks whether the electrical path has sufficiently low resistance.
Low Test Current vs Higher DC Test Voltage
Continuity
Normally uses a low test voltage/current intended to establish whether a conductive path exists.
Insulation Resistance
Uses a higher DC test voltage specifically to stress and evaluate the insulation system.
Important: the actual IR test voltage must be selected according to the equipment/cable rating, applicable standard, manufacturer instructions, and approved project test procedure. Do not select an arbitrary Megger voltage.
Working Safely With an IR Tester
An IR tester can apply a significant test voltage.
ISOLATE EQUIPMENT
↓
VERIFY DE-ENERGIZED
↓
DISCONNECT SENSITIVE EQUIPMENT
↓
PERFORM IR TEST
↓
DISCHARGE CABLE
↓
VERIFY SAFE CONDITION
For AGL circuits, particular care is required because CCRs, transformers, monitoring equipment, surge protection, and other connected components may have specific test requirements. Follow the approved AGL test procedure before applying an IR test voltage to an installed circuit.
A Typical Commissioning Sequence
VISUAL INSPECTION
↓
CONTINUITY TEST
↓
INSULATION RESISTANCE TEST
↓
FUNCTIONAL TEST
↓
RECORD RESULTS
The actual sequence should follow the project's approved commissioning/maintenance procedure.
Think of a Water Pipe
Is the pipe physically
connected end to end?
A ═════════════════ B
CONTINUOUS
Is the pipe wall
preventing water leaking out?
A ═════════════════ B
│
│ NO LEAKAGE
↓
GOOD
So: continuity checks the intended path. Insulation resistance checks unwanted paths.
Full Engineering Parameters
| Parameter | Insulation Resistance | Continuity |
|---|---|---|
| Main question | Is insulation healthy? | Is the path complete? |
| Desired resistance | High | Low |
| Unit | MΩ / GΩ typically | Ω |
| Test voltage | Higher DC test voltage | Low test voltage/current |
| Main fault | Insulation breakdown/leakage | Open circuit/broken conductor |
| Instrument | IR tester (Megger) | Multimeter/continuity tester |
| Cable application | Very important | Very important |
| Can one replace the other? | No | No |
Insulation Resistance vs Continuity
Insulation Resistance measures the resistance of the insulation system to confirm that unwanted leakage/current paths are sufficiently restricted.
Continuity verifies that the intended conductive path is complete and has acceptably low resistance. For AGL circuits, a broken primary cable is found with a continuity test, while damaged primary cable insulation is found with an IR test. Both tests are complementary, not alternatives.
A cable can pass continuity and still have failed insulation, and it can have excellent insulation while being completely broken. Always perform and record both tests as part of the approved AGL commissioning/maintenance procedure.
Engineering & Expert Notes
Run Both on Every Circuit Section
ScopeA circuit can be continuous and leaking, or well insulated and open. One result never substitutes for the other.
Discharge Circuits After Insulation Tests
SafetyTest voltage can leave stored charge. Follow the approved discharge procedure before touching conductors.
Trend Insulation Values Over Time
TrendingThe direction of change often matters more than a single reading.
Record Conditions With Each Reading
RecordsTemperature, humidity, test voltage and duration belong in the record.
Frequently Asked Questions
Insulation resistance testing checks whether the insulation around a conductor is preventing unwanted current leakage, usually to earth. Continuity testing checks whether the intended conductor path itself is electrically complete, end to end.
Yes. A cable with damaged insulation can still have a fully continuous conductor, so continuity passes while the IR test reveals the leakage path caused by the damaged insulation.
Not reliably. An open circuit can show a high IR reading simply because the circuit is not connected, which does not confirm that the insulation itself is in good condition.
Insulation resistance testing typically uses an insulation resistance tester, commonly called a Megger. Continuity testing typically uses a multimeter, a dedicated continuity tester, or a low-resistance ohmmeter.
The IR test voltage must be selected according to the equipment/cable rating, the applicable standard, manufacturer instructions, and the approved project test procedure — never an arbitrary Megger voltage.
A broken primary cable is identified by a continuity test, while damaged primary cable insulation is identified by an IR test. Since neither fault type is reliably detected by the other test, both are performed as complementary checks.
Governing Documents
For an actual project design or commissioning activity, always verify the current edition of the applicable documents and the approved project test procedure.
FAA AC 150/5340-26 / Related AGL Specs
Airfield lighting equipment and circuit specifications referencing acceptance testing, including insulation resistance and continuity checks for primary and secondary circuits.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — general electrical reliability and maintenance expectations for visual aids, supported by IR and continuity testing during commissioning.
DGCA / AAI Requirements
For Indian airport projects, also verify applicable DGCA requirements, AAI requirements, airport-specific technical specifications, project drawings and approved test procedures.
Final Engineering Note
Do not select an IR test voltage from a generic web reference. The actual test method, voltage, and acceptance criteria must always follow the current governing standard, cable/equipment rating, and approved project test procedure.
Insulation resistance is not a substitute for continuity, and continuity is not a substitute for insulation resistance — each answers a different engineering question about the same circuit.
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