AGL-CS-019 — Very High vs Low IR Test Result Investigation
The AGL Knowledge Hub | Engineering Case Study Series. A diagnostic-interpretation case where two circuits produced dramatically different IR readings while both remained operational — from test verification through trend analysis to condition-based decision-making.
Important: This is an engineering simulation created for training. It is not presented as an actual airport incident. All readings, values, dates and outcomes are illustrative, and no airport, organisation, person or manufacturer is identified or implied.
1Case Study Overview
During a preventive maintenance program, two runway edge lighting circuits were tested using a 5 kV insulation resistance tester.
Circuit A produced an insulation resistance reading of greater than 1000 MΩ, while Circuit B produced a reading of 6 MΩ.
Surprisingly, both circuits were operating normally, and neither CCR reported an operational fault.
The maintenance team debated whether the very high reading indicated a perfect circuit and whether the low reading automatically required immediate cable replacement.
A detailed investigation revealed that IR values must always be interpreted together with historical trends, environmental conditions, circuit configuration, and sectional testing results.
This case demonstrates that the interpretation of an IR reading is often more important than the reading itself.
2Learning Objectives
After completing this case study, the learner should be able to:
- Correctly interpret very high and low insulation resistance readings.
- Understand the limitations of a single IR measurement.
- Use historical IR trends for condition assessment.
- Differentiate healthy insulation from misleading measurements.
- Apply sectional IR testing.
- Evaluate transformer and cable insulation separately.
- Develop condition-based maintenance decisions.
3System Background
Insulation resistance testing evaluates the integrity of:
- Underground primary cables.
- Cable joints.
- Connectors.
- Isolating transformers.
- Associated insulation systems.
The test measures leakage current through insulation.
A simplified relationship is:
However, context is essential.
4Problem Statement
The reported condition was:
- Circuit A: Greater than 1000 MΩ.
- Circuit B: 6 MΩ.
- Both circuits operational.
- CCR healthy.
- No visible cable damage.
- Testing performed during dry weather.
Which circuit presents the greater reliability risk, and how should each IR result be interpreted?
5Initial Observations
| Parameter | Circuit A | Circuit B |
| CCR status | Healthy | Healthy |
| Circuit operation | Normal | Normal |
| IR result | Greater than 1000 MΩ | 6 MΩ |
| Historical IR | Greater than 1000 MΩ | 450 MΩ six months earlier |
| Weather | Dry | Dry |
The historical data immediately became the most important factor.
6Initial Engineering Hypotheses
The maintenance team considered:
Circuit A is perfect.
Circuit B requires immediate replacement.
Circuit B has progressive insulation deterioration.
Circuit A may require additional verification.
Trend analysis is required before conclusions.
7Understanding IR Interpretation
A useful engineering interpretation is:
| Greater than 1000 MΩ | Generally healthy insulation |
| 200–1000 MΩ | Acceptable with trend monitoring |
| 50–200 MΩ | Early deterioration possible |
| 10–50 MΩ | Significant deterioration |
| Less than 10 MΩ | Serious insulation degradation |
However, the rate of change is often more important than the absolute value.
8Test Equipment
The investigation used:
- 5 kV insulation resistance tester.
- True-RMS multimeter.
- Approved AGL circuit test equipment.
- Transformer isolation equipment.
- Circuit drawings.
- Historical maintenance records.
- Airport-approved PPE.
9Investigation
Step 1: Verify Test Procedure
Both circuits were tested under identical conditions. The measurements were confirmed as valid.
Step 2: Review Historical Trends
Circuit A
| 12 months earlier | Greater than 1000 MΩ |
| 6 months earlier | Greater than 1000 MΩ |
| Current | Greater than 1000 MΩ |
Circuit A showed stable insulation behavior.
Circuit B
| 12 months earlier | Greater than 1000 MΩ |
| 6 months earlier | 450 MΩ |
| 3 months earlier | 120 MΩ |
| Current | 6 MΩ |
Circuit B showed rapid deterioration.
Step 3: Sectional Isolation Testing
Circuit B was divided into sections.
Results:
| CCR–S1 | Greater than 500 MΩ |
| S1–S2 | 8 MΩ |
| S2–S3 | Greater than 500 MΩ |
The deterioration was localized.
Step 4: Transformer Isolation
Transformers in the affected section were disconnected. The IR remained low. This indicated that the primary cable section was the dominant source of insulation deterioration.
Step 5: Cable Investigation
Excavation found:
- Moisture contamination.
- Damaged cable sheath.
- Insulation discoloration.
- Early electrical tracking.
Circuit B had progressive underground cable deterioration.
10Root Cause
Circuit A represented a stable healthy insulation system.
Circuit B represented progressive moisture-assisted cable insulation deterioration.
The low IR reading was important not because it was low, but because it had fallen dramatically over time.
11Corrective Action
Circuit A
- No replacement required.
- Continue routine monitoring.
- Maintain historical trend records.
Circuit B
- Isolate affected section.
- Replace deteriorated cable.
- Install new approved cable joints.
- Improve drainage.
- Perform post-repair IR testing.
- Resume trend monitoring.
12Post-Repair Verification
| Item | Circuit A | Circuit B |
| CCR operation | Normal | Normal |
| Circuit continuity | Normal | Normal |
| Insulation resistance | Greater than 1000 MΩ | Greater than 1000 MΩ |
| Maintenance documentation | Updated | Updated |
Circuit B returned to healthy insulation values after cable replacement.
13Root-Cause Analysis
Low IR detected.
Valid measurement.
Rapid deterioration identified.
Fault section localized.
Moisture damage confirmed.
Progressive cable insulation deterioration.
Replace affected cable section.
14Why This Case Was Important
A common maintenance mistake is making decisions based only on the latest IR reading.
Correct engineering practice considers:
- Historical trend.
- Rate of deterioration.
- Environmental conditions.
- Circuit configuration.
- Sectional localization.
- Associated electrical symptoms.
A stable 100 MΩ circuit may be healthier than a rapidly declining 300 MΩ circuit.
15Engineering Lessons
A single IR reading should never be interpreted in isolation.
A single IR reading should never be interpreted in isolation.
Trend analysis is the most powerful insulation diagnostic tool.
Trend analysis is the most powerful insulation diagnostic tool.
Rapid deterioration is often more significant than the absolute value.
Rapid deterioration is often more significant than the absolute value.
Sectional testing should always follow abnormal IR results.
Sectional testing should always follow abnormal IR results.
Condition-based decisions require both measurement and history.
Condition-based maintenance decisions require both measurement and history.
16Engineer's Decision Point
You observe: Circuit A: Greater than 1000 MΩ. Circuit B: 25 MΩ. Circuit B was 400 MΩ three months earlier. What should you do?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about interpreting very high and low insulation resistance results, based on this case study.
Q1What is AGL-CS-019?
AGL-CS-019 is an engineering training case study on interpreting very high versus low insulation resistance readings from two operational runway edge lighting circuits. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2What were the two IR results?
Both circuits were tested with a 5 kV insulation resistance tester. Circuit A read greater than 1000 MΩ and Circuit B read 6 MΩ, yet both circuits were operating normally and neither CCR reported a fault.
Q3Does a very high IR reading mean a circuit is perfect?
Not automatically. Circuit A represented a stable, healthy insulation system that needed no replacement, only routine monitoring and trend records, but a reading always needs context.
Q4Does a low IR reading always require immediate replacement?
No. Circuit B was important not because the reading was low, but because it had fallen dramatically over time. IR values must be interpreted together with historical trends, environmental conditions, circuit configuration and sectional testing.
Q5Why should a single IR reading not be read in isolation?
A single IR reading should never be interpreted in isolation. A trend of rapid decline is the critical diagnostic factor and shows deterioration that one snapshot cannot.
Q6What was done for each circuit?
Circuit A required no replacement, with routine monitoring and trend records continuing. For Circuit B the team isolated the affected section, replaced the deteriorated cable, installed new approved joints, improved drainage, tested post-repair IR and resumed trend monitoring.
Q7What was the root cause for Circuit B?
Progressive moisture-assisted cable insulation deterioration. After cable replacement, Circuit B returned to greater than 1000 MΩ.
Q8What should you do if Circuit B reads 25 MΩ after being 400 MΩ three months earlier?
Investigate Circuit B immediately using sectional insulation testing. The rapid decline is the critical diagnostic factor, and ignoring or replacing both circuits is not the right response.
Engineering & Expert Notes
Compare Circuits Like With Like
InterpretationLength, age and temperature affect insulation values. Compare against similar circuits or the same circuit's history.
Look at the Rate of Change
TrendingA rapid fall is more concerning than a stable low value. Plot readings over time.
Test in Sections After an Abnormal Result
Follow-UpIsolate the portion responsible before making repair decisions.
Keep Raw Readings and Method Notes
RecordsTest voltage, duration and conditions are part of the result. Without them readings cannot be compared.
20Case Study Conclusion
The value of an insulation resistance test lies not only in the numerical reading but in its trend, location, and relationship to the operating condition of the AGL circuit.
Systematic historical analysis, sectional testing, transformer isolation, and cable investigation provide a far more accurate assessment of insulation health than a single measurement alone.
In AGL engineering, the trend of insulation resistance is often the most reliable predictor of future circuit failure.
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Please read before relying on any content on this page or on any record downloaded from it.
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- Fictional training simulation. This case study is an engineering simulation created for training. It is not a report of an actual airport incident. Scenarios, sequences, dates, readings, measurements, outcomes and numerical values are illustrative and invented for teaching. No fault, failure, cause or liability is attributed to any person, company, airport, regulator, manufacturer or product, and any resemblance to a real event is coincidental. The content is not a forensic, legal, insurance, regulatory or accident-investigation finding and must not be cited as evidence, as a record of an event, or as a basis for disciplinary, contractual, insurance or legal action.
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